3 replies to this topic

[kpa]

Dear All,

This is regarding the low low discharge pressure set point for the pumps.
The interlock used is as follows,

There are two pumps (1 working + 1 standby). If the discharge pressure of working pump is low low then the standby pump will get autostarted. In this case how to select the low low dischage pressure set point? Is it the pressure corresponding to highest allowabel operating capacity of the pump?

Please reply.
Pradip

[pleckner]

I hope I'm understanding the question. If you are concerned enough about the pressure in the piping system (such as in a circulating loop or such) then you must have some idea what the lowest pressure is before it starts to affect the process. You would then set the autostart just below this pressure. It should be no more complicated than this.

[Zauberberg]

The lowest allowable discharge pressure for the pump is always set by downstream minimum pressure requirements.
In your case, this would be the minimum discharge pressure at which pump can overcome total downstream pressure loss. And, of course, the set point is based on maximum pump capacity.

[Art Montemayor]

Pradip:

Phil Leckner is precisely correct. It isn't complicated.

However, this can get complicated very, very easy if you are generalizing and not giving us all the details. For example, a normal Fire Water system (employing centrifugal pumps) is set up generally very much as you describe - but it may be for other reasons. You are accentuating the flow conditions (since the pump will pump more gpm at a lower TDH) when the pressure in the discharge header is reduced. A Fire Water system is instrumented under another principle: Keep the fire main pressure constant and no lower than a minimum pressure. In order to precisely answer your question we really need all the process details - in the form of a detailed P&ID - with a scope of work. Otherwise, we don't know if we are addressing what you are dealing with.

I believe we have assumed you have centrifugal pumps. It would help if you were specific, just to make sure we are all talking about the same thing. Perhaps if you fully describe your process and scope we can specifically address what you are proposing.