When designing a control strategy you may be faced with the challenge of there being an extra degrees of freedom. One of the most common examples is where one control parameter may be maintained at setpoint through the adjustment of two manipulated parameters. Often the solution is to address the control design using split range or valve position control. Through the use of these techniques, the two actuators appear as one actuator to the PID control. However, there are some significant differences in the resolution and dynamic response that may be achieved using either technique. An alternate approach is to implement a strategy that combines the best of split range and valve position control.
I once was responsible for the design of the 400 # header pressure controls for a new power house in a pulp and paper mill. Under normal operating conditions, the header pressure was to be maintained by the turbo-generator extraction to the 400# header. However, if the turbine was to trip or be taken off-line for maintenance, then two pressure reducing valves (normally closed) were to be used to let down steam from the 1475# header to the 400# header. Under a trip condition, it was important that the full dynamic range of the pressure reducing valves be used to make up for the steam that had been supplied by the turbine extraction. This objective could be achieved through the use of split range control. However, if the turbo-generator was to be off-line for an extended period of time for maintenance, then it would be advantageous to provide the precise pressure control that may be achieved by taking advantage of the operating characteristics of valve position control. After some work, I came up with a network that combined the best of split range and valve position control. I commissioned and tested the header controls as the power house was brought on-line. The 400# header control proved to be quite effective and after over 20 years is still in use at the plant- migrated to new controllers.
The work I did on the revised header pressure control strategy was documented in a paper that I wrote and presented shortly after the power house startup, “Improving PRV Pressure Control”, ISA 31st Annual Southeaster Conference, April, 1985. The technique was later used within Emerson’s pulp and paper group to address a variety of applications e.g. furnace draft control variable speed ID fan in combination with damper, variable speed pump in combination with a regulating valve for recovery boiler liquor flow control, forced-draft fan control pressure control using a variable speed fan with inlet vanes. Many of these applications were documented by Bill Love, Forney International, in an article “Innovative control technique that improves control rangeability and resolution in paper mill applications”, Tappi Journal, February, 1994.
The tools that are available in most modern control systems are sufficient to implement the network that I originally designed for the header pressure control. The basic network design is shown in the following:
Also, this material includes an example of how the network may be implemented as a re-usable composite block in DeltaV. Some process examples are iprovided that allow you to compare the dynamic response of this network to that achieved using spilt range control and valve position control. If your control objectives can not befully met by valve position or split range control, then you may want to consider this network for your application.