Increase pump efficiency by reducing running clearances

[FFV5]

I have come across that pump efficiency can be increased by reducing wear ring clearances. However from what I understand, when wear ring clearances are reduced, the pump head will increase, meaning the pump has to be throttled more. In that case, how is it possible that efficiency of the pump is improved? Can someone please enlightened me on this? Thank you.

 

[Artisi]

A new properly toleranced pump shouldn't need any change to wear ring clearances to otain increased efficiency - the discussions you hear about reducing clearances to improve efficiency are usually refering to pumps in service that may have suffered wear with increasing clearances of the wear rings.

 

[FFV5]

Thank you Artisi. But in that case if I understood you well, as pumps runs in service, clearance will increase, which leads to inefficiency. Then we have to bring running clearance back to bring back efficiency to original for the given flow.

So cutting clearance is has no impact on improving efficiency but rather just mitigating efficiency losses.

What if the pump is not operating at BEP now? would reducing clearances help to improve efficiency in some way?

 

[Artisi]

The leakage across properly toleranced wear rings should be minimal even when running away from BEP. In actual fact if runing away from BEP you may need to increase clearance so that the wear rings don't clash as a result of shaft deflection.

 

[CH5OH]

in theory thats true, of coarse...
the pump is designed to establish flow from suction to discharge.internal leakage will reduce that flow (from discharge to suction)but still require shaft power.
the wear ring clearances are a compromise to coop with:
-shaft imbalance and thermal expansion
-internal leakage

 

[FFV5]

Thanks for the kind explanation

 

[raronen]

I apologize for being a bit late to the thread, so I hope FFV5 will get this answer.

Reducing wear ring clearance has several positive impacts on pump performance. True, the head curve increases; however, the efficiency increases by a greater amount.

If we look at the pump horsepower equation, head is in the numerator, efficiency is in the denominator. For example, if head increases by 2% and efficiency increases by 4%, total power consumption will drop by about 1.8%. The actual results will vary as a function of specific speed. Low-flow, higher-head pumps will exhibit larger efficiency gains. High-flow, lower-head pumps will exhibit smaller efficiency gains.

For a specific example, Pumps & Systems published a paper last summer:

http://www.pump-zone.com/pumps/centrifugal-pumps/effect-of-internal-clearances-on-npshr.html

"With the tighter rings, the head increased about 20 ft (3 percent). The reduction in power was more significant-about 6 bhp (7 percent). Efficiency increased about 5 points (10 percent). The reduction in NPSHR ranged from 0 to about 10 ft."

So, not only do you increase potential pump capacity (similar to the impact of a slightly larger impeller), you do so at lower power consumption and lower NPSHR. (Note: in the example, they made special floating rings and cut the clearance by 75%. It is more common to use composite materials like DuPont Vespel CR-6100 and reduce the clearance by 50%.)

I hope you find this helpful.

raronen

 

[dabluffrat]

head increase will be minimal. most users want additional TDH anyway, and it will decrease over time as the clearances open.

API-610 lists a minimum running clearance for metal on metal rings. Look into non-metallic wear rings such as Peek or Vespel when wanting to reduce running clearence below these to increase efficiency. these are more forgiving and designed specifically for close clearance pump applications.

Did you know that 76.4% of all statistics are made up...

 

[Artisi]

There are other means of increasing pump efficiency other than reducing runing clearances, points that can be addressed are, improving the surface finish on the pump internals - especially the impeller, changig the shape of the impeller blades at the discharge, ensuring the impeller blade leading edges are clean and uniform, ensuring the pump is running at its BEP, make sure the flow onto and into the impeller eye is optimised - no pre-rotation or disturbences at the pump entry.

 

[raronen]

Artisi,

The modifications you note are all additional methods of improving efficiency. A user looking to maximize efficiency would do everything you note AND reduce the wear ring clearance. They are mutually exclusive improvements.

Reducing wear ring clearance also provides benefits beyond any of the modifications you note. Specifically, reducing wear ring clearance will typically reduce the overall vibration levels of pumps with long, slender shafts (typically pre-API 610 7th Edition/pre-1990 designs), leading to substantial reliability improvements (fewer seal failures). Reducing wear ring clearance also reduces the NPSHR (as noted in the link above). Also in the link, reduced wear ring clearance increases the potential pump capacity.

Impeller filing, gap optimization, and so on are good things to do, but they are not a substitute, nor do they negate the benefits of reduced clearance.

 

[dabluffrat]

Artisi,

Working for a pump OEM I can tell you all of those things you mentioned are considered before a pump efficiency is ever published, as you want to publish the highest efficiency possible.

Since most industry standards and pump OEM's have standard running clearances, reducing these below non-standard amounts will increase efficiency above published values.

I agree reducing clearances needs to be done on an application basis... if you're running off BEP radial loading will increase... if you're in a hot service the difference in thermal expansion is an issue... personally I do not reduce clearances below API-610 unless non-metallic rings are being used, and even then I ask the mfg (Peek or Vespel) for their recommendation.





Did you know that 76.4% of all statistics are made up...