Figuring out Fixed Pressure Loss

[FireFreak08]

This is more of a hydra-calc question but I would also like to hear general answers.

We frequently do high-rise tenant fitup projects and one question came up which I've heard multiple answers to.

There are two standpipes. The floor is looped with a PRV at each outlet from the S.P. The PRV is set to 90 psi. In hydracalc I run a source calc for the entire system which runs the system down to 0. I take the pressure at the point of the PRV and I subtract the 90. This gives me the loss at the PRV.

Now, I've also heard other ways of doing it and am curious what you may have to say.

How would you do it without hydracalc. I know my method is conservative but I'd rather it be worst case.

 

[stookeyfpe]

Well, water is an incompressible fluid and P = 0.433 x H, where H is measured in feet. So what are you really asking?

By the way, that's one heck of a pressure loss through a pressure reducing valve given that such a valve is not required for system pressures generally that are less than 175 PSIG.

Is it me or am I missing something?

 

[FireFreak08]

This is for an existing high rise building with a fire pump which has pressures in excess of 175 on the lower floors.

The fixed loss would be the loss of which happens from the point of the prv, to the point after the PRV. For example, if you have 200 psi at the prv, the prv is set to 100. You have a 100 psi fixed loss (not factoring in actual valve loss).

In our calculations, we need to show that loss. The way I do it is on the high side (which is better) but I'm looking for different ways people may do this. I have also done it by

Static at pump - Feet in elevation (x .434) = pressure at prv
pressure at prv - prv setting = fixed loss