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When we run hydrant flow tests, we usually use both 2-1/2" side-outlets of a typical dry fire hydrant. We hook up one, threaded, swivel 45-elbow on each side to divert water in a direction that won't destroy anything.
What is the appropriate Coefficient of Discharge when measuring the pitot on the centerline of the elbow? Traditionally flowing straight out of the side outlet of a hydrant, NFPA 291 gives three Coefficients (0.90, 0.80, and 0.70) based on how the outlet projects into the barrel. NFPA 291 also states that a coefficient of 0.85 is suggested for stream straighteners, unless the coefficient of the tube is known. Is there a known Coefficient for a single 45-degree elbow? Any help is greatly appreciated. Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
4 Comments
Dan Wilder
6/22/2021 08:41:26 am
The pitot tube is in the 45 at the bend? I would think this would cause an issue as the center of the flow would offset toward the outside of the bend due to the increase in velocity and is it truly perpendicular to the flow?
Mitchell O'Bryant
6/22/2021 11:28:58 am
Agree with Dan's question, seems odd the pitot tube is in the end of the 45 bend. We use a similar setup with a 2-1/2" barrel (straight) with an internal pitot tube and pressure gauge, then we attach a 45 elbow with diffuser. The elbow and diffuser have little/no effect on the upstream pitot reading with this arrangement.
ASKER
6/22/2021 11:35:20 am
Yes, it's actually this swivel piece -
Mitchell O'Bryant
6/22/2021 04:50:57 pm
Does not appear this was intended for hydrant testing use (or as a pitot). If so, they should have provided a pitot to flow conversion chart which would include the pressure loss on that 8-10 inch pipe section. Leave a Reply. |
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