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Mmajner
April 7, 2017, 08:09 AM
What are some factors that may cause a pressure sustaining valve to have positive pressure on the upstream side and an immediate negative pressure on the downstream end?

Patrick Moore
April 7, 2017, 08:57 AM
Hi Marty!

A Pressure Sustaining Valve is meant to maintain the upstream pressure without regard to the downstream conditions. Thus a PSV will allow flow through the valve unrestricted if the upstream pressure is greater than the setting, but it will restrict flow through the valve once the upstream setting reaches the upstream pressure. If the upstream pressure falls below the PSV setting it should close the valve and not allow any flow through the valve.

If you had a condition where the upstream pressure was below the valve setting, the valve would be closed and it would not care about the downstream pressure condition. In this case it is possible to have a positive upstream pressure and a potentially negative pressure on the downstream. Even if the valve was open but active it is possible that the upstream pressure could be positive and the downstream pressure be negative. The PSV is only concerned with maintaining it's setpoint on the upstream and has no controls changes made based on the downstream pressure. Generally it is other factors that are controlling the HGL downstream that you would need to look at to determine why the pressures are negative.

Recall the model solves for the hydraulic Grade Line (HGL) and then calculates the pressure as Pressure_psi = (Head_ft-elevation_ft)/ 2.31 ft/psi and so negative pressures represent locations where the HGL is lower than the element elevation. Usually if your HGL is that low there is a good chance a zone intertie may be occurring that causes a large outflow which can depress the hydraulic grade line. Look for pipes with high headloss and flow as possible indications of this or look at what else controls the HGL for the downstream zone to explain why the PSV discharge is negative pressure.

Patrick Moore