So, what about the BEP?

[Alanator]

We have been running a lot of centrifugal pumps (liquid nitrogen service) through the repair shop. I began reading up on centrifugal pumps and discovered the BEP (best efficiency point). I also learned how a pump can be damaged if one strays too far to either side of this point. Here are a couple of questions.

How close should one stay to the BEP to avoid damage? Is the safe range symmetrical about the point (assuming efficiency vs. flow is on a separate curve)?

To satisfy my curiosity, I took a pump curve with me to the field and watched part of a transfer. The suction gauge read 15 psi and the discharge read 35. My curve had a statement on it "Shut down pump if differential pressure drops below 23 psi." I mentioned this to one of the techs. Let's just say he was skeptical.

I do think people are right when they say most pump problems are due to bad operating practices. I plan to suggest some changes to operating procedures, but I have a feeling I'll run into quite a bit of resistance.

 

[BigInch]

Centrifugal pumps can be damaged by operating for extended periods below approximately 60% of rated BEP flow. You should stay as close as possible to BEP flow, as the mfgr has optimized the pump to run at that rate. It will maximize pump life and minimize your pumping cost.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[pmover]

Alanator,

provided that the gauge readings are reasonably accurate, the fluid properties are near design conditions, the pump rpm is near design condition, & the pump impeller diameter is at design conditions, then operating that pump with that low of differential implies a higher flow (i.e. operating point to right of bep). thus, a higher requirement of npsha for the pump to operate without cavitation. what sort of damage has been done to these pumps?

a website to aid in your diagnosis and educate you, other than this one is at:

http://www.mcnallyinstitute.com/home-html/Technical_paper_index.html

the author has many articles that you may find helpful.

like BigInch states, operating the pump outside of the mfg recommended flow range (i.e. min/max continuous flow range) is not healthy at all.

good luck and we'll be glad to further assist you.
-pmover

 

[JJPellin]

The recommended range about BEP depends on the suction specific speed of the pump. The higher the Nss, the more unstable the pump will become if operated away from from BEP. We have a table that we use to determine minimum flows for our pumps. For very low Nss, we can go as low as 45 percent of BEP. For very high Nss, we require 90 percent of BEP as a minimum. We don't tend to have as many problems at high flow. But I am working on a high energy water injection pump that had cavitation damage in the diffuser throat at the impeller discharge from running above about 110 percent of BEP. By the way, this was still well within the operating range recommended by the manufacturer. But this is a very unusual pump. Most pumps can run pretty well from about 50% to about 120% of BEP.

 

[BigInch]

API 610 suggests that the minimum steady continuous flow within the Preferred Operating Range is 70% of Best Efficiency Point (BEP), while the maximum flow within the POR is 110% of BEP. Typical Allowable Operating Range, based on pump studies to determine acceptable limits on pump vibration have been found to vary from the lower end of the POR at 70% to approximately 50% of BEP.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

In addition to what has been said concerning instability effects (vibration, hydraulic thrusts, shaft deflection, resulting in premature bearing and mechanical seal failures) when working away from the BEP, one can also consider that since this is cryogenic service operating close to the boiling point of nitrogen, any loss of efficiency would probably induce vaporization and vapor binding and even separation cavitation.

Beside the non-symmetrical forces appearing when working to the right or left of the BEP, thermal expansions and contractions may affect the internal dimensions of the moving and stationary pump components. Thus, many users prefer to operate within 80 to 110% of BEP for optimum performance.