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Don't Take Control – Cascade It (Part 1)

Flow control can be challenging at times – that fact keeps all of us here at Brooks employed! Many users who need to control gas flow in their process don't want to spend a lot of time setting up their own flow control scheme, and instead purchase a mass flow controller. Installation simplicity and flow control accuracy are major benefits of this approach, as you can see in this short video: Unfortunately, some users who purchase mass flow controllers are confused when they are not able to achieve similar flow control performance like you just saw once the device is installed in their process. These users commonly encounter one of two problems. Problem #1: Unstable Flow Control The actual flow rate measured from the mass flow controller is not steady as shown in the video. The actual flow rate may rise and fall erratically when no change in flow setpoint was sent to the device. Alternatively, a change in flow setpoint sent to the mass flow controller may result in oscillation (also called droop) that causes the actual flow rate to fail to settle onto the desired flow rate for an extended time. This result is particularly troublesome for continuous processes, such as selective catalytic reduction or combustion control, as it can reduce the efficiency of the entire process – which can result in safety incidents, fines, and numerous other undesirable outcomes. Problem #2: Lagging Flow Control Response Instead of taking less than one second as shown in the video, the actual flow output from the mass flow controller installed in the process may not settle onto a new flow setpoint for ten or even thirty seconds. The length of this lag may not be consistent between occurrences, and the slope of the flow response curve may not be consistent either. This result is very troublesome for batch processes, like vacuum deposition or heat treating, as the flow response lag time can cause inconsistent treatment quality on the parts – which can increase the number of parts rejected for poor quality, or shorten the installed lifetime of the treated parts once they're being used in their intended applications. Is the Mass Flow Controller to blame? Since the mass flow controller was purchased to control the gas flow in these applications, many users believe these problems are caused by the flow controller itself.  However, this is rarely the case. These flow control problems are typically symptoms that boil down to the same problem: the mass flow controller does not control the actual process flow rate, the entire control loop does. We'll discuss this in further detail in part 2 of this series, including a process control concept known as cascade control which can help minimize the impact of these flow control disturbances on the entire process. In the meantime, post a comment, email me or give us a call if you'd like to discuss gas flow control in your application or troubleshoot any of these issues.

About The Author

Brandon Kulp
Applications Engineer

Brandon is applications engineer at Brooks Instrument.

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