9 GAS MEASUREMENTS

9.1 PROPERTIES OF PERTOLEUM

9.1.1 True vapour pressure (TVP)
All petroleum products and crude oil are essentially mixtures of a wide range
of hydrocarbon compounds. The boiling points of the compounds range from
162oC
(methane) to well in excess of +400oC, and the volatility of any particular
mixture of compounds depends primarily on the quantities of the more volatile
elements. The volatility is characterised by the vapour pressure. When
transferring a petroleum product to a gas-free tank it begins to vaporise, that
is, it liberates gas into the space above it. This gas has also a tendency to
re-dissolve in the liquid. The pressure exerted by this gas is called the
equilibrium pressure of the liquid, usually referred to simply as the vapour
pressure.
The vapour pressure of a pure compound depends only upon its temperature. With
a mixture of compounds, the vapour pressure depends on the temperature,
elements and the volume of the gas space in which vaporisation occurs.
The true vapour pressure (TVP) or bubble point vapour pressure is the
equilibrium of vapour pressure of a mixture when the gas/liquid ratio is
effectively zero. The highest vapour pressure is possible at any specified
temperature. As the temperature of a petroleum mixture increases, its TVP also
increases. If the TVP exceeds atmospheric pressure, the liquid begins to boil.
The TVP of a petroleum mixture gives a good indication of its ability to give
rise to gas, but unfortunately it is a property which still is extremely difficult
to measure.


9.1.2 The Reid Vapour Pressure (RVP)
Testing is a simple and generally used method for measuring the volatility of
petroleum liquids. Measurement of the RVP is conducted at 37,8oC (100oF). The
greater the RVP value, the more volatile is the oil. Normally crude oil has a
RVP of between 0,1 and 0,8kg/cm2.
A sample of liquid is put into the test container at atmospheric pressure. The
volume of liquid should be one fifth of the containerłs total volume. Then the
container is sealed and immersed in a water bath, which is heated to 37,80C.
The container is then shaken in order to mix the liquid properly and the rise
in pressure due to vaporisation can be read on the attached pressure gauge.
This pressure gauge gives a close approximation in bars.
Because the liquidłs vapour pressure is at 37,8oC, RVP is useful for generally
comparing the volatility of a wide range of petroleum liquids. However, it has,
small value as a means of estimating the likeliness of gas evolution in
specific situations, mainly because the measurement is made at the standard
temperature of 37,8oC and at a fixed gas/liquid ratio. For this purpose TVP is
much more useful. As mentioned, in some cases, correlation exists between TVP,
RVP and temperature.
For safety measures against fire on ships, the Norwegian Maritime Directorate
in the Regulation of December 3rd1979 uses 61oC as limit value for flash point
and 2,8kg/cm2 for vapour pressure at 37,8oC. The oil referred to in this
regulation is:

· Mineral oils with a flash point below 61oC, such as kerosene, benzene,
gasoline and crude oil or other flammable liquids with a flash point below said
limit.

· Mineral oils with a flash point of 61oC or higher, such as marine gas oils,
fuel oil, diesel oil, lubricating oil, which give off flammable gases when
heated.

· Oils and fats of animal or vegetable origins, such as whale oil, groundnut
oil, linseed oil etc., which give off flammable gases when heated.

The liquid chemicals referred to are:
· Chemicals with an absolute vapour pressure lower than 2,8kg/cm2 at 37,8oC.

The condensed gases referred to are:
· Chemicals with an absolute vapour pressure of 2,8kg/cm2 or higher at 37,8oC.

9.1.3 Flash Point
The flash point for an oil product is the temperature at which it is possible to
ignite the vapour above the liquid. In other words, the flammable gas
concentration above the liquid is close to the lower explosive limit.
Determination of the flash point is done with a special apparatus and according
to specific rules.
A sample of liquid is gradually heated in a special pot and a small flame is
repeatedly and momentarily applied to the surface of the liquid. The
temperature is recorded when a small flame initiates a flash or flame across
the liquid surface, thereby indicating the presence of a flammable gas.
In this test the space above the liquid is kept closed except for the brief
moments when trying ignite the liquidÅ‚s surface. This test is called “Closed
cup Flash Point".
When we do the test with the liquid surface permanently open to the atmosphere,
the result of such a test is called “Open cup Flash Point".
Because of the greater loss of gas to the atmosphere in the open cup test the
open cup flash point is always a little higher (about 6oC) than the closed cup
flash point. The restricted loss of gas in the closed cup apparatus also leads
to a much more consistent result than can be obtained in open cup testing. For
this reason, the closed cup method is generally favoured. However, open cup
test figures, still may be found in the registration of various national
administrations, in classification society rules and other such documents.
If the temperature is increased further beyond the flash point, the liquid will
obtain a temperature so high that the evaporation will take place fast enough
to support a flame. This is called “The Burning Point"“.


9.1.4 Burning Point of some hydrocarbons






Product  
Burning point in degree Celsius



Asphalt
+204



Benzene
-50



Benzene 
-11



Butane
-35



Crude oil
-10/+30



Diesel oil
+70



Ethan
-125



Fuel oil (no. 1&2)
+38



Fuel oil (no.4&5)
+54



Fuel oil (no.6) 
+65



Hexane
-28



Methane (LNG)
-175



Mineral oil 
+193



Naphtha (mixtures)
+38/+60



Paraffin wax
+320



Pentane 
-40



Propane
-105



Lub.oil (motor oil)
-149/+232



Propylene
-108



Ethylene
-150





For refined products, the flash point increases from light to the heavy
hydrocarbons, for gasoline it is about
50oC and for kerosene over +60oC. The
flash point for liquids is used in rules and regulations for transportation and
storage.
Crude oil from various sources may have quite different flash points, usually
between
10oC and +30oC.

9.1.5 Flammability
The burning process means that hydrocarbon gases react with the oxygen in the
air to produce carbon dioxide and water. This reaction gives enough heat to
form a flame which goes through the mixture of hydrocarbon gas and air. When
the gas above a liquid hydrocarbon is ignited, the heat that is produced is
usually enough to evaporate sufficient fresh gas to maintain the flame and the
liquid is said to burn. In fact, it is the gas that is burning and continuously
being replenished by the liquid.
9.1.6 Flammable Limits
A hydrocarbon gas mixture and air cannot be ignited and burned unless its
composition lies within a range of gas-in-air concentrations, known as the
“flammable range".

The lower limit of this range is known as the “LEL" (lower explosive limit).
The “LFL" (lower flammable limit) is also used. This level means that
hydrocarbon concentration has an insufficient amount of hydrocarbon gas to
support and propagate combustion. The mixture is “too lean".

The upper limit of the range known as the “UEL" (upper explosive limit), or
also known as “UFL" (upper flammable limit). This level means that the
hydrocarbon concentration has an insufficient amount of air to support and
propagate combustion. The mixture is “too rich".

Between these two areas, the mixture is flammable and results in a fire or
explosion, if ignited. With hydrocarbon gases from crude and sediments, it is
usually assumed that the upper explosion limit lies at about 10% by volume of
hydrocarbon gas-in-air and the lower explosion limit at about 1% by volume of
hydrocarbon gas.

9.1.7 Explosion limits in % flammable gas in mixture with air.

                              
                                      





PRODUKT
LEL, volume %
UEL, volume %



Methane (LNG) 
5,3
14,0



Ethan
3,1
12,5



Propane
2,1
9,5



Butane
1,5
9,5



Pentane
1,5
7,8



Hexane
1,2
7,5



Hepthane
1,2
6,7



Octane
1,0
3,2



Nonthane
0,8
2,9



Dechane 
0,8
5,4



Hydrogen
4,0
75,0



Hydrogen sulphide
4,3 
45,0



Carbon monoxide
12,5
74,0



Crude oil 
1,5/2,5
8,0/11,0



Benzene
1,4
7,6



Naphtha
0,9/1,1
6,0/6,7



Propylene
2
12



Ethylene  
2,5
34



VCM
4
31






9.1.8 Air
The mixture of gases found in the atmosphere is given the name air. The ratio
of mixture between various gases is the same, independent of time and place,
except for the water vapour content, which can have great variations.






ELEMENTS in air 





Nitrogen, N2 

78,09%




Oxygen, O2

20,93%




Argon, A

0,93%




Carbon dioxide, CO2

0,03%




Other gases

0,02%




AIR

100%






There may be a significant amount of water vapour in the air. Different results
are measured depending on whether water or moisture is removed or not. The
amount of water vapour, which the air may contain, will depend very much on the
temperature. The air is saturated with water vapour when the air contains a
maximum amount of water vapour at a certain temperature. Saturated air being
cooled will release the excess water in droplets.
At high humidity and high temperature, there will be a reduction of oxygen and
other gases that is caused by the increased water vapour content.
The atmospheric pressure will influence the measurement result when using gas
measure instruments. For example, when using a portable oxygen analyser that is
calibrated to read 21% oxygen by volume in clean air at atmospheric pressure,
the reading will increase as the atmospheric pressure increases.
To compensate for the changes in atmospheric pressure, the instrument has to be
calibrated with clean air from time to time. The instruments used for measuring
hydrocarbon gases will also be influenced by the atmospheric pressure,
depending on the instrumentłs measuring principle.






TEMPERATURE

WATER VAPOUR CONTENT




-200C

0,1 volume %




00C

0,9 volume %




200C 

2.3 volume %




400C

7,3 volume %




600C 

19,7 volume %




800C

46,7 volume %




1000C

100 volume %







The risk of fire or explosion is drastically increased if air is replaced by
pure oxygen. As known, oxygen leakage during welding has resulted in several
fatal accidents. However, when reducing the oxygen below 21% by volume, the
fire and explosion hazard is reduced. When reducing the oxygen content to below
10,8% by volume, fire and explosion cannot take place even though both
hydrocarbon gas and ignition sources are present.
9.1.9 Hydrocarbon gases
Crude oil is formed from plants and animals residues and contains several
thousand different chemical compounds. Most of these materials consist of only
the element hydrogen (H) and carbon (C) called by the common name hydrocarbons.
The simplest hydrocarbon is methane, which is the main element of natural gas.
Butane, propane and ethane are also composed of hydrogen and carbon atoms and
they are all called hydrocarbon gases. For example butane, C4H10 means that
this gas contains a total of 4 carbon atoms and 10 hydrogen atoms. Hydrocarbons
with up to 4 carbon atoms are liquefied gases at room temperature and atmospheric
pressure. From 5 to 16 carbon atoms the hydrocarbons are liquids, and above 16
carbon atoms, the hydrocarbons are solid materials such as wax and asphalt.
When the crude oil is taken out of a well, hydrocarbon gases and solid
materials are dissolved in the oil. When reducing the pressure, gases will
bubble out. To separate these liquefied gases the crude must pass through one
or more processing units (stabilisers). The crude oil is called “stabilised
crude", but even stabilised crude oil will give off hydrocarbons from the
surface.
Methane gas is lighter than air. Ethane gas has approximately the same density
as air. The gases butane and propane from higher hydrocarbons are heavier than
air. The gas mixtures given off from crude oil, sludge and sediments are all
heavier than air. Until such gas mixtures have been mixed with air inside inert
gas, the highest hydrocarbon concentration will appear near the bottom.
“Spiked crude oil" (also called “enriched" or “tailored" crude) is crude oil,
which has had hydrocarbons, added in gas or liquid form. The spiked crude may
contain rather large amounts of added hydrocarbons and therefore emit heavy
gasses under certain conditions (during loading, crude oil washing,
discharging).



9.2 TOXICITY HAZARDS
The toxic hazards to which personnel are exposed in tanker operations arise
almost entirely from exposure to various kinds of gasses.

TLV (Threshold Limit Value) has been in use within the industry for a number of
years, and is often expressed as a “Time Weighted Average" (TWA). The use of
the term “PEL" (Permissible Exposure Limit) is becoming more commonplace and
refers to the maximum exposure to a toxic substance that is allowed by an
appropriate regulatory body.

The PEL is usually expressed as a Time Weighted Average, normally averaged over
an eight hour period, or as a “Short Term Exposure Limit" (STEL), normally
expressed as a maximum airborne concentration averaged over a 15 minute period.
The values are expressed as parts per million (ppm) by volume of gas in air.

1 ppm corresponds to one-millionth part by volume pollution in air. Compared
with a value quoted in percent by volume, we find that 1% by volume = 10000
ppm.

List of TLV (PEL) are adjusted from time to time, so take into consideration
the experience gained. Keep the list up to date at all times.

9.2.1 Ingestion
There is a very slight risk of swallowing significant quantities of liquids
during normal tanker operations. The oral toxicity from petroleum is low, but
if swallow it causes acute discomfort. Liquid petroleum may be drawn into the
lungs during vomiting resulting in serious consequences.

9.2.2 Skin contact
Petroleum products cause skin irritation and remove essential oils from the
skin, leading to dermatitis. Oil can also cause serious skin disorders from
repeated and prolonged contact. The effects of a gas mixture from crude oil
include headache, eye irritation, reduced sense of responsibility and dizziness
similar to drunkenness. Higher concentrations may lead to paralysis, numbness
and death.

To avoid direct contact, always wear appropriate protective clothing and
equipment!!!!! 

9.2.3 Petroleum gases
The toxicity of petroleum gases has a wide variation depending on the major
hydrocarbon constituent of the gas. For a short period of time the human body
can tolerate a somewhat higher concentration than the corresponding TLV.
Toxicity can greatly be influenced by the presence of some minor compounds,
such as benzene and hydrogen sulphide.
A TLV of 300ppm, corresponding to about 2% LEL is established for gasoline
vapours. Such a figure may be used as a general guide for petroleum gases, but
must not be used for gas mixtures containing benzene or hydrogen sulphide.

The following are typical effects found at higher concentrations:






Concentration

% LEL

Effects




0,1% vol. (1.000ppm)

10%

Irritation of the eyes within one hour.




0,2% vol. (2.000ppm)

20%

Irritation of the eyes, nose and throat, dizziness and
unsteadiness within half an hour.




0,7% vol. (7.000ppm)

70%

Symptoms as of drunkenness within 15 minutes.




1.0% vol. (10.000ppm)

100%

Rapid onset of “drunkenness" which may lead to
unconsciousness and death if exposure continues.




2,0% vol. (20.000ppm)


Paralysis and death occur very
rapidly







9.2.4 Benzene
Aromatic hydrocarbons include benzene, toluene and xylene. These substances can
be found in varying amounts, in many typical petroleum cargoes, such as
gasolinełs, naphtas, special boiling point solvents, turpentine, substitutes,
white spirits and crude oil.

Benzene primarily presents an inhalation hazard. It has poor warning qualities.
Benzene can be absorbed through the skin and is toxic if ingested.

For handling cargo that contains benzene, use the described operation
procedures for this kind of hydrocarbon.

9.2.5 Hydrogen sulphide (H2S)
If the vessel is carrying sour crude, it is absolutely essential to check the
tank(s) atmosphere for hydrogen sulphide before entering.

A lot of crude oil comes out of the well with high levels of hydrogen sulphide,
but is usually reduced by a stabilisation process before the crude oil is
delivered to the vessel. This stabilisation may, however, decrease over time.

The nose has no trouble detecting the smell from hydrogen sulphide at low
concentrations, which is like the smell of rotten eggs, but the sensory cell in
the nose is immediately put out of function if higher concentrations are
inhaled.

The effects of the gas at concentrations in air in excess of the TWA (Time
Weighted Average) are, as follows:






Concentration

Effects




50 - 100 ppm

Eye and respiratory tract irritation after exposure of one hour.




200 - 300 ppm 

Marked eye and respiratory tract irritation after exposure of one
hour.




500 - 700 ppm

Dizziness, headache, nausea etc. Within 15 minutes, loss of
consciousness and possible death after 30-60 minutes exposure.




700 - 900 ppm

Rapid unconsciousness, death occurs a few minutes later.




1000 - 2000 ppm

Instantaneous collapse and cessation of
breathing






Persons over exposed to H2S vapour should be taken to clean air, as soon as
possible. The adverse effects of H2S can be reversed and the probability of
saving the persons life improved, if prompt action is taken.
For handling cargoes containing hydrogen sulphide follow the operation
procedures described for such a cargo.

9.2.6 Toxic Elements in Inert Gas
Inert gasłs low oxygen content is the main hazard. Inert gas produced by
combustion, either in a steam boiler or in an inert gas generator, contains a
various amounts of toxic gases, which may increase hazard to the personnel
exposed to it. Follow the precautions to protect personnel against toxic
hazards. These precautions do not include the requirements for direct
measurement of the trace flue gas elementłs concentration. This is because when
gas that is freed from a tank, the hydrocarbon gas concentration is about 2% by
volume to 1% LEL. Until there is a steady 21% by volume oxygen reading, it is
sufficient to dilute these elements to below their TLVÅ‚s.

9.2.7 Nitrogen Oxides
Flue gas contains approximately 200ppm (0,02%) by volume of mixed nitrogen oxides.
Nitrogen oxide (NOx) is generally removed in the water scrubber in the inert
gas plant. The NOx gas is colourless with a weak smell at its TLV of 25ppm.
Nitrogen dioxide is even more toxic with a TLV of 3ppm.

9.2.8 Sulphur Dioxide
Flue gas produced by the combustion of high sulphur content fuel oils typically
contains about 2,000 ppm of sulphur dioxide (SO2). Inert gas system water
scrubbers remove this gas with an efficiency, which depends upon the design and
operation of the scrubber, giving inert gas with sulphur dioxide content
usually between 2 and 50 ppm.
Sulphur dioxide produces irritation of eyes, nose and throat and may also cause
breathing difficulties in sensitive people. It has a distinctive smell at its
TLV of 2 ppm.

9.2.9 Carbon Monoxide
Carbon monoxide (CO) is normally present in flue gas at a level of only a few
parts per million, but at abnormal combustion conditions and slow running it
can give rise to levels in excess of 200ppm. This gas is an odourless gas with
a TLV of 50ppm. It is insidious in its attack, restricting the blood to absorb
oxygen, causing a chemically induced form of asphyxiation.

9.2.10 Oxygen Deficiency
For several reasons the oxygen content in enclosed spaces may be low. On oil
tankers the most obvious one is that the space is in an inert condition. Also
it can be due to a lack of oxygen based on chemical reactions, such as rusting
or the hardening of coatings.
When the available oxygen decreases below 21% by volume, breathing tends to
become faster and deeper. Symptoms indicating that an atmosphere is oxygen
deficient may not give adequate notice of danger. Most persons would fail to
recognise the danger until they were too weak to be able to escape without
help. This is especially so when escape involves the exertion of climbing.
Entry into spaces with oxygen less than 21% by volume must never be permitted.


9.3 INERT GAS
In principle, inert gas is used to control the tank atmosphere in order to
prevent the formation of flammable mixtures. Inert means inactive and the
primary requirement for an inert gas is low oxygen content. The composition of
inert gas can vary. The following table provides an indication of typical inert
gas components from flue gas, expressed as a percentage by volume:






Inert gas

Before
scrubber 
After scrubber




Nitrogen, N2

Approx. 80% vol.

Approx. 80% vol.




Carbon dioxide, CO2. 

Approx. 14% vol

Approx. 14% vol.




Oxygen, O2

2 - 5% vol.

2 - 5% vol.




Water vapour, H2O
Approx. 5% vo

20oC: approx. 2% vol.

40oC: approx. 7% vol.




Carbon monoxide,

CO Approx. 0,01% vol.

Approx. 0,01% vol.




Nitrous gases, NOX

Approx. 0,02% vol.

Approx. 0,02% vol.




Sulphur dioxide, SO2  

Approx. 0,3% vol.

Approx. 0,005% vol.




Ash and soot 

300mg/m3

30mg/m3




Density

1.044

1.044






When hydrocarbon gas burns in air, the oxygen in the air reacts while the
nitrogen gas is inert and does not take part in the reaction. Examples of inert
gases are nitrogen, carbon dioxide or combustion gases.
On a crude oil tanker, the production of inert gas is done with flue gas from
the shipłs boilers or by a separate inert gas generator. The flue gas being
produced, before being transferred to the cargo tanks, is first cooled and
cleaned of soot and corrosive gases. This prevents fire and explosion. The
maximum permissible oxygen content in the inert gas delivered to the cargo
tanks is 5% by volume (all kinds of tankers).
Approximately the content of carbon dioxide in the inert gas is 14% by volume
depending to some extent on quality of the oil being burned and on the air
supply.
The carbon monoxide contained in the supplied inert gas is approximately 0,01%
by volume, but if the excess air is reduced too much in hopes of reducing the
oxygen content, the concentration of carbon monoxide could increase
significantly.
The concentration of nitrogen in inert gas will more or less be the same as for
the concentration in air, broadly speaking, about 80% nitrogen by volume.
A small amount of Nitrous Gases (NO and NO2) is formed, following the reaction
between nitrogen and oxygen in the air at higher temperatures. It will be
approximately 0,02% NOX by volume.



The concentration of sulphur dioxide in the inert gas depends on the sulphur
content of the oil being burned. It will be approximately 0,3% by volume in the
flue gas. After passing the scrubber, depending on the efficiency of this, the
content is reduced to approximately 0,005% by volume.
Flue gas contains soot as high as 300mg/m3, but is reduced to below 30mg/m3
after passing the scrubber.
The oxygen concentration in flue gas will be different, before the scrubber,
than in the inert gas, after the scrubber. Some ships use the same fixed
instrument for measuring the oxygen content in the flue gas, before passing the
scrubber, and the inert gas, after the scrubber. This is done by providing a
choice of sampling lines from two different places into the same instrument.
The main problems are in the flue gas measuring with greater reading and
guarding against instrumentation error. It is strongly recommended to have a
separate oxygen-measuring instrument for inert gas, after the scrubber.
When recalculating inert gas through the scrubber beware of the oxygen content
increase due to the evolution of oxygen from the seawater.
The figure to below shows an example of design of a scrubber for cooling and
cleaning of the flue/ inert gas.




9.4 GAS INDICATORS
9.4.10 Sampling lines and pumps
It is very important to realise that the quality of the sampling hose has
influence on the measuring result, and that correct use and maintenance are
important. If the hose is not properly chosen, it is likely that a poor quality
hose will absorb hydrocarbon gases.
Make sure that the quality of hoses being used on your ship is approved and in
good condition. Examples of hoses which have proved acceptable:
1. Teflon inner hose, neoprene outer hose. This hosełs inside diameter is 3mm,
which corresponds to an inner volume of about 7cm3 per meter length.
2. “Tanol" (Trade mark of MSA). This hose is marked: “Tanol" - synthetic rubber
sampling line, low solvent absorption, anti-static. Note: In an enclosed
container use adequate electrical bonding. The inside diameter is 5mm
corresponding to an inner volume of about 20cm3 per meter length.
When ordering a measuring hose make sure you are getting an approved one.
Always ask the deliverer for a certificate, which shows the authorisation. It
is very important to -m
9.4.2 Pumps
The hand pumps used are often in a rubber form with a volume of 40cm3 or more.
When using long hoses, it is important to know the number of pump strokes from
the sampling point that are necessary for the gas to reach the instrument. The
number of strokes depends on the hose length, as well as, the inside diameter
of the hose.
The number of strokes may vary from 6 to 15 for a hose length of 30 metres,
depending on the inner diameter. The numbers mentioned are based on a pump
volume of 40cm3. Some types of instruments are fitted with built-in pumps.
Follow the user instruction for such a pump.

9.4.3 Cleaning of hose
If the sampling hose gets dirty with oil on the outside, immediately clean it
with a dry cotton rag. If the hose is dipped by accident in oil and oil is
drawn into the hose, discard the hose because it is very hard to clean it.
Always follow this rule: Each gas measuring instrument has its own hose only
for using with the specific instrument. Do not mix hoses with hoses, which
belong to another instrument.

9.4.4 Leakage, plugging, contamination
Always check the hose, instruments and pumps before use, in order to detect any
leakage, plugging or contamination. Follow the procedure check for the
instrument being used.
Place a finger on the hose opening and check that the hand pump remains
squeezed together for about 1 minute. If there is a built-in pump, the flow
indicator gives an alarm. See the illustration to your right.



Carry out measurements with and without the sampling hose to check that the
hose does 
not influence the measurement by absorbing or releasing gases. For this purpose
use clean air and a calibration gas, depending on the type of gas measuring
instrument being checked.
Also carry out a leakage test on the instrument, and if applicable, on a drop
catcher or other optional equipment that has been fitted. See the illustration
to your right.


 


9.4.5 Maintenance
Make it a rule to always purge the hose by pumping clean air through it after
use. And blow the measuring hoses with compressed air from time to time to
remove water droplets and dust. As the analysers are of vital importance,
they must be carefully maintained and tested strictly in accordance with the
manufacturer instructions.

9.4.6 Filters
Normally used in hydrocarbon gas meters are cotton filament type filters,
catalytic or non-catalytic. Additional filters are not normally needed. In
extremely moist or wet conditions, for example during tank washing, excessive
water can be removed from the gas sample using materials that retain water,
but do not affect the hydrocarbons.
Materials for this purpose are granular calcium chloride or sulphate. If required,
soda asbestos will selectively retain hydrogen sulphide without affecting
the hydrocarbons. However, it also retains carbon dioxide and sulphur dioxide
and must not be used in tanks, which are inerted with scrubbed flue gas.
The use of water retaining filters is essential when using an oxygen analyser,
especially the analysers based on the paramagnetic principle. This is because
the presence of water vapours in the sample can damage the measuring cell.
Use only manufacture recommended filters.

9.4.7 Calibration gas
Always have the appropriate calibration gas for the instruments on board.
This calibration gas has to be the right type and the availability has to
be good. Also, knowledge how to use the different types of calibration gas
must be properly understood. Always follow the manufacture's recommendation
when ordering calibration gas. Also demand a certificate on the ordered calibration
gas to be sure that you are receiving a gas of high quality.
Explosimeters use a mixture of hydrocarbon gas and air, approximately 50%
LEL or lower, as a calibration gas. (It is important to have a certificate
on the specified hydrocarbon gas, showing the exact percent of LEL).
Various types of hydrocarbon gas measuring instruments may have different
requirements of calibration gas. Make sure you have the right one on your
vessel.
Oxygen analysers used at low concentrations usually use nitrogen as the calibration
gas in order to get a zero adjustment and dry air is used for the 21% O2 by
volume adjustment.





9.4.8 Attention 
Those using the measuring instruments on board must have sufficient knowledge
about the instrument, and all such instruments must have the operating instructions
attached to the instrument. Also keep a log for each instrument, where records
are made of the calibration performed, replacement of parts or other repairs,
faults and irregularities. Always have additional spare parts in supply, which
may have to be replaced from time to time.
If the instrument not is in use for a long period of time, remove the batteries;
even the leak proof ones.

Warning
For the sake of safety, all instruments must be operated and serviced by qualified
personnel only. Read and make sure you fully understand the instruction book
before using or servicing the instrument.


9.4.9 Volume % hydrocarbon gas measuring instruments
We are going to discuss various principles for the measurement of hydrocarbon
gases given off by crude oil.
In order to measure hydrocarbon gases in a mixture with other gases, for example
inert gas, an instrument is used, which measures the absorption of infrared
light. Such infrared absorption instruments are found both as laboratory instruments
and as instruments for fixed installations on board ships.
In the early 1970Å‚s, when trying to find portable gas measuring instruments
for the determination of % by volume HC in a tank atmosphere, there were few
commercial instruments, which appeared suitable. An interferometer was modified
and the Riken Interferometer Type 17HC, with the measuring ranges 0-5% by
volume and 0-30% by volume HC, was developed in collaboration with Riken Keiki
Fine Instrument Co., Japan. At this time, only a few ships had an inert gas
system on board. The instrument was used for measuring hydrocarbon concentrations
in air, which were higher than the lower explosive limit, to check for freeing
gas with air before tank washing in a “too lean" atmosphere. Later on, the
instrument also came to be used for the measurement of hydrocarbon gas concentration
in an inerted atmosphere.


9.4.10 Riken portable indicator Model 17HC
9.4.10.1 Operating Principles
This instrument measures volume by percent of hydrocarbon gases above crude
oil using an optical registration at the speed of light, which passes through
the air and gas/air mixture respectively. The gas in question is sucked into
two chambers that are placed in sequence and equipped with glass end walls
enabling the light to pass through. The volume percentage is registered on
a double scale that is graduated 0-5 and 0-30 and is read through an adjustable
lens.
With Riken 17HC one can measure concentrations of hydrocarbon gases by utilising
the difference between the speed of light through air and the gas. The difference
increases with increasing hydrocarbon gas concentration.
The refractive index for a gas is an expression of the ratio between the speed
of light in vacuum and in the gas. The speed in the gas will depend on pressure
and temperature. The refractive index is normally quoted at a pressure of
1 atmosphere and either 0oC or 20oC.
Compared with the refractive index for the various hydrocarbon gases, the
hydrocarbon mixture index used by Riken is closest to butane.
The instrument is tested for working within a temperature range of +113oF
(45oC) to
22oF (-30oC). Hotter gases should be cooled down to come within
this range.
The interferometer was originally chosen to determine hydrocarbon gases in
air. The conditions become more complicated if the interferometer is used
for measuring hydrocarbon gases in inert gas. There will be a difference between
the zero adjusts for air without hydrocarbon gas and inert gas without hydrocarbon
gas.
Oxygen and nitrogen have rather similar refractive indexes, but there will
be a positive deviation in relation to air when the oxygen content decreases
from 21% by volume. If the oxygen content is reduced from 21% by volume to
5% by volume, the reading on the interferometer increases from 0% by volume
HC to 0,5% by volume HC.
Carbon dioxide has a higher refractive index than air, so the reading on the
interferometer for 1% by volume CO2 is approximately 0,15. Inert gas, which
contains close to 14% by volume CO2 and approximately 5% by volume O2, will
therefore give a reading on the interferometer of 2,5. (Approximately 2,0
is due to carbon dioxide and about 0,5 is due to low oxygen content). When
the interferometer is used for measuring hydrocarbon gas in inert gas, a correction
is therefore necessary for the difference between zero setting in clean air
and zero setting in inert gas.
Previously a method was used whereby carbon dioxide was removed from the gas
mixture before the introduction to the interferometer. The gas mixture was
passed through a tube filled with soda lime, as an absorption material. Experience
has shown that the absorbent often is not very efficient, so that measurements
with the interferometer have given too high values. It is therefore recommended
to correct for the difference between zero setting in clean air and in inert
gas by using a method, which does not include the use of the external filter.
Inert gas contains 12-14% CO2. To remove such a large concentration by means
of the external filter has proved difficult. Instead of using the filter the
measurement is read directly and the values read are reduced by 2,5%. If there
is a risk of sucking in water vapour/condensate, one can use a moisture collector
(which usually accompanies the instrument) and install it between the suction
hose and the instrument.
When measuring hydrocarbon gases in an inerted tank atmosphere with an interferometer
without the soda lime, the reading must be corrected by subtracting 2,5 from
the values read. For example, the correct value will be 2,5% by volume HC
for a reading of 5,0.
Optical diagram



9.4.10.2 How to use the instrument
Function of parts:
1. Inlet port to which the sampling tube is connected.
2. Outlet port to which the aspirator tube is connected.
3. Push button switch to illuminate the scale.
4. Screw off cover to protect zero setting from any disturbance in handling
the instrument during tests.
5. Zero adjusting knob for setting interference fringe to zero position in
fresh air.
6. Cock to change the sampling route either HIGH RANGE or LOW RANGE.
7. Eyepiece lens and protecting push on cover (on chain) to the right. The
lens can be focused for personal vision by turning in either direction.
8. Aspirator bulb.
9. Screw on covers, replaceable moisture absorbent cartridge and single cell
flashlight battery.
10. Cover for electric bulb for the light source.
9.4.10.3 Preparation:

1. a) Secure auxiliary filter in leather strap. Connect rubber tube to gas
inlet port (1) through auxiliary filter.
2. b) Connect rubber aspirator to gas outlet port (2).
3. c) Place cock (6) in position 5 and squeeze aspirator (8) at least five
times in fresh air to clean gas chamber.
4. d) Press the switch (3) and observe interference fringe through eyepiece.
5. e) Remove protective cover (4) of zero setting knob (5). Adjust the right
one of two black lines, just on the zero position of scale, by rotating the
zero setting knob.
6. f) Put the cover back on, in order to protect the knob from any accidental
movement.
Reading: 
1. Suck the gas to be examined into instrument by squeezing aspirator about
5 times or more if extension tube is used.
2. Press the switch and examine amount of shift of marked black line through
eyepiece, which gives percentage of gas on 0 - 5% scale.
3. If the marked black line or fringe is beyond scale, gas concentration is
higher than 5%. In such case, change cock position to 0 - 30% scale.
4. Suck clean air into instrument by squeezing aspirator 3 to 5 times.
5. Press the switch and examine amount of shift of marked black line through
eyepiece, which gives percentage of gas on scale 0 - 30%.
After reading: 
Place cock position to 5 and clean gas chamber with fresh air.




9.4.10.4 Taking readings:
In gaseous atmosphere draw in test sample by squeezing bulb at least 4 times
for each meter of sampling hose in use.
Press the switch (3) and observe new position on scale of RIGHT HAND EDGE
of INDEX STRIPE.
The reading indicates the percentages of hydrocarbon gas. Repeat for further
gas tests.

9.4.11 Percent LEL measuring instruments & Explosimeters
Most types of instruments giving concentration of flammable gas in air in
%LEL use catalytic combustion as the measuring principle. Such instruments
are usually called exsplosimeter.
A catalyst is a substance, which helps a chemical reaction to take place.
Exsplosimeter normally use platinum metal or platinum alloyed with other metals
as a catalyst. To make the reaction take place, the catalyst has to be heated
to a high temperature.
Certain types of Explosimeters use a platinum wire as a catalyst and the reaction
between flammable gas and the oxygen in the air takes place on the surface
of the metallic wire.
The temperature of the wire may then be 1000oC. Other types of Explosimeters
have a coating on the outside of a heated metal wire, and it is the coating
which catalyses the reaction. The reaction takes place somewhat easier on
this coating, and a temperature of 500oC may be sufficient. The part of the
instrument where the reaction takes place is normally called a sensor or detector.



The flammable gas to be measured is burned on the surface and the heat generated
results in a temperature increase. The electrical resistance of the metallic
wire increases with the temperature. The change in resistance is proportional
to the increasing temperature and to the concentration of flammable gas in
the air. This applies only to a lean mixture below the lower explosive limit.
The instruments are usually designed in such away that they first have to
be adjusted to zero with clean air. Then the atmosphere that should be measured
is sucked into the instrument where the sensor is located and a reading is
made. Finally, clean air is sucked in again and the zero setting checked.
Some types of instruments are intended for monitoring and are designed so
that the sensor is located at the spot where the measurement is to be performed.
Explosimeters are calibrated with a certain gas, for example butane. It should
be marked on the instrument, which gas is used for calibration gas. To some
extent the explosimeter will also be suitable for measurement of other flammable
gases, and many manufactures of instruments quote the correction factors for
various gases other than the calibration gas. The most frequently used calibration
gases for commercial explosimeters are methane, propane, butane, pentane,
hexane or nonane. For ships carrying crude oils, it is recommended to use
butane in air or alternatively propane in air.
Theoretical calculations of the sensitivity of an explosimeter for various
flammable gases show that the reading for 100% LEL of the gas mixture is proportional
to the heat of combustion, to the diffusion coefficient of the flammable gas
and to the gas concentration at the lower explosive limit. The diffusion coefficient
is an expression for the speed at which the molecules can move to the catalyst
surface where the reaction takes place, and the lighter molecules move faster
than the heavy ones. For example, the methane molecules move faster than the
propane molecules.
Theoretical calculations of sensitivity have been performed for nearly 100
different flammable gases, and the value for hydrocarbon gases are given in
the table below:






Type of HC gas 

Sensitivity




Methane

100




Ethane

68



Propane

55



Butane, n

59



Butane, i

52



Pentane, n

46



Hexane

37



Heptane

38



Octane

38



Nonane

31





The above figures are given in arbitrary units. As an example, an exsplosimeter
calibrated with propane will theoretically give a deflection for 100% LEL of
hexane which is (37:55) x 100 = 67% LEL. There is however, some difference
between theory and practice.


In practice there will not be the same conversion factors for different types
of Explosimeters, since the details of how the instruments are designed are of
great importance. There may also be a large difference from one instrument to
another instrument of the same type, which is greatly dependent on how good of
a control the manufacturer has over own production.
From what we have seen so far, explosimeters calibrated with butane should show
higher values for methane, lower values for pentane, hexane and the other
heavier hydrocarbon gases.
There is a complicating factor, however, in that methane is a gas, which requires
a more efficient catalyst and/or a higher catalyst temperature. On the market
there are some types of explosimeters with low sensitivity for methane and
several types of explosimeters which have been investigated showing that the
sensitivity to methane may drop after a short period of time of using the
instrument. However, it still gives a correct reading for the heavier
hydrocarbon gases.
For explosimeters being used on board LNG-carriers, methane must be used as the
calibration gas. Explosimeters to be used on ships carrying crude oil, butane
is recommended to be used as calibration gas, alternatively propane. This is
because the gas mixture given off by crude oil contains relatively small
amounts of methane gas and the gas given off from sediments and oil residues
contain quite negligible concentrations of methane. Be aware that the
exsplosimeter will give somewhat misleading low values for the hydrocarbon
gases that are heavier than the calibration gas.
The catalyst will, when used gradually, lose its ability to bring about
combustion, and all types of explosimeters have, to a greater or lesser extent,
the regrettable characteristic that the sensitivity is reduced.

All explosimeters must therefore from time to time be checked with its
calibration gas.

Certain gases may poison the catalyst, and it is known that hydrogen sulphide
from sour crude may act in this manner. A poisoning will lead to the properties
of the catalyst being temporarily or permanently damaged so that the
sensitivity of the instrument to flammable gases is greatly reduced or vanishes
altogether. The best-known catalyst poisons are silicones and vapours from
leaded gasoline, which give a solid deposit on the outer surface of the
catalyst.
We have mentioned that the reading of the explosimeter depends on the
concentration and diffusion coefficient of the flammable gas. This only applies
when we have a lean mixture of flammable gas in air. For high concentration of
flammable gas, the reading will instead depend on the concentration and diffusion
coefficient of oxygen. Very high concentrations of flammable gas, in relation
to oxygen, at the catalyst surface may result in the combustion reaction being
completely prevented, so that the explosimeter gives reading of close to zero
for such a high concentration.
High concentrations of flammable gas and/or low concentrations of oxygen give
misleading, ambiguous readings and may also damage the catalyst in that a sooty
layer is formed.

Therefore, never use the explosimeter at concentrations of flammable gas higher
than 100% LEL, and never at lower oxygen concentrations than approximately 10%
O2 by volume


9.4.12 Riken, portable combustible detector, model GP-204
9.4.12.1 General description
The model GP-204 hand held portable gas detector is a compact battery operated
portable instrument used for taking an air sample and indicating the presence
and concentration of combustible gas. Samples of the air under test are drawn
by means of a rubber aspirator bulb and analysed for combustible gas content on
a heated platinum filament in a Wheatstone bridge measuring circuit. A built-in
meter indicates combustible gas content in units of explosibility. Power for
operation of the instrument is provided by built-in dry cells. A probe and
extension hose permit sampling from remote locations and the instrument fits in
a compact leather case with an over the shoulder-carrying strap. The model
GP-204 is suitable and recommended for testing tanks, manholes, vessels other
spaces to determine presence or absence of combustible gas in pressure
cylinders, pipe lines and other closed systems. It is a valuable aid to safety
of operations whenever combustible gases or vapours are handled.







"D" is exposed to the gas.
"C" is isolated from the gas.
The resistance "D" increases during catalytic combustion.


Samples of air , which may contain flammable gases or vapours, are sucked
through the instrument by means of a suction bellow.
The content of flammable Gases effects a heated platinum filament (D =
detecting element) which forms part of a Wheatstone bridge measuring circuit as
shown in the circuit diagram on the right hand side. Besides the measuring
filament “D", this circuit includes a compensating filament “C" and two fixed
resistanceÅ‚s “R1 & R2".
The flammable gases or vapours in the air are oxidised and burn at the surface
of the measuring filament “D", and the evolution of heat causes a change in the
resistance of the platinum wire which gives rise to an imbalance in the
Wheatstone bridge. This corresponds to the content of flammable gases in the
sample.

9.4.12.2 Operation 
In a gas hazardous area the instrument should always be in the carrying case
and strapped to this.
Before taking the instrument to the hazardous area, check the battery voltage.
To check the voltages, put the switch in “VOLT ADJ:" position. Meter should
rise to the “check" position near top of the scale. Lift and turn VOLT ADJ.
Control clockwise to determine maximum voltage setting. If the needle cannot be
set beyond the VOLT ADJ mark, batteries need recharging or replacing for full
capacity. Do not attempt to use instrument at all if reading cannot be set up
to the mark or beyond the mark.
Do not replace batteries in a hazardous area; bring the instrument to a safe
area before changing taking place.

If the voltage is satisfactory, continue with the next steps of preliminary
adjustment, as follows:
1. Confirm operating of pilot light/meter illuminating lamp.
2. With sample inlet in fresh air, squeeze bulb several times to flush out any
remaining gas from the instrument.
3. Check zero setting by turning the switch in “ON" position. Meter should read
close to zero. If not, lift and turn the “ZERO" knob to bring the reading
exactly to “0". 
4. Couple the sampling hose to the instrumentłs inlet pipe, which is located on
the left-hand end, and also connect the probe to the end of the hose.
5. Admit a sample of some combustible gas to the end of probe and confirm that
the meter rises upscale. 
Instrument is adjusted and ready to use. Now it may be turned off and carried
to the job area. To run a gas test, proceed as follow:

1. Turn the instrument to VOLT ADJ. position, adjust voltage if necessary
2. Turn the instrument to ON position, zero adjust if necessary.
3. Hold probe within space to be tested. Squeeze bulb several times (4 times
for each metre of sampling hose being used) while watching the meter and
observe maximum reading.
4. After completion of test, remove probe from test space. Flush the instrument
with fresh air and turn it off.
The sampling hose being used for this instrument should not be used for
sampling with other instruments. Make it a rule that a specific measuring
instrument has its own sampling hose.

9.4.12.3 Interpretation
Meter readings are taken on a scale graduated 0
100% LEL. The abbreviation
LEL stands for Lower Explosive Limit and represents the lowest concentration
which can be ignited by a source of ignition, hence the lowest concentration
which can produce an explosion. This quantity is also spoken as the LFL
Lower
Flammable Limit.
The mode GP
204 is calibrated before shipment to read directly in percent of
LEL of iso-butane in air, based on the known LEL for iso-butane of 1,8% by
volume. This 1,8% by volume will produce a reading of 100% LEL and lower
concentrations will be read in proportion.

Other combustible gases will read approximately correctly in terms of
explosibility, but for the maximum accuracy a calibration curve for various
gases has to be used. This curve is delivered together with the instrument.
This curve is drawn in terms of percent LEL for both co-ordinates. See the
table below.


9.4.12.4 Maintenance

Calibration and adjustment - In addition to the normal operating controls found
on the top of the panel, the following auxiliary controls are available.

Calibration potentiometer - This adjustment is used to set meter reading to the
desired level, while sampling a known concentration of combustible gas. In the
GP-204 the top plate must be removed by taking out the screws in each corner.
The calibration potentiometer is a slotted-shaft control located above right
upper corner of meter. Turn clockwise to increase meter reading.
Element replacement - The element assembly, consisting of an active filament
and a similar but enclosed reference filament, should be replaced if zero
cannot be set within range of “ZERO ADJ.", or if reading cannot be set high
enough on a calibration gas, using calibration potentiometer.
1. Loosen the two panel hold-down screws, remove and invert top panel.
2. With switch off, loosen (do not remove) the three screws holding the
terminals for red, black and white wires. Pull wires from terminal.
3. Remove the two Phillips head screws holding cross-shaped element retainer in
place. Pull out both filaments and replace with new ones in same position.
4. Check that gaskets are in place on element before installation. Be sure that
the active (black wire) filament is in the cavity with the flame arrestor.
Install wires on terminals as before.
5. Turn instrument on and adjust zero.
6. If a calibration gas is available reset span.
9.4.12.5 Batteries
The model GP-204 is furnished with two standard size “D" dry cells. These dry
cells (UM-1/1,5 size D/R 20 Maxell 100) will give 3 hours (maximum) of operating
time.
When meter cannot be set as high as the “Check" line with switch in “VOLT ADJ:"
position and “VOLT ADJ." knob all the way clockwise, batteries require
replacement or recharging.
To replace batteries, remove instrument from hazardous area. Take the
instrument out of the leather case, and loosen the coin slotted captive screw
found in centre of bottom plate. Remove bottom plate, exposing batteries in
their spring contact holders. Pull old batteries out and install new ones in
the same position. Observing polarity as marked on holder.

9.4.12.6 Sample system 

Hose
The hose used is Teflon lined synthetic rubber jacketed and immune to
absorption or attack by any combustible vapours or solvents. Keep hose clean
and are sure that couplings make airtight contact. Check occasionally by
holding finger over hose inlet. Bulb should remain flattened after squeezing if
there is no leak. Extension hoses in various lengths are available.


Flame arrestor
The active filament is installed within a sintered bronze porous metal cup,
which acts as a flame arrestor to retain explosions that may occur when
sampling explosive gas/air mixtures. The flame arrestor may be removed by
taking out the four screws that hold the plate in which the elements are
installed. If flame arrestor is dusty, wet, oily or corroded, it must be
cleaned or replaced.
Preferred cleaning method is by washing in detergent solution, rinsing from the
inside out, and drying thoroughly in air. Before re-installing flame arrestor
in instrument, be sure that the reaction chamber cavity and incoming lines are
clean and dry.


Meter Lamp
The meter lamp is on whenever the instrument is on, and provides illumination
to permit reading meter in dark places. If lamp fails, replace it as follows:
1. Remove four screws holding top plate to the top panel. Take off top plate
exposing lamp. Loosen set of screws, which lock lamp wires to terminal and pull
the lamp out. Install new lamp in the same position.

9.4.12.7 Precautions and notes on operation
1. Heated samples - When sampling spaces such as hot tanks that are warmer than
the instrument remember that condensation can occur as the sample passes
through the cool sample line. Water vapour condensed in this way can block the
flow system and corrode the flame arrestor. A water trap can be used to control
this, and is available as an accessory. If heated hydrocarbon vapours of the
heavier hydrocarbons (flash point 90oF or above) are present, they may also
condense in the sample line and fail to reach the filament. Thus an erroneous
low reading may be obtained.
2. Element poisoning - Certain substances have the property of desensitising
the catalytic surface of the platinum filament. These substances are termed
“Catalyst Poison" and can result in reduced sensitivity or in failure to give a
reading on samples containing combustible gas. The most commonly encountered
catalyst poisons are the silicone vapours, and samples containing such vapours
even in small proportions should be avoided. Occasional calibration checks on
known gas samples are necessary, especially if the possibility exists of
exposure to silicones. A calibration check on a known iso-butane gas is the
most dependable as an indication of normal sensitivity. A convenient
calibration accessory is available and described under “Accessories".
3. Rich mixture - When high concentrations of gas are sampled, especially those
above the LEL, considerable heat is liberated at the filament. This heat may
cause damage to the filament or tend to shorten its life, so sustained testing
of samples beyond the meter range should be avoided. When sampling rich
mixtures, the following instrument action may be expected.
· Mixture up to 100% LEL reading on scale.

· Mixtures between LEL and Upper Explosive Limit (UEL) readings at top of
meter.

· Mixtures above UEL
When a sample is introduced, the meter is sent to the
top of scale, then comes back down on scale or below, depending upon
concentration. Very rich mixtures will give a zero or negative reading. The
alarm circuit thus insures that a very rich sample will not be overlooked, as
it could otherwise be with a simple indicating instrument.


4. Oxygen deficient mixtures - Samples, which do not have the normal proportion
of oxygen, may tend to read low if there is not enough oxygen to react with all
combustible gas present in the sample. As a general rule, samples containing
10% oxygen or more have enough oxygen to give a full reading on any combustible
gas sample up to the LEL.
5. Oxygen enriched mixtures - Samples having more than the normal proportion of
oxygen will give a normal reading. However, they should be avoided because the
flame arrestor used is not dense enough to arrest flames from combustible gas
in oxygen, which can be much more intense than those in air can. Do not attempt
to use the model GP-204 on samples of combustible gas in oxygen.
6. Accessories - Additional lengths of extension hoses may be used for sampling
from deep tanks and spaces. The polyurethane hoses are satisfactory for most
samples including natural gas, hydrogen, and gasoline vapours. Where there is
danger of water being drawn into the instrument, a water trap should be used.
This glass-bodied trap, with sintered metal filter, couples to the indicator
inlet and will collect water that is drawn into or condensed in sample hose.
Inspect trap periodically while in use, and empty or clean bowl and filter
whenever visible water or dust accumulate. Regular sample hoses connect to
inlet of trap when it is installed on the instrument.

9.4.13 Servomex, oxygen analyser, type OA 262


WARNING! To ensure safe operation in hazardous applications, the analyser must
be used to comply with the conditions of certification, relevant standards and
codes of practice. Failure to do so may invalidate the certification. Any
modification to the standard analyser, or repairs or servicing using parts that
are not specified or approved by Servomex, will invalidate certification. In
case of doubt contact Servomex or their agents.

9.4.13.1 General description
The Servomex portable oxygen analyser type 262A is a robust lightweight
instrument built for industrial, marine and laboratory applications.
The oxygen content of the gas is indicated directly on a 70mm scale taut band
meter after suitable zero and span adjustments. The ranges, 0-100, 0-25, and
0-10% are selected by a rotary switch on the front panel. Battery checks are
also selected with this switch.
This analyser is used on marine applications throughout the world. The front
panel controls are symbolic, such that engineers from many different nations
can understand them.
All analysers are supplied with a hand aspirator and silica gel dryer.
Batteries are not supplied with the analyser.
The 262A is powered by dry cells batteries which are housed in a waterproof
compartment at the rear of the analyser.
The analyser is supplied with a filter, elements of which are and simply
replaced from the front of the instrument.

9.4.13.2 Hazardous area and shipboard use
Hazardous area - For hazardous areas the 262A is certified by BASEEFA as
intrinsically safe code Ex ia s IIC T4 to SFA 3012, SFA 3009. Certificate BAS
No. 74149.

Instruments up to serial no. 2983 are approved by “Factory Mutual" for use in
class 1, division 1, groups B, C and D hazardous locations. Report 25243 dated
August 30th, 1975 applies.
Seaworthiness - Lloyds has approved the analyser as suitable for shipboard use.
Certificate Lon. 409515.574 applies.

The Norwegian Maritime Directorate (Sjłfartsdirektoratet) has also approved the
analyser for use on board ship. (Reference letter A-44140/75.AGR/MI dated
24.10.75 applies).







Specification
Specification



Oxygen ranges
0-10%, 0-25%, 0-100% O2. Selected by front panel switch.
Indication on front panel meter.



Accuracy
Range:
0-100% O2. +/- 3% F.S.D.0-25% O2. +/- 3% F.S.D.0-10% O2. +/- 3% F.S.D.




Effect of ambient temperature .

The analyser will operate between the temperature
of
10oC to 50oC (14 to 122oF). The accuracy will be maintained for a
temperature change of +/- 10oC (18oF) of the calibration temperature




Effect of tilt 

0,01% oxygen per degree.




Weight (net)

3kg. (6,5Ib).




Sample pressure

Maximum inlet pressure, 2 psi. (14kPa).




Flow rate pressure 

0 to 3 I/min, depending on sample.




Materials contact with sample gas

Acetal copolymer, Glass micro fibre, Nickel,
Platinum, Polypropylene, Pyrex glass, Quartz glass, Stainless steel 316,
Synthetic rubber, Viton.




Calibration gases

Zero on O2 free nitrogen (N2). Span on clean dry air or high
purity O2 if desired.




Accessories

Waterproof case with shoulder strap. Drying tube. Two hexagon
wrenches (2,5 and 3mm).




Case material

Polypropylene. The case is splash proof and sealed against
ingress of water, provided the sealing gaskets around the front panel and
battery compartment are in good condition.





9.4.13.3 How the Servomex oxygen analyser works
The physical property, which distinguishes oxygen from most other gases, is its
paramagnetism. Faraday discovered this in 1851, who demonstrated that a magnet
attracted a hollow glass sphere at the end of a horizontal rod supported by
silk fibres when filled with oxygen.






In portable oxygen analysers, the convenience and sensitivity of Faradayłs
arrangement are increased by having a sphere at both ends of the bar, forming a
“dumb-bell", which seals the gas surrounding it. The dumb-bell is suspended in
a symmetrical non-uniform magnetic field, and being slightly diamagnetic, it
takes up a position away from the most intense part of the field. When the
surrounding gas contains oxygen, the dumb-bell spheres are pushed further out
of the field by the relatively strongly paramagnetic oxygen. The strength of
the torque acting on the dumb-bell will be proportional to the paramagnetism of
the surrounding gas: it can therefore be used as a measure of the oxygen
concentration.




The only common gases having comparable paramagnetic susceptibility are NO, NO2
and CO2. A magnetic oxygen analyser cannot therefore be used where these gases
occur in the mixture other than in trace amounts. It is important to note,
however, that in the direct method of measuring susceptibility no other
physical property of the gases has any significant effect.

The heart of the Servomex analyser is a measuring cell using these principles,
but having a rare metal suspension in place of the delicate materials used in earlier
designs. The “zero" position of the dumb-bell is sensed by a split photocell
receiving light reflected from a mirror on the suspension. The output from the
photocell is amplified and fed back to a coil wound on the dumb-bell, so that
the torque, due to the oxygen in the sample, is balanced by a restoring torque,
due to the feedback current. The measuring system is thus “null-balanced", and
has all the inherent advantages of this type of system.



Because of the extremely linear relationship between the feedback current and
the susceptibility of the sample, a proportional output voltage can be
developed, and various ranges can be obtained by means of a switched
attenuator. Linearity of scale also makes it possible to calibrate the
instrument for all ranges by checking at two points only. For example, accurate
calibration is obtained by using nitrogen for zero and air for setting the span
at 21%




9.4.13.4 Operating procedures
Installation and changing of the batteries. The following batteries are
required:
3 of 1,5V Type IEC LR6 (HP7)
1 of 9V Type IEC 6F22 (PP3).
It is recommended that alkaline batteries be used.
The batteries are housed in a waterproof compartment at the bottom of the
analyser. This compartment is opened using the 3mm-hexagon wrench supplied with
the analyser.

A battery strap is provided for easy removal of old batteries. The batteries
must be installed with the correct polarity, as indicated by + and - signs
moulded into the plastic holder.

Various resistors are potted into a recess in the battery compartment. 
Under no circumstances should these components be removed or tampered with.

The stud of a 1,5V battery is “+" and the base “-". These batteries will not
make contact if fitted the wrong way round. The 9V battery has a terminal clip
that can only mate when the battery is correctly positioned.
Care must be taken, when fitting new batteries, not to damage the gasket
sealing the edge of the battery compartment. If the analyser is to be stored
for a longer period of time, remove the batteries.
Do not replace batteries in a hazardous area 

9.4.13.5 Battery checks 
Check that the batteries are fully operational:
Select switch position “B1". The reading should be greater than 60 on the 0-100
scale. Change the 9V battery if the reading is low.
Select switch position “B2". The reading should be greater than 60 on the 0-100
scale. Change the 1,5V batteries if the reading is low.

9.4.13.6 Calibration
Frequency of calibration - Check the zero adjustment weekly. If there is a large
difference in ambient temperature between the point of measurement and the last
calibration, it is advised that calibration should be rechecked.
The span adjustment should be checked daily when in use, due to variance in
atmospheric pressure.
Set Zero - Switch the control to 10% range. Introduce oxygen free nitrogen into
the instrument at a pressure between 1 to 2 psig. (7 to 14 kPa). Stop the gas
flow. Adjust the screw for zero adjustment so that the meter reads 0% oxygen.

9.4.13.7 Span
Switch the control to the 25% range. Introduce dry air into the instrument at a
pressure between 1 and 2 psig (7 to 14 kPa).
The hand aspirator and a drying tube are convenient for this. Stop the gas
flow. Adjust the screw for the span adjustment so that the meter reads 21%
oxygen on the 0-25% scale.
When changing from air or oxygen to nitrogen or vice versa, ensure that the
filter, cell and sample lines have been purged thoroughly. One minute with the
standard hand aspirator should be enough. With long sample lines a pump is
recommended.
When using the instrument for higher concentrations of oxygen it is recommended
that pure oxygen is used on the 0-100% range for optimum accuracy.
To prevent possible damage, it is not recommended that air or pure oxygen be
put into the analyser when it is switched to the 0-10% range.


9.4.13.8 Measuring sample gas
Connect the hand aspirator to the sample inlet by means of the drying tube.
Connect sample tube to the aspirator and place in space to be checked.
Check the battery voltage. Set switch to range required.
Pump the hand aspirator until the reading is steady. Ensure that sufficient
sample gas has been taken to flush out the sample lines.

CAUTION.
The drying tube must always be used, unless the sample is known to be dry. The
analyser will be damaged if water or liquids are allowed to get into the
instrument.

However, the crystals can be regenerated by removing from the drying tube and
drying in an oven at about 110-1200C.

9.4.13.9 Maintenance







WARNINGS




Only qualified personnel who are familiar with good workshop practice should do
maintenance of the analyser.




Replacement parts should be to the quality specified by Servomex in the part
lists. The use of inferior replacement components may degrade the performance
of the analyser and invalidate any certificates, which may apply.







9.4.13.10 Replacement of measuring cell
1. Remove the six hexagon socket
stainless steel screws holding the front panel into the case and keep them in a
secure place.
2. Remove the chassis by placing one hand over the front, and turn the analyser
upside down. This will prevent the chassis falling out accidentally. Should the
chassis not come out very readily, bring the analyser sharply down on the flat
of the hand, which is guarding the front. Never substitute a hard surface for a
hand.
3. Unscrew the nuts on the cell supporting the gas connections (use
non-magnetic spanners).
4. Unsolder the electrical leads. Apply minimal heat to the pins on the cell.
5. Remove the two hexagon socket screws, which retain the cell and slacken the
third retaining screw, which is situated between the inside of the lower magnet
space and the chassis wall.
6. Withdraw the measuring cell and replace it with a new cell type 286. When
fitting a new cell, ensure that the ball of the dumbbell, which is nearest to
the cell window, is nearest the front panel.
7. Tighten the remaining screws in the reverse order described for the removal
of the cell.
8. Solder the electrical connections to the solder pins on the cell. Black to
the pin with a black spot near it and yellow to the pin with a yellow spot.
9. Reconnect the cell gas connections.
10. Adjust the zero and span of the analyser. 
Should the analyser not zero or the adjustment is at one end of its travel,
readjust the photocells. It may not be possible to span the analyser, in this
case change R23 on the printed circuit board 00262905, to a value, which gives
a reading with air between 20 and 22 % oxygen. For circuit diagram, see the
instruction manual.

9.4.13.11 Replacement of photocells
1. The photocells are located to the side of the magnet assembly, just in front
of and above the measuring cell.
2. Release the two screws, which fix the retaining plate to the photocell
mount.
3. Remove the screws and plate and manoeuvre the photocell mount through the
springs of the support.
4. Unsolder the leads.
5. Replace the new photocells on their mount in reverse order.
6. Leave the two retaining screws slack and pass nitrogen into the analyser.
7. Ensure the zero adjustment is at the centre of its travel and move the
photocells until the analyser reads as near to zero as possible. 
8. Tighten the screws and make a final zero adjustment.
9. Adjust the span.
10. Confirm with the analyserłs instruction book.

9.4.13.12 Replacement of LED
1. Remove the two screws, which hold the photocell mount to the control magnet
assembly.
2. Allow the photocell assembly and mount to lay away from the magnet.
3. Remove the two screws holding the LED mount. Withdraw the LED and mount and
unsolder the leads to the LED.
4. Remove sleeving from old LED and discard lamp. Replace with new LED and
sleeve and solder the leads.
5. Replace the LED and secure the retaining strip.
6. Replace the photocell assembly and mount.
7. Replace the cell.
8. Adjust the zero and span.
9. Confirm with the analyserłs instruction manual.
For replacing the amplifier board, meter, filter block and circuit description
do confirm with the instruction manual.

Any doubts about the analyser or its equipment, contact the manufacturer or any
of the manufacturer's agents. 



9.4.14 Riken portable oxygen indicator, Model Ox - 226



1. Summary
Riken portable oxygen indicator, Model OX-226 and OX-227 provide a quick,
convenient method for determination of oxygen content of any atmosphere. It is
intended primarily as an indicator of oxygen deficiency, with good readability
from 0
25%. The instrument is routinely calibrated on normal atmospheric
oxygen concentration (21%). These models are most suitable and recommended for
testing tanks, manholes, vessels and other spaces to determine safety from the
standpoints of oxygen deficiency before entering and while work is in
progress. 


2. Principle
The oxygen cell operates by an electro-chemical process in which a voltage is
set up between two electrodes. Under a test where one electrode is exposed to
the atmosphere, a change in oxygen concentration on this electrode produces a
proportional change in the cellłs output voltage.
Therefore, an increase in oxygen concentration will “speed up" the
electro-chemical process, producing a higher output voltage, and a decrease in
oxygen concentration will “slow down" the process, lowering the output voltage.
The centre electrode is exposed to the atmosphere by means of a Teflon membrane
placed directly in contact with the polished top surface. This Teflon membrane
serves two functions simultaneously. First, it has the ability to pass oxygen
molecules freely, thus placing the electrode in direct contact with the
atmosphere and secondly, it keeps the electrolyte contained in the cavity
between the two electrodes.


3. Measurement procedure
a). Preparation - Connect the sampling hose (6) to the gas sampling probe (7)
and then connect it to the gas inlet of the instrument.
b). Voltage checks of battery - Turn the control switch (1) to “Batt" zone and
check the meter needle marks inside of “Batt" zone. If the case of model
OX-226, the battery drop can be heard as a buzzer sound.
c). Span adjustment - Turn the control switch (1) to “25" and make span
adjustment by spanning adjusting knob so as to bring the meter needle to 21%.
When making span adjustment of Model OX-227, try it with 0-25% range. 



4. Measurement
After finishing the above procedure items 1, 2 and 3, the instrument is ready
to run. Introduce the sampling probe to the source and start measurement. In
the case of Model OX-226, when the oxygen concentration is less than 18% by
volume, alarm light (4) illuminates and it gives us the warning of oxygen
deficiency by buzzer sound.
Caution
1. Check the flow pump by the flow monitor during operation.
2. Operate the instrument in leather case when in use.
3. The replacement of batteries and recharging procedure must be done in
non-hazardous areas.


5.Maintenance procedure
The replacement of batteries and recharging procedure.
a). Take off the leather case from the instrument and turn the battery box knob
(11) to “open" position.
b). Pull out the whole battery box and replace the batteries with new ones.
c). When the replacement of batteries is finished, put back the battery box in
correct position and turn the battery box knobs (11) to “Lock" position with
finger press.
Replacement procedure (Ni-Cd battery).

When Ni-Cd batteries are used for the instrument, detach the label (12) of
charging inlet and insert the exclusive charger to the charging jack, and plug
the charger into AC 100V. The recharging takes 15 hours.


Replacement of sensor.
When the meter needle can not be adjusted to 21% by turning the span adjusting
knob and the indication of meter needle gets unstable, this is the sign to
replace the sensor. In this case, take off the bottom screws of the instrument
and remove the cover. The cover comes off by sliding it sidewise. Turn the
sensor to left and adjust the mark to “open". Now the sensor can be removed.
Insert the new sensor and turn it in clockwise direction to the mark “lock".
Place the cover back.





Replacement of filter
The filters are filled in the gas-sampling probe and in instrument. When they
appear dirty, replace them with new ones.




Take off the tip of the sampling probe by turning the metal part of roulette
and replace the cotton filter with a new one.





Pull out the filter holder (10) of the instrumentłs flank and take out the
filter. Replace it with a new one.

Zero adjustment
As the zero adjustment is factory set, there is no need of zero adjustment
procedure in normal operation. But, when it is high sensitive type instrument
such as Model OX-227A with 0-5 and 0-25% etc., make zero adjustment. Induct
100% clean nitrogen and turn the adjusting screw to bring the needle to zero.


9.4.15 Detector tubes for health hazardous gases
Health hazardous gases may be detected through chemical colour reactions, and
several manufacturers make metering pumps and accompanying detector tubes for a
great number of various gases.
Probably the most convenient and suitable equipment to use for measuring very
low concentrations of toxic gases on board tankers are chemical indicator
tubes.
These tubes consist of a sealed glass tube containing a proprietary filling which
is designed to react with a specific gas and to give a visible indication of
the concentration of that gas. To use the device, the seals at each end of the
glass tube are broken, the tube is inserted in a bellows-type fixed volume
displacement hand-pump, and a prescribed volume of gas mixture is drawn through
the tube at a rate fixed by the bellowłs expansion rate. A colour change occurs
along the tube and the length of the discoloration, which is a measure of the
gas concentration, is read off a scale integrated with the tube. In some
versions of these instruments, a hand operated injection syringe is used
instead of a bellow pump.
It is important that all the components used for any measurement should be from
the same manufacturer. It is not permissible to use a tube from one
manufacturer with a hand pump from another manufacturer. It is also important
that the manufacturersł operating instructions are carefully observed.
Since the measurement depends on passing a fixed volume of gas through the
glass tube, if an extension hose is used it should be placed between the glass
tube and the hand pump.
The tubes are designed and intended to measure concentrations of gas in the
air. Thus measurements made in a ventilated tank, in preparation for tank
entry, should be reliable.
Under some circumstances errors can occur if several gases are present at the
same time, as one gas can interference with the measurement of another. The
manufacturer should be consulted for guidance.
For each type of tube the manufacturer must guarantee the standards of accuracy
laid down by national standards. Tanker operators should consult the regulatory
authority appropriate for the shipłs flag.

9.4.16 Dräger Multi Gas Detector 
In our experience, detector tubes and metering pumps made by “Dräger" are the
most frequently used. A more detailed description is given in the instruction
book for “Dräger Multi Gas Detector".
Various chemical substances are used for tube fillings, depending on the gas to
be analysed. For some gases there are several types of tubes, so that there are
tubes for measuring very low concentrations and for measuring larger
concentration ranges. In some cases two scales will be marked on the tube,
corresponding to different numbers of pump strokes.
It is important that the pump is checked to see if it is tight before it is
being used, sealing the opening with an unused detector tube does this. The
bellows should then use more than 10 minutes to expand for the pump to be
satisfactory. Cleaning the valves, according to the instructions accompanying
the instrument may usually eliminate any leakage that has arisen.
To avoid corrosion, the pump must be purged with air by performing a number of
pumping strokes each time after use.
To perform measurements with difficult accessibility, an extension hose may be
used. The detector tube is placed in the suction of the hose.



9.4.16.1 Opening of the tubes
Both ends of the tube are opened in the hole, which is provided for that
purpose in the pump. A breaking socket accompanying the apparatus can also be
used for this. This prevents glass fragments from falling down.



9.4.16.2 Installation of the tube in the
pump
The opened sampling tube is inserted into the pump head so that the arrow on
the tube points toward the pump. The tube must be attached firmly and tightly
in the pump head so that false air is not sucked in.





9.4.16.3 Suction of a gas sample
The bellow is pressed together completely and is then released. During the
compression the air is squeezed out of the bellow through an exhaust valve. The
suction action of the pump takes place when the compression springs inside the
bellow expand after the compression. The air (to be measured) flows through the
sampling tube and into the bellow while this again expands to its original volume.
The suction movement comes to an end when the distance chain is tight once
again, and at this stage 100cm3 has been sucked through the tube.
The operating instructions, which accompany each packet of tubes, give i.e. the
approximate time for each pump stroke, for example 15 - 25 seconds. The time
will depend on how tightly the powder is packed in the tube.
The specified number of pump strokes, indicated in the operating instructions,
should be used for each sampling tube.


9.4.17 MSA
Detector Tubes and Kwik-draw Pump


Features
1. Quick and inexpensive to use.
2. A reliable method of testing more than 120 hazardous gases and vapours.
3. Kwik-draw pumps offer accurate one-handed automatic stroke counter and
unique end of stroke indicator on deluxe version.
4. Tubes are printed with easy-to-read scales.
5. Specialised kits are available for use in HAZMAT work and underground
storage tank applications.



Description
SAÅ‚s Kwik-Draw and Kwik-Draw Deluxe Pumps can be used with an assortment of MSA
detector tubes to spot-test the atmosphere for a wide variety of toxic
substances. Kwik-Draw Pumps are designed for one-hand operation and consistent
delivery of a sample draw volume of 100 millilitres (ml). The pumps are
constructed with a shaft-guided compression system for a more consistent and
replicable flow rate and volume per stroke than may be available with
hand-guided pumps. MSA offers detector tubes for measuring more than 150 gases
and vapours.





Kwik-Draw Detector Tube Pumps
Kwik-Draw Pumps allow detection of gases and vapours with the squeeze of a
handle. To obtain a precise (100ml) sample volume, the user simply grasps the
handgrip and pushes the knob. The pumpłs compression system provides the
guiding action to drive a spring-loaded bellow pump.
An internal easy-to-read stroke counter shows the exact number of strokes
performed and provides a positive stop when the stroke is fully compressed.
A second model, the Kwik-Draw Deluxe Pump has a unique end-of-stroke indicator
that “winks" after the precise volume of air is drawn, confirming that enough
air has been sampled for a successful reading.


Detector tubes.
MSA/Auer detectors are made of glass, have break-off tips and are filled with
treated chemical granules for sampling a variety of substances. Most MSA/Auer
detector tubes are packaged 10 in a box.
For ordering information, see the Detector Tube Summary Chart which follows the
Detector. After selecting the appropriate tube, the user would break off the
tubesł end tips and attach the tube to the sampling pump. After air is drawn
through the tube by the pump, the chemical layer in the tube changes colour if
the test gas or vapour is present in the air.
The length or shade of the colour-change, indicates the concentration of the
gas or vapour in the air. A scale is printed on each tube for interpretation of
data.

Controlled Interchange ability of MSA/AUER Detector Tubes and Pumps with Other
Manufacturersł Tubes and Pumps.

As long as a pump meets the following criteria, it may be used with any
detector tube designed for use with that kind of pump. Pumps meeting these
criteria are interchangeable.
1. The characteristics of the pump- volume per stroke, sampling time and flow

must be within the same accuracy range.
2. The detector tubes must have an outer diameter of 7 mm and be
factory-calibrated with a pump that meets the criteria of (1) above.
3. The manufacturer of tubes and pumps must operate under a certified quality
assurance program. 
Based on these criteria, the following pumps are interchangeable:
· MSA's Kwik-Draw Pumps.
· AUER's Gas Tester II H Pump.
· Dräger's Model 31 Bellow Pump.
· Dräger's Accuro Pump.


Sampling Pump Operation and Maintenance.

The Kwik-Draw Pump is designed to measure concentrations of gases and vapours
when used with AUER/MSA Detector Tubes.
Description - The Kwik-Draw Pump is a one-handed, manually operated bellow pump
of 100cc capacity.
Tube Holder - This rubber part permits mounting of detector tubes, remote
sampling lines or other detectors.

Filter Disc - This porous plastic disc mounted in the rubber tube holder
protects the pump from dirt and dust particles, which may alter the flow or
damage the pump.

Exhaust Valve - Located under the valve cover, this valve closes as the bellow
re-inflates, and readily opens on the exhaust stroke so that blow-back through
the tube holder is negligible.

Stroke counter - For convenience, a stroke counter is incorporated into the
pump handle.

End-of-stroke indicator - As the bellow begins to re-inflate, and after the
knob is released, the indicator eyeball turns high visibility green. As the
vacuum decreases, the eye begins to roll back to black. The stroke is over when
the eye is all black.

Note! - Kwik-Draw Pump (part no. 488543) does not have an end-of-stroke
indicator.

Operation
· Using the breaker on the pump, break off both tips of the detector tube.

· Using a twisting motion, insert the tube into the rubber tube holder. The
arrow on the tube should point toward the pump.

· Re-zero stroke counter.

· With all four fingers on the handle, depress the knob with your palm.

Note! Watch the stroke counter to ensure proper sample volume, the counter will
only advance if a full pump stroke is taken.
· Release the knob.

· As the pump re-inflates, the end-of-stroke indicator turns to high-visibility
green. The stroke is over when the eye returns to the all black state.

Note! If your pump does not have the end-of-stroke indicator, wait 30 seconds
after full bellow inflation to ensure that all 100cc of the sample has been
drawn through the tube. The detector tube must be held in the sampling area
during this period.

· To evaluate the stain, follow the instructions provided with the detector
tubes.

Remote sampling
Remote sampling is accomplished by putting the pump, connecting tube, remote
sampling line and detector tube together, in this order.

Maintenance
Under conditions of normal use, this pump should require little maintenance.
Depending on the frequency of use, periodic cleaning and checks for correct
performance as recommended.
Tube holder - Replace tube holder when it shows signs of wear or loss of
elasticity. If filter is not clogged or cracked, save the filter discs for
re-use in new tube holder.
Filter disc - Periodically remove the filter disc for cleaning or replacement.
Remove filter disc from tube holder by rolling flange part of tube holder down
and away from the disc.
Gently tap or blow on the surface to remove any foreign matter.
Replace disc so previously exposed surface is once again facing away from pump.

Shaft
If shaft becomes dirty or if bellow inflation is jerky, remove shaft by
unscrewing, then clean with auto wax.

Valves
1. With the valve cover removed, check the valves for dirt or debris.

2. Remove dirt with a gentle puff of air or by using a soft brush.

3. Replace valve(s) if necessary.

Pump performance test
After extended idleness and periodically during use, check the pump for proper
performance with the following test:

1. Plug pump inlet by inserting an unbroken detector tube into tube holder.

2. Deflate pump fully, release, and wait 10 minutes. The pump is leak-free if
the distance from the bellow to the frame is ½ inch or greater after 10
minutes. If the pump leaks check the tube holder and, if necessary, the valves
(see Maintenance). After repair, re-test for leakage.

Warning! Use of a pump that leaks may result in the under-estimation of a
hazard and could result in property damage, injury or death.

Read the instruction book following the Detector!


9.5 FLAMMABILITY COMPOSITION DIAGRAM
In 9.4, we saw that when measuring oxygen content we use the instrument
“Servomex OA-262". For measuring hydrocarbon gases in percent of volume the
instrument, we use the “Riken 17HC", and an explosimeter, “Riken GP-204" is
used for measuring combustible gas below LEL in air. Finally, the “Drager Multi
Gas Detector" is used for measuring low concentrations of toxic gas.
When an inert gas, typically flue gas, is added to a hydrocarbon gas/air
mixture, the result increases the lower flammability limit hydrocarbon concentration
in order to decrease the upper flammability limit concentration.
The effects illustrated on the following two diagrams, one for crude oil and
one for propane gas, should only be regarded as a guide to the principles
involved, and should not be used for deciding acceptable gas compositions.
Every point on the diagram represents a hydrocarbon gas/air/inert gas mixture,
specified in terms of its hydrocarbon and oxygen contents.
The first flammability diagram is for hydrocarbon gas above crude oil where the
UEL is 11% and the LEL is 1,5%. The left side of the diagram (vertical) gives
the hydrocarbon gas value. The bottom line of the diagram (horizontal)
represents the oxygen content from 0% to 21%. Note that 21% oxygen represents
100% clean air and, as mentioned before, 10,8% by volume of oxygen is the
minimum oxygen content present in a mixture's ignition.
Also when measuring 21% oxygen, the atmosphere contains 100% clean air, so the
ratio will be close to one to five. For example when measuring the oxygen
content at 20,5% volume, 0,5% volume oxygen is still missing in the atmosphere
in order to call it clean air. When multiplying 0,5% with 5, the result will be
2,5%, and the amount of clean air in the atmosphere will be (100-2,5)= 97,5%.
The missing 2,5% from the clean air contain unknown gas concentrations. So,
proper tank venting is extremely important.
The following example is based on a cargo of crude oil. During discharging the
cargo tanks where refilled with inert gas. The quality of the supplied inert
gas was in accordance to the regulations in force. After discharging is
completed the tank atmosphere contains a mixture of inert gas and “cargo gas".
During the ballast voyage, the arrival ballast tanks are cleaned. Water washing
takes place in an inerted atmosphere and before any tanks are vented with air,
they have to be re-inerted, in order to avoid entering the flammable zone.





Follow
example “1" on the this page:
After water washing is completed the measurements in point “A" give the
following values: HC=15%, O2=3%. These values are plotted on the diagram and
give the point “A".
Point “B" represents 21% oxygen and 0% HC.
From point “B" a line is drawn against the left side of the diagram by keeping
sufficient clearance from the flammable zone. The inert gas that is supplied
contains an O2 of 4,5%.
Point “C" on the diagram is the point, which the re-inerting is heading toward.
All measurements will follow a straight line from point “A" to point “C".
Where the line from point “A" crosses the line from point “B" the following
measurements are found: HC= 2,6% and O2= 4,2%, point “D".
In point “D" we stop the inerting and start venting with air. All measurements
taken from now on will follow a straight line from point “D" toward point “B".
If we had started to vent with air at point “A", all the measurements taken
would have followed a straight line from point “A" toward point “B", through
the flammable zone involving a great deal of danger. Avoid all contact with the
flammable zone!
At point “D" there it is still too early to use the explosimeter, because the
content of hydrocarbon gas is above LEL. Continue to use the instrument, which
measures hydrocarbon gas by volume until the HC-gas content is below 1,5% by
volume. This is the LEL value in this example. Also, to repeat, the hydrocarbon
concentration of 1,5% by volume corresponds to 100% LEL.

In point “E", the measurements are HC= 1% and O2= 15,3%. The hydrocarbon gas
concentrations are now below LEL and the explosimeter can be used. It is also
possible to calculate the explosimeter reading ahead of the measuring by using
the formula where measured HC gas is multiplied by 100 and divided by LEL. In
our example, the explosimeter in point “E" will show (1 x 100): 1,5 = 66,67% of
LEL.
After sufficient venting the measuring in point “B" will be HC= 0% and O2= 21%.
The tank is ready to enter.
After inspection/repairing, the tank(s) must be re-inerted before arrival at
loading port in order to achieve the required tank atmosphere according to the
regulations in force.

Example “2" .






The above example is based on propane cargo. In the example, nitrogen and air
are used for tank purging and air venting. Follow the diagram for propane.
After some time purging with nitrogen, a measurement is taken at point “A": HC=
12% and O2= 0%.
Just like in example “1", a line is drawn from point “B" toward point “C" on
the left side of the diagram by keeping sufficient clearance from the flammable
zone. Continue to purge with nitrogen until reaching point “C" where the
measurements are HC= 3,75% and O2= 0%.
Stop purging with nitrogen at point “C". Start dilution with air. All
measurements will now follow the straight line toward point “B".
Take a measurement at point “D" which is HC= 2,4% and 02= 8%. When the LEL for
propane is 2,1% by volume, it is too early to use the explosimeter.
At point “E", the measurements are HC= 1% and 02= 15,25%. The explosimeter will
in point “E" show (1x100) : 2,1 = 48% of LEL.
After sufficient dilution, point “B" end up with measurements of HC= 0% and 02=
21%.
After inspection/repairing etc., the tank must be treated according to routines
and regulations.