Centrifugal pumps - Cavitation

If the pressure in the eye of the impeller falls below the vapor pressure of the fluid, then cavitation can begin.

Cavitation results in : Pitting marks on the impeller blades and on the internal volute casing wall of the pump, premature bearing failure, shaft breakage and other fatigue failures in the pump, and premature mechanical seal failure.

Types of cavitation: 1. Vaporization cavitation 2. Internal re-circulation cavitation 3. The vane passing syndrome cavitation 4. Air aspiration cavitation 5. Turbulence cavitation

#1. Vaporization cavitation also know as inadequate NPSHa Net Positive Suction Head(available).

It occurs 70% of all cavitations. If NPSHa is lower than the pump NPSHr then vaporization cavitation occurs. Hence to avoid this type of cavitation it is important to have NPSHa always higher than the NPSHr.

#NPSHa > NPSHr + 3ft

NPSHa = Ha + Hs - Hvp - Hf - Hi

where,

Ha= Atmospheric Head (14.7 psi X 2.31) 33.9 ft at sea level

Hs= Static head in ft of the fluid level in the suction vessel to the pump center line

Hvp= The Vapor pressure of the fluid expressed in ft of head

Hf= Friction Head or friction losses in ft in suction piping & connections

Hi= Inlet head or losses in ft in the suction throat of the pump

It means NPSHa can be increased either by increasing Ha & Hs or decreasing Hvp, Hf & Hi.

Following are the ways for increasing NPSHa:

1. Lower the temperature of fluid: This will help in reducing Hvp

2. Raise the liquid level in suction vessel: This will elevate the Hs

3. Change the pump to:

a. Reduce the speed : this will reduce Hf

b. increase the diameter of the eye of the impeller: this will reduce Hf & Hi

c. Use an impeller inducer: this will increase Ha & reduce Hi

d. use to lower capacity pumps in parallel: Reduce Hf & Hi

e. use a booster pump to feed the principal pump: This will increase the Ha.

#2. Internal re-circulation Cavitation:

This cavitation is a low flow condition where discharge flow of the pump is restricted. The liquid is forced to recirculate from high pressure zones in the pump & low pressure zones across the impeller.

Sources:

1. The liquid is circulating inside the volute of the pump at the speed of the motor & it rapidly overheats

2. The liquid is forced to pass through tight tolerances at very high speed.

The heat & the high velocity will cause the liquid to vaporize.

Identification:

1. In open impeller, leading edge damage on the impeller blade towards the eye of the impeller

2. In enclosed impeller, damage on the web ends between impeller & volute casing

Correction:

1. In open impeller, It is necessary to perform impeller adjustment to correct strict tolerances between blades & volute. Some back pull out pumps designed with check Valve at power end of the bearing housing to easily perform this adjustment without pump dissembling.

2. In closed impeller, Impeller Adjustment is not possible. We need to relax the restricted discharge flow on the pump, could be due to a clogged downstream filter, closed discharge valve, check valve installed backwards or operating the pump close to the shutoff head. Variable motor could be used in certain circumstances.

#3. The vane passing syndrome cavitation

This type of cavitation exists when the blade tips at the outside diameter of the impeller are passing too close to the cutwater on the pump casing.

Source:

This is caused by exchanging an impeller with larger diameter impeller OR from re-metallising or coating the internal housing of the pump.

Identification:

In maintenance shop, the damage is seen on the blade tips at outside diameter of impeller & just behind the cutwater on the internal volute wall.

Prevention:

To prevent this, clearance between the tip of blade at OD of the impeller & cutwater on the internal volute wall should be 4% of the impeller outside diameter.

e.g. For 13" impeller, the free space between tips of blade at OD of the impeller & cutwater should be 0.5" to prevent this type of cavitation.

#4. Air Aspiration Cavitation

Air can be drawn into the pump & piping through

1. Diverse forms & different points

2. When pump is in vacuum.

3. Pump shaft packing

4. Valve stem packing on valve in suction piping

5. Joint rings on suction piping

6. Flange face sheet gaskets at pipe joints

7. O-ring & threaded fittings on instrumentation in the suction pipe

8. O-rings & other secondary seals

9. The faces of single mechanical seals

10. Bubbles & air pockets in the suction piping

11. Products that foams

Identification:

The damage on the impeller appears like Vaporization Cavitation

Prevention:

To prevent this, you need to seal all points of entrances & escapes by

1. Tightening all flange faces & gaskets

2. Tightening all pump packing rings & all valve stem packing on suction piping

3. try to keep the velocity of the fluid in the suction piping at less than 8 ft/sec. It may be necessary here to increase the diameter of the pipe.

4. Consider using dual mechanical seals with a forced circulation barrier fluid between both sets of faces on vertical pumps, pumps in vacuum, lift pumps, pumps operating to the right of their best efficiency point (BEP)

#5. Turbulence Cavitation

Sources

1. Formation of vortexes in the suction flow
2. Inadequate piping, sharp elbows, restrictions, filters and strainers in the suction
3. The waterfall effect in suction vessels
4. Violating or not respecting the submergence laws.

# Preventing Cavitation:

Most of the cavitation is caused or induced by operation & design. System design is responsible for much of the cavitation. Maintenance person has always to deal with cavitation problem.

Minimum requirement to prevent cavitation is,

NPSHa > NPSHr + 3ft

Hence either increase NPSHa or Decrease NPSHr

NPSHa = Ha + Hs - Hvp - Hf - Hi

To increase NPSHa:

1. Raise the level in the suction vessel which will increase Hs
2. Elevate the suction vessel & lower the pump. This will increase the Hs.
3. Reduce the friction in suction piping. This is the most creative way to deal with cavitation by reducing Hf.

• Take larger diameter suction pipe or change the pipe schedule. Choose pipe schedule based on discharge pressure and not suction pressure.
• Change pipe to lower friction characteristics. for example, change CI piping for PVC or even 4th grade stainless.
• Move pump closer to the suction vessel and convert globe valve into gate valve if possible OR quarter turn butterfly valves into globe valves. Also make sure that all ball valves are full port design & all suction valves are totally open.
• Think of reducing multiple elbows. Convert 2 or 3 close fitting elbows into a flexible S.
• Convert short radius elbows into long radius elbows.
• try to keep suction pipe inside diameter surface clean and scale free.
• Change filters & strainers with more frequency.
• Ensure all gaskets & ring seals are perfectly centered within the flange faces.

4. Lower the temperature of the fluid in the suction vessel to lower the Hvp.

5. Pressurize the suction vessel to increase Ha. By rule of thumb, Ha increases by 23 ft for every 10 psi.

To decrease NPSHr:

1. Use a pump with a larger suction flange to reduce Hi.
2. Machine & polish the suction throat of the pump. this will help in reducing Hi.
3. Increase the inside diameter of the eye of enclosed impeller.
4. Use a larger/slower pump to reduce Hi as well as Hf
5. Use a small booster pump to feed principal pump. this will increase Ha.
6. Smaller capacity pumps in parallel to reduce Hi as well as Hf.
7. Use a double suction impeller so to convert your end suction centrifugal pump into split case horizontal design.
8. use an impeller inducer.

In some cases, you will have to leave pump with cavitation. many pumps suffer cavitation due to inadequate design. For examples,

1. operating only one pump in parallel system. This pump for sure tends to go into cavitation.
2. Pumps that perform more then one duty through the valve manifold tends to suffer cavitation.
3. Pumps that feed & drains from the bottom tends to suffer cavitation.
4. Vacuum pumps & pumps in high suction lifts are good candidates for cavitation.

In this way, many of the solutions to prevent cavitation are impractical and those practical solutions are uneconomical but the deal is to apply those solution to prevent bearings, seals & shaft from failure.