Pump Shut off head
Pump Shut off head
(OP)
I would like to learn what is pump shut off head? how it is calculated? is it a characteristic determined by manufacturer and etc..
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RE: Pump Shut off head
BobPE
RE: Pump Shut off head
RE: Pump Shut off head
Not sure if I'm being pedantic, semantic or pragmatic but
I don't think we can ever assume that a pump can operate safely at shut-off.
Naresuan University
Phitsanulok
Thailand
RE: Pump Shut off head
RE: Pump Shut off head
bobPE
RE: Pump Shut off head
I agree that one can't arbitrarily assume that a centrifugal can run safely at zero flow shutoff for any time period. However, the real fluid temperature rise limitation is definitely time-dependent and can be calculated using the known volume of trapped fluid in the pump casing and the suction/discharge piping between the isolation valves and employing the fluid heatup rate. If structural design temperature of the piping system material is the limiting temperature, for example, many minutes of safe running at shutoff may be possible. Pumps designed for pressurized, high temperature heat transfer fluid systems often employ isolation and even check valves with seat bypass orifices of high resistance for the purpose of equalizing temperatures in idled parallel pumping loops to avoid piping component thermal shock. Pump specifications for such systems might require safe thermal operation for some limited time at flowrates as low as 1% of rated pump flow and tests have to be run on the lead pump to prove this capability. A more difficult question is whether safe "stable" operation is possible at shutoff or near shutoff. This is virtually uncalculable and short-time testing may be necessary to prove this. Generally, the seriously unstable low flowrates are considerably above shutoff in the range of say 25 to 60% rated flow which happens to be about the same place where incipient cavitation curves rise to and descend from near infinity. If one has determined where the unstable flow regimes are located by finding the telltale dips, kinks or slope changes in power,current, axial thrust, and radial thrust curves (head curve is poor for this because pressure taps are usually too far fron the flow channels where the action occurs). Mapping these unstable flow regimes during engineering tests of each design reduces the risk of stability damage during shutoff tests.
RE: Pump Shut off head
My comment was "tongue-in-cheek" - as you and I know and many others fully understand, short time operation at shut-off in some cases is acceptable all things being considered - and as BobPE has pointed-out - be careful.
Naresuan University
Phitsanulok
Thailand