NFPA 14, 2013 ed. FIGURE A.7.2.4 1

[bmlxd40]

The figure referenced in the title shows two pressure reducing valves to comply with the requirements of 7.2.4. I am in a situation where we will have multiple hose valves down stream of a PRV, so we will be adding a second PRV to comply.

Does the first PRV have to have a higher inlet pressure than the second PRV? Part of me thinks yes, but I am a bit torn on this because if the second (downstream) valve fails, then the system will be exposed to higher pressures which defeats the entire purpose of having a redundant valve.

 

[skdesigner]

PRV manufacturers publish the minimum required pressure differential in the valve datasheets. The outlet pressure of the first PRV does have to be higher than the outlet of the second PRV to account for this differential. Singer and Cla-Val both have models with a 10 psi minimum differential, which is pretty tight. I think the Tyco PRV-1 differential is 50 psi, so I've never used it in this scenario.

I have always designed systems with possible failure of the second PRV in mind. I generally set the outlet pressure of the first PRV for 175 psi, and the outlet of the second PRV for 165 psi. That way you're covered if the second valve does fail.

 

[bmlxd40]

skdesigner, thanks for your help. Unfortunately I need right at 170 PSI at the outlet of the second PRV. If the second valve were to fail, I would expose the systems to about 180 PSI because of the pressure differential.

So, as an alternative, I am looking at using pressure regulating hose valves on my standpipes with a single Tyco or Cla-Val PRV downstream of the pump. What are the thoughts from folks here? Does that meet the redundancy required by 7.2.4? I think it does because if the main PRV fails, the hose valve is a regulating hose valve and provides the redundancy.

Thanks in advance for your help folks.

 

[skdesigner]

Do you have a combined system, or just standpipe? Are you using the PRV's to create low and high zones, or just flatten out the pump curve?

If you do have a combined system, NFPA 13 is going to apply as well. Specifically, 8.16.1.2.1

In portions of systems where all components are not listed for pressure greater than 175 psi and the potential exists for normal water pressure in excess of 175 psi, a listed pressure-reducing valve shall be installed and set for an outlet pressure not exceeding 165 psi at the maximum inlet pressure.

Another thing to be aware of is - even though the big regulating valves are fairly precise, the downstream pressure usually sits about 10 psi higher than what the valve is set for (give or take a few psi - nothing in this world is perfect) under non-flowing conditions.

 

[bmlxd40]

Yes, it is a combined system. Thanks Skdesigner. I missed that reference in 13. Now I gotta pick up 5 PSI somewhere, but I'm glad I asked here!

 

[skdesigner]

 

[bmlxd40]

I ended up moving the Cla-Val type PRV up a floor and making my standpipe interconnections on that level. That got me down to just below a 165 PSI required pressure at the oulet of the PRV, and is overall a better design for multiple reasons. I'll use Potter Roemer 4085 Pressure Regulating Hose Valve on the standpipes as my redundancy.

 

[LCREP]

Do not forget to install the required piping to flow test the devices as per NFPA 25. Running fire hose down 10 stories to the ground floor is not the best way to go.

 

[skdesigner]

4085's are pressure restricting hose valves, not pressure reducing. All pressure restricting valves do is prevent the operator from opening the valve completely (same premise as a wood block under a gas pedal). They only restrict pressure when water is flowing, but do nothing to reduce static pressure (which is the reason they are only rated up to 175 psi).

A pressure reducing hose valve like a Potter-Roemer 4023 reduces pressure below 175 under both flow and no-flow conditions. Unfortunately, they are probably going to cost quadruple what the restricting valves cost. Also, direct-acting valves like the 4023 have a fairly high differential, which shouldn't be ignored.

The fact that Potter-Roemer's names and descriptions don't quite match up with the definitions in NFPA 14 does make things a bit more confusing.

To answer the question in your second post, a main PRV + hose PRV's would more than meet the requirements of 7.2.4. Why? 7.2.4 states "Where more than two hose connections are used downstream of a pressure-regulating device..."

If each hose valve is itself a pressure regulating device, then you would never have more than two hose valves downstream of said device. This means that NFPA 14 is satisfied, and the main PRV would just be for the sprinkler system components.