15.0.0.1
The TwinPipe system
Overview
Introduction This section describes:
- the principles
- the special design rules
- the properties
of the TwinPipe system.
If the design rules and other properties coincide with those of the single pipe system, referen-
ce will be made to this.
Pipe dimensioning, pressure loss calculation etc. are not included in this section.
Contents System description 15.1
Installation methods 15.2
Straight pipe runs 15.3
Directional changes 15.4
Branches 15.5
Other components 15.6
Excavation of trench and expansion zones 15.7
Heat loss 15.8
Surveillance 15.9
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15.1.0.1
TwinPipes
System description
The TwinPipe Like the single pipe system ALSTOM TwinPipe system is a bonded system, in which
system service pipe, insulation layer, and casing pipe are cast together in a sandwich construction to
a firm unit.
In this system flow and return pipe are embedded in the same outer casing.
The bonded pipe principle means that the expansions or contractions, occurring in the steel
pipes as a result of temperature variations are transmitted through the insulation to the outer
casing.
Consequently, the actual movements take place between the outer casing and the sur-
rounding sand and due to the friction the movements are considerably reduced, compared to
non-buried systems.
The frictional forces are converted into axial stresses in the steel pipes and the ability to
absorb these is the basis of the function of the bonded TwinPipe system.
Control of expan- The exposure of the two steel pipes to
sion movements varying temperature influence results in an
irregular longitudinal expansion of the two
pipes.
In order to ensure the pipe system against
too large movements between the steel
pipes, fixing bars are welded onto the ends
of all straight pipe runs.
The fixing bars are only placed at the ends of
the pipe runs where the expansion move-
ments are largest.
On the rest of the pipe run the two steel
pipes are secured in relation to each other by
means of power transmission through the
insulation foam.
The TwinPipe system has built in fixing bars
in all preinsulated fittings components except
preinsulated venting valves.
However, on preinsulated branches there are
only fixing bars on the branch pipes (See
15.5.0.1).
Straight pipes and curved pipes are not
delivered with fixing bars.
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15.2.0.1
TwinPipes
Installation methods
Overview of ALSTOM bonded TwinPipe system is installed after 2 different installation methods:
installation
- Heat prestressing (I)
methods
- High axial stress installation (IV)
The installation methods influence the first movement of the pipe system and the maximum
axial stress in the TwinPipe system.
Detailed description of the principles of the installation methods are found in section 1.3 of this
manual.
Conditions of the Due to the influence between the steel pipes the basis for calculating first movements and fric-
calculation tional lengths is another than the one for single pipes.
Basis of calculation for table values:
- Flow temperature (Tf) 130°C
- Return temperature (Tr) 70°C
- Soil temperature 10°C
- Average temperature difference 90°C
- Heat prestressing temperature 55°C
Values, corresponding to a tempeature variation of Ä… 45°C are used as key figures.
The bonded TwinPipe principle means that the PUR-foam bonds the casing and the two iden-
tical steel pipes together in a secure sandwich construction.
When calculating frictional length and expansion movement an average temperature for flow
and return (Tf + Tr) / 2 is applied.
This simplification is possible, because the two steel pipes have the same dimension.
To calculate the stress in the individual steel pipe use the actual medium temperature in the
steel pipe.
Prestressing (I) Due to prestressing to mean temperature the
first movement after backfilling will be a con-
traction, corresonding to "l45.
The subsequent movements within the tem-
perature difference range will be 2 x "l45.
L45
Pipe runs longer than L45 are locked by fric-
+112,5 N/mm²/10ÚC
tion and consequently without expansion
movement.
The stress in steel pipes is 2.5 x "T (N/mm2).
Prestressing to 55°C makes the TwinPipe
stress-free when flow as well as return pipe
-112,5 N/mm²/100ÚC
l45
have this temperature.
If both pipes are cooled to 10°C, then both
the average and the actual temperature are
10°C.
l45
So the stress is:
2.5 x (55 - 10) = 112.5 N/mm2.
Design · 2003.01 ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone]
15.2.0.2
TwinPipes
Installation methods
High axial If high axial stresses without relief are
stresses (IV) allowed, then heating within the temperature
difference range will result in a first move-
ment, corresponding to 4 x "l45.
As is the case with other installation methods
L45 L45
the subsequent expansion movements will
be 2 x "l45.
+112,5 N/mm2/10ÚC
Pipe runs longer than 2 x L45 are locked by
friction and consequently without expansion
movement.
Calculation of stresses, see the installation
method heat prestressing. 4 x "l45
- 225 N/mm2/100ÚC
Full load, flow = 130°C and return = 70°C
means that the average temperature is
(130 + 70) / 2 = 100°C. "l45
However, here there is no heat prestressing,
so the average stress is:
2.5 x (10-100) = -225 N/mm2.
References Design: General, system definitions 1.2
installation methods 1.3
Frictional lengths and first movements 15.3.0.1
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone] Design · 2003.01
15.3.0.1
TwinPipes
Straight pipe runs
Installation Long straight TwinPipe runs may be installed after the following installation methods:
methods - Heat prestressing (I)
- High axial stress installation (IV)
Pipe dimension Frictional Expansion
Frictional lengths From the table the frictional lengths and first
length movement
L45 and expan- movements, described in section 15.2 appe-
Å‚ out. mm L45 m "l45 mm
sion movements ar.
33.7/140 28 8
"l45
"
42.4/160 32 9
48.3/160 37 10
60.3/200 42 11
76.1/225 47 13
88.9/250 54 15
114.3/315 62 17
139.7/400 61 17
168.3/450 73 20
Fixing If a straight TwinPipe run is terminated without connection to preinsulated components, fixing
bars must be welded on (on both sides of the pipe pair). See page 15.1.0.1.
References Product Information: TwinPipes 6.0
Design: Straight runs, general 2.0
Handling and Installation: TwinPipes, pipe trench 14.1
TwinPipes, straight pipes 14.2
Design · 2003.01 ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone]
15.4.0.1
TwinPipes
Directional changes
Possibilities of Dependent on pipe dimension and joining method horizontal directional changes on the
directional TwinPipe system can be made in the following ways:
change - Preinsulated bends
- On-site curved pipes
- Curved pipes
- Bevellings (in general not recommended)
Preinsulated Preinsulated bends may be divided into 2
bends groups:
- 90° bends
- 5-75° bends
- 90° bends:
Leg lengths are carried out according to
the table on page 3.2.1.1.
- 5-75° bends:
Here the curve on page 3.2.2.3 for short
pipe runs applies.
However, the values are % of L45 in stead
of L60.
Dependent on the project it may be neces-
sary to protect the bends with anchors.
On-site curved Preinsulated TwinPipes Å‚ 33.7/140 - Å‚
pipes 60.3/200 mm can be bent to on-site curved
pipes in connection with installation of the
pipes when using the ALSTOM bending tool.
Only horizontal bending.
Steel pipe Tool no.
Å‚ out. mm
39.7-48.3 3
60.3 4
Application range
Steel p=0,05 p=0,058 p=0,065 p=0,073 p=0,09 Elastic
pipe h=0,4 h=0,5 h=0,6 h=0,7 h=0,8 radius
for on-site curved
Å‚ out.
pipes
mm v° Rp v° Rp v° Rp v° Rp v° Rp v° Re
max max max max max e
33,7 9,5 8 7 6,5 5 35 19,5
42,4 10,5 9 8 7 6 27 25,5
48,3 12 10,5 9,5 8,5 6,5 25 27,5
60,3 14 12 10,5 9,5 7,5 18 38
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone] Design · 2003.01
15.4.0.2
TwinPipes
Directional changes
Curved pipes Preinsulated TwinPipes Å‚ 76.1/225 - Å‚ 168.3/315 mm can be delivered prebent on a machine.
Conditions:
- Only horizontal bending.
- Installation method I (or IV, when relieved, max. 150 N/mm2).
- Average stress on steel pipes: 125 N/mm2.
- p: earth pressure in N/mm² each pipe
- h: soil cover (m)
- v°
max: max. angular displacement each 12 m
- Rp: min. design radius (m)
v° and Rp refer to the centre line of the trench
max
- v°
e: max. elastic angular displacement each 12 m
- Re: min. elastic radius (m)
Application range
Steel p=0,05 p=0,058 p=0,065 p=0,073 p=0,09 Elastic
pipe h=0,4 h=0,5 h=0,6 h=0,7 h=0,8 values
for 12 m Twin
Å‚ out.
curved pipes
mm v° Rp v° Rp v° Rp v° Rp v° Rp v° Re
max max max max max e
76,1 20 34,5 20 34,5 20 34,5 20 34,5 20 34,5 14 49
88,9 23 30 23 30 23 30 23 30 23 30 12 57
114,3 31 22 31 22 31 22 31 22 31 22 10 69
139,7 33 21 37 19 37 19 37 19 37 19 8 85
168,3 27 25,5 32 21,5 35 20 39 18 39 18 6 115
As regards 16 m TwinPipes the limitations
Application range Steel pipe Elastic
Å‚ out. mm values
are due to the bending machine and not the
for 16 m Twin
mm v° Rp v° Re
earth pressure and soil cover. max e
curved pipes
76,1 - - 19 49
88,9 3 305 16 57
114,3 12 76 13 69
139,7 18 51 11 85
168,3 22 42 8 115
Bevellings Avoid bevellings in TwinPipe systems i.a. due
to adaption of steel pipes. It is however
recommended to use the elasticity of the
pipes after straight welding instead and/or to
use preinsulated bends or curved pipes.
References Product Information: TwinPipes 6.0
Design: Bends 3.2
On-site curved pipes and curved pipes 3.3
Handling and Installation: TwinPipes 14.0
Tools for on-site curved pipes 24.6
Design · 2003.01 ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone]
15.5.0.1
TwinPipes
Branches
Branch possibiliti- Branches on/from the TwinPipe system can be made in the following ways:
es
- Preinsulated branches
- On-site made branches
Common to both ways is that they can be carried out without expansion bend.
Preinsulated Preinsulated branches for the TwinPipe
branch system have welded fixing bars and reinfor-
cement plates on the branch pipes.
If the branch is built in at the end of a pipe
run without e.g. prefabricated bend, then fix-
ing bars must be welded on to the main pipe
of the branch.
On-site made In connection with on-site made branches
branch reinforcement plates should in some cases
be welded on to the branch pipes (A).
Consequent use of reinforcement plates
(A)
complies with the more stringent require-
ments in accordance with standard draft
prEN 13941:2000.
When branching to TwinPipe also weld on
fixing bars (B).
(A)
(B)
References Product Information: Branches 6.6
Design: Reinforcement plates, general 4.2
Handling and Installation: TwinPipes, branches 14.4
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15.6.0.1
TwinPipes
Other components
Relief Due to special piping and differences in temperature and stresses some components must be
placed in the TwinPipe system so that they are relieved.
Preinsulated iso- Place preinsulated isolation valves in the pipe
lation valves run as illustrated.
Max. 1/3 x L45
Note! There must be sufficient expansion
possibility in the valve chamber.
Straight transition Secure straight transition pipes against over-
pipe load by placing them as shown in the illustra-
Twin - single pipe tion.
Min. B
Max. 1/3 x L45
90° transition Secure 90° transition pipes against overload
pipe by placing them as appears from the illustra-
Min. M
Twin - single pipe tion.
Max. 1/3 x L45
References Product information: Isolation valves 6.7.1.1
Transition pipe 6.9
Design: Design with 90° bends 3.2.1.1
Design · 2003.01 ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone]
45
Min. M
Max. 1/3 x L
15.7.0.1
TwinPipes
Excavation of trench and expansion zones
Basis In order to protect the pipes against sharp stones etc. and establish a uniform friction against
the outer casing on the entire pipe run, make the trench as described in this section.
Cross section of Make the cross section of the trench as
trench shown on the illustration as a minimum.
Min. 100 mm stoneless sand around the
min.
400
!
pipes.
mm
100 mm
In areas with heavy traffic the 400 mm are
measured from the top of the pipe to the
bottom of the asphalt or concrete layer.
100 mm
In areas with no or only light traffic the 400
100 mm
100 mm
mm are measured from the top of the pipe to
the top of the grass or gravel layer.
Allowable surface pressure, max. 900 kPa.
The instructive measurements do not take
slopes and possible local requirements to the
trench into account.
Do not forget warning tape (!).
Welds and muffs Excavate additional 250-300 mm at welds
and muffs under and along the pipes to
ensure sound working conditions.
When using a preinstalled muff ensure that
there is sufficient space for it, min. 1000 mm.
1000 mm
Branches As branches on the TwinPipe system are
always made at the same level as the main
pipe, the TwinPipe system can always be
installed with a minimum cover.
Expansion zones Due to the limited expansion movements of the TwinPipe system special expansion zones are
normally not required - not at bends nor at branches.
References Design: Single pipe 9.0
Handling and Installation: TwinPipes, trench 14.1
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15.8.0.1
TwinPipes
Heat loss
Heat loss The calculation of heat loss from TwinPipes is much more complicated than the calculation for
systems with two single pipes. However, the values are often used to compare the heat loss
between different systems and insulation thicknesses and therefore it is essential that the con-
ditions and basis of calculation are known.
ALSTOM Power FlowSystems has chosen the  first-order multipole formula, described by Mr.
Peter Wallentén in  Steady-state Heat Loss From Insulated Pipes . (Lund Inst. of Techn.,
Sweden 1991).
The formulas contain only two materials: Insulation and soil (surroundings). The influence of
steel pipes and outer casings are not considered which is completely acceptable for normal
pipe dimensions.
Wallentén s formula has been compared to FEM, the Finite Element Method, and has given
good identical results when calculating the total heat loss and the heat loss of the flow pipe
alone. The deviations were from 1 to 1.5%.
On return pipes the variations were higher; but they only have small influence on the total
result due to the essentially lower heat loss.
Calculation Formulas (1) and (2) are used to calculate the heat loss of flow and return pipes.
formulas, general
Formulas (3) to (10) are used as an input to formulas (1) and (2).
Design · 2003.01 ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone]
15.8.0.2
TwinPipes
Heat loss
Symbols Q Heat loss per m pipe (W/m)
g Formula, used as preliminary result
à Formula, used as preliminary result
Å‚ Formula, used as preliminary result
T Temperature
 Thermal conductivity
Ä„ <" 3.14
H Depth of pipe centre
D Outer diameter of circular geometry
C Distance between steel pipe centres
Indices twin TwinPipe
sy Symmetrical problem
as Anti-symmetrical problem
a Ambient temperature
i Insulation
s Service pipe
TwinPipe versus Only when the individual heat loss of the pipes is known is it possible on the basis of the given
pipe pair operating conditions to assess whether TwinPipes or pipe pairs are the optimum choice as
regards heat loss.
ALSTOM Power FlowSystems has therefore chosen to show the results for TwinPipes, partly
as the total loss for flow and return pipe and partly individually for flow pipe.
Summary of conclusions, drawn on the basis of the shown illustrations:
- The use of TwinPipes instead of pipe pairs with standard insulation results in considerable
savings both on the flow and the return pipe.
- The heat loss from a Twin flow pipe is similar to that of a plus-insulated flow pipe pair up to
and including DN 100.
- However, the total heat loss of the TwinPipe is lower than that of a corresponding plus insu-
lated pipe pair. This means that most of the savings in heat loss is on the return pipe.
- Consequently, it is primarily the value of the savings on the Twin return pipe which are deci-
sive for the choice on the basis of heat loss criterions.
Not much energy must be added to the return water, before it is sent out in the system
again. Lowering the flow temperature or water velocity will however not be possible when
using TwinPipes compared to a Plus pipe pair.
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone] Design · 2003.01
15.8.0.3
TwinPipes
Heat loss
Example 1 Total heat loss in W/m for pipe pair (two single pipes in standard or plus insulation) and for
Total heat loss TwinPipes
W/m
45
40
35
30
25
20
15
10
5
0
33.7 42.4 48.3 60.3 76.1 88.9 114.3 139.7 168.3 Å‚ mm
Standard pipe pair
Plus pipe pair
Twin pipe
Example 2 Heat loss in W/m for flow pipe of pipe pair in standard or plus insulation and for TwinPipe.
Heat loss
W/m
Flow pipe
45
40
35
30
25
20
15
10
5
0
33.7 42.4 48.3 60.3 76.1 88.9 114.3 139.7 168.3 Å‚ mm
Standard pipe pair
Plus pipe pair
Twin pipe
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone] Design · 2003.01
15.8.0.4
TwinPipes
Heat loss
Calculation con- The above curves are based on the following conditions:
ditions
Soil cover: 0.6 m
Thermal conductivity of the soil 1.6 W/mK
Thermal conductivity of the insulation 0.027 W/mK
Flow temperature 80°C
Return temperature 40°C
Soil temperature 8°C
Distance between single pipes 0.15 m
If specific calculations are required due to essential changes in the conditions, then contact
ALSTOM Power FlowSystems.
References Design: Heat loss, general 24.1
Internet Heat Loss Calculation www.flowsystems.dk
Design · 2003.01 ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone]
15.9.0.1
TwinPipes
Surveillance
General All preinsulated TwinPipes and preinsulated components to match are as a standard delivered
with embedded copper wires for surveillance of the pipe system against:
- Leakage: Welding fault
Installation fault
- Damages due to excavation
Moreover, the embedded wires ensure the possibility of systematic maintenance of the pipe
system.
(Flexible pipes are delivered without surveillance wires).
Design In principle the surveillance system for the TwinPipe system is designed as the one for the
single pipe system, but it is more simple as there is no coupling between the flow and the
return pipe.
References Product Information: Surveillance system 16.0
Design: Surveillance system 25.0
Handling and Installation: Surveillance system 23.0
Design · 2003.01 ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. [phone]