Booster Pump Operating Point

No, but t e sy tem pres sure wil b ha f th tot l pr ssure re uired f r the total s stem.

Sor ry, I'm ha ing trou le wi h my ke boar .

Yo wil ne d a big er pu p too.

What do you mean by a static pressure of 170 PSI? Is that without any flow?

The pressure in a supply line controlled by another may further deteriorate by adding more users to it and may drop even lower.

If you are sure about the minimum pressure of 120PSI you may choose a pump to deliver the Q at 170 - 120 = 50 PSI.

This could be for a 'fire water system', wherein the firewater header is expected to maintain at an elevated pressure even there is no requirement. This elevated static pressure is an indication that there is no leak in the system and could through water readily when there is a sudden demand. Still there could be some spillage in the system through leaky valves or occasional testing of hydrant etc. Such spillage may deplete the header pressure and would be re-pressurized with a relatively smaller pump compared to actual firewater pumps. This is referred here as 'booster pump', also widely referred as 'jockey pump'.

" System pressure drops below 140 psi the booster pump should kick-in to raise the system pressure back to 170 psi."

As indicated this booster pump kicks-in by 140 psi and pumps to a closed system. And would run only a short time to rise to 170 psi and would cut-off. During this run the pump will run mostly around 140 psi rather than 170 psi. So your assumption of pumps operating point around the lower limit sounds appropriate

Do you want to select a pump for this application or pump is already installed?

Pump should have discharge head of about 200 psi. Cut-off points for start & stop shall be 140 & 170 psi. System resistance is continuously changing till it reaches higher cut-off point. So operating point is also changing with an average of 155 psi.

Hi All, i am the OP didnt notice my login was as guest.

Thanks for the valuable comments from all.

I would like to put a rule for selecting these type of pumps that booster pumps in closed system should be selected such the shut-off head is higher than the max pressure in the system regardless of the flow rate the pump will deliver.

Hope to hear your opinions.

This rule does not seem possible in the way you stated it.

Centrifugal pumps pressures depend on their rpm and the flow rate.

What you might be saying is: You want the pump to be able to produce a Static Pressure (Static means no flow, when there is no water moving any more) higher than the required System maximum pressure, so that there is a cut-off pressure switch(or the like) that will operate at the rquired pressure (in your case: Static head ~ 200psi to cut off at 170psi...?).

Your original question: Yes, when looking at the curve, what ever is the oressure read at the pump outlet, you will read the equivalent flow rate. And as the pressure changes (here, increasing towards 170psi), the flow will change as per the charted curve. the precision accuracy of the curve is not 100% and is normally stated by the maker.

In any case, the maximum (Static) pressure developped by the pump must not exceed the system maximum admissible pressure (designed at) as much as possible. This is in case the pressure switch controlling the pump fails! Extra safety mechanisms like pressure relief valves will need to be installed...

What you are describing to me sounds like the jockey pump on a fire water system.

If this is the case, flow is of little to no concern.

Jocky pumps usually aren't designed to feed water into the fire water system while it is active. They are there merely to pump the system up to about 170 PSI so you don't have to start up the much larger feed pump. You dont want to start up a 200 HP motor to compensate for pressure drops associated with a small drip or thermal expansion.

I said the same in the post 4, but no feed back from OP...