DRAFT - Cavitation and NPSH quiz - LMS

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Question 1 of 7

1. Question
NPSHa (available) curve:

How is the effect of the fluid’s vapor pressure different from the effect of the tube diameter?
- There is no difference, vapor pressure and tube diameter affect NPSHa in the same way
- Higher vapor pressure causes a steepening of the NPSHa parabola, while tube parameters (lenght and diameter) affect the vertical offset
- The diameter of the tube affects the pressure losses, which affect the steepness of the parabola. The vapor pressure contributes to the vertical offset only
- Tube diameter affects both the vertical offset and the steepness of the parabola, vapor pressure only affects NPSHr (not NPSHa)
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Question 2 of 7

2. Question
Question 1 of 6

Is the pump working at a stable operating point during cavitation?
- yes
- yes, if it's a temporary condition
- no
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Question 3 of 7

3. Question
Question 2 of 6

Which parameters affect the NPSHa (available)?
- Inlet tube diameter
- Fluid temperature
- Fluid level in the tank
- All of the above
- None of the above
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Question 4 of 7

4. Question
Question 3 of 6

How would you recommend to maximize the NPSHa of the system below?
- place the pump in the highest point of the system
- Insert a control valve at the suction of the pump to control the inlet pressure
- Put the tank under low vacuum to de-aerate the fluid and remove the microbubbles
- Move the heater downstream of the pump
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Question 5 of 7

5. Question
Question 4 of 6

A LPS-600 has the following flow pressure curves

And the following NPSHr curves

Since the pump can operate over an extended flow and speed range, why was NPSHr measured for a reduced range only? Multiple options possible
- Because at certain working points the pump became unstable before cavitation was reached
- At the low flow rate range it was not possible to obtain an accurate measurement
- because at low speeds to impeller was too unstable to perform the measurement
- Because at low speeds the 3% cavitation criterion was not reached
- None of the above
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Question 6 of 7

6. Question
Question 5 of 6

In conventional centrifugal pumps, the NPSHr has the shape of a parabola.

Consider now the NPSHr plot of a LPS-4000 (Figure below). At 4000 rpm, the line has the shape of a “bathtub” instead of a parabola.

Can you imagine why?
- At 4000 rpm the pump became too unstable and therefore the measured pressured was lower.
- Levitronix pumps are not susceptible to cavitation, hence the low NPSHr
- The hydraulic geometry of Levitronix pumps change with the operating point, therefore the characteristic is not as regular as for conventional pumps
- The fluid heated due to cavitation, so the flow properties changed and caused an irregular NPSHr plot
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Question 7 of 7

7. Question
Question 6 of 6

Do Air Operated Diaphragm Pumps cavitate?
- No, that's why they can be installed above the suction tank
- Yes, the tight gaps within the check valves cause cavitation
- It follows the same guidelines of the centrifugal pumps: depending on the NPSHa it may or may not cavitate
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Physical Basics: Pumps

DRAFT – Cavitation and NPSH quiz

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