To achieve best overall plant operation, it is often necessary to coordinate the operation of equipment in multiple process areas. In Chapter 16 of Control Loop Foundation – Batch and Continuous Processes, some examples are given where multi-loop techniques may be effectively used for such coordination. One of the examples from this chapter is power house steam generation. This example also illustrates how PID is used to coordinate the operation of boilers and steam generaton the importance of PID response for recovery from a Process Saturation Condition.
In many process plants, one or more power boilers are used to meet the process steam requirements of a plant. Also, this steam may be used in turbine generators to meet some or all of the plant requirement for electricity. Any imbalance between the quantity of steam generated by the power boilers and that required to operate the plant will be reflected in the pressure in the main steam header. Thus, a pressure control loop known as the plant master is used to set the power boiler steam demand as illustrated below.
The high pressure steam generated by the power boilers is used to feed the turbine generators. The pressure of the steam that is extracted from the turbines at various points is automatically regulated by the turbine control system to meet the lower pressure steam requirements of the process. If a turbine goes off-line, then the pressure in the lower pressure steam headers will drop. Once the pressure drops to the setpoint of a header pressure controller then a pressure reducing valve is opened to maintain the pressure at setpoint. It is critical that the valve open before the pressure drops below the controller setpoint. The tuning of the controller has a large impact on the response of the pressure control. Also, the way the PID internally handles recovery from process saturation has a direct impact on how quickly the controller responds.
At Emerson Exchange 2011 I will be co-hosting a workshop, “Recovery from a Process Saturation Condition”, that addresses this important aspect of PID control. If you attend Emerson Exchange this fall then I hope you will be able to attend this workshop. After Emerson Exchange I will be blogging more on this workshop.