Pump Design Flashcards
What are the 2 main pumps?
Centrifugal & positive displacement
How does a centrifugal pump work?
Raises pressure by providing the fluid with kinetic energy when converting work.
How does a positive displacement pump work? Give 2 examples.
Fluid flows into enclosed space where there is a change in volume displacement.
eg. piston & diaphragm
What is ‘head’?
Pressure measured in terms of the height of liquid used.
Why is head equal for different fluids in the same system?
Pressure is proportional to density.
When does the maximum pressure occur in a centrifugal pump?
When flow rate = 0
When does the maximum flow rate occur in a centrifugal pump?
When pressure = 0
How do you alter flow rate in a centrifugal pump? (By changing something on the pump itself).
Change the number/diameter/width impeller blades.
What is net positive suction head (NPSH)?
The amount by which the pressure at the inlet to the pump must exceed the vapour pressure of the liquid.
What is NPSH(R) & NPSH(A)?
NPSH(R) = net positive suction head required NPSH(A) = net positive suction head available
What happens if NPSH(A) < NPSH(R)?
Liquid may vapourise in the pump causing cavitation.
What is cavitation?
The formation of gas bubbles in flowing liquid occurring when local pressure falls to vapour pressure. Bubble collapse causes pump failure.
Where does the minimum pressure occur in a centrifugal pump?
The middle of the pump.
How do you calculate pump efficiency?
Efficiency = (power out)/(power in)
How do you calculate the power of the pump?
Power of pump = power/efficiency = (PxQ)/efficiency
How do you calculate NPSH?
NPSH = (P/density x g)tank - suction lift - pipe drop - (Pv/density x g)vapour
What is the relationship between NPSH & temperature?
As temperature increases, vapour pressure increases so NPSH(A) decreases.
How do the losses differ if a pump system is on one level as oppose to arranged vertically?
A system on one level will only have frictional losses which is cancelled out by suction losses.
A vertical system will have frictional & suction lift losses which should be accounted for.
What happens to the value of Q and the pressure in a multi-stage centrifugal pump system?
Q is constant. Total pressure is the sum of each individual pump.
How do you calculate total pump head?
Total pump head = P column + pipe losses + delivery head + suction lift - air P
What happens if the tank level with fluid going into the pump decreases by 2m?
2m of ‘push’ has disappeared so pump needs to provide 2m more then previously.
What happens when a centrifugal pump is primed? What is the purpose?
The pump casing is filled with liquid before pump is switched on. It prevents gas/vapour build up which will break the pump.
What is the gas/vapour threshold for a centrifugal pump?
Approximately 10%.
What liquids are positive displacement pumps used for?
High pressure, high viscosity, abrasive & difficult liquids (eg. delicate or solid content)
How does a positive displacement pump work?
Solid object physically pushes liquid out of the way. For a fixed speed, flow rate is fixed.
What are the 2 types of moving, solid part in a positive displacement pump? Give 2 examples of each.
1) Reciprocating - piston & diaphragm
2) Rotary - gear & screw
What is the problem with reciprocating positive displacement pumps compared to rotary? What does it create?
Reciprocating action creates pulsation creating fluctuation between suction & delivery so not a continuous pressure.
How can pulsation be prevented in reciprocating positive displacement pumps?
Put multiple pumps in series to create continuous pressure. Using multiple cylinders & dampers also reduces pulsation.
How can the flow rate of a centrifugal pump be varied? (By not altering the pump directly.)
Using a control valve.
Partially closing increases the pressure drop on the discharge side, reducing the flow rate
How can the flow rate of a piston positive displacement pump be varied?
It cannot.
It can only be diverted using a by-pass.
How can the flow rate be varied in a air-driven diaphragm pump?
Adjusting the air supply or varying pressure.
What is the problem with varying the rotational speed in a pump?
It is expensive
What are pump seals used for?
Used around the rotating shaft to prevent escape.
What is an auxiliary fluid used for on a pump seal?
Lubricating the seal or keep particles away from critical areas.
How can the pump seal be eliminated in centrifugal pumps?
Using magnetic coupling.
How do you calculate the pressure required in a pump?
P = density x gravity x (height up - height down + height losses)
How do you calculate pressure drop across a pump?
P drop = density x gravity x (height into pump + height out of pump + losses)
