Recently I came to know that even if NPSH required is less than NPSH available, cavitation can occur. And cavitation goes on incresing as NPSH available increases until at two times NPSH required, worst cavitation is faced. Any explanation as to how cavitation can occur even if NPSH available is greater than NPSH available. This ionformation also exists on the website of Hydraulic Institute.

Vortexing; poor or bad piping configurations and design problems. A conical reducer should not be used at the suction but I saw two this week.

SAM. YES, THE REASONS YOU MENTIONED MAY BE TRUE BUT THEY CAN NOT BE GENERALISED AND IT IS A GENERAL STATEMENT ABOUT ALL PUMPS ON HYDRAULIC INSTITUTE'S WEBSITE FAQ'S. ALSO PLEASE TELL WHEN CAVITATION IS MORE DANGEROUS . WITH HOT FLUID OR COLD FLUID. IS IT SOMETHING TO DO WITH VAPOUR PRESSURE?

Then you need to be more specific. What is the fluid pump and what kind of pump with which metalurgy. Is it ANSI standard or a slurry pump or?????

Couple of points to keep in mind:
(1) The NPSH required is based upon loss of pump performance - typically the criterion is a certain percentage of head loss relative to the single-phase liquid H-Q curve.
Before that limit is reached (ie at a NPSH available above the NPSH required) there can be bubble formation and collapse that can cause damage without noticeable loss of performance.
(2) Cold fluid can be worse, ie very intense cavitation and damage before performance degrades. This has to do with the specific volume of the vapour, ie high specific volume leads to larger bubbles and more intense collapse / damage. I have seen very intense dynamic effects with cold water for example.

I would like to add several points:
1. Cavitation curve is developed on 3% drop in pump head. It means the value on the cuves indicate situation when cavitation is already present.
2. The temperature affects vapour pressure. Adding cold liquid lowers vapour pressure and can create enough margin that cavitation stops.
3. The most damaging effects of cavitation is created by water. The water specific volume is much higher than any hydrocarbon fluid. You can have pump with HC fluid cavitation but no damage occures to the impeller.

This also can be temperature related as HC's specific gravity can vary with temperature. Example:sp gr may normally be 0.88 but at 60 degrees F may be higher than 1.0 depending on which fluid is pumped.

quote:

Originally posted by Sam Pickens:
Then you need to be more specific. What is the fluid pump and what kind of pump with which metalurgy. Is it ANSI standard or a slurry pump or?????

SAM, THE LIQUID PUMPED IS WATER AS IN BFP. THE PUMP IS A 3 STAGE CENTRIFUGAL PUMP TAKING SUCTION FROM DEAERATOR. CAN CAVITATION HAPPEN IF NPSH REQUIRED IS HALF OF THAT AVAILABLE. IF YES, WHY?

Vortexing can occur causing cavitation. This can happen when suction is too close the tank/pot/etc... if a vortex breaker isn't installed (not to say that's your problem). Piping configuration can also cause vortexing. Improper cut-water clearance can also cause problems. Ensure proper setup between stages.