Intermediate liquid storage tanks (surge tanks) are commonly installed between processing areas of the plant. Under normal operating conditions, these storage buffers allow each process area to be operated independently of the other process areas. Any imbalance in the area production rates within a plant will be reflected by a change in level of the surge tanks between process areas. When a downstream process area is made up of a continuous process, then its throughput may be automatically adjusted based on the surge tank level, as illustrated below.
To avoid abrupt changes in production rate in the downstream process, the level controller (LC 202) may be configured for proportional-only control, with the bias setting based on the normal plant production rate. As described in Chapter 11 of Control Loop Foundation – Batch and Continuous Processes, the range over which this floating-level control adjusts the downstream flow is determined by the controller gain and bias.
For this tank level example where the PID output directly adjusts the tank outlet flow valve, the BIAS value would be set equal to the valve position needed to achieve normal flow rate through the tank. The proportional gain in this example may be used to determine the outlet flow rate change necessary to compensate for a change in level. For example, if the proportional gain is set to 1 and the BIAS is set to 50% (assuming an outlet valve position of 50% is normally needed to maintain the tank level), then the control output to the valve will be full open if the level reaches the upper limit and full close if the level reaches the lower limit of the transmitter range. If the proportional gain is set to 2 with a BIAS of 50% and a setpoint of 50%, then the control output would be full open when the level reaches 75% and full closed when the level reaches 25%.
The proportional gain in this example determines over what range the level is allowed to vary (float). The use of proportional-only control for tank level control is often called floating level control. That is, the level is allowed to vary or float within a certain range, and the amount it floats is determined by the proportional gain. In this case, the objective is to take full advantage of the surge capacity of the tank, that is, to not immediately pass upstream process throughput changes to the downstream process. If applied in this manner then proportional-only control can be quite effective.