Sundyne pumps and cavitation

[Engen1]

thread407-188371

Hi,

I have a Sundyne pump model LMV-311. Cavitation damage is evident on the inducer, between vanes at the impeller eye and on the diffuser bowl suction edge. The NPSH margin is 5m. The flow does at times exceed 105% of BEP flow.

Regarding a discussion held some years back titled: Sundyne Model LMV-341, the discussion ended with Johnny Pellin referring to an experience with cavitation damage between vanes at the impeller eye even though there was adequate NPSH available.

Johnny Pellin,
How did you resolve the cavitation problem?

Has anyone else experienced similar cavitation as I have described?

Darryl Engen1

 

[Pumpsonly]

The Sundyne inducer is like a archimedean screw pump and behave like a PD pump.It is selected based on the design flow rate and RPM.If you are operating much beyond the designed flow, it can cause starvation to the impeller and hence cavitation.

 

[GPRnD]

Inducers will tend to cavitate outside of a narrow band of preferred operation. You didn't give all the details with regard to how wide your operating range is, % time at each flow point, speed, fluid etc, so it is hard to specifically talk to your application.

However in general I've heard feedback from many refineries where Sundyne pumps are used. Usually the maintenance people will identify those pumps as one of their top headaches/costs. Cavitation, loss of balance, vibration and seal damage tend to be the things they report.

This is not to say they are bad pumps. But they need to be operated close to their rated point. In practice that almost never happens, hence the high maintenance cost.

 

[Engen1]

The starvation theory could explain the diffuser and eye cavitation. Exceeding narrow band preferred operation could explain the inducer cavitation. Thanks for the inputs!

My plan of action is to setup alarms for the preferred operating range (90 to 105% of BEP flow), monitor trends over the next few months and lift the gearbox (with impeller) from the diffuser bowl to inspect for damage.

Would appreciate any success stories of similar cavitation of the inducer, eye and diffuser, with the preventive actions that have worked.

 

[JJPellin]

Sorry I was not able reply earlier. I was out of the country on vacation. We are still working to understand and resolve the cavitation problem that I described in the thread that you referenced. Sundyne redesigned the impeller to smooth the transition to the inlet of the vanes. Then they completely redesigned the hydraulics of the pump. They slowed it down from about 21,000 rpm to about 14,000 rpm. They built a test rig with all parts made out of aluminum and ran it for a week and then checked for cavitation damage. They declared the problem solved. However, after about a year of running with the new parts, we took one of the pumps apart for a seal leak and found new damage in a new location. We are now getting damage on the upper surface of the diffuser near the start of the inlet end of the vanes. For our pumps, the diffuser is integral, so damage in this area will require that we replace the case. I am predicting we will need new cases every 5 years or so.

The most common cavitation problem I see in our big Sundynes is in the diffuser throat. This is a result of operation at excessively high flow rates. Even with NPSH margins of 10 or 20 meters, the diffuser throat can be eaten away if the flow exceeds about 120% of BEP. For cavitation damage on the inducer, I have usually found a plugged inlet strainer. In more than one of these cases, the strainer was a temporary start-up strainer that was accidentally left in place. The insulator was kind enough to fold the handle over and insulate over it in one example.

Cavitation at the impeller eye when using an inducer can also be a result of high flow as noted in a previous replay. At high flow, the inducer becomes a choke and limits the flow into the impeller.

I would check for a strainer, limit the flow to 105% as you already planned, and explore better materials to resist the cavitation.


Johnny Pellin