FDC Used for Forward-Flow and as Drain Outlet?

11/20/2023

A fire department connection can be used with a bypass to facilitate a forward flow test of the backflow preventer as described in NFPA 25 Annex A.

Is there anything prohibiting the FDC from being used as a drain outlet, as well?

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

13 Comments

Pete H

11/20/2023 07:43:02 am

Wouldn't the check valve prevent the FDC being used as a drain outlet?

Anthony

11/20/2023 08:08:19 am

The forward flow line ties in after the check and you have a butterfly valve at the riser. This is very common in my community. Great for anytime you need more than a 2 1/2'' hose valve or the riser room isn't close to an exterior wall or in a basement.

Glenn Berger

11/20/2023 08:10:15 am

With the correct piping and valve arrangement, the potential is there.

Dan Wilder

11/20/2023 08:31:39 am

Are you talking the piping to the FDC, not the using the actual FDC correct? Using the piping to the FDC, while unconventional, would be OK if it served the purpose of an actual drain....however, the less manipulation of the FDC itself the better in my mind.

Would love to see the detail on the proposed piping to confirm everything.

Chris

11/20/2023 08:34:34 am

I have tried this a few times and have also ran into many issues with it.

1) If Knox Locking Caps are used, then it would prevent any water from discharging at all in most cases. MOST inspector's will not have the FDC key, so they would be unable to remove the caps and flow as needed.

2) A standard siamese FDC has a flap that only allows 1 outlet to be flowed at a time. In a case where the required system demand being used for forward flow is >250gpm, then more discharge would be required than what a siamese FDC would allow.

3) An FDC is usually 3'-6" +/- off the ground and a siamese FDC would be facing horizontal. When flowing large amounts of water, it is usually best to be closer to the ground and pointed downwards to minimize any potential damage from the flow.

Patrick S

11/20/2023 08:52:24 am

To control the flow from the FDC, plug one outlet, grab a double male adaptor on the other, throw 10' of hose with a Hose Monster.

Works great to measure a Foward Flow if no other larger flow available.

Chris

11/20/2023 09:32:52 am

Patrick, I would agree that this is a way to control it for option #3. Each of my comments has a workaround that can be done to avoid the issue, however the issue still creates a problem for most inspectors.

1) An inspector with a Knox FDC key could unlock the cap.

2) Remote the FDC head entirely for the forward flow test to use the 4" pipe.

3) Attach a hose (as you mentioned).

My point in this is that all of these have problems that can be avoided with a better design. All of these issues that I originally proposed do have workarounds, but the workaround are not as ideal.

Andrew

11/23/2023 03:55:10 am

Fun fact, Knox wrenches come up on eBay from time to time ;)

But overall, for systems without hydrants on site, I think the simplest option is a hose valve or two on the riser/manifold, if a 2" main drain for sure can't provide the flow (it probably can most of the time). An FDC bypass, for which I've even seen references to the disassembly of the FDC's clappers being part of the intended procedure, really shouldn't be accepted as good design.

Alex

11/20/2023 08:45:11 am

If you use the same setup as described above with the bypass around the check valve, I do not see an issue. As Chris mentioned, you may want to use additional sections of hose to prevent property damage

Eric R

11/20/2023 09:09:55 am

/edit: My comment below turned into a rant, so adding a quick answer to the top. You can do this in certain circumstances but you'd want to insure that you are not creating an issue with trapping water in the FDC line that may freeze or corrode the pipe./

This section of NFPA 13 and 25 is one of my biggest pet peeves.

There are limited circumstances where this approach can be useful, but in general terms it should be avoided if possible.

The only times we have personally found it useful is for small buildings with risers/backflows sized 2"-3" where the main drain is only 1.25" and the FDC can be a single 2.5" line without a clapper. In this situation you can install a 2" or 2.5' angle valve to bypass the FDC check and get good flow volume without running a dedicated line.

On 4" and larger risers/backflows however this approach really shouldn't be acceptable. Like Chris mentioned, the first big obstacle is that a normal siamese FDC has a clapper so you'd only be able to flow from a single outlet, and larger FDCs generally have individual clappered-snoots which make this approach impossible altogether. Add in the issues with removing caps, plugging automatic ball-drips, and potentially having trapped water corrode the FDC piping, and you are really creating a lot of trouble for very little gain.

I think a big part of this issue is a lack of clarity on how much water you can flow through a 2" main drain. For example, my office has a 4" backflow with a system demand of 325gpm, and we have a BOR residual pressure of around 50psi at that flow rate. The main-drain for the riser is a 2" ball valve with four threaded 90s and roughly 8' of sch-40 pipe. Attaching a 2"x2.5" fire hose adapter to the outlet and running 25' of fire hose to a hose monster allows us to flow the system demand without any trouble at all.

Obviously every system will be arranged differently and have unique water supply considerations, but a 2" main-drain should cover a large majority of light and ordinary hazard systems on its own, and if you need more than that then you would be much better served running a dedicated test manifold with two or more outlet connections.

Chris

11/20/2023 09:42:18 am

Love this response Eric, you did a great job explaining it! Have you or anybody else in the community come up with a way to approximate the discharge of a drain? If anybody came up with an approved chart or rule of thumb for this, it would likely fulfill the forward flow requirements as you mentioned. Then, the added costs/design wouldn't be required for simple LH/OH systems.

Casey Milhorn

11/20/2023 09:28:14 am

Everyone else has covered the "gotchas" pretty well, but I just wanted to add that this would be a great product to develop by one of the manufacturers out there. There's got to be a way to avoid so many penetrations through the wall and use the FDC connection as the FDC, forward flush connection, drain, inspectors test, PRV drain, etc....

Something to consider anyway...

sean

11/26/2023 09:30:10 pm

forward flow yes, main drain is harder

FDC Used for Forward-Flow and as Drain Outlet?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES