Dear Skyler, There are a couple of situations that could present data in this manner. Some municipalities have pumps that will start when a certain demand is placed on the system. The hydrant supply loop at the University of Illinois' Fire Service Institute are on such a supply. This will cause the pressure to drop and then rise during a flow test.
I have also seen this happen in the flatland of Central Illinois when the water supply is solely from an elevated tank and the land changes elevation. In those cases, the elevation was just enough to overcome the friction loss in the piping. This situation has larger pipes and lower flows. Please feel free to call me with any questions you may have. :-D It should be recognized that the above is my opinion as a member of the NFPA Committee on Sprinkler System Installation Criteria, and has not been processed as a formal interpretation in accordance with the NFPA Regulations Governing Committee Projects and should therefore not be considered, nor relied upon, as the official position of the the NFPA, nor any of their technical committees. Sincerely, Cecil Bilbo Academy of Fire Sprinkler Technology Champaign, IL 217.607.0325 www.sprinkleracademy.com<http://www.sprinkleracademy.com> [email protected]<mailto:[email protected]> ?? OUR STUDENTS SAVE LIVES!! ________________________________ From: Sprinklerforum <[email protected]> on behalf of Skyler Bilbo <[email protected]> Sent: Wednesday, January 2, 2019 11:19 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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