Centrifugal Pump: Faults & Overhauling! Centrifugal Pump Classification by Flow

Centrifugal Pump Overhaul

1. Switch off supply from the control room and isolate breaker with masking tape and place a safety board (Men at work).

2. Switch off supply from local panel and isolate by masking tape, place a safety board on the panel (Men at work).

3. Close suction and discharge valves and lash them with rope.

4. Drain the suction pipe and ensure water is not coming, then loosen or crack open the joints and ensure that pump is safe for opening .

5. Fix chain block and ensure its cable length and strength is satisfactory.

6. Mark on coupling and casings and then remove the coupling bolts.

7. Now motor input supply wire can be marked and disconnected as per drawing.

8. Motor can be lifted by chain block.

9. Disconnect all external fittings and pump casing e.g. cooling pipe, pressure gauge, purging cock.

10. Remove top cover and jack it for removal.

11. Now the pump shaft with impeller can be lifted.

12. Dismantle the impeller, liner and remove the wear ring.

13. Remove the gland packing.

INSPECTION PROCEDURE

1. Impeller, pump shaft and casing to be inspected for erosion, pitting and wear down etc.

pic courtesy: pumpfundamentals.com

2. If required rectify defects with brass putty.

3. Excess worn out parts can be replaced with new ones.

4. Check wear down ring clearance, general practice is to replace with new ring.

5. Check shaft trueness, remove sleeve from the shaft replace sleeve with ‘o’ ring if required.

6. Check key, key slots, nuts and its threads are OK.

7. Assemble all the pump parts.

PROCEDURE TO START THE PUMP

1. Open inlet and out let valves.

2. Purge air and prime the suction line.

3. Turn the shaft coupling and ensure shaft is free to rotate.

4. Connect motor wire R, Y, B.

5. Start and check current drawn by the motor while staring and running.note down the pr and feel the pump casing for any noise or rise in temp of motor.

Fault Finding & Trouble Shooting

• Does not deliver any liquid

Insufficient priming liquid

Insufficient pump speed

Excessive discharge pressure (such as a partially closed valve or some other obstruction in the discharge line)

Excessive suction lift Clogged impeller passages

Wrong direction of rotation

Clogged suction screen (if used)

Ruptured suction line

Loss of suction pressure

• Insufficient capacity and Crackling noise from the pump casing

Air leakage into the suction line

Insufficient speed of the pump

Excessive suction lift

Clogged impeller passages

Excessive discharge pressure

Mechanical defects (such as worn wearing rings, impellers, stuffing box packing,or sleeves)

• Does not develop enough discharge pressure

Insufficient speed of the pump

Air or gas in the liquid being pumped

Mechanical defects (such as worn wearing rings, impellers, leaking mechanical seals, and sleeves)

• Works for a while and then fails to deliver liquid

Air leakage into the suction line

Air leakage in the stuffing boxes

Clogged water seal passages

Insufficient liquid on the suction side

Excessive heat in the liquid being pumped

• Takes too much power and the motor overheats

Operation of the pump at excess capacity and insufficient discharge pressure

Misalignment

Bent shaft

Excessively tight stuffing box packing

Worn wearing rings

Other mechanical defects

• Vibration

Misalignment

Bent shaft

Clogged, eroded, or other-wise unbalanced impeller

Lack of rigidity in the foundation

Insufficient venting (Positive suction head)

Insufficient suction pressure may also cause vibration, as well as noisy operation and fluctuating discharge pressure

Centrifugal Pump Classification by Flow

Centrifugal pumps can be classified based on the manner in which fluid flows through the pump. The manner in which fluid flows through the pump is determined by the design of the pump casing and the impeller. The three types of flow through a centrifugal pump are radial flow, axial flow, and mixed flow.

Radial Flow Pumps
In a radial flow pump, the liquid enters at the center of the impeller and is directed out along the impeller blades in a direction at right angles to the pump shaft. The impeller of a typical radial flow pump and the flow through a radial flow pump are shown in Figure 6.

Fig 6 Radial Flow Centrifugal Pump

Axial Flow Pumps

In an axial flow pump, the impeller pushes the liquid in a direction parallel to the pump shaft. Axial flow pumps are sometimes called propeller pumps because they operate essentially the same as the propeller of a boat. The impeller of a typical axial flow pump and the flow through a radial flow pump are shown in Figure 7.

Fig 7 Axial Flow Centrifugal Pump

Mixed Flow Pumps
Mixed flow pumps borrow characteristics from both radial flow and axial flow pumps.
As liquid flows through the impeller of a mixed flow pump, the impeller blades push the liquid out away from the pump shaft and to the pump suction at an angle greater than 90o. The impeller of a typical mixed flow pump and the flow through a mixed flow pump are shown in Figure 8.

Fig 8 Mixed Flow Centrifugal Pump

Multi-Stage Centrifugal Pumps
A centrifugal pump with a single impeller that can develop a differential pressure of more than 150 psid between the suction and the discharge is difficult and costly to design and construct. A more economical approach to developing high pressures with a single centrifugal pump is to include multiple impellers on a common shaft within the same pump casing. Internal channels in the pump casing route the discharge of one impeller to the suction of another impeller.

Figure 9 shows a diagram of the arrangement of the impellers of a four-stage pump. The water enters the pump from the top left and passes through each of the four impellers in series, going from left to right. The water goes from the volute surrounding the discharge of one impeller to the suction of the next impeller.

A pump stage is defined as that portion of a centrifugal pump consisting of one impeller and its associated components. Most centrifugal pumps are single-stage pumps, containing only one impeller. A pump containing seven impellers within a single casing would be referred to as a seven-stage pump or, or generally, as a multi-stage pump.

Fig 9 Multi Stage Centrifugal Pump