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Minimum Flow using Suction Specific Speed

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SaadAly

Mechanical
Jul 29, 2014
6
Hi Guys,

I am new here and just been looking to find some guidelines to have a consevative minimum flow for pumps. I came across this thread and there is a table by JJPellin which uses Suction Specific Speed and Specific Speed to define the minimum flow at percentage of BEP. So I have two questions in particular:

What are the units used for in calculating Suction Specific Speed and Specific Speed?

Do you use NPSHR for water or do you determine the NPSHR for the liquid that is being pumped?

Thank you.

P.S I am having relibility issues operating a turbine driven pump in parallel to an electric motor driven pump with both operating simultaneously. Its quite similar to this thread
 
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Use water, if you are fabricating a series of pumps for any product, for sale to the general community, or if only for one fluid, use the fluid.

You can use any unit of flow, but the number you get must correspond to whatever scale you are using to gauge your pump's performance. If you use USGPM, the scale is already a standard.

you must get smarter than the software you're using.
 
Ah thanks a lot! That answers my questions. I suppose for the units I would need to ask JJPellin since he is the one whose table I will be using.
 
No you don't ask Johhny Pellin Probably you get a pump book and have a read, all will be revealed. What you are asking is pumps 101 - no black magic or any big secrets.

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.)
 
Thanks for pointing me in the right direction. I will do that. His table is based on Pump User's Handbook so I will try get a hold of that. It will probably have the units that I need to be working in order to be in same scale as the table.
 
I took a look at JJPellin's numbers. The units look to be US, that is to say USGPM, and ft.

I think they are good for the specific speed ranges stated.

I think they are rather too conservative for the suction specific speed ranges used. Particularly at low combinations of low specific speed and high suction specific speed, they are significantly off of the mark. The other missing component is running speed which has a significant effect.

A couple of papers that might be of interest as to where we've been (in the 1980's) vs. 2014 are linked to below.


 
Thank you so much GPRnD! This is exactly what I was looking for.

Yes, you're absolutely right. I had a look at the graph and it is a bit less conservative.

I am new to pumps and the pumps I have been looking at have almost double the MTBF as others. The pumps have suction specific speed of 17050 (USGPM and ft) which is quite high. They are being operated at 75% flow rate of BEP. So no wonder there are more issues with these pumps.

Thanks a lot, I will have a read through the papers and explore the subject a bit more.
 
You would be better off giving all details of the pump, curve etc and the operating conditions. Knowing the specific suction speed will not overcome your operating problems, only the actual/designed conditions will give a result.

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.)
 
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