NFPA 13D and pressure regulators/reducers 3

[chopsnrox]

Hello

What’s your thoughts on PRV’s/regulators on 13D systems? I know 6.5.3 talks about water softener’s, etc., but what about PRV’s? I would imagine, the friction loss should be taken into consideration anyways, but should the sprinkler piping be tied in upstream of the regulator anyways to be safe? I know there are a lot of “what ifs”…like if they adjust the PRV setting way low which now could affect the flow/pressure requirement for the two head calc, or what if they install a different PRV later on with a different setting? Am I reading too much into this or am I justified in being a little concerned? I know a majority of houses will use a PRV of some kind to regulate their domestic water anyways.

Thanks.

 

[TravisMack]

I believe the fire sprinkler system should be upstream of the domestic PRV. Just a way of keeping some one from changing the settings later on and possibly reducing the pressure too much for the fire sprinkler system to work.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[chopsnrox]

Thanks, TravisMack, that was my thought exactly......

 

[stevenal]

In my experience when a PRV fails, it fails closed.

 

[wafflefryer]

If its a stand-alone system. Put the PRV downstream of the tee branching off to the fire line.

On a multipurpose system, you'll need to have a prv for a bypass line. (That is , unless the water softener/filter can handle the full flow of the system.)
The prv has to be set low, but not too low.
You want it set low so that when there is domestic water use, a small downstream pressure drop will not cause the PRV to open up. This will cause hard water issues and will become a 'return call' headache.
You don't want it set too low because you need enough avail. psi downstream for the sprinkler system to work adequately.

So what is the right set pressure for a prv?
Unfortunately there isn't a generic number for this. It is different for every system and every PRV.

For example if the residual pressure needed at the base of the riser is 25psi and you have a flow of 30gpm, then we know our starting point is 25psi.
The next step is including the friction loss across the prv in the set pressure. In the example we are flowing 30 gpm. You'll want to select a prv that demonstrates a low friction loss (around 5 psi). Remember, we need the set pressure to be low to avoid hard water issues.
You must include the loss across the valve in the set pressure. So now we're looking at 25psi plus 5psi for a total of 30psi for the set pressure.
I think it is a matter of 'best practice' to add 20% to the flow when figuring your friction losses since most PRV friction loss charts are created using a 50psi differential. (Meaning that the static psi is 50lbs higher than the prv set pressure)
So now in our above example we may have a set pressure that is around 32psi. THe last step is rounding to the nearest 5 lbs.

So now our system that had a demand of 25psi at the base of the riser will need a PRV that is set at 35psi.

It is very, very, very, very important to use a pressure gauge that is only slightly higher than the static pressure to set the PRV. If your static is 75psi, use an 80psi gauge. DO NOT use a 300 psi gauge to set a prv. A 300 psi gauge measures so inaccurately at those low pressures that 1 of 2 things will happen.

1. The set pressure will be too high and there will be hard water issues. (a headache)
2. The PRV will be set too low and you will have a sprinkler system that doesn't work. (a nightmare)

Lastly it's important that the residence has a water softener that is sized adequately. If the domestic demand is going to nomally be 10gpm and they install a 8gpm softener, then the prv will open often. This is because the downstream pressure will drop enough to cause the prv to open up and let the extra 2 gpm (of hard water) passed it.

 

[chopsnrox]

Wafflefryer,

Thanks for the detailed post......