Water Systm Flow Test Variations-Diffuser Correction?

[akciv]

We would appreciate the benefit of this forums member’s vast experience. We are having trouble calibrating a water model due to differing hydrant flow tests. One set was done by an independent company, and we performed some. We then did simultaneous tests to discover what was different. It turns out whereas we are measuring flow using the GFE HK-25 apparatus which consists of a 2 ½” x 10” tube with fixed pitot and oil filled pressure gauge, a 30 degree elbow, and a ‘beehive’ screen diffuser.

The other test apparatus is a hand held knife blade pitot with oil filled pressure gauge.

The difference is that our pitot is fixed in the center of the flow tube, while the knife blade tester moves his pitot around to find the highest reading in the port flow.

In testing three hydrants the knife blade pitot pressures averaged 140% of the fixed pitot pressures.

We were not able to check the knife blade pressure gauge accuracy as its mounting was different than all of our gauges. We compared our three oil filled gauges: all agreed within 1 psi.

Questions:

1. Can pitot pressure vary that much (10 psi) across the flow stream?
2. Is the apparatus we are using acceptable for these tests, or is the knife blade preferred?

 

[btrueblood]

What size pipe are you putting your pitot/screen device into?

The single pitot in general presents less blockage to the flow, and thus may provide higher flow rates.