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Jul
04

Commissioning Control Systems

The long-term performance of a plant is directly impacted by the care with which the control system is commissioned. Establishing control loop tuning is an important part of the work needed to commission a new plant or an area of the plant in which measurement or control changes have been made. Such work can be rewarding since the impact of a change made in
control setup and tuning is immediately seen in the plant operation. However, loop tuning can, at times, also be challenging because of problems in establishing tuning and stable operation of the plant when there is a failure or degraded performance of field devices. To resolve these problems, some investigation and often a trip to the field are required to observe valve movement or the physical condition of a field device and its associated wiring installation. Even when commissioning a new installation, it is common to encounter many problems. A person who does this type of work quickly learns some valuable lessons about commissioning. In Chapter 14 of Control Loop Foundation – Batch and Continuous Processes I address this topic in some detail. Some of the lessons learned that are address in this chapter may be summarized as follows:

Lesson 1 – Be aware of problems in field installation.
Many of the problems associated with control system commissioning have nothing to do with the setup or tuning of the PIDs. It is important to be aware of problems in field installation that may prevent a control loop from working correctly.

Lesson 2 – Observe loop response.
Establish an understanding of the process gain and dynamics by trending the control block PV and OUT using trend screens that are standard in most control systems and by introducing a small step change in the manipulated parameter while the loop is in Manual mode.

Lesson 3 – Validate loop setup.
It is never a good idea to just place a control loop in automatic mode using the default tuning and setup. The positive feedback caused by any unaccounted- for reversals in the transmitter and control system output can cause the manipulated valve, damper drive, variable speed drive to quickly go to a full open or closed position. In an operating plant, such a mistake can cause a process disruption or even a plant shutdown. Whether the PID option for Direct acting and the AO block option for Increase to Close are correctly set can be quickly determined by changing the PID output in manual mode and observing the response.

Over the next month I will go into more detail on some aspects of control system commissioning.