Calc Backflow Retrofit for Pipe Schedule System?

7/18/2024

When retroactively adding a Double Check Valve Assembly (DCVA) to an existing system, we need to verify that the new DCVA does not adversely affect the system's performance.

If no significant adjustments are required, I usually use the hydraulic placard information and compare it to a new hydrant test to determine the system's available pressure. I then subtract the pressure loss from the DCVA at the flow rate indicated on the placard.

How do people deal with adding a DCVA to a pipe schedule system with no existing documentation?

Thanks in advance!

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9 Comments

Pete H

7/18/2024 06:35:41 am

With no existing documentation?

I rerun the calc for the system. I get a new flow test, I draw the system to the most remote area, and I calculate said remote area, now with backflow.

If you got the placard (and the placard still has all the information legible), the method proposed in the OP sounds good, imo. But I could see an AHJ requesting that it's at least directly stated with a note dating the hydraulic placard you're using, the available pressure/demand, the cut for the backflow you're installing with a reference to the pressure loss graph and the number you got from it to reach your current safety factor. Heck even a photo of that somewhere in your submission wouldn't hurt. Just to keep everything on the up and up.

Dan Wilder

7/18/2024 07:36:55 am

As most of my AHJ's require a letter submittal when adding in backflows retroactively, we perform a new flow test, calc the existing system via survey or as-builts (it is rare that the hydraulic placard is still on the valve), and provide those results in a writeup to the AHJ...inspector comes out to verify the calc plate being installed.

What typically happens is the owner gets caught for other work completed without permits, or some poor tenant that was sold on a suite that was not up to current requirements (racking, remodels, change in storage criteria from originally approved plans)....then we get to have that conversation.

Anthony

7/18/2024 07:53:13 am

A pipe schedule system "calc" must be completed which is just a the elevation loss of the highest sprinkler and the loss through the backflow subtracted from the curve at 850 gpm (For OH). if you have 20 psi left you're compliant.

see 11.2.2 through 11.2.2.5 in NFPA 13-2016

Anthony

7/18/2024 08:07:17 am

I miss typed I mean though 11.2.2.7 * in NFPA 13-16ed

Also see NFAP 13-19 ed:
Section 19.3.2 though 19.3.2.7

7/18/2024 08:07:30 am

Pete and Dan hit the nail on the head. The system needs to be recalculated with a current flow test to ensure the hydraulic calculations aren't compromised by adding a new backflow preventer (either based on field verified as-built fire sprinkler drawings or by resurveying the existing system).

Jesse

7/18/2024 10:20:32 am

I'd definitely calc the system. An insurance company I did a lot of work for in the past never would accept placard data (the VP of engineering was on 13), so we calc'd EVERYTHING.

Glenn Berger

7/18/2024 12:25:04 pm

Like others have said - Get new hydrant flow test and then hydraulic calculate the system. There used to be assumption information in some of the old FM Design Guides.

Jack G

7/18/2024 05:03:10 pm

The AHJ usually wants you do survey , new flow test and calculate.
What I d like them to accept is a method I use to determine if the water supply will accept a density upgrade, etc.
First determine the systems k factor from the hydraulic plate.
Say 500 gpm = K x sq root of p. Use 36 psi. K = 500/6,
System k = 83.33
Substitute— here we are keeping density same and adding the psi for the bfp and valves. Say 8 psi. So
GPM= 83.33 x sq root of 44 psi
Gpm = 83.33 x 6.63 = 552.47 gpm.
Check the water curve to see if it’s under, flow increased and pressure increased o the water supply curve. In hydraulic calcs we do the same thing for paths, etc.
It’s pretty accurate, and I use it to first estimate the need for a pump on a water supply or an increased density say a whouse goes from palletised storage to rack storage etc. try it here.

Kyle R

7/18/2024 08:03:50 pm

I agree w/Anthony above. For an existing Pipe Scheduled system, NFPA 13 is pretty descriptive of how to retroactively add a BFP and the 'Calc' that is required.

For an existing hydraulically calculated system, I agree with Pete/Dan, the system would need to be as-built and hydraulic calcs performed to ensure the system can support the addition PSI loss of the BFP.

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Calc Backflow Retrofit for Pipe Schedule System?

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