# 78 PUMP & PUMPING OPERATION
A pump is to be used to pump fresh water at 25 degrees C. The vapour pressure of water at 25 degrees C=3 kN/m2.
If the atmospheric pressure is 105 kN/m2 ,the maximum suction lift is 6,7m, and the pipeline losses on the suction side are equivalent to a head of 3,2m.What will the pump NPSH available be ?
+ 0.5 m
If the NPSHA is less than the NPSHR then ?
+ The pump will suffer damage.
The efficiency of a pump is not dependent on ?
+ NPSHA
By starting a centrifugal pump with the delivery valve closed then ?
+ The starting load on the motor will be minimum.
If NPSHA is too small then it can be increased by ?
+ Reducing the length of the suction side pipe work.
If a pump had available NPSH of 0.5m would a NPSH requirement of 0.48 m. would be suitable for this purpose?
+ No.
Positive displacement pumps must not have then delivery restricted because ?
+ A constant volume is displaced.
What is the advantage of a double entry centrifugal pump ?
+ It has a lower NPSH required characteristic, giving advantages in poor suction conditions
Centrifugal pump need priming because ?
+ It is the movement of the liquid from the eye of the impeller to the discharge that causes a low-pressure region at the suction, if the pump is started full of air this movement of liquid does not occur therefore on suction pressure is created
Which one of the these option is NOT a cause of excessive vibration in centrifugal pumps ?
+ Discharge valve partly closed
The energy transformation within a centrifugal pump is as follows ?
+ The impeller creates centrifugal force, with increases the liquid velocity, an increase in velocity means an increase in kinetic energy, the increased kinetic energy is converted into pressure by reducing the velocity in the volute casing
Why is the axial flow pump ideal for trimming and heeling duties ?
+ It is reversible and has a high capacity flow
Cavitations of the fluid in a centrifugal pump is caused by ?
+ The vapour pressure in the suction pipe falls below the vapour pressure of the liquid at a given temperature
Positive displacement pumps are ideal for fuel transfer duties because ?
+ They can handle large volume of vapours and gases
Which of the following statements is correct?
+ Positive displacement pumps can require priming
In order to pump liquids positive displacement pump rely on ?
+ Change of volume of the pump chambers
Positive displacement screw or vane pumps are used for oily water separator supply because ?
+ They do not cause oil droplet size to change
Positive displacement pump can only vary their delivery rate by changing the speed of the drive motor ?
+ False
Positive displacement pump require a relief valve because?
+ The pump gives a constant flow rate
In centrifugal pump series operation the flow through all the pumps is equal ?
+ True.
The main reason for operating pumps in parallel ?
+ Is to allow a wider range of flow than for a single arrangement
Under what system condition does it turn out that operating pumps in parallel achieves very little benefit ?
+ If the piping system is undersized or some component in the system act as a bottleneck
The main reason for operating pumps in series is ?
+ So that commercially available pumps ( i.e. small ) can be used and to avoid extremely high discharge pressures which would be necessary with only one pump
When operation pump in parallel the increments of flow from doubling the number pumps is correspondingly doubled, tripled for three pumps etc?
+ False.
Which one of the following statements is true ?
+ Centrifugal pumps cover the lower ranges of specific speeds, axial flow the higher, and mixed flow the intermediate values.
The main reason for operating pumps in parallel ?
+ Is to allow a wider range of flow than for a single arrangement