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Two Parallel Pressure Reducing Valves in System Supply per FM Global.

Two Parallel Pressure Reducing Valves in System Supply per FM Global.

Two Parallel Pressure Reducing Valves in System Supply per FM Global.


Per the title I am working on a job that has two parallel pressure reducing valves in the system supply which is a first for me, and I specifically have some questions on the FM Global pressure relief valve requirements.

Some job info:
-Building is on a college campus supplied by a private high pressure main from a central pump house, hydrant flow test and recent pump test are not yet available.
-Job must meet NFPA & FM Global requirements and products must be UL & FM listed.
-Building is five stories with a mix of HC-1 classroom space and HC-2 lab space and mechanical rooms.

For those who don't know all FM Global Data Sheets are available for free on FMs website for further reading, but I will do my best to provide the relevant text from FM DS 3-11 Pressure Reducing Valves below.

I am comfortable with the sizing of the two pressure reducing valves. FM DS 3-11 Section discusses how larger pressure reducing valves may not be accurate or cavitate at low flows, and recommends that the smaller valve be sized for a flow range of no flow to ~20% of max system flow, and the larger valve be sized for ~20% of max system flow to max system flow. Section goes on to state that pilot-operated pressure reducing valves should be sized to operate within 20% to 80% of the valves max flow capacity to avoid instability and damage.

Using an assumed automatic standpipe demand of ~1100gpm (1000gpm design demand + 10% for losses) and the Tyco PRV-1 for a valve I arrived at a 6" and 3" valve. The 6" PRV-1 is rated for 0gpm to 2200gpm, and would handle flows between 440gpm and 1760gpm for this application. The 3" PRV-1 is rated for 0gpm to 550gpm, and would handle flows between 0gpm and 440gpm. The standards and data sheets are all very straight forward with this, and it all jives with things I have heard in a past life from the process engineers as a pipe stress analysis guy in Heat Recovery Steam Generators.

The bid drawings lack any indication of size on the pressure reducing valves, but based on the sizes available of other equipment, the use of reducers, and the end preps used, it looks like the engineer developed the bid drawings using 4" and 2" valves. I am confident that these are undersized per my above analysis.

The vague bid drawings go on to show what scales to be another 2" line off the discharge of the pressure reducing valves with the note "Extend to relief valve, sight orifice and drain per FM Global and NFPA requirements." FM DS 3-11 Section requires a pressure relief valve, water flow alarm switch, and check valve on a separate tee off with the relief valve sized per Section 3.1. Section Sizing Drains and Relief Valves only goes on to state why proper sizing is important, and offers no sizing guidelines.

I have never provided anything larger than a 1/2" relief valve per NFPA 13, and the Tyco PRV-1 data sheet notes, "FM approval is based on the use of the 1/2 inch Watts Regulator, Series 530C Pressure Relief Valve". The Watts 530C is only available in 1/2" and 3/4" and the data sheet does not provide any sizing guidelines. Part of me thinks that this is it, provide a 1/2" relief valve like this was installed in a floor control assembly and call it a day, it is how the valve was listed. Another part of me thinks that the FM requirements for a flow switch and check valve indicate the intend use of a larger line and relief valve for this type of installation, at least a 1" line to use those paddle flow switches that thread into a 1" tee. The bid drawing almost looks like a reducing tee that you would put an angle pattern pressure relief valve on with a waste cone and sight glass for a diesel fire pump discharge line.

Is a 1/2" pressure relief valve large enough for the installation of a 6" pressure reducing valve in the system supply line for an automatic standpipe? Or is it really only adequate for use in floor control assemblies with lower demands?

If the 1/2" is large enough when do you move to the 3/4" valve?

If the 1/2" and 3/4" valves are too small and I need to move to something like the TYCO RV-1 how do you select an appropriate size? Specifically the RV-1 is targeted at pump discharge lines and the data sheet only includes guidance from NFPA 20.


RE: Two Parallel Pressure Reducing Valves in System Supply per FM Global.

Not sure if I got the idea clear. But, if there is a pump and the pump goes on churn, will the p relief valve at the pump work fine?, or do you need another preliefv at this pipe section?. I understand that the small PReliefV is for no flow relief when valves are closed for example. Just a point meanwhile FM experts appear.

RE: Two Parallel Pressure Reducing Valves in System Supply per FM Global.

1/2" valve is too small, and the supplied info on this topic is a bit of a mess.
My experience from using the Cla-Val pilot type PRVs is that one size less than 1/2 the size of the main valve will get the job done. I have noticed that if you look up listing on the FM Global Approval Guide, the requirements are not always the same as the product data sheets. Had a 8" valve say both 1/2" relief on FM Site, but had a note indicating 1-1/2" on all sizes, then the data sheet called for 3" relief. We went with a 3" relief valve, thinking that if the main valve was to fail open, that a 3" relief valve would have a much better chance of keeping the pressure down. Make sure your relief is a quality pilot type to handle short term surge pressure that get past the main valve.

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