CAVITATION 3

[khaoula]

i have a cavitation problem in my centrifugal pumps.
in the bigining, i thought that it was a problem of NPSHA which was in fact bearly lower than the NPSHr but our pump supplier told me that the problem of cavitation could have many rasons not only NPSHA and proposed an expensive pump.
can someone explain the other reasons of cavitation?

 

[zdas04]

Cavitation is the formation and subsequent collapse of vapor bubbles in a continuous phase liquid. It is caused by a localized phase change so the only way to stop it is to prevent the liquid from ever crossing a phase boundary. It can be prevented by lowering temperature, raising pressure, or slowing the control surface. There may be other ways, but I don't know what they are.

"Cavitation resistant" materials are mostly snake oil because no material can stand up to cavitation for long. Smooth transitions in control surfaces can allow you to get closer to the phase boundary, but success with that requires that you don't allow any process changes (a couple of degrees can push you over the boundary).

The only sure way to prevent cavitation is to make sure that there is an adequate safety margin between your NPSH-a and your NPSH-r.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Everyone is entitled to their own opinion, but not their own facts" Patrick Moynihan

 

[JJPellin]

I believe that the "other" forms of cavitation you may be asking about are the ones often classified as recirculation cavitation. If you take a pump with a fairly low NPSHr, it necessarily has a higher Suction Specific Speed (Nss). A higher Nss pump will be more prone to suction recirculation cavitation if it is operated at too low a flow. There can also be pump configurations that are prone to discharge recirculation cavitation at low flow. These forms of cavitation are still cavitation in the sense described by Mr. Simpson. But, simply increasing suction head to the pump cannot usually suppress recirculation cavitation. The solution to these forms is to run the pump closer to the Best Efficiency Point flow rate. A simple minimum flow spill-back line can be used to run the pump out at a higher flow in some cases.

The other causes of cavitation can include inadequate submergence in a vertical pit pump. Even if the NPSHr is satisfied, the pump could cause vortices and ingest gas. Any pre-rotation of the fluid coming into the pump suction can cause cavitation in a pump that that otherwise has adequate NPSH margin. Turbulence coming into the suction of a pump with a double suction impeller can result in an imbalance in flow between the two impeller eyes and localized cavitation.

If you can give more details about the configuration of your pump and how it is operated, we could provide more specific comments.


Johnny Pellin

 

[raronen]

Cavitation is a complicated issue, and you should be very careful about identifying the root cause before spending big money on new pumps.

There are 4 common suction problems which can lead to cavitation-like symptoms: inadequate NPSHA, air entrainment, vortexing, and turbulence. It sounds like you have the NPSHA tracked down, so I'll briefly touch on the others.

Air entrainment and vortexing are most common in cooling water systems. An example of air entrainment would be where the suction basin was being filled by a pipe that was pouring water into the basin from above, creating a lot of air bubbles in the liquid.

Vortexing is a result of the level in the suction basin being too low--like when you drain your sink and the air vortex forms when the water level falls. (By the way, there are examples of vortexes being pulled into pumps several hundred feet away from the source, so it really is a source issue.)

Either of these problems can be fixed with appropriate baffles or modifications to the piping intake at the suction basin.

Turbulence usually results from inadequately sized suction piping or poor piping practice on the suction side. Here is a good discussion of this topic:

http://www.eng-tips.com/viewthread.cfm?qid=140819&page=1

There is also flow-related phenomena. If the pump is running way above or way below the best efficiency point, cavitation-like damage will occur, but at different locations on the impeller.

Note: Flow related phenomena are one of the main reasons it is important to identify the root cause before buying a new pump. Chances are, your vendor is offering a pump with a very low NPSHR. In general, if the NPSHR goes down, the suction specific speed of the pump will go up. When the suction specific speed is higher, the pump needs to run closer to BEP to avoid recirculation cavitation phenomena. Therefore, unless the reason for the cavitation is understood, a new pump could potentially make things worse.

Finally, there is the condition of the pump. If the wear rings have excessive clearance, the pump will begin cavitating (due to increasing NPSHR).

Conversely, you can explore the option of using composite materials (like Vespel CR-6100) for the wear rings. Composite materials allow you to reduce the clearance to 50% of the typical clearances for metal rings, which will reduce the NPSHR below the original design value (the amount of reduction depends on the type of pump).

So, I would check the suction basin to make sure there isn't a vortex or another pipe discharging into the system where this pump is pulling suction, check the flow velocity in the suction piping, check where the pump is running in relation to BEP, and verify the wear ring clearance.

If none of that is conclusive, I'd try installing composite wear rings with minimum clearances before I bought a new pump.

In the spirit of full disclosure: I represent Vespel CR-6100 in Europe. All the same, I believe you will find my answer technically correct.

 

[EdStainless]

The first step is always to check the operating point. Are you near BEP? If not then you need to look at sizing and flow control.
The only source of trouble that I can think of that hasn't been covered is when you are pumping a fluid near its boiling point. Localized boiling along the low pressure side of the vanes can result in cavitation damage.
Cavitation resistance is largely a matter of the quality of the build. Good surfaces, tight clearances and proper geometry all go a long way in improving operation.
The only alloys that are cavitation resistant are some of the Co based alloys. Trust me, it is less expensive to buy the right pump and fix system flow issues.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[wimple]

khaoula

You did not provide enough details on the fluid handled, the type of pump, etc? This can provide orientation toward the right diagnostic

For cavitation issues generally speaking, I would check three main points :
1/ NPSHa vs NPSHr as per Manufacturer curves
2/ Check that suction configuration is appropriate and will not lead to creation of Vortex (distance from suction point to free surface, or bottom of the reservoir etc.)
3/ The pipe diameter upstream, presence of convergent at pump suction and the fluid velocity at inlet (turbulence)

If your configuration is correct, then it is interesting to see where are you operating the pump with regard to the BEP. In some centrifugal pumps, the NPSHr rise drastically at the right of the curves (110 - 120 % of BEP). In some case, the resistance of the hydraulic network is overestimated, and in operation, it is nessary to operate with permanent throttling in order to not destroy the pump (keep in mind that a pump that cavitates can be destroyed in less then few hours...).

rgds
Wimple

 

[khaoula]

the fluid handled is condensat,the pumps used are horizantal pumps(in line).They are between the scrubber and the tank.
We didn't use a complicated configuration in that area.
i opted for the solution of a vertical canned pump but it was too expensive.Have u an idea of the prices of this type of pumps.

 

[BigInch]

If you have condensates at high temperature, you'll be needing that can pump.

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