Centrifugal pump troubleshooting

Centrifugal Pump Troubleshooting Procedure

Centrifugal pumps are one of the world’s most commonly used devices. Moreover. their field of application is continuously expanding. However. new applications often bring about new problems. Consequently. it is impossible to foresee the problems that may turn up in the future. The only alternative is an indepth knowledge and understanding of how different factors may affect pump performance.


However. even with this knowledge. it is not easy to determine which of the over 120 known causes of trouble with centrifugal pumps is the most likely source of a given malfunction. Unique cases may also occur. Therefore. one of the first steps in
diagnosing the source or sources of a given problem is to reduce the number of factors to be checked. This can be best accomplished by studying ready-made checklists that enumerate the most probable causes of a given problem.

INTERNAL PARTS AND EXPLANATION FOR MAINTENANCE

1.       Pedestal

A heavy, rigid cast-iron pedestal supports the pump body and keeps the bearings aligned. Overhang is kept to a minimum to reduce vibration, and packing and bearing wear. Notice that there is plenty of room between the front pump bearing and the rear casing plate to adjust and replace the packing, yet this exceptionally large maintenance space has not been gained at the expense of rigidity. A tapped hole in the pedestal below the packing gland allows a pipe to be installed to carry away the drippings.

2.       Bearings

Bearings may be of either single-row or double-row grease-lubricated type. Open type bearings with pipe taps and plugs for lubrication are standard equipment. They may be easily removed if replacement becomes necessary.

The pump will be constructed of material to suit the application. Parts are available in practically any of the materials commonly used in pumps. These materials include iron, bronze, aluminium bronze, stainless steel, and high-nickel alloys. The manufacturer will recommend exactly the right material, or combination of materials, to give the lowest cost possible per litre pumped.

3.       Shaft and Sleeve

The shaft is made of either a high-grade carbon steel or stainless steel, depending upon the requirements. Most pumps are furnished with shaft sleeves of cast bronze or a special material, extending through the stuffing box to protect the shaft from scoring, corrosion, or other damage. However, in many instances where mechanical seals are used or where special temperature problems must be overcome, the shaft is furnished without a sleeve.

4.       Stuffing Box or Seal

A number of sealing arrangements are available to meet all types of problems and conditions. Figure 23 shows a standard internal sealing arrangement with a packing gland, where the liquid being pumped is used as the sealing medium. The internal liquid passage for this sealing method is built into the pump together with a needle valve to regulate the flow of the sealing liquid.

5.       Impeller and Wearing Ring

A high-efficiency enclosed type impeller is used in this pump. It may be made of any of a number of materials depending upon the service for which it is intended. The impeller can be removed and replaced quickly and easily where this may be required. Wearing rings in the casing protect the casing from wear and can be easily and inexpensively renewed when worn. They, too, may be made of any material to suit the application.

The following is a list of problems that I have encountered during my practice.
Following this list are checklists of causes that might have generated the given problems.

LIST OF PROBLEMS WITH CENTRIFUGAL PUMPS

1. Pump does not develop any head. nor does it deliver liquid

2. Pump develops some pressure. but delivers no liquid

3. Pump delivers less liquid than expected

4. Pump does not develop enough pressure

5. Shape of head-capacity curve differs from rated curve

6. Pump consumes too much power

7. Pump does not perform satisfactorily, although nothing appears to be wrong with pumping unit or system

8. Pump operates satisfactorily during start, but performance deteriorates in a relatively short time

9. Pump is operating with noise, vibrations, or both

10. Stuffing box leaks excessively

11. Packing has short life

12. Mechanical seal has short life

13. Mechanical seal leaks excessively

14. Bearings have short life

15. Bearings overheat

16. Bearings operate with noise

17. Pump overheats, seizes, or both

18. Impeller or casing, or both, has short life

19. Loud blow is heard each time pump is started or stopped

20. Casing bursts each time pump is started or stopped

21. Gaskets leak during pump operation

22. Flow-rate periodically decreases, or stops completely, then returns to normal

23. Pump develops cavitation when the available NPSH is increased

CHECKLISTS OF CAUSES OF PROBLEMS

1. Pump does not develop any head, nor does it deliver liquid
Possible Causes

1. Pump not primed (not full of liquid)

2. Shaft is broken

3. Broken or disengaged connection between driver and pump

4. Impeller key broken or missing

5. No impeller in pump

2. Pump develops some pressure but delivers no liquid

Possible Causes

1. Air pockets in pump or pipelines

2. Suction line clogged

3. Foot valve stuck to seat or clogged

4. Strainer covered with solid, usually stringy, matter

5. Strainer filled with solid matter such as sand

6. Discharge pressure required by system is higher than maximum

pressure developed by pump

7. Operating speed too low

8. Wrong direction of operation

9. Available NPSH inadequate

10. Excessive amounts of gas or air entrained in pumped liquid

11. Outer diameter of impeller machined to a too small diameter

3. Pump delivers less liquid than expected

Possible Causes

1. Air enters pump during operation, or pumping system not deaerated before starting

2. Insufficient speed

3. Wrong direction of rotation

4. System requires higher pressure than that developed by pump

5. Measuring instruments not properly calibrated or incorrectly installed

6. Available NPSH too low

7. Excessive amount of air or gas entrained in pumped liquid

B. Excessive leakage through wearing rings or other sealing faces

9. Viscosity of liquid higher than that for which pump has been designated

10. Impeller or casing partially clogged with solid matter

11. Fins, burrs, or sharp edges in path of liquid

12. Impeller damaged

13. Outer diameter of impeller machined to a smaller dimension than specified

14. Faulty casting of impeller or casing

15. Impeller incorrectly installed

16. Pump operating too far out of the head-capacity curve

17. Obstruction to flow in suction or discharge piping

18. Foot valve clogged or jammed

19. Suction strainer filled with solid matter

20. Suction strainer covered with fibrous matter

21. Incorrect layout of suction or discharge piping

22. Incorrect layout of suction sump

23. Excessive leakage through stuffing box or seal

24. Excessive amount of liquid recirculated internally to stuffing box lantern or seal

25. Excessive leakage through hydraulic balancing device

26. liquid level in suction tank or sump lower than originally specified

27. In a system with more than one pump, operation of one pump may affect operation of others

6. Pump consumes too much power

Possible Causes

1. Speed too high

2. Pumped liquid of higher specific gravity than originally quoted

3. Pumped liquid of higher viscosity than originally quoted

4. Oversized impeller

5. Total head of system either higher or lower than anticipated

6. Misalignment between pump and driver

7. Rotating parts rubbing against stationary parts

S. Worn or damaged bearings

9. Packing improperly installed

10. Incorrect type of packing

11. Mechanical seal exerts excessive pressure on seat

12. Gland too tight

13. Improper lubrication of bearings

14. Too much lubricant in bearings

15. Bent shaft

16. Uneven thermal expansion of different parts of pumping unit

17. Faulty power-measuring instruments

18. Power-measuring instruments incorrectly mounted or connected

19. Wrong direction of rotation

20. liquid not preheated to keep viscosity below specified limits

21. Impeller or casing partially clogged with solid matter

22. Wetted surfaces of impeller or casing very rough

23. Damaged impeller

24. Faulty casting of impeller or casing

25. Impeller incorrectly located in casing

26. Impeller inversely mounted on shaft

27. Pump operating too far out on head-capacity curve

2S. Incorrect layout of suction sump

29. Breakdown of discharge line

7. Pump does not perform satisfactorily, although nothing appears to be wrong with pumping system

Possible Causes

This is usually due to incorrect testing. The reasons for this may be as follows:

1. Incorrect measuring instruments

2. Measuring instruments damaged during installation

3. Measuring instruments mounted in wrong locations

4. Tubing that leads from pipelines to measuring instruments clogged

5. Instrument-connecting tubing that should be full of liquid not deaerated completely

6. Instrument-connecting tubing that should be full of air contains some liquid

7. Leakage in instrument-connecting tubing or in its fittings

8. Burrs or fins at mouth of connections between tubing and piping

9. Incorrect connections of wiring to electrical instruments

10. Connections of wires to terminals too loose

11. Dirty electrical terminals or connections

12. Dust or dirt in torque bar

13. Torque bar incorrectly mounted

14. In a dynamometer, misalignment or dirt in bearings produces false readings

15. In a dynamometer, excessive friction in pivots or pulleys that guides the levers and cables produces false readings

16. In a dynamometer, weight and stiffness of the electrical cables affect torque readings

17. Cavitation in measuring instruments

18. Cavitation in pipelines where instruments are hooked up

19. Actual inner diameter of piping different from nominal diameter

8. Pump operates satisfactorily during start, but performance deteriorates in a relatively short time

Possible Causes

1. Air leaks into pump

2. Pumped liquid contains high percentage of entrained air or gas

3. Waterfall-like supply of liquid into suction sump draws air into pump

4. Air pocket in suction line has moved into pump

5. Air funnels in suction sump

9. Pump is operating with noise or vibrations, or both

Possible Causes

1. Misalignment between pump and driver

2. Rotating parts rubbing against stationary parts

3. Worn-out bearings

4. Wrong direction of rotation

5. Available NPSH too low

6. Impeller or casing partially filled with solid matter

7. Fins, burrs, or sharp edges in waterways causing cavitation

8. Damaged impeller

9. Impeller incorrectly mounted

10. System requirements too far out on head-capacity curve

11. Suction strainer filled with solid matter

12. Strainer covered with fibrous matter

13. Incorrect layout of suction sump

14. Air enters pump during operation

15. Mutual interaction of several pumps within one common system

16. Incorrect layout of suction or discharge piping

17. Piping imposes strain on pump

18. Pump operating at critical speed

19. Rotating elements not balanced

20. Excessive radial forces on rotating parts

21. Too small distance between impeller outer diameter and volute tongue

22. Faulty shape of volute tongue

23. Undersized suction or discharge piping and fittings causing cavitation somewhere in system

24. Loose valve disc in system

25. Bent shaft

26. Impeller bore not concentric with its outer diameter or not square with its face

27. Misalignment of pump parts

28. Pump operates at very low flow rates

29. Improperly designed base plate or foundations

30. Resonance between pump speed and natural frequency of base plate or foundations

31. Resonance between operating speed and natural frequency of piping

32. Resonance between operating speed and valve discs

33. Loose bolts

34. Uneven thermal expansion

35. Improper installation of bearings

36. Damaged bearings

37. Improper lubrication of bearings

38. Obstruction to flow in suction or discharge piping

39. Total head of system either higher or lower than expected

40. Excessive amount of air or gas entrained in liquid

41. Waterways of impeller or casing badly eroded or rough

42. Cavitation in pipelines

10. Stuffing box leaks excessively

Possible Causes

1. Worn out bearings

2. Improperly installed packing

3. Incorrect type of packing

4. Rotating element not balanced

5. Excessive radial forces on rotating parts

6. Bent shaft

7. Bore of impeller not concentric with outer diameter, or not square with face

8. Misalignment of pump parts

9. Rotating parts running off-center

10. Water-seal pipe clogged

11. Seal cage improperly located

12. Shaft sleeve worn or scorched at packing

13. Failure to provide cooling liquid to water-cooled stuffing boxes

14. Excessive clearance at bottom of stuffing box (between shaft and box bottom)

15. Dirt or grit in sealing liquid

11. Packing has short life

Possible Causes

1. Worn bearings

2. Improperly installed packing

3. Incorrect type of packing

4. Gland too tight

5. Rotating element not balanced

6. Excessive radial forces on rotating parts

7. Bent shaft

8. Bore of impeller not concentric with its outer diameter or not square with its face

9. Misalignment of pump parts

10. Rotating parts running off-center from damaged bearings or other parts

11. Water-seal pipe clogged

12. Seal cage improperly located in stuffing box, preventing sealing fluid from entering

13. Shaft scorched where it contacts packing

14. Failure to provide cooling liquid to water-cooled stuffing box

15. Excessive clearance at bottom of stuffing box, between shaft and stuffing box’s bottom

16. Dirt or grit in sealing liquid

17. Improper lubrication of packing

18. Space in stuffing box where packing is located is exocentric to the shaft

12. Mechanical seal has short life

Possible Causes

1. Worn out bearings

2. Rotating elements not balanced

3. Excessive radial forces on rotating parts

4. Bent shaft

5. Misalignment of pump parts

6. Rotating elements running off-center from damage to bearings or other parts

7. Dirt or grit in seal-flushing liquid

8. Sealing face not perpendicular to pump axis

9. Mechanical seal has been run dry

10. Abrasive particles in liquid coming in contact with seal

11. Mechanical seal improperly installed

12. Incorrect type of mechanical seal

13. Misalignment of internal seal parts preventing proper mating between seal and seat

13. Mechanical seal leaks excessively

The same factors as in checklist 12, plus the following

1. Leakage between the seal seat and gland from faulty gasket or O-ring

2. Leakage between seal and shaft from faulty O-ring or lip seal

14. Bearings have short life

1. Damaged impeller

2. Impeller partially clogged

3. Rotating elements not balanced

4. Excessive radial loads on rotating parts

5. Excessive axial loads

6. Bent shaft

7. Bore of impeller not concentric with outer diameter or not square with hub face

8. Misalignment of pump parts

9. Misalignment between pump and driver

10. Pump operates for prolonged time at low flow rate

11. Improper base plate or foundations

12. Rotating parts

13. Improper installation of bearings

14. Bores of bearing housing not concentric with bores in water end

15. Cracked or damaged bearing housing

16. Excessive grease in bearings

17. Faulty lubrication system

18. Improper workmanship during installation of bearings

19. Bearings improperly lubricated

20. Dirt finds access to bearings

21. Water has entered bearing housing

22. Excessive wear of impeller sealing rings reducing the effects of balancing means

23. Excessive suction pressure

24. Too tight fit between line bearing and seat (may prevent it from sliding under axial load, transferring this load to the line bearing)

25. Inadequate cooling of bearings

26. Inadequate cooling of lubricant

27. Source of cooling media shut-off from bearing housing

16. Bearings operate with noise

A. Steady high-pitch tone

1. Excessive radial load

2. Excessive axial load

3. Misalignment

4. Too much clearance between bearing and shaft, and/or housing

B. Continuous or intermittent low-pitch tone

1. Bearing brinelled

2. Pitted raceway, from dirt

3. Resonance with other structural pump parts

C. Intermittent rattles, rumbles, and/or clicks

1. Loose machine pats

2. Dirt in bearings

3. Clearance between balls and races too large for given application

4. Bearings that require preloading not adequately preloaded

D. Intermittent squeal or high-pitch tone

1. Balls skidding from excessive clearance between balls and races

2. Balls skidding from insufficient preloading (whenever required)

3. Shaft rubbing against housing from improper mounting of housing

4. Shaft rubbing against housing from bent shaft

5. Shaft rubbing against housing from having been machined excentrically

17. Pump overheats or seizes, or both

1. Pump allowed to run dry

2. Vapor or air pockets inside pump

3. Pump operates near shut-off

4. Simultaneous operation of poorly matched pumps

5. Internal misalignment from too much pipe strain, poor foundations, or faulty repair work

6. Internal rubbing of rotating parts against stationary parts

7. Worn or damaged bearings

8. Poor lubrication

9. Rotating and stationary wearing rings made of identical, galling-prone materials

18 Impeller or casing, or both, has short life

1. Corrosion from chemical interaction with pumped liquid

2. Electrochemical corrosion from difference of electrochemical potential of different materials of which wetted pump parts are made

3. Abrasion from solids contained in pumped liquid

4. Fatigue from thermal shocks

5. Fatigue from vibrations

6. Erosion from cavitation

7. Excessive transient stresses during starting or stopping

8. Pump used at excessively high temperatures

9. Excessive stresses imposed on pump by piping

10. Excessive stresses imposed on casing by

11. Pump mishandled during installation

19 Loud blow heard each time pump is started or stopped

1. Water hammer

2. Air or gas entrapped between pump discharge and nonreturn valve

3. Slam pressure

20 Casing bursts each time pump is started or stopped

1. Water hammer

2. Slam pressure

21 Gaskets leak during operation

1. Uneven thermal expansion of pump parts

2. Loose bolts

3. Unevenly tightened bolts

22 Flow rate periodically decreases, or stops completely, then returns to normal

1. Periodic fluctuations of liquid level in suction tank

2. During operation pump removes more liquid from suction tank than rate at which liquid enters the tank

23 Pump develops cavitation when the available NPSH is increased

This may happen when the increase in the available NPSH has reduced the system resistance so far that the pump operates far out on the QH curve. This happens when

1. Oversized impeller installed in pump

2. Pump operates at excessive speed

3. Breakdown or serious leak in discharge line

4. Open bypass in discharge line

5. Extremely large clearances between impeller and casing

6. Hole in casing allowing liquid

4.1     Vapour Lock

CAUSE OF VAPOUR LOCKTROUBLESHOOTING PROCEDURE
Low liquid level in feed vesselRaise liquid level
Low flow rate through pumpRaise flow rate by recycling some discharge back to pump suction
Valve in suction line is partially closedCheck position of all valves in suction piping for full open
Pump suction line obstructed with pieces of wood, dirt, slag, etc.Clean strainer in suction line. Disassemble suction piping and remove obstruction
Pump casing is heated from sun or other source of heat which causes liquid inside pump to boilInsulate pump Cover pump Cool pump with fan or water
TABLE 1 VAPOUR LOCK

4.2     Low Flow Rate

CAUSE OF LOW FLOW RATETROUBLESHOOTING PROCEDURE
Excessive head pressure. Discharge pressure is above normal, or suction pressure is below normal.Measure head pressure (discharge pressure – suction pressure). If it is more than normal, determine if suction pressure is low or discharge is high. If discharge pressure is high, check discharge piping for partially closed valves. If control valve is in discharge piping, check to see that is not stuck in a partially closed position. Check for pressure rise in vessel or discharge line. If suction pressure is low, check suction vessel for low level or blocked strainer.
Impeller or casing is worn.Replace worn parts
Pump is vapour lockingRaise suction vessel level. Recycle some discharge liquid back to suction end if flow is less than 20% of design.

TABLE 2

LOW FLOW RATE

4.3     Liquids Not Delivered

  • Pump not primed
  • Air or vapour pocket in line
  • Pump not up to rated speed
  • Wrong rotation
  • Strainer, Impellar or passages blocked

4.4     Failure to Deliver Rated Capacity and Pressure

  • NPSHA not sufficient
  • Pump not up to rated speed
  • Wrong rotation
  • Strainer, Impellar or passages blocked
  • Wear rings or impellar damaged
  • Air or gas in liquid
  • Viscosity or specific gravity not as specified
  • Air or vapour pocket in suction line
  • Air leak in stuffing box
  • Total head greater than head for which pump designed
  • Injection of low vapour pressure oil in latern ring of hot pump

4.5     Pump Loses Prime

  • Air leak in suction line
  • Air leak in stuffing box
  • Air or gas in liquid

4.6     Pump Overloads Driver

  • Speed too high
  • Specific gravity or viscosity too high
  • Packing too tight
  • Misalignment
  • Total head lower than rated head
  • Low voltage or other electrical problem
  • Trouble with engine, turbine, gear or other applied equipment

4.7     Pump Vibrations

Available NPSH not sufficient

  • Air or gas in liquid
  • Misalignment
  • Worn Bearings
  • Damaged rotating element
  • Foundation not rigid
  • Pump operating below minimum recommended capacity
  • Impellar clogged

4.8     Stuffing Box Overheats

  • Packing too tight
  • Packing not lubricated
  • Incorrect type packing
  • Gland cocked

4.9     Bearings Overheat or Wear Rapidly

  • Incorrect oil level
  • Misalignment or piping strains
  • Insufficient cooling water
  • Bearings too tight or preloaded
  • Oil rings not functioning
  • Suction pressure appreciably different than specified
  • Improper lubrication
  • Vibration
  • Dirt or water in bearings

4.10    Caution

Never run a centrifugal pump continuously with the discharge valve completely closed. The mechanical power applied to the impeller is dispelled as friction to the fluid trapped and churned about in the casing. This friction causes overheating of the fluid to the point where it turns into vapour which may result in damage to the pump.

Always operate a centrifugal pump with its suction valve wide open. Never use it for flow control. Throttling or closing of this valve starves the impeller of its fluid supply, the casing becomes partially empty resulting in cavitation and excessive vibrations which may ruin the bearings or impeller. The lack of liquid may also damage mechanical seals and stuffing boxes.

The difference between the power put into a centrifugal pump ( brake or shaft power) and the useful power performed by the pump ( water or liquid power) is converted into heat which increases the temperature of the fluid within the pump. This temperature rise occurs very rapidly at low flows. It may cause the impellers to become loose on the shaft and the fluid within the pump may flash with pump seizure the result.  To prevent this rapid temperature rise at low flows, a recirculation line from the pump discharge to the suction is used. The recirculation line has an orifice designed to pass the minimum flow required to prevent overheating in the pump. The valves in the recirculation line may be operated manually or automatically, but they must be open on starting and stopping the pump, and when the pump is operating at low flows.

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