Pumps Flashcards
Describe with a sketch, the operating principles of a centrifugal pump fitted with a diffuser?
Static vane ring is placed around the impeller. The vanes for a series of diffusers which act to convert
the high velocity of the pumped fluid to pressure energy. The design of the casing can further assist
with this by have a volute casing around the diffuser.
Describe why it is necessary good practice to start a centrifugal pump with the discharge valve
closed?
Characteristics of a centrifugal pump are such that when the discharge valve is shut on the pump,
then flow will be zero and the head pressure will be high. It is also noted that since the flow is zero
the power required to power the pump is also low. This is why we often start the pumps with the
discharge valve closed as to keep the starting current low.
Explain why the delivery valve must not be left closed on a centrifugal pump after starting?
Water can boil which can lead to cavitation.
State why it is not necessary to fit a relief valve on a centrifugal pump?
They can run against a closed outlet and then produce the locked outlet pressure. But no harm will
be done. The liquid will circulate inside the pump, the power requirement will drop as there is no
displacement, but that’s it. The liquid inside the pump body will be heated up by the friction so it
should not last to long. But there is no need for a relief valve as on PD pumps.
Why are centrifugal pumps no self-priming?
The main principle of a centrifugal pump is centrifugal force that throws water through an exit and
takes water through the central area because of the vacuum created in the vortex. So when there is
no water, it is not able to create a vacuum force strong enough to suck the water. Once filled full of
water the pump will be able to suck as a low pressure or vacuum is created at the impellers inlet.
Describe how cavitation damage occurs within a pump?
Cavitation is the formation of bubbles or cavities in liquid, developed in areas of relatively low
pressure around an impeller. The imploding or collapsing of these bubbles trigger intense
shockwaves inside the pump, causing significant damage to the impeller and/or the pump housing.
These can occur in two cases. Suction cavitation and Discharge cavitation. When the pump is being
starved of flow bubbles or cavities will form on the eye of the impeller as the boiling point of water is
rapidly decreased with the reduction of pressure. As bubbles carry over to the discharge side the
flud conditions change compressing the bubble into the liquid and causing it to implode against the
face of the impeller.
How operational causes will increase this in a centrifugal pump?
Increased Suction (blocked strainers)
Blockage in the pipe (suction or discharge side)
Poor Piping Design
With reference to a constant speed single stage centrifugal pump, explain the following?
The effect of increasing the diameter of the impellor?
As the diameter of the impellor increases so does the centrifugal force acting on the liquid that
it is pumping and as such the velocity (kinetic Energy) of the liquid leaving the impellor
increases.
As this liquid passes through the volute section (forming a divergent nozzle) of the pump
casing the resulting kinetic energy is converted to pressure energy higher than before with a
corresponding increase in flow rate
State five inductions that a faults and causes has occurred on a centrifugal pump?
Vibration - Imbalance from cavitation or damage to the impeller or bearing failure
Reduced Pump Flow - Damaged or worn impeller, wear ring or wear plate
Overheating - Clogged suction strainer obstructed discharge line
Pump Leakage - Excessive pressures temperatures or corrosion can loosen the seals and joints. Plus
mechanical seal failure
Slow re-prime - Impellor clearances
Noise - Rattling from bearing failure
Ammeter Reading -
State the types of pumps that should be used with the following?
Main sea water/fresh water circulation
OWS
Emergency fire pump
Steering gear
Main engine lube oil supply pump
Bilge main
Fuel Supply
Single Stage Volute Centrifugal Pump
b) OWS
Scroll Positive Displacement Pump
c) Emergency fire pump
Self-priming centrifugal pump
d) Steering gear
Variable Delivery Pump
e) Main engine lube oil supply pump
Gear Pump
f) Bilge main
Centrifugal Pump
g) Fuel Supply
State 4 pumps suitable for the use in a hydraulic system and why they are suitable?
Hele Shaw Variable Delivery Pump
Scroll Pump
Swash Plate (piston Pump)
Gear Pump
Vane Pump
All produce high pressure which is constant,
Reference to positive displacement pumps?
Pulsation Damper?
Why do some positive displacement pumps do not require them?
Relief Valve why?
a) Pulsation Damper?
To smooth the flow of the pumped fluids (certainly if a piston type is used)
b) Why do some positive displacement pumps do not require them?
If they are continuous flow then these are not required are the flow output is constant. There
is no cyclic behaviors
c) Relief Valve why?
If the output port of a positive displacement pump was blocked, then the pressure would
increase instantaneously to the point that the pumps pumping element or its case would fail,
hence the need for a relief valve.
Explain 3 checks that need to be done after the motor has been aligned?
- No noise or rubbing
- Direction is correct
- Pump is operating accordingly
With reference to an emergency fire pump above the vessels water line
Why a positive displacement I more effective?
How a centrifugal pump can overcome the situation explained in Q2a)
a) Why a positive displacement I more effective?
Self-priming characteristics
b) How a centrifugal pump can overcome the situation explained in Q2a)
Liquid Priming Ring
Eductor through a venturi
Describe with the aid of a sketch, a priming system which is driven by the centrifugal pump it is
intended to prime?
Primer consists of an elliptical casing in which a vane rotor revolves. The vane rotor revolves forcing
an elliptical ring to take shape. The water ring advances and recedes from the central hub causing a
pumping action to occur. The suction piping is connected to the inlet and it removes the air from the
line. A reservoir for the water is provided when necessary.
