Sunday, February 7, 2010

17 Ways to Reduce Likelihood of Pump Cavitation

In recent project, there was a topic being discussed. What are the measures can be considered to reduce the likelihood of pump cavitation ?. To answer to this question, we may need to understand some background about pump cavitation phenomenon. Previous posts as follow may probably provide some background information :
Recommended :

Key to ensure no detrimental cavitation is the ensure NPSHa is higher than NPSHr. One may have to take extra note that pump cavitation can exist eventhough NPSHa is above the NPSHr of a centrifugal pump. However, it suction energy is sufficient low (below 3% head drop) and will not results cavitation which is detrimental to pump internal (discussed in "Facts About NPSH - Cavitation Even NPSHa More than NPSHr ?". Therefore, two main parameters we may have to focus are NPSHa and NPSHr.

Increase NPSHa
Previous post "How to Increase NPSHa to a Pump ?" , several points have been highlighted. Will further expand. Following equation define NPSHa :

NPSHa = Hp + Hs - Hf - Hv - Hvp

where
Hp - pressure head
Hs - static head gain
Hf - frictional loss
Hv - velocity head
Hvp - vapor pressure head
The key is to increase Hp and Hs whilst decrease Hf, Hv and Hvp
(1) Increase suction line size to reduce frictional loss (decrease Hf ) and velocity head (decrease Hv)

(2) Rearrange and /or redesign suction pipe work to minimise bends, valves and fittings to reduce frictional loss (decrease Hf )

(3) Reduce suction pipe length to reduce frictional loss (decrease Hf )

(4) Use smoother pipe (lower friction factor) to reduce frictional loss (decrease Hf ) e.g. SS  instead of CS

(5) Raise suction vessel to increase static head (increase Hs )

(6) Lower pump elevation to increase static head (increase Hs)

(7) Increase pressure in suction vessel to increase suction pressure head (increase Hp) e.g. pressurize suction drum with inert gas

(8) Reduce fluid vapor pressure to decrease vapor pressure head (decrease Hvp) e.g. subcool fluid by dropping it temperature





Decrease NPSHr
Suction specific speed ( Nss) of a pump is a dimensionless number expressed as

Nss = ( N* Q 0.5 ) / (NPSHr)0.75

Where

Nss : Suction Specific speed
Q : Flow rate (gpm) at the Best Efficiency Point
N : Pump rotational speed (rpm)
NPSHr : Net Positive Suction Head required (ft)


(9) Use low speed pump
Decrease pump speed reduce NPSHr. Therefore use low speed pump required lower NPSHr.

(10) Use double suction impeller
Double suction impeller as shown in below image will reduce flow to each impeller by half and will reduce the NPSHr by approximately 36%-37%.



(11) Increase impeller eye area to minimize inlet pressure drop. The downside is introduction of suction recirculation. There shall be a balance in eye area selection.

(12) Use cavitation resistance material like SS (discussed in "Stainless Steel SS316 resist to CAVITATION ?"
(13) Use Suction Inducer to streamline suction flow to pump and reduce suction pressure drop

Process Treatment
Besides above measures, process engineer may also consider other process measures :
(14) Use of pump in series to reduce pump capacity of pump and reduce pump NPSHr
(15) Use of booster pump to provide sufficient head for main pump NPSHr
(16) Vortex in suction results vapor entrainment into liquid and lead to pump cavitation. Install vortex breaker at vessel outlet to avoid vapor entrainment (discussed in "Vortex Breaker to Avoid Vapor Entrainment") and;

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