Pressure Reducing Valves - Friction Loss in Calcs

5/17/2019

Pressure Reducing Valve are commonly used to prevent excess pressure upon systems when supplies exceed the NFPA limit of 175 psi.

There has been much discussion regarding the 'application' of these valves when hydraulic calculations are being provided to the discharge of the PRV and determining the pressure margin available.

Currently, we have been instructed to calculate the system back to the PRV, use the PRV's friction loss calculator to determine friction loss through the valve and subtract that from the outlet setting to establish outlet pressure. This is then used as the available pressure at the outlet; minus the demand and you're left with the margin.

I'm not sure that I agree that the valves 'friction' loss need to be considered unless you are at a point where the PRV can no longer provide the set pressure (when the supply is unable to provide sufficient inlet pressure to overcome the internal friction loss).

Are there any 'papers' written for direction or other sources anyone might recommend?

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9 Comments

FIREPE25

5/17/2019 10:22:54 am

We see these calculated incorrectly very often. You are absolutely correct about the friction loss. If the valve is set for 150PSI and you have 190PSI before the valve the friction loss will be built in. The only time the friction loss is needed is when the valve is wide open (assuming we are talking about pilot operated PRV). This gets a bit tricky if the valve is set for 150PSI and you have 155PSI before the valve. We use SprinkCAD and we calculate them using the following steps. This may be a bit confusing but i will do my best.

1) Run a calculation from the remote area back to the source.

2) Insert a standpipe demand node at the PRV.

3) Run a separate calculation using the standpipe demand node set at the required flow. The pressure needs to be set at whatever makes the safety factor zero. This will show you what you have for pressure before the PRV at your required flow.

4) You then can look at your original calculation and determine what pressure is required at the PRV location and compare it to what you actually have for pressure before the PRV and what you plan on setting the PRV for.

*If your pressure available before the PRV is less than the PRV is set for then you can run a normal calculation all the way back to the source with the friction loss through a wide open PRV at the location of the valve.*

PETE

5/17/2019 10:41:07 am

The PRV will output an entirely new water supply curve based on it's friction loss curve. One way to consider the losses is to plot the new water supply points based on the inlet water supply adjusted for friction loss at various points, and input that as your water supply. The other way is to add a set friction loss at your flow for each calculation. Either way is acceptable, but you can't just ignore it. The PRV will have friction loss even when the supply pressure is lower than the outlet pressure setting. Look at p.11 of this Cla-Val PRV. https://www.cla-val.com/documents/Technical%20Manuals/TM-90-21.pdf

Also consider that PRVs have a minimum flow for the diameter of the valve, before which the PRV won't flow at all. The scenario is something like a pump with an 8" discharge has an 8" PRV with a 390 gpm minimum flow. You want to calculate a light hazard system (0.1 gpm/ft^2 x 1500 ft^2 + 100) at 250 gpm. At 250 gpm, you have not reached the minimum. So your PRV must include a smaller diameter bypass (say 2") piped in parallel that has a smaller diameter 2" PRV on it, to accommodate lower flow rates. An alternative arrangement would be to put PRVs on the individual risers or riser manifolds, because most pipe and fittings are UL listed for up to 300 psi.

5/17/2019 10:52:38 am

I think I agree! In most instance the pressure after a pilot operated PRV will be a flat curve at whatever the set point is until the flow gets so large that the water pressure deteriorates to less than what the valve is set at.

M Akram M Ibrahim

6/20/2020 01:33:45 pm

Yes,if we see curve of inlet verses outlet pressure setting .though globe valve pressure drop is not more than 10 psi

Clifford Schulze

5/17/2019 04:20:44 pm

Be very careful when using the 2 1/2" brass pressure regulating valves. The pressure loss across those, especially at the higher flows, is substantial.

Nimal Tissa Wijetunga

5/18/2019 12:42:12 pm

PRVs are highly susceptible to failure and high maintenance requirements and therefore not recommended to utilize in Fire Protection Systems due to reliability considerations.

5/18/2019 02:23:59 pm

^^Citation needed. There are UL listed and FM approved PRVs on the market. Backflow preventers are high maintenance also, but they're required in some jurisfictions invariably.

Mike

5/19/2019 01:22:47 am

When installed correctly and maintained per NFPA 25, these valves are highly reliable. Most often if they fail, they were set incorrectly or not maintained.

5/19/2019 01:30:17 am

I think the delineation needs to be made between a pressure reducing valve and pressure regulating valve.

The pilot operating regulating valves are designed to deilver a constant pressure on all flows within it's range. Therefore, i don't think friction loss is a factor.

Friction loss would be determined as a fixed flow back to the source from the PRV as mentioned.

I think two curves is the correct way as stated FIREPE25.

Pressure Reducing Valves - Friction Loss in Calcs

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