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Under Pressure, Part 4: More on Pressure Regulator Droop and Flowmeters

Sep 27, 2012

If my previous post on droop, the least apparent pressure regulation issue, wasn’t enough of the low down for you, let me share some more about this common issue that arises with flowmeters.

Droop is the difference between the outlet pressure at low inlet pressures and at high inlet pressures along the ‘operating range’ of the regulator – some 5%-95% of capacity. Below 5%, you have the seat-drop lock phenomenon, where the relationship between pressure and flow rate are sharp. Therefore, below some 0.5%, the seat will drop over the orifice and lock the regulator closed.

Above 95%, you start to hit the choked flow phenomenon, wherein the flow rate and the outlet pressure are again sharply Pressure Regulation and Droop with Gas Mass Flowmetersrelated, and thus, outlet pressure drops off drastically as the flow rate increases. Conversely, for a given maximum outlet pressure, the flow rate cannot increase past a given point, and increasing the outlet pressure by further regulation will only result in a cutoff in flow, not additional outlet pressure. And so droop is the difference between the maximum outlet pressure available at some 5% of maximum flow rate, and the maximum outlet pressure available before choked flow cutoff, somewhere around 95% of max flow rate.

Hopefully, this information will help you move in the right direction with gas mass flow control in your system. If not, don’t hesitate to contact us for more information and assistance.

Morgan Zealear, Engineer
Written By:
Morgan Zealear, Engineer
Sierra Instruments

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