Pumps, Piping, and Valves Flashcards

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

Answer: Cavitation from insufficient NPSH available

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

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

Answer: 33%

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

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

Answer: A slight steady drip to lubricate and cool the packing

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

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

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

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

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

Answer: Positive displacement pump

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

After a motor rewire, a centrifugal pump runs but produces low flow and low head with normal amp draw. What should be checked first?

Answer: Direction of rotation

A centrifugal pump running backward still moves some liquid but delivers greatly reduced head and flow; swapping two motor leads fixes it.

What component keeps a centrifugal pump primed when it draws from a sump below the pump centerline?

Answer: Foot valve at the suction line inlet

A foot valve is a check valve at the bottom of the suction line that prevents the column of liquid from draining back when the pump stops.