Page 246 - Know and Understand Centrifugal Pumps

Common Sense Failure Analysis

the shop?’ And someone says, ‘Because it was making noise’, or ‘The
seal failed’.
The noise and the seal failure are actually symptoms and not the
problem. This is like the electrician blaming the fuse for an overloaded
electrical circuit. The problem is the overloaded circuit and the
symptom is the burned fuse. Likewise, in the maintenance shop, the
noisy pump, the failed seals and bearings are the ‘Symptom’ of a
problem that probably occurred outside the pump.
In this book, we’ve dedicated whole chapters to seals and bearings.
However, there are some other complaints (symptoms) that send
pumps into the shop. We have listed below some of those reasons. We
present them in table form with the symptom and the possible
hydraulic and/or mechanical cause for the symptom. We hope this
helps someone.

SYMPTOMS AND POSSIBLE ROOT-CAUSES
Symptom Possible Hydraulic Cause Possible Mechanical
Cause

Noisy Pump. Cavitation Bent Shaft
Aspirated Air Bound Rotor
Excessive Suction Lift Worn Bearings
Not enough NPSHa
Not enough discharge Excessive discharge Head Worn or damaged impeller
flow Not enough NPSHa Inadequate foot valve size.
Air aspiration or air pocket
in the suction line.
Plugged impeller or piping
No discharge pressure. Pump improperly primed. Plugged impeller or piping.
Inadequate Speed. Incorrect rotation.
Not enough NPSHa. Closed discharge valve
Air aspirated or air pockets
at the suction line.
Pressure Surge. Not enough NPSHa. Air aspirated or air pockets
at the suction line.
Entrained Air.
Plugged impeller.
Inadequate Pressure. Not enough velocity. Impeller diameter too small
Air or gases in pumped Worn or damaged impeller
liquid. Incorrect rotation
Excessive Power Head too small, excess flow. Bent shaft.
Consumption High specific gravity or high Bound shaft.
viscosity. Incorrect rotation.

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