HEAD RISE TO SHUT OFF.
HEAD RISE TO SHUT OFF.
(OP)
WHY WE CONSIDER A 5% MINIMUM HEAD RISE FOR SINGLE STAGE LONE OPERATING AND 10% FOR PARALLEL OPERATIONS?NEED DETAIL EXPLANATION
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RE: HEAD RISE TO SHUT OFF.
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RE: HEAD RISE TO SHUT OFF.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: HEAD RISE TO SHUT OFF.
Although pumps with unstable curves can work well in many applications, they are not suited for operation in parallel. Why? Pretty simple, if the primary pump is operating at a head that is higher than shut off, the secondary pump may not be able to produce enough head to come on line. This is especially true for larger pumps that may be started against a closed valve. Several agencies recommend that all pumps should have stable H/Q curves. API goes even further and mandates them for parallel operation. They also recommend a minimum head rise from rated capacity to shut off of 10 – 20%.
RE: HEAD RISE TO SHUT OFF.
http://www.pumped101.com/parallel%20part%202.pdf
RE: HEAD RISE TO SHUT OFF.
BIGINCH AND STANIER
RE: HEAD RISE TO SHUT OFF.
RE: HEAD RISE TO SHUT OFF.
Centrifugal pumps are controlled based on discharge pressure. If your head rise is too low, a very small change in pressure can mean a very large change in flowrate. Even more important for pumps in parallel operation, due to allowable test tolerances, one pump could always be operating at a higher flow than the other. Or flow can surge as one pump overtakes the other, and vice versa. These effects are minimized with a steeper (more head rise to shutoff) curve.
When you have a problem (vibration, premature wear) you will ask the pump vendor for assistance. They will ask for operating conditions. You will provide pressure data and it will not be easy to determine the flowrate. Even if you have flow data (not as common as having pressure) the accuracy of the flowmeter may be called into question. 9 times out of 10, you will have inadvertently operated the pumps outside of the allowable operating region and repair may not be covered under warranty.
Having said that, an unstable curve (where a given pressure can have two potential flowrates) makes it impossible to determine the flow from pressure data alone even for a single pump. With two or more pumps in parallel, forget it.
RE: HEAD RISE TO SHUT OFF.
% of BEP
Head % BEP
Rise Flow
to Change
ShutOff
5 8%
10 4%
15 3%
20 2%
30 1%
A pump with BEP Q = 1000 gpm,
and a 250 psig BEP discharge pressure,
30% rise to shutoff = 325 psig,
has a variance of only 10 gpm per psi.
10% Rise to shutoff gives 40 gpm/psi
If it was a 5% rise pump and it goes off just a couple psi, you're looking at a flow variation of as much as 15%. If you're trying to hold BEP flow, maybe you'd be at only 85% on one pump and 115% on the other, then, if your wires arn't getting hot and your process still keeps going, power inefficiency starts creeping up the bill and pump maintenance can start becoming troublesome.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: HEAD RISE TO SHUT OFF.
RE: HEAD RISE TO SHUT OFF.
I guess I have seen a lot of "bad" pumps in my time.
RE: HEAD RISE TO SHUT OFF.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: HEAD RISE TO SHUT OFF.
Yes, there are some "bad" pumps round but not as many as "bad" engineers who can't apply a basic pump to a simple operation.
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.)
RE: HEAD RISE TO SHUT OFF.
RE: HEAD RISE TO SHUT OFF.
RE: HEAD RISE TO SHUT OFF.
Sorry, Artisi...my "bad".
RE: HEAD RISE TO SHUT OFF.
Nothing implied with my response - just an observation after years of trouble shooting "bad" pumps.
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.)
RE: HEAD RISE TO SHUT OFF.