New Fire Hydrant Flow Test Charts (PDF)

7/14/2021

One of my wanted items on my personal to-do list was a full lineup of flow charts that incorporate discharge coefficients coming off a fire hydrant. These (in my line of work) are most commonly used in fire hydrant flow tests, where flow comes out of the side (commonly 2-1/2") outlets or the main pumper outlet (single 4" or larger).

So what is a discharge coefficient?

When we flow out of an orifice, water is restricted and forced into a smaller cylindrical shape. The discharge coefficient is what helps us adjust the measured pitot pressure, which is taken along the centerline of the stream, and adjust for about how much flow is coming through the opening.

It's simply comparing the size of the actual stream against the size of the opening.

When a long straight stream-straightener is used, this coefficient is high. The stream is less tightly condensed as compared to the opening dimension.

When water takes a more difficult approach and is restricted in its flow, this coefficient is lower.

Discharge Coefficients as Identified in NFPA 291

We'll discuss this in more detail later. For today, we've created flow charts for these three different discharge coefficients. See the PDF link below to get access to all three flow charts, broken out across a number of different orifice sizes.

GET FLOW CHART PDFS

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Kevin LaRose

7/14/2021 01:51:59 pm

Thank you for this chart. I always apply the correct coefficient on my flow tests but I still don't understand why we would take the results from our flow test which is after the hydrant outlet and use a coefficient to make the results worse. Aren't our readings already worse than what we have available before the hydrant outlet?

Joe Meyer

7/14/2021 02:51:37 pm

I've thought about this quite a bit myself.

In the coefficients we are accounting for the method in how we are measuring the stream. If it were otherwise easier to measure flow, we could just fill up a 5,000 gallon tank exactly and time how long it takes. It either isn't consistent or isn't practical, so we take the pitot instead.

We are not factoring the loss through the pathways that the water takes to flow out of the hydrant. Usually, there would be a tee, and elbow up, and effectively a 90-degree turn that water has to take just to go from a city main to flow out the hydrant. We're not accounting for that here through this coefficient.

I'd want to do some more study/calcs to see how much we actually are losing due to how we test a hydrant (and not just a giant opening in the main itself). I would think if our flows are relatively low compared to the size of the pipe to the hydrant, then these effects are not huge. Not inconsequential, but not huge.

Brian Gerdwagen FPE

8/3/2021 02:45:25 pm

Do the larger diameter pumper outlets include the extra reducing coefficient per Table 4.8.2?

8/3/2021 03:02:50 pm

No - the pumper coefficient needs to be in ADDITION to the coefficients applied above.

Good clarification Brian.

David Kulbacki

9/22/2021 03:22:52 pm

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?

Moderator

9/22/2021 03:24:46 pm

Great question David - we're going to throw this in the Forum queue as I think it'd be a good discussion point. Thanks for commenting!

Comments are closed.

New Fire Hydrant Flow Test Charts (PDF)

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