Where are PRV Hose Valves Req'd in a Highrise?

10/30/2024

How would you recommend determining when PRV hose valves should be used in a highrise?

I am designing a 12-story hotel. The highest outlet is at 125'-5".

Our flow test is mid-50's static and residual at 1,500 gpm. I did the standpipe calculation using a combined source of the city and fire pump. I sized the pump to what the demand was at 750gpm @ 174.26 psi. In turn I put in a 750gpm @ 125psi fire pump and had a 6.45lb buffer.

Do I then do another calculation to find out the maximum static pressure available on the supply side at each level and minus out the elevation loss?

Can anyone give me a guide on how to find out which level needs a PRV FHV and which doesn't?

Thank you in advance.

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8 Comments

Dan Wilder

10/30/2024 07:39:42 am

I would highly suggest reaching out to the PRV MFG that you plan to use for guidance.

I know AutoSPRINK has a detailed document on the process. Explaining it here will be difficult.

You are on the right track.
-Do the floor calculation to define your needed residual pressure at what flow. Round that up to the next available PRV setting.
-Run a demand calc to determine supply side residual pressure (before PSI reduction)
-Run a supply calc to determine static pressure, at churn, for the fire pump (any floor >175 PSI needs the PRV)
-Use those numbers on the supply side to determine the PRV bonnet setting based on what you need on the residual side for the system.
-Redo the floor calc with the new Static/Residual on the discharge side of the PRV to confirm system is compliant with any safety factors

The Standpipe calc is similar however your need to run it for both a single hose valve flowing (250 GPM), and all 3 hose valves flowing at the same time (assumption being the 750 GPM pump is for 2 standpipes). Verify the bonnet sizing works across both supply side residual pressures giving you the required discharge side residual pressure at 250 GPM.

Watch out for the top mechanical level, many times those systems exceed the max GPM for a PRV FCV and you'll need to switch it out to a pressure reducing station (larger valve).

Glenn Berger

10/30/2024 08:21:55 am

Per NFPA 14 Paragraph 7.2.3.2 PRVs are required where the pressure exceeds 175 psi.

Based upon information presented, you might need a PRV at the lower floors depending on the fluctuation of the incoming water supply.

Anthony

10/30/2024 08:23:38 am

A PRV is required any time any pressure is higher than working rated pressure either at static or residual conditions.

For hose valves a PRV hose valve will be required if the static OR residual pressure is over 175 psi. the hose valve should be set as close to 100 psi as possible.

Yes a static calculation needs to be done: churn pressure + static incoming - elevation = static system pressure.

As a general note I'd allow for no less than a 10 psi safety factor on all new high rise pumps. (probably more like 15 psi). You have no idea what the city water pressure will look like in 15 ~25 years especially if this building is located in a developing community.

Jack G

10/30/2024 08:38:52 am

That was an excellent synopsis by Dan.
The only thing I would add is I use the hose valve manufacturer when picking the settings for the PRV Hose valve for the floor.
When I redo the floor calc with the new static and residual for the floor I try to pick a setting that will put the most water to the floor ( keep the valve as open as possible at the design pressure) as the “safety factor). I also do a supply and demand calc for the floor( and regulate the sizes so the demand calc matches as close as possible to the supply calc—- so the pipe sizes control the flow with as little overages as possible— like calculating a deluge system). ( habit from old days- special hazards)
In my opinion, The supply calc is actually the best model of how the system would work when opening the entire MRA.

Jack G

10/30/2024 08:52:58 am

I might also add that adding too much safety factor ( putting 10-15 extra psi ) would increase the floor demand so that it would not work— unless you balanced the demand calc to the supply calc. Try it on your next hydraulic exercise.
If you put the safety factor at the pump, you will need extra PRV s when you physically turn the water on. 15 psi might construe to 3 or more floors actually needing prv s . ( exceeding 175 psi static pressure) .

Anthony

10/30/2024 01:09:37 pm

Try explaining to a building owner why their fire pump is under sized 10 years after its installed, 2 extra floor PRVs are worth the expense.

Ken Thackery

10/30/2024 10:16:11 am

What is the demand at the roof? That will drive pump size and everything else. You will need PRV for HV and zone controls, possibly high pressure couplings depending on the size/listing.

Jack G

10/30/2024 11:20:26 am

Agree with Ken also. On government buildings you might find a .15/ 2500 or 3000 sq feet. Probably make one of the standpipes to 6 inch so that the friction loss is reduced. No prv.

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Where are PRV Hose Valves Req'd in a Highrise?

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