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Practical Process Control System Questions & Answers – 17

A very useful technique for testing process control loop response is to subject it to a “step-change” in controller output. In other words, the process is perturbed (the highly technical term for this is “bumped”) and the results recorded to learn more about its characteristics.

What practical concerns might surround “bumping” a process such as this? Remember, the process variable (PV) is a real physical measurement such as pressure, level, flow, temperature, pH, or any number of quantities. What precautions should you take prior to perturbing a process to check its response?

Answer :

Some processes may not take well to “bumps,” especially large bumps. Imagine “bumping” the coolant flow to a nuclear reactor or the fuel flow to a large steam boiler: the results could be catastrophic! Not only is it a potential problem to exceed an operating limit (PV too high or too low) in a process, but it may be dangerous to exceed a certain rate of change over time.

Short of catastrophe, unacceptable variations in product quality may result from perturbations of the process. Again, these may be functions of absolute limit (PV too high or too low), and/or rates of change over time.

Remember, the purpose of regulatory control systems is to maintain the PV at or near setpoint. Any time the control system is disabled and the process purposely “bumped,” this purpose is defeated, if only momentarily. It is essential that operations personnel be consulted prior to manually perturbing a process, so that no safety or operating limit is exceeded in the tuning process.

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