How to Verify Flow for Underground Flush Test?

7/31/2020

Many sprinkler systems in our area (I'm a fire marshal) have aboveground pipe installed by a fire sprinkler installer and the underground installed by an underground pipe contractor. NFPA 24 requires a minimum flow rate from underground pipe in order to remove rock and debris from the underground pipe.

Many of the underground contractors simply open up the pipe and wait until there's consistent clear water and/or stop hearing the rocks ping around. There's no measurements taken for the underground flushing and when I ask for them to verify the flow rate I get blank stares.

Is there a way to tell just by static pressure in the area if opening up the pipe flange is enough flow to satisfy the underground flushing? Or is there a measurement I should be seeing to verify the flow rate?

Thanks in advance.

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9 Comments

Brian Gerdwagen FPE

7/31/2020 10:06:49 am

Just put a handheld pitot on the flow after it is clear, like you would when flow testing a hydrant.

You know the orifice and the pitot pressure will give you the flow.

FIREPE25

7/31/2020 10:08:00 am

We generally use hosemonsters that we make ourselves with friction clamps welded to them to hold play pipes. Once the water is clean we then pitot the playpipes to ensure we flowed enough.

James

7/31/2020 10:34:38 am

NFPA 13 seems to indicate that flowing the maximum available may be acceptable. But using a pitot tube should be able to measure the flow.

10.10.2.1.3.1 Where the flow rates established in Table
10.10.2.1.3 are not attainable, the maximum flow rate available
TO the system shall be acceptable.

Bruce Verhei

7/31/2020 10:34:16 pm

You can calculate the flow required.

I’d be concerned that you are flowing long enough. How long is the new connection? I’d require a longer flush after last debris for 1200’ than for 75’.

I might do 5 minutes for the shorter length, 15 for the longer. That’s after last debris. Another piece of debris, clock starts again.

You might want to talk with your water department about how clear the want the water system after a flush. My water departments used a styrofoam cup to take a sample. They are super white and quite tiny pieces of scale are visible. Way whiter than a Starbucks paper cup. The water department wanted the water slowly turned off and any microscale flushed before we were done.

.......

More opinion. The way the fire service, the sprinkler industry, and the insurance industries expect flushing to be done is antediluvian. Pigging is the way of the future. Much cleaner. More assured obstruction removal. Much less of the citizen’s water poured on the ground.

Best.

Franck

8/3/2020 05:05:21 am

Just a question (maybe stupid) from Europe.

Is pigging a recognized solution by NFPA and/or FM to clear pipe systems in the US?
We still use to recommend the damn old flushing solution, but pigging seems more efficient (and environmental friendly).

Bruce Verhei

8/4/2020 02:43:09 am

I talked with survey bureau and local FM contact a little more than a decade ago. I had to teach them what pigging was in terms of water system. Back then the fire protection engineers I talked with weren’t aware just how widely the public water systems were cleaned using pigging.

Most of our fancy hydraulically calculated sprinkler systems are completely, directly, dependent on on those public water systems. We normally focus only the flow-pressure curve. We don’t consider debris in those mains. Mains that are kept debris free by pigging.

Pigging is the gold standard.

To my knowledge it is not widely accepted by the various stakeholders once the water passes the last underground ‘T’ on the way to the fire sprinkler system(s).

Best.

Franck

8/3/2020 05:16:16 am

Another possible solution to guess the flow without a pitot tube or hosemonster, is to use the water supply flow curve at the area of the flushing.
If you make a flow curbe using Q1.85 graph (this is normally a straight line), you have a direct relationship between the pressure in the mains and the flow.
This means, for example that if you have a static pressure of 65 psi and a residual pressure of 55 psi when flowing 500 gpm, for example, you can expect that if your pressure dropped to 25 psi, you probably flow around 1000 gpm, or around 1200 gpm when the pressure drops to 15 psi (this is not a linear scale, so use Q1.85 graph).
This is not as accurate as with a flow measurement, as indicated above, but gives you at least a guess.

Q1.85 graph can be found on the web at the following website:
https://www.pingfire.com/Default.aspx
or
https://glendhoward.files.wordpress.com/2009/11/flowtestplotcropped.jpg

ELSA LOPEZ

8/4/2020 04:11:07 pm

For me the flowrate measurement for a 4" or 6" pipe is not an Easy solution to be done at Site with an open pipe flowing huge quantity of water, for small risers it could be a good solution. For me the most accurate way is to measure the flow with the pumps discharge pumps Curve. Measuring pressure at pumps discharge will give you the flowrate you are using if any other system is being used at the same time.

8/6/2020 10:12:10 am

The requirement is found in NFPA 24:Table 10.10.2.1.3 (2013). The contractor should be using a pitot gauge to verify the minimum required gpm is reached during the flush test.

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