Pressure Reducing Valves

[gfire]

Has anyone dealt with pressure reducing valves? This is the first time I've had to deal with them. I'm in the process of upgrading a system to a new fire pump. The AHJ requires that the pump be able to fully supply the standpipe system (which the old pump didn't do), resulting in pressures higher than the 175 psi limit that the system is designed for.

I'm looking at running a new feed main throughout the ground floor with high pressure fittings on the ground floor main and pressure reducing valves on the risers going up.

I can knock the pressure down to 165 at the base of each riser and still have enough pressure to supply the standpipe, but is there a 6" pressure reducing valve out there that is listed for fire pro? I'm trying to keep the number of pressure reducing valves to a minimum rather than having a pressure reducing control valve on each of the lower floors (there are several systems on each floor which would result in multiple pressure reducing valves per floor).

Also, since I haven't dealt with them before, how reliable are pressure reducing valves? As with anything there is a chance of failure, but are these valves pretty reliable or are there problems associated with them? Do they need to be tested annually?

 

[PEDARRIN2]

Try Tyco

 

[b1ueshift]

you can get UL listed pressure reducing valves larger than that. I sourced some 8" PRV's for a project recently from Singer.

Reliability is a concern. I think NFPA 20 requires that when pressure reducing valves are installed on a pump outlet, two should be installed in series incase one lets pressure through.

 

[TravisMack]

Also, you need to check the wording of PRV's in the latest edition of NFPA 14. You can't have failure of a single PRV affect more than 2 hose stations (or something like that as I don't have the standard in front of me this weekend). You will either want to have (2) PRV's in parallel at the bottom of each standpipe, or you will need PRV's at each floor. Remember that you need to have a way to perform a full flow test on each PRV, so figure that in as well.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[DavidCR]

There are two groups of type of valves, the "small" ones you can find from Potter-Roemer, Zurn, or the hydraulic control valves from Watts, OCCV, Claval, Bermad, ...

The type of valve depends on whether you are controlling pressure for sprinklers and hose connections for small systems, or if you are thinking of hydrants, monitors, deluge systems, combined systems...

PRVs have a working range between the minimum and maximum flow that they are capable of working without pulsating flow.

In my criteria depending on the case a PRV should be selected to work from a minimum flow (eg.from just one open sprinkler) and a maximum (eg. a design flow of the system and the hose demand), that would need a PRV that work fine on a wide flow range or two on parallel that work together on that wide range.

NFPA requires in some cases that there shall be an expensive independent drain riser to test and adjust the PRV.

Maybe strainers should be considered.

Cavitation limits should be checked.

For hose connections sometimes cheap restricting devices may work.

 

[chillylulu]

In addition to the previous responses I would add the following:

Why does your pump exceed 175 PSI, yet you can knock the pressure down to 165 PSI at the base of the standpipes? Is the pump located at an elevation below the base of the standpipes? Is the problem the pump static pressure, or does the water supply vary that much?

Sometimes you can use a pump rated for lower flow to limit static issues. For instance, if the standpipe requires 1000 gpm, use a 750 gpm pump. NFPA 20 4.8.1 allows you to use 150% of the rated flow. Check out the curves from different manufacturers - you may be able to avoid PRV's altogether.

NFPA 20 also allows the use of "Variable Speed Pressure Limiting Control" I've never seen it, but maybe your problem can be solved by the controller.

Regarding PRV's:

See NFPA 14 2010 7.2.4 - & check out the appendix for the proper configuration.

You need to be careful with the design of pressure reducing valves. You almost always should also have a pressure relief valve located downstream of the pressure reducing valve. The reason is that most pressure reducing valves are designed to operate under flowing conditions. They don't react quickly and are not designed to catch a spike or surge in pressure. The pressure relief valve does not have to be as big as the line size, just big enough to relieve pressure surges.

Another option you have is to install pressure reducing hose valves for the fire fighters on the lower levels, and use 300 psi sprinklers where sprinkler system pressure exceeds 175 psi. You will need a 3" drain riser with hose connections to test the PRV's every 3 years. I know that this might be an expensive option in an existing building, but with the head testing requirements of NFPA 25 - it might be time to replace the sprinklers regardless.

Remember that NFPA 13 allows the use of standard weight fittings per section 6.4.4.1 through 6.4.4.4.

Another problem with having higher system pressures is leaks. Even on a system that has been service for decades, if you add pressure, there are likely to be some leaks show up.

 

[inspectortx2]

try a cla-valve right after your pump set at 175 psi as long as you dont have a high rise with to many floors you should not have a problem but you need to check your calc's. prv's need to be tested every 5 years with a full flow test.

 

[TravisMack]

Also, don't forget that the FDC must be connected downstream of a PRV according to NFPA 14. If these PRV's are spread throughout a project, then that is a lot of bulk pipe to get back to the FDC locations.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com