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INVESTIGATION REPORT: ADV/CNN/12 Sep. 15th, 2013

INVESTIGATION REPORT

1. INCIDENCE : Pump casing damage

Requested By : Area incharge & Head-Mechanical

Dept. : Mechanical

Date : 15/09/2013 Location : Chennai

2. INVESTIGATED BY:

Manojkumar.M

3. INTRODUCTION:

Cavitation: Cavitation is the localized formation and subsequent collapse of cavities, bubbles,

in a liquid. The formation of cavities (or bubbles) is induced by flow separation, or non-uniform

flow velocities, inside a pump casing that result in localized low pressure points.(This is

analogous to the effect of air flowing against the leading edge of an airplane wing where flow

separation and difference in air velocities reduce the pressure at the top of the wing causing the

plane to be air-lifted.)

Cavitation in a centrifugal pump has a significant effect on pump performance.

Cavitation degrades the performance of a pump, resulting in a fluctuating flow rate and

discharge pressure. Cavitation can also be destructive to pumps internal components. When a

pump cavitates, vapor bubbles form in the low pressure region directly behind the rotating

impeller vanes. These vapor bubbles then move toward the oncoming impeller vane, where

they collapse and cause a physical shock to the leading edge of the impeller vane.

This physical shock creates small pits on the leading edge of the impeller vane. Each

individual pit is microscopic in size, but the cumulative effect of millions of these pits formed

over a period of hours or days can literally destroy a pump impeller and casing. Cavitation can

also cause excessive pump vibration, which could damage pump, bearings, wearing rings, and

seals.

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Design Specifications-

PUMP:

Design flow – 18.5 M3/hr

Head - 21.6 m

NPSH – 2.2 m

Rpm – 2900

Make -VBB

Tank:

Size - 1500 dia × 2000 mm ht

4. OBSERVATIONS:

It was found that there is a leakage from the casing of pump

It was observed that the pump runs continuously, though there is no liquid in the tank

5. REASONS OF INCIDENCE:

Input Flow coming to the tank is very less(Approx-7m3/hr) than the pump

discharge-8m3/hr.So pump running most of the time with bottom level of

liquid.This causes cavitation to occur and leads the pump casing to damage

There is no level switch for the tank to control the pump based upon the liquid

level present in the tank

No VFD to control the pump rpm

NPSHR>NPSHA- Pump NPHR is 2.2 Metre but the total height of the tank is

only 2 Metre

Suction pipe choking due to foreign materials

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6. RECOMMENDATIONS:

S. No. Recommendation Action By

1 Level switch to be fixed in the cloth wash tank to control the pump

based upon the level

Instru

2 Tank height to be increased by another 2 metre to have the total tank

height of 4 metre

Mech

3 Foreign materials(mostly polythene) deposition to be avoided and

regular cleaning to be done

Process

Enclosed Photos

1. Damaged pump casing

2.

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3.Cavitation effects on impeller

4.

Pits on casing

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5. Tank

Pump

Damaged vanes