I found two below formula for estimation of Shut-off pressure of Motor Driven Centrifugal pumps in absence of any Performance Curves:
Shut-off pressure = 1.25 x Diff. Pump Pressure + Design Pressure of Upstream Vessel + Static Press. (From HLL).
Shut-off pressure = 1.25 x Diff. Pump Pressure + Static Press. (From HLL).
I want to know which one of above formula is correct. In this regard, I want to know how those formula is developed.
Having read the contributions to the subject, useful indeed, I would like to add following views.
1. Centrifugal pump shutoff (shut-in per Norsok) head falls below 1.2-1.25 x operating head at the selected operating point. Of course the latter should not be far from best efficiency point. This rule is statistical, has exceptions, but has been verified for most curves of refinery pumps. We have to use such a rule in a new project, seeing that piping design proceeds in advance of pump orders / data.
2. Max pump shutoff pressure comes directly from this rule and the above formulae compute it, as explained by shan. Actually we can consider the first formula only, since Design Gauge Pressure=0 for an atmospheric tank.
Datum is suction centerline, so static pressure shall consider the perpendicular distance between it and HLL (HHLL can be used instead, being more conservative).
3. Let us try to feel discussed "Design Pressure of Upstream Vessel" by example: Forced circulation boiler, operating during safety valves blowing off at steam drum (I think this is requirement by code, but not certain). Max shutoff pressure of circulation pump includes design pressure of steam drum (this is operating pressure in the example)+ max static pressure of water to pump centerline+pump differential shutoff pressure.
4. We can add the requirement "shutoff head = 1.25 operating head, or lower" in our pump data sheet. For refinery pumps transferring hydrocarbons, the factor 1.25 can be even a bit lower. Some pumps may not be able to realize the requirement, and this will be assessed during bid evaluation.
This is usually preferable to no shutoff requirement, which would increase checking labor, time and design changes.
5. Shell formula (brought by Ankur) is understood as SOP=SP+HPO+HPS, only if HPS represents the static pressure due to height difference between suction relief device and suction centerline; SP actually representing design pressure of Upstream vessel. The negative HPD is not understood, but at any case it is a rather small quantity and neglecting it would be on the conservative side.
One can only imagine that if the closed discharge valve lies at a height creating HPD static pressure in respect to suction centerline, pressure on this valve would be given by Shell formula.
5. I wonder why the flat pump curve is preferable, while less flat curves would give more satisfactory control, e.g. combined with a control valve downstream of the pump. Probably this matter (not known to me) is related to stable pump operation in a wide range of flows beyond its best efficiency point.