Pitot measures velocity pressure, residual is atmospheric pressure. There’s not a fixed correlation between the two values – I’m guessing that the main supplying the test hydrants is a very large diameter one?
Steve Leyton From: Sprinklerforum [mailto:[email protected]] On Behalf Of Skyler Bilbo Sent: Wednesday, January 02, 2019 9:20 AM To: [email protected] Subject: Pitot Pressure Above Residual Pressure I apologize if this has been discussed already, but I would like to clear my own mind, if nothing else. I can't recall a hydrant flow test, that I performed, where my pitot pressure has been higher than the residual pressure on the test hydrant. I have received two tests in the last year from city water departments where this has been the case. I can't make sense of the physics. This *should* not be possible, correct? I can dream up a scenario where it would happen, but essentially, the flow test would have to be taken backwards, where the test and flow hydrants are flipped, and someone is using a decent amount of water, either past the test hydrant, or between the test hydrant and flow hydrant... It seems like if this happens (if your pitot reading is higher than your residual pressure), you should flip the flow and test hydrants and repeat the test. Am I thinking about this correctly? Has anyone else had this issue, and is it addressed in NFPA anywhere (perhaps it should be, if it isn't)? Of course, one of the gauges could be bad as well (which is probably just as likely, if not more so, than the hydrants being flipped). Thanks for any help with this, Skyler Bilbo
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