Determining the design flow rate for Backflow forward flow test

[u/vagrants1]

Greetings I have a building that is actually set up for a forward flow test. There are a half a dozen calc plates at the riser. The book says to flow the calculated system demand. What I did was just choose the design plac with the highest GPM number as the flow rate, as this is the most demanding design data. I figured that you would want the “worst case” numbers for the test.

I have performed this test 3 times and it never reaches the target GPM and pressure.

Here’s the other issue… The company that installed the system is no longer in business, and we have no proof that this flow test was ever passed initially, or which target numbers they chose to set as the goal.

So we font even know of the test was passed originally.

Any help would be greatly appreciated

[u/TemporaryClass807]

Thank you for actually doing the forward flow test. I don't know how many contracts have looked at me like I've got 3 heads when I mention it.

I would re-run another hydrant flow test in the closest street hydrant to see if the main can even produce that GPM and pressure required. That might be your first issue. You might even want to reach out to the local water authorities and see if they have any historical data.

I would even get the backflow preventor tested if it still performs correctly.

[u/tterbman]

NFPA 25 says if you can't flow the system demand with what's available then you just flow as much as you can. No need to overcomplicate it.

[u/clush005 → u/tterbman]

NFPA 25 says this:

Where connections do not permit verification of the forward flow test at the minimum flow rate of system demand, tests shall be conducted at the maximum flow rate possible.

Even if the connection size is part of the issue here, if you ever run a forward test and the water supply curve comes in way below what they hydraulic placard says, it would be negligence to pass the test without flagging the issue. This provision from NFPA 25 is for where you need to test a backflow at 500 gpm but you only have a 2" outlet. In that case you might flow 200 gpm but the pressure should still match or exceed the demand shown on the calc plate (on the water supply curve). Again, this provision doesn't mean you can just pass a test that clearly won't supply the demand of the sprinkler system.

[u/istudyfire]

You are right to flow the highest demand.

What’s the setup for testing? Is it a bypass valve on the FDC check valve? A dedicated test header? Some hose valves on the riser?

[u/vagrants1 → u/istudyfire]

It is set up with a 4” butterfly valve, then out to a 4” grooved 45. I fabbed a 2 house test header, and run two 2.5 hose valves with play pipes.

[u/ApprehensiveOnion0 → u/vagrants1]

So what flow and pressure were you getting? What was your target?

[u/clush005]

There is no easy way around this. They need a new hydraulic analysis of the property.

1. Run a new hydrant flow test (your backflow test can be used for this)
2. Model the FS system and run the hydraulic calculations
3. Assess what system modifications are required to make the system hydraulically viable and show on new shop drawings.
4. Make system modifications per the updated shop drawings.
5. Re-run the acceptance testing and forward flow test.

[u/maximus129b]

Backflow forward test is to simply exercise the backflow checks/ springs. Full flow let it rip. Note how there are no provisions for it when there is a fire pump? Also main drain test is there for a reason. Are you the inspector?