Why is this pump in the shop

Did you ever notice that the building or area in the plant called the 'maintenance shop', is actually the Pump Hospital? The shop may have twelve workbenches, but ten benches have a pump in some stage of surgery. You go into the shop and ask someone 'Why is this pump in 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 fiise. 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

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.

Although about half of all pumps manufactured in the world are centrifugal (the other half are positive displacement), industry tends to use a higher quantity of centrifugal pumps. For that reason, much of this book has dealt with pump theory, applications, and problems, from a centrifugal point of view. You may think that we have abandoned PD pumps in this book. You would be wrong.

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency are as applicable to PD pumps as centrifugal pumps.

So in this chapter of failure analysis and corrective methods, we decided to consider some problems, symptoms, and remedies particular to PD pumps. We're using two tables. The first table lists the few symptoms that send a PD pump into the shop. These symptoms are mated to another column of possible causes listed in numerical order. The numerical causes are on the second table starting with the source of the problem in the left column and the probable cause/suggested remedy in the right column. As you go through the list, you'll see again that PD pumps and centrifugal pumps have a lot in common. Enjoy.

SYMPTOMS AND CAUSES OF FAILURE FOR POSITIVE DISPLACEMENT PUMPS Symptom Possible Cause

Pump fails to discharge liquid.

1,2,3,4,5,6,8,9

Noisy pump.

6,10,11,16,17,18,19

Pump wears rapidly.

11,12,13,16,20,23

Pump not up to capacity.

3,5,6,7,9,21,22

Pump starts, then loses suction.

1,2,6,7,10

Pump consumes excessive power.

14,16,17,20

Source of Problem

Suggested Cause/Remedy

1. Suction problem.

Not properly primed

2. Suction problem

Suction pipe not submerged

3. Suction problem.

Clogged strainer

4. Suction problem.

Foot valve leaks

5. Suction problem.

Suction lift too high

6. Suction problem.

Air leak in suction piping

7. Suction problem.

Suction piping too small

Source of Problem Suggested Cause/Remedy

8.

System problem.

Wrong rotation

9.

System problem.

Low speed

10.

System problem.

Insufficient liquid supply

11.

System problem.

Excessive discharge pressure/resistance

12.

System problem.

Grit or dirt in liquid.

13. System problem.

Pump running dry

14.

System problem.

Viscosity of liquid being pumped is higher than specified.

15.

System problem.

Obstruction in the discharge line

16.

Mechanical problems.

Unbalanced or misaligned coupling.

17.

Mechanical problems.

Bent motor shaft

18.

Mechanical problems.

Chattering relief valve

19.

Mechanical problems.

Pipe strain distorting the pump casing.

20.

Mechanical problems.

Air aspiration thru the packing/seal.

21.

Mechanical problems.

Inadequate relief valve.

22.

Mechanical problems.

Packing is too tight.

23.

Mechanical problems.

Corrosion.

AUTHOR'S NOTE

Oh yes, 'Common Sense Maintenance' is likely to be the title of our next book.

0 0

Post a comment