Pump Head Calculations

[Michengr]

I am trying to find the minimum flow rate for a set of Sour Water Pumps that have been in existence for some time in the refinery. I have two pump curves. One is the performance curve from the vendor which shows various impeller sizes, hp, etc.. and the second is a characteristic / test curve for the current impeller size of the pump, head and efficiency. Readings for the same capacity from both curves yields different avail. head readings. The test curve gives less head than the performance curve at the same capacity. Which curve should I be using??

 

[chief]

To find the head that the pump is capable of, close the discharge valve and read off the discharge gauge pressure. Convert to head allowing for the Sp.Gravity of the fluid medium. This is the max head the pump is presently capable of. To increase head, either fit a larger dia impeller or increase pump speed (presuming that you have sufficient redundant power capacity in your drive motor)

Offshore Engineering&Design

 

[RPilch]

Chief, to get the maximum head you need to subtract the suction pressure from the discharge pressure.

 

[Artisi]

The test curve should be the one to use - assuming it is the test curve for the units installed and that they are running at the duty point given to which the pumps were tested and approved as part of the supply, you also need to establish that they haven't had any changes to the build since the tests were undertaken.

 

[roker]

Hello,

I assume you want the head for a certain flow and not at zero flow (shut off pressure) at zero flow you can compare if it is as the test curve or the vendor curve.

If you mean the pump minimum flow that below it the pump should not be operated it is specified in vendor's data sheet it is not part of the curve.

regards,
roker

 

[Artisi]

If you are in fact trying to establish the minimum allowable flow for these pumps you might find it on the publish (catalogue curve) it might be shown on the test curve although not all manufactures show minimum allowable flow.
Many people will tell you 20% or some such percentage of BEP as a rule of thumb for the minimum flow BUT the only way to be sure is to ask the manufacturer as it is based on a number of factors.

 

[chicopee]

Rpilch-At max. head whereby flow is nearly nil would suction pressure be nearly zero?.

 

[dcasto]

chicopee, no, the suction pressure is what the process upstream feeds the pump less friction from flowing in the suction line. At nearly zero flow, friction is zero, so the suction pressure is what the process upstream supplies.

Rember the upstream process may be just athe hieght of the liquid over the pump suction or it may be the process operating presssure +/- liquid height.