A pump is to be used to pump fresh water at a temperature, where the vapour pressure of the water is equivalent to the pressure caused by 0,2 m head of fresh water, and atmospheric pressure is equivalent to 10,6 m head of fresh water. The maximum suction lift is 6,0 m, and the pipeline losses on the suction side are equivalent to a head of 3,0 m. What will the pump NPSH available be?
Cavitation of the fluid in a centrifugal pump is caused by:
Too high a speed of impeller rotation creating adverse heat.
The vapour pressure in the suction pipe falls below the vapour pressure of the liquid at a given temperature.
The viscosity of the fluid is too high, the extra power absorbed being converted into heat.
If NPSHA is too small, how can it be increased? Select the ONE correct answer:
Reduce the length of the suction side pipe work.
Throttle the suction valve.
If a pump had available NPSH of 0,5 m would a NPSH requirement of 0,48 be suitable for this purpose?
If a pump has a NPSH requirement of 0,5 m, would an available NPSH of 0,45 m be suitable?
Is the following statement True or False? No types of positive displacement pump can change their delivery rate, unless the speed of the drive motor is changed.
Is the following statement True or False? When operating pumps in parallel, if we use three pumps we will get three times the flow.
Is the following statement True or False? In centrifugal pump series operation, the flow through all of the pumps is equal.
Is the following statement True or False? When operating pumps in parallel, if we double the number pumps the flow is also doubled.
In this pump a single acting pump or a double acting pump?
Is this statement True or False? Positive displacement pumps can only vary their delivery rate by changing the speed of the drive motor.
Under what system conditions would operating pumps in parallel achieve very little benefit? Select the ONE best answer:
If the pumps are of different size.
If the piping system is oversized, causing a lack of discharge pressure.
If the piping system is undersized or some component in the system acts as a bottleneck.
What is the advantage of starting a centrifugal pump with the delivery valve closed? Select the ONE correct answer:
The delivery pressure will be large causing the relief valve to open.
The starting load on the motor will be minimum.
Noise and vibration will be reduced.
The pump will prime itself.
What is the main advantage of a double entry centrifugal pump? Select the ONE best answer:
It has a lower NPSH required characteristic, giving an advantage in poor suction conditions.
It gives double the flow rate.
It uses only half the input power for the same flow rate.
What is the main advantage of starting a centrifugal pump with the delivery valve closed? Select the ONE best answer:
There will be no surge of pressure in the pipeline, causing the relief valve to open.
The starting load in the motor will be minimum.
There will be less vibration.
The pump will be self-priming.
What is the main cause of cavitation of the fluid in a centrifugal pump? Select the ONE best answer:
Too high a speed of impeller rotation creating adverse heat.
The vapour pressure in the suction pipe falls below the vapour pressure of the liquid at a given temperature.
The viscosity of the fluid is too high, the extra power absorbed being converted into heat.
What is the main reason for operating pumps in parallel? Select the ONE best answer:
A greater discharge pressure can be achieved.
It allows a larger range of flow than for a single arrangement.
The combined electrical load on the pumps is lower than an equivalent single pump.
What is the main reason for operating pumps in series? Select the ONE best answer:
To avoid cavitation in high-pressure systems.
To share the power absorbed using many small motors instead of one large motor, which is more energy efficient.
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.
What operating principle do positive displacement pumps rely on? Select the ONE best answer:
High speeds giving the fluid Kinetic energy.
Speeding up the fluid and then slowing it down.
Changes of volume of the pump chambers.
Compressing the fluid and then re-expanding it.
What will happen if the NPSHA is less than the NPSHR? Select the ONE best answer:
The pump will not work at all.
The pump will suffer cavitation damage.
The pump needs to run faster.
Which ONE of the following does NOT affect the efficiency of a pump?
Which ONE of the following statements is correct?
Positive displacement pumps are always self-priming.
Positive displacement pumps can require priming.
Positive displacement pumps always require priming.
Which ONE of the following statements is correct?
Centrifugal pumps cover the lower ranges of specific speeds, axial flow the higher, and mixed flow the intermediate values.
Centrifugal pumps cover the higher ranges of specific speeds, axial flow the intermediate, and mixed flow the lower values.
Centrifugal pumps cover the intermediate ranges of specific speeds, axial flow the higher, and mixed flow the lower values.
Which ONE of these statements best describes the energy transformation within a centrifugal pump?
Pressure is converted into kinetic energy by the impeller; this kinetic energy is converted to an increase in velocity by the volute casing.
The impeller 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.
Decreasing the velocity in the impeller decreases the kinetic energy, whilst increasing the velocity of the fluid in the volute casing increases its pressure.
Which one of the following does not directly affect the efficiency of a pump? Select the ONE best answer:
Which one of the following statements best describes the energy transformation within a centrifugal pump? Select the ONE best answer:
Pressure is converted into kinetic energy by the impeller; this kinetic energy is converted to an increase in velocity by the volute casing.
The impeller increases the liquid velocity, increase the fluid’s kinetic energy. This kinetic energy is then converted into pressure by reducing the velocity in the volute casing.
Decreasing the velocity in the impeller decreases the kinetic energy, decreasing the kinetic energy whilst increasing the velocity of the fluid in the volute casing increasing its pressure.
Which one of these options is NOT a cause of excessive vibration in centrifugal pumps? Select the ONE correct answer:
Discharge valve partly closed.
Why are positive displacement pumps ideal for fuel transfer duties? Select the ONE best answer:
They can deliver high pressures.
They have very small clearances.
They are usually self-priming.
They require the pumped liquid to be used for lubrication.
Why are positive displacement pumps well suited for fuel transfer duties? Select the ONE best answer:
They can deliver high pressures.
They have very small clearances.
They are self-priming and can handle some amount of vapours and gases.
They require no maintenance.
Why do centrifugal pumps need priming? Select the ONE correct answer:
An excellent suction lift causes the surface of the liquid to vaporise.
They must be started with the discharge valve open to reduce the starting load, but this causes the pump to run backwards.
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 no suction pressure is created.
Why do positive displacement pumps require a relief valve? Select the ONE best answer:
It can regulate the delivery pressure.
Because the pump will always try and displace a constant volume of fluid, it can generate dangerously high pressures against a closed discharge valve.
It allows priming liquid to return to the suction.
A high pressure could overload the motor.
Why is the axial flow pump ideal for trimming and heeling duties? Select the ONE best answer:
It is reversible and has a high capacity flow.
It has excellent suction lift.
It discharges pressure is increased by the speed of the ship.
Why should positive displacement pumps not have their delivery restricted? Select the ONE best answer:
Because a constant volume is displaced, very high pressures would be created.
Internal losses lead to inefficient operation.
The pump delivery volume will be reduced.
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