CPM Pumps, Piping, and Valves 2

Question 1

A centrifugal pump makes a crackling noise like gravel passing through it, and discharge pressure fluctuates. What is the most likely cause?

Accepted Answer

Cavitation from insufficient NPSH available

Answer

When NPSH available drops below NPSH required, vapor bubbles form and collapse, producing a gravel-like noise and unstable pressure.

Question 2

According to the pump affinity laws, if a centrifugal pump's speed is increased by 10%, brake horsepower increases by approximately:

Accepted Answer

33%

Answer

Power varies with the cube of speed, so 1.1^3 ≈ 1.33, an increase of about 33%.

Question 3

What is the correct leakage condition for a properly adjusted packed pump stuffing box in service?

Accepted Answer

A slight steady drip to lubricate and cool the packing

Answer

Packing needs a small controlled leak to lubricate and cool it; zero leakage burns the packing and scores the shaft sleeve.

Question 4

Why should a centrifugal pump not run for long periods against a closed discharge valve?

Accepted Answer

The trapped liquid heats up and can flash, damaging seals and bearings

Answer

At shutoff, nearly all input energy becomes heat in the trapped liquid, which can vaporize it and damage internal parts.

Question 5

Which pump type must always have a pressure relief valve on its discharge side before any isolation valve?

Accepted Answer

Positive displacement pump

Answer

Positive displacement pumps deliver a fixed volume per stroke and will build pressure until something fails if discharge is blocked.

CPM - Certified Plant Mechanic Practice Test