Colorado Springs, Colo., USA -- June 15, 2020 -- Thank you so much to Chemical Engineering Magazine who worked with AFT's Walt Prentice to publish this beautiful cover story, "Gas Handling: Process Control for Compressible Flow." 

There are many factors to keep in mind when designing for flow or pressure control in gas handling processes. Pressure is the driving force of the system, even when you are designing for flow control. By changing the frictional losses in the system, you change the operating mass flow rate. Mass conservation is a fundamental law, but volume conservation is not. Volumetric flow, velocity, density, and temperature will all change according to the fluid’s equation of state even though mass flow may be constant. Down the length of an adiabatic and constant cross-sectional area pipe, you will see pressure and density decrease, volumetric flow rate and velocity increase, and most often will see temperature drop.

When pressure is your controlled variable, you affect both the upstream and downstream pressure (and flow) with the same valve operation. Closing a valve will create larger pressure losses, which implies a greater upstream pressure, a lower downstream pressure, and a lower mass flow rate. A pressure reducing valve controls the downstream pressure, while a pressure sustaining valve controls upstream.

Depending on the process application, engineers design a wide range of flow and pressure control methods, but it all boils down to changing the pressure to change the momentum transfer. That is the beautiful consistency of chemical engineering: there is always a driving force behind transport phenomena. Controlling that driving force is how you ultimately control your process variables.

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