Pump Design Flashcards

1
Q

What are the 2 main pumps?

A

Centrifugal & positive displacement

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2
Q

How does a centrifugal pump work?

A

Raises pressure by providing the fluid with kinetic energy when converting work.

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3
Q

How does a positive displacement pump work? Give 2 examples.

A

Fluid flows into enclosed space where there is a change in volume displacement.
eg. piston & diaphragm

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4
Q

What is ‘head’?

A

Pressure measured in terms of the height of liquid used.

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5
Q

Why is head equal for different fluids in the same system?

A

Pressure is proportional to density.

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6
Q

When does the maximum pressure occur in a centrifugal pump?

A

When flow rate = 0

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7
Q

When does the maximum flow rate occur in a centrifugal pump?

A

When pressure = 0

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8
Q

How do you alter flow rate in a centrifugal pump? (By changing something on the pump itself).

A

Change the number/diameter/width impeller blades.

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9
Q

What is net positive suction head (NPSH)?

A

The amount by which the pressure at the inlet to the pump must exceed the vapour pressure of the liquid.

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10
Q

What is NPSH(R) & NPSH(A)?

A
NPSH(R) = net positive suction head required
NPSH(A) = net positive suction head available
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11
Q

What happens if NPSH(A) < NPSH(R)?

A

Liquid may vapourise in the pump causing cavitation.

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12
Q

What is cavitation?

A

The formation of gas bubbles in flowing liquid occurring when local pressure falls to vapour pressure. Bubble collapse causes pump failure.

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13
Q

Where does the minimum pressure occur in a centrifugal pump?

A

The middle of the pump.

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14
Q

How do you calculate pump efficiency?

A

Efficiency = (power out)/(power in)

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15
Q

How do you calculate the power of the pump?

A

Power of pump = power/efficiency = (PxQ)/efficiency

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16
Q

How do you calculate NPSH?

A

NPSH = (P/density x g)tank - suction lift - pipe drop - (Pv/density x g)vapour

17
Q

What is the relationship between NPSH & temperature?

A

As temperature increases, vapour pressure increases so NPSH(A) decreases.

18
Q

How do the losses differ if a pump system is on one level as oppose to arranged vertically?

A

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.

19
Q

What happens to the value of Q and the pressure in a multi-stage centrifugal pump system?

A

Q is constant. Total pressure is the sum of each individual pump.

20
Q

How do you calculate total pump head?

A

Total pump head = P column + pipe losses + delivery head + suction lift - air P

21
Q

What happens if the tank level with fluid going into the pump decreases by 2m?

A

2m of ‘push’ has disappeared so pump needs to provide 2m more then previously.

22
Q

What happens when a centrifugal pump is primed? What is the purpose?

A

The pump casing is filled with liquid before pump is switched on. It prevents gas/vapour build up which will break the pump.

23
Q

What is the gas/vapour threshold for a centrifugal pump?

A

Approximately 10%.

24
Q

What liquids are positive displacement pumps used for?

A

High pressure, high viscosity, abrasive & difficult liquids (eg. delicate or solid content)

25
Q

How does a positive displacement pump work?

A

Solid object physically pushes liquid out of the way. For a fixed speed, flow rate is fixed.

26
Q

What are the 2 types of moving, solid part in a positive displacement pump? Give 2 examples of each.

A

1) Reciprocating - piston & diaphragm

2) Rotary - gear & screw

27
Q

What is the problem with reciprocating positive displacement pumps compared to rotary? What does it create?

A

Reciprocating action creates pulsation creating fluctuation between suction & delivery so not a continuous pressure.

28
Q

How can pulsation be prevented in reciprocating positive displacement pumps?

A

Put multiple pumps in series to create continuous pressure. Using multiple cylinders & dampers also reduces pulsation.

29
Q

How can the flow rate of a centrifugal pump be varied? (By not altering the pump directly.)

A

Using a control valve.

Partially closing increases the pressure drop on the discharge side, reducing the flow rate

30
Q

How can the flow rate of a piston positive displacement pump be varied?

A

It cannot.

It can only be diverted using a by-pass.

31
Q

How can the flow rate be varied in a air-driven diaphragm pump?

A

Adjusting the air supply or varying pressure.

32
Q

What is the problem with varying the rotational speed in a pump?

A

It is expensive

33
Q

What are pump seals used for?

A

Used around the rotating shaft to prevent escape.

34
Q

What is an auxiliary fluid used for on a pump seal?

A

Lubricating the seal or keep particles away from critical areas.

35
Q

How can the pump seal be eliminated in centrifugal pumps?

A

Using magnetic coupling.

36
Q

How do you calculate the pressure required in a pump?

A

P = density x gravity x (height up - height down + height losses)

37
Q

How do you calculate pressure drop across a pump?

A

P drop = density x gravity x (height into pump + height out of pump + losses)