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Anonymous Poster

### Operating pressures for high rise buildings

05/11/2007 12:08 PM

I have a contractor asking for the operating pressure of my HW, Primary Condenser Water, and Secondary Condenser Water systems. He needs this info to order his PRVs.

I have a cooling tower on the roof of a 21 story building, the plumber has provided a fill on the roof. My heat exchanger, primary and secondary pumps and secondary condenser make-up water fill is in the cellar. My city steam to HW heat exchanger, HW pumps and the make-up is also in the cellar.

So what is the operating pressure of these systems (for purposes of sizing the PRVs for the makeup water)? Do I add the static head (roughly 90 psi) to the dynamic head? Is that the pressure that the make-up water needs to exceed in order to inject make-up water into the system? What should my circ pumps be sized for? The static head, the dynamic head, both or some percentage in the middle?

I have a similar question on the domestic service. The owner wants to change some fixtures in the penthouse. These fixtures require a min. pressure to operate. If I know how much pressure I have at the POE and I know how much pumping capacity I have, how do I determine what is left at the penthouse? Is it the static plus the dynamic head minus my available pressure?

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Join Date: Sep 2006
Posts: 24
#1

### Re: Operating pressures for high rise buildings

05/12/2007 12:36 AM

First of all, you are asking a whole bunch of engineering questions at one time. In order to even start, I must know what the operating pressures of the various systems are. The static AND dynamic pressures must be know (the piping may be too small to yield sufficient flow). Generally speaking, the required pressure at the point of use must be greater than the system operating pressure so that there is positive flow into the system. For domestic (potable) water systems, you must know the minimum operating pressure of the plumbing fixtures. The systems must be engineered,as they cannot be answered in a single response such as this.

Anonymous Poster
#3

### Re: Operating pressures for high rise buildings

05/15/2007 4:25 PM

The opperating pressure is what I am trying to find. I know the static and dynamic pressures (95psi and 60' of dynamic head) and I am trying to figure out how to determine the opperating pressure of the system. I understand that the domestic booster pump and city water combined pressures must be greater than the opperating pressure of my system in order to inject water into the system. With regards to the plumbing, again I know the min required pressure of the fixtures. The question is really, how do I determine the opperating pressure of a system if I know the static and dynamic pressures.

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#2

### Re: Operating pressures for high rise buildings

05/13/2007 10:32 PM

You need to total up the static, dynamic and fricitional resistant heads in order to know the required pressure for your pumps. The static head is the height the water must be raised.

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Anonymous Poster
#4

### Re: Operating pressures for high rise buildings

05/16/2007 1:22 AM

Pressure = density x acceleration due to gravity x height of liquid column.

So what is the operating pressure of these systems (for purposes of sizing the PRVs for the makeup water)? What should my circ pumps be sized for? The static head, the dynamic head, both or some percentage in the middle?

For circulating water, the inlet pressure will be the height of the water column, less some dynamic losses due to flow down the return pipes. The outlet pressure will be the height of the water column plus the dynamic losses up through the supply pipe and the equipment it supplies. The chosen pump may have a dynamic delivery pressure that is rather low, say, with the capability of withstanding a much higher system static pressure. Try looking at some similar systems, deterining the pump manufacturers and talking to them about the specific needs.

Do I add the static head (roughly 90 psi) to the dynamic head? Is that the pressure that the make-up water needs to exceed in order to inject make-up water into the system?

Yes, otherwise it won't go in. It will be easier to get water to inject into the upstream side of the pump as the pressure is lower at that point.

I have a similar question on the domestic service. The owner wants to change some fixtures in the penthouse. These fixtures require a min. pressure to operate. If I know how much pressure I have at the POE and I know how much pumping capacity I have, how do I determine what is left at the penthouse? Is it the static plus the dynamic head minus my available pressure?

It is the pressure at the POE [Point of Entry?], minus the static head to the penthouse (allow for the elevation of each one above the POE for good measure), minus a bit more to allow for other users on the pipe drawing water at the same time. If this pressure exceeds the minimum pressure of the new fixtures then there will be no problem. Sometimes a minimum pressure criterion is there to protect the manufacturer from "it's not working properly - please help" claims. A recently-installed instantaneous water heater for a domestic application has a manufacturer's minimum pressure criterion of 1 bar. It is working satisfactorily at 0.6 bar as installed, as chance would have it. If one cannot take chances then play safe, do the research and do the sums.