Playpipe without Tip OK for Testing?

10/10/2023

I have observed forward-flow tests of backflow preventers, and fire pumps tests, that are conducted through 2.5-inch nozzles identical to the image below, but without the playpipe tip screwed onto the end of the nozzle.

Image Link

The pitot reading was taken off the 1-3/4" threaded end of the nozzle where the playpipe tip gets screwed on.

Is this approach acceptable, and would the GPM table for the 1-3/4" butt for converting the pitot pressure to flow be acceptable?

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8 Comments

Dan Wilder

10/10/2023 08:46:06 am

As long as the measurements are taken from the correct location per NFPA 291 for the discharge orifice size and the corrected Coefficient is used, this is acceptable. Just make sure that the hose between the two points isn't too long and creates excessive friction loss and skewing results.

Eric R

10/10/2023 08:53:25 am

I think it would depend on what kind of test you are performing, and what level of precision is needed.

You can use the playpipe as a generic "stream straightener", but I would not recommend using the GPM flow rates listed with the device based on an actual 1.75" tip. NFPA-291 4.9.2 recommends that you use a 0.95 coefficient of discharge if there is no specific listing information. I did a quick check online, and it looks like all the major brands sell a 1.75" tip as opposed to listing the cD of the tube without a tip.

So if the flow test is for a hydrant or backflow you should be able to comfortably use the 0.95 cD and at worst case get a slightly conservative test result.

For a fire pump though, that margin of error could put the pump off it's factory curve enough to fail the test, so I would be much more hesitant in that case.

Anthony

10/10/2023 08:58:31 am

NFPA 291 will say the reading from the hydrant should come from:

Q = 29.83 * c * d2 * √P

So you'll have to get a proper coefficient of friction for you nozzle and use the proper diameter.

I'd also assume you have some loss through your hose you'll need to account for. I don't think this is a table look up problem.

David Kendrick

10/10/2023 09:22:02 am

Curious, I've read this comment several times about hose length.

If there is a known pitot pressure (flow) and a known supply gauge (residual) pressure points. Using this along with a static pressure these two points would create a flow curve.

If the hose is "long" the resistance would lower the flow. It would be a known flow reduced none the less. With a short hose the flow resistance would decrease and flow increase.

A flow test with a static and residual from the same consistant supply should plot along the curve.

If the goal is to flow a specific quantity of water as in forward flow or fire pump test larger or multiple flowing sources might become more important.

Fairly confident that flows from a consistant source will generate the curve whether it is from a direct connection to the outlet or through hoses.

Really looking for input. I may have been misunderstanding the physics of this for a long time.

DAVE

10/10/2023 01:33:46 pm

That has been my belief as well, and have argued the same. I think we purched 3" diameter hose material just to have the ability to crank the gpm to #11.

Ivan Humberson

10/11/2023 07:31:39 am

David, I totally agree. As long as the static and residual pressures are taken at the source, where the flow characteristics need to be determined, the length of hose will have no effect on the characteristic flow curve, other than the data point while flowing water may be a lower gpm due to the friction loss in the hose.

Glenn Berger

10/10/2023 04:03:38 pm

As long as the calculations are based upon the true flowing orifice the test results should be acceptable.

James Art. FPE

10/21/2023 08:59:38 pm

Using a Playpipe is fine! Great Idea.

Esp. useful if the flow is weak and the pitot readings are low.
See Meyerfire's write up on flow testing.
If pitot readings from the hydrant butt are 10 psi or below, great time to use a playpipe or other nozzle.
to reduce the opening (take your choice of the 1 3/4 " without the last reducer or the 1 1/2" if that is the size of your reducer. Even with a low flow, that will get your pitot readings up where they are more accurate, more reliable.

And while it is easiest to just screw it to the hydrant butt, a bit of hose doesn't really hurt. You need to secure it to something, like running under a pickup and tying to the bumper.
A hose will reduce the Flow a little, but the curve is the same,
( the Residual at the lower Flow will be higher) and the results will still show the same water supply curve.

When I tested often for the HPR Insurance industry, I had exactly that in the trunk.
I also carried a Test Nozzle made of 2.5 inch pipe, about 10 diameters long. Produced a straight stream with less bouncing of the gauge.
It had a 2.5 inch hose coupling, with a 2.5 inch female pipe thread, cut off a fitting, welded to it. Piece of cake for a fab shop.
I also carried a 2.5 inch threaded "street" 45 degree elbow: Male on one end female on the other.
So I either just screwed the pipe into the pipe threads for a straight shot
OR
used with the elbow to aim away from the landscape, etc.
Could point it up or to either side. You gotta be careful!

These nozzles were mine, so they came with me.
But when I parked in San Francisco to survey a warehouse, I found a parking space out front, where my car had been!
When the stolen car was recovered, after weeks in their $torage yard (And they didn't tell me) all my tools were gone.
Please let me know if anyone can help me get more!

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Playpipe without Tip OK for Testing?

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