keyhanafshar
Mechanical
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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Pump Shut off head
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Centrifugal pump shutoff head is the pressure rise at shut valve discharge conditions where flow is recirculating in the pump impeller and diffusing spaces and brakehorsepower output of the pump driver is being dissipated by heating up trapped water in the casing. It is difficult to calculate because of the unpredictable flowpaths of the fluid. Whether or not it is higher or lower than the head at best efficiency flowrate depends on the specific speed of the pump and the tendency toward separation/stalling tendencies of the impeller/diffuser combination that produes either rising or drooping head curve shapes from bep to shutoff flows. Manufactures may or may not test at full shutoff flow, depending on the extent of the instability there and the danger of damage from overheating the fluid in the casing. If manufacturers tests uncover one or unstable flow regimes at low flowrates or fluid temperature rise warrants, the highest of these flowrates may be designated (or simply shown by ending the plotted head-flow curve at some below-bep flowrate)as the minimum flowrate. One can extend the foreshortened curve graphically to zero flowrate at his own risk since there is no guarantee that the pump can operate there safely.
In assessing whether a pump can operate at shut-off for a short period for testing purposes, I usually refer to its start up instructions. If these specify venting the pump and running up against a shut discharge valve, and then gradually opening up the valve, it should be reasonable to assume that is possible to run at shut-off for a short period. Another clue would be at what point on the curve maximum motor current draw occurs. If its right hand side of curve, you should be ok, left hand side of curve could be a problem. Of course this is a general rule, and each instance on a case-by-case basis.
jet1749 is right, if you choose to test shut-off head on your own, understand what you are doing and consider each case independently. Shutting a pump discharge in the field means that you will be relying on the system to accomplish this task. The system was designed by others and how comfortable are you with their design to expose it to a max pressure condition??????
bobPE
bobPE
To Artisi,
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.
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.
Vanstoja
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
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
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