Centrifugal pump air injection
Centrifugal pump air injection
(OP)
I'm working on a project to replace three large centrifugal sewage pumps. Among their many features are suction air injection ports. The existing pumps use air injection, but due to design changes with the new pumps, I'm not confident that the existing air flow rates will be appropriate for the new pumps.
Unfortunately I have not had great support from the pump manufacturer to comment on this issue. I will continue to pursue this avenue, but I can't count on it.
I've found very few references regarding this issue. Here are two of them:
Pumping Machinery Theory and Practice, by Hassan Badr & Wael Ahmed says "...a small amount of entrained air (below about 1%) has been found to cushion the pulsation effects of cavitation and consequently reduces pump noise and minimizes erosive damage."
The other reference is easy enough to read for yourself:
http://www.waterworld.com/articles/print/volume-25...
I am unable to find a consistent and quantifiable estimate of the air which may be used to control cavitation. The most common values I can come up with show air use rates between 0 and 2% of pump flow. However, none of these are specific about the units used for air. The pump flow is surely a volume flow, but what of the air? Can I assume that this is a volume flow also? If yes, what version of a volume flow? (i.e. scfm or acfm)
Does anyone have good information on this subject?
Thanks for reading and considering my issue.
Unfortunately I have not had great support from the pump manufacturer to comment on this issue. I will continue to pursue this avenue, but I can't count on it.
I've found very few references regarding this issue. Here are two of them:
Pumping Machinery Theory and Practice, by Hassan Badr & Wael Ahmed says "...a small amount of entrained air (below about 1%) has been found to cushion the pulsation effects of cavitation and consequently reduces pump noise and minimizes erosive damage."
The other reference is easy enough to read for yourself:
http://www.waterworld.com/articles/print/volume-25...
I am unable to find a consistent and quantifiable estimate of the air which may be used to control cavitation. The most common values I can come up with show air use rates between 0 and 2% of pump flow. However, none of these are specific about the units used for air. The pump flow is surely a volume flow, but what of the air? Can I assume that this is a volume flow also? If yes, what version of a volume flow? (i.e. scfm or acfm)
Does anyone have good information on this subject?
Thanks for reading and considering my issue.





RE: Centrifugal pump air injection
Most people are more concerned about removing the air. Small amounts of air have been introduced in the pump suction on purpose to alleviate certain site problems However, in general, entrained air reduces efficiency, head, and capacity and it increases maintenance. It may cause the pump to lose prime.
What type of pump is this and what is the process conditions.?
RE: Centrifugal pump air injection
The reference noted "Hassan Badr & Wael Ahmed says "...a small amount of entrained air (below about 1%) has been found to cushion the pulsation effects of cavitation and consequently reduces pump noise and minimizes erosive damage." this is nothing new and an accepted practice used when NPSHa is (usually) less than NPSHr resulting in suction cavitation.
There is no constant or quantifiable magic number, it is derived by trail and error for each pump / installation and could well be different between "identical" pumps operating in a common installation, due to slight variances between the pumps.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Centrifugal pump air injection
Given that the air entering will be at atmospheric pressure as the inlet pressure will be less than atmospheric ( I would suppose), then the flow rates, if anyone ever measured them, would essentially be in scfm.
I would just run them and see if it is required then use the minimum amount tat stops the "crackling" noise, but this is not a good way to continue.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Centrifugal pump air injection
I don't think there's any real guide or calculation to predict such values, but if I had a good range of values to use, it might be helpful later.
Once everything gets plumbed in for these pumps it's a pain to change it. They're critical for operation of the treatment plant, being the raw sewage pumps feeding the primary treatment areas. They're 600 or 800 hp units depending on which is discussed, flowing anywhere from 20 to 110 Mgd. They'll be on VFDs of course to get the flow rate variations. Luckily, the process conditions are pretty stable, with the exception of suction head. That varies by as much as 20 ft. The temperature doesn't vary much through the year, the wet well and discharge location are both open to atmosphere, and the sewage properties are very much like water (for density, vapor pressure, and viscosity anyway). NPSHA is expected to be more than twice NPSHR over the full operating range.
Bimr, your answer gives the same interpretation of those air flow numbers that I had. It's nice to see that I'm not alone in this.
LittleInch may have the right idea here. IF air injection is needed, assume the reference flow values are a percent of volume flow. Calc the flow rates needed and put a small safety factor on it to get the highest flow I might have to use.
Thanks for your contributions and concerns.
RE: Centrifugal pump air injection
The use of VFD will more than likely overcome your problem.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Centrifugal pump air injection
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Centrifugal pump air injection
The question I posted here is a matter of preparation. I would much rather spend the money and time to have this set up and ready and never use it, than find out I need it and not have it for a while. I need to make sure I can meter the air appropriately if it's needed. I already have an air supply in the pump room, air injection ports are included on the pumps, and the incremental cost of the air injection system is small.
RE: Centrifugal pump air injection
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Centrifugal pump air injection
RE: Centrifugal pump air injection
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)