Wednesday, June 17, 2020

Cavitation in centrifugal pump

Cavitation in the centrifugal pump

Cavitation in centrifugal pump is the formation of bubbles because the liquid suction pressure fall below its vapour pressure or any other reasons. There are many cases of cavitation in centrifugal pumps these are discussed below. The formation of the bubble mostly happens in a pipe inside, equipment, valves, and impellers, etc. The cavitation is developed where the low pressure is available like a near impeller eye. There are two reasons for cavitation in the Centrifugal pump if at the impeller eye fluid suction pressure falls below its vapour pressure.  NPSH

In the other case if the absolute pressure of the liquid falls below the vapours pressure of the liquid in the same system. The valves and elbow are also suffered from cavitation like a pump. The pump doesn't design to deliver the gas its delivers only liquids but the compressor can. So, bubbles start to collapse with some energy inside the Pipe and equipment casing. That generates high suck waves due to this inside the pump body of start eroded and then cavity takes place inside casings of Centrifugal pump. The cavitation reduces the life and performances Centrifugal pump. Cavitation in centrifugal pump is very danger. Cavitation,


During the cavitation in Centrifugal pump, If it is going a long time it's damaged wearing, bearing, seal another part of the Centrifugal pump, etc. You can learn more



cavitation in centrifugal pump

Why cavitation bad for the Centrifugal pump?


Cavitation is bad for the Centrifugal pump because due to the cavitation many parts of pumps can damaged and life of that parts can reduce, performance and efficiency of the pump  also goes down. That's why cavitation is bad for the Centrifugal pump.

What is the effect of cavitation?

There are many effects of cavitation in centrifugal pump.
  1. High noise and high vibration is generated.
  2. Equipment life decrease, the shaft can break and fatigue failure could happen in the pump.
  3. The efficiency of equipment decreases.
  4. Performance of equipment decrease.
  5. Failure of the pump casing and formation of pitting marks on the impeller and inside the casing.
  6. Premature failure of the impeller and bearing.
  7. Reduction of mechanical seal and bearing life, reason for premature failure.
  8. Power consumption is increase.
  9. Decrease the pump flow and pressure.

What happened during cavitation?

During the cavitation bubble formation in the system liquid start because suction pressure goes below to its vapour pressure. The cavitation is very danger for pump and its parts.
What pressure does cavitation occur?
The pressure for cavitation does not fix depends on suction head, vapour pressure, and temperature of the liquid.

How many types of cavitation in centrifugal pump?

There are 5 types of cavitation in the centrifugal pump.
  1. Vaporization type.
  2. Turbulence type.
  3. Vane passing syndrome types.
  4. Internal circulation-type.
  5. Air aspiration types.
Vaporization cavitation
It is very common, happens in the system more than 70 percent of all cavitation. This type of cavitation pumping fluid suction pressure falls below to its vapour pressure and due to this bubble formation starts this is called vaporization cavitation or (classical cavitation). 
This is the most common type of cavitation that happens in the system. To minimize the risk of this type of vapour cavitation should be maintained.
 = (NPSHA > NPSHR + 3 feet (0.91 m) min) this is the thumb rule.
To prevent this, maintains the temperature of the liquid also.
Turbulence cavitation
It happens if the pump system is not designed properly or pipeline equipment not installed properly like valve, reducer, gaskets, Stainer, NRV, filters, etc. Because of this, the flow of pipelines is disturbed and that creates turbulence cavitation.
Vane pass syndrome cavitation
In this type of cavitation inside casing fluid velocity increase and  reduced the suction flow of the pump. Cause generates vane pass syndrome cavitation. The main reason of this type of cavitation clearance of the impeller tip and casing is less, impeller oversize, or casing over coated. To prevent this type should maintain the gap between impeller and inside casing 4 percent of the impeller size, it means if the impeller size is 20 inches (ca. 51 cm) the gap maintained between them least 0.8 inches (ca. 2 cm).
Internal recirculation
This type of cavitation in this type of cavitation pump is not able to discharge fluid. This is happening due to many reasons like not design properly, either pump is not able to suck liquid from the suction. Due to this, these two phenomena have occurred internal circulation in the pump casing increased and fluid start heating and fluid flow velocity inside the casing also increased due to this fluid bubble formation start.
Air aspiration cavitation
This type of cavitation generates when sucking the air by the pump from the leaking flange, valve gland, vent points, etc with liquid. Air dissolves into the liquid and create air pocket. Air pocket collapse in the inside casing surface with some energies it forms air inspiration cavitation.

How does cavitation eliminate?

We will see one by one to prevent Cavitation in a centrifugal pump.
Prevent Vaporization cavitation
  1. Reduce the motor RPM and provide an extra seal cooling system.
  2. Reduce the motor RPM provide an extra seal cooling system.
  3. To increase the section area of the pump As well as increase the impeller eye size.
  4. Maintain the suction head pressure above the vapour pressure.

Prevent turbulence cavitation
  1. Replace the valve if any issue
  2. Always clean the suction strainer
  3. Change the reducer to big area diameter
  4. Minimize the number of elbows used in the line
  5. Use short angle elbow
Prevent vane pass syndrome
  1. To prevent this type of cavitation
  2. Maintain the clearance between the pump casing and impeller tip should be maintained by almost 4% of the total diameter of the impeller.
Prevent the internal circulation cavitation
  1. Suction and discharge valve should be fully open otherwise internal circulation takes place
  2. If you require low-pressure and flow change the drivers.
  3. Operate a pump at low pressure and flow, throttling is not a good idea.
  4. Because of throat valve loss of energy and reduce pump performance.
Prevent the air aspiration cavitation
  1. Check the inline all flanges tighten properly.
  2. Suction and discharge line, Vent point points valve’s should be fully closed.
  3. Check pump seal and valve gland tightened properly.

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