Coefficient of Discharge for Forward-Flow in FDC?

10/5/2021

In my area many designers use the main test/drain or one of the FDC inlets to conduct forward flow for the double check backflow assembly. They use the velocity pressure pitot equation to demonstrate that they will achieve the system demand flow. During the acceptance testing, I verify that the system demand is achieved by measuring with a pitot gauge and calculating the flow.

I generally use a coefficient of 1 when a hose monster or the main drain piping is flowed from, because they are long runs of smooth continuous pipe.

Is this appropriate or should I be using some other coefficient?

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6 Comments

Alex

10/5/2021 05:18:25 am

Hi Anonymous,

When using any Hose Monster device, their coefficients are published online. Link below.

Link: https://hosemonster.com/wp-content/uploads/2021/09/HYDRAULICS-ENGINEERING-INFORMATION.pdf

Thanks,
Alex

Jesse

10/5/2021 08:10:27 am

If its a Hose Monster branded device, they have coefficient data on their website.

If we're using "hose monster" in a ubiquitous / generic way like "coke" or Kleenex" the hydraulic data should be on the actual manufacturers website.

If its a home made hose monster type device, then I don't really know.

Cordell

10/5/2021 08:30:11 am

In many instances the 2" main drain or a single 2-1/2" outlet will not achieve the desired flow rate. I have added a manifold with six 2-1/2" outlets to achieve the system flow. To answer your question regarding the outlet coefficient, a coefficient of 1 would not be suitable since this is not a smooth outlet. 0.9 or 0.8 would be a better option.

Randy Kimbro link

10/5/2021 10:38:56 am

As an AHJ, I agree with Cordell. The 2" main drain or a single 2-1/2" outlet will not achieve the desired flow rate. We require the installation of one to many 2-1/2" outlets to achieve the system flow.

As an aside, starting with the 2019 version of NFPA 13, there is a requirement that the riser be permanently plumbed for forward flow. 16.14.5 ".. shall be serviceable without requiring the owner to modify the system to perform the test".

There is also an exception that says where connections do not permit full flow you can conduct the test "at the maximum flow rate possible". NFPA 25 13.7.2.3. Again as an AHJ it is rare that a riser cannot be altered to add outlets for this required annual test.

sean

10/5/2021 07:08:32 pm

i think the main issue is 1 is a super efficient delivery a play pipe does not achieve that and it is significantly smoother and las more laminar flow a .9 is the max I would personally find acceptable.

Dan Wilder

10/6/2021 09:04:33 am

I recently had an AHJ redline a plan for this reason.

We use a 4" Hose Monster for all of our flow tests with calibrated gauges with results coming from the gauge/chart (the coefficient comes out different based on the pitot/flow when using the formula. in my case it resulted in a just below .8) . The AHJ said that their hydrants were all a .9 in that area and that our flow test was invalid.

We ended up getting a letter from Hose Monster Techs with a great explanation that the point of the flow where the pitot tube was taking measurements became the effective point for the coefficient, i.e. the equipment itself not the internals of the hydrant. Had we used a pitot on the hydrant itself the .9 would be accurate but as we used a remote piece of equipment that their charts became the approved result (regardless of coefficient).

Even if you are using pipe, NFPA 291-19' Section 4.7.2 suggests a C of .95 or as others have suggested use the MFG data sheets indicating the effective C.

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Coefficient of Discharge for Forward-Flow in FDC?

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