GUIDELINES FOR COMMON CONTROL LOOPS
Guidelines for selection of controller type (P, PI, etc) and controller setting are available for common common proces processs variabl variables es such such as flow flow rate, rate, liquid liquid level, level, gas press pressure ure,, temper temperatu ature re and composition. F!" #$%&' Flow control loops are widely used in the process industries. For eample, half of the control loops loops in oil oil refi refine neri ries es are are used used for for flow flow contr control ol.. Flow Flow and and pres pressu sure re cont contro roll loop loopss are are characteried by fast responses (on the order of seconds), with essentially no time delay. %he process dynamics result from compressibility compressibility (in a gas stream) or inertial effects effects (in a liquid) plus control valve dynamics for large diameter pipelines. *isturbances in flow control systems tend to be frequent but generally small. +ost of the disturbances are highfrequency noise (periodic or random) due to upstream turbulence, valve changes and pump vibration. For flow control loops, PI control is generally used with intermediate values of the controller gain. %he presence of recurring highfrequency noise discourages the use of derivatives action, because flow control loops usually have relatively small settling times (compared to other control loops), there is little incentive to use derivatives action to ma-e the loop respond even faster. G$' P#&''#& %he control of gas pressure is very analogous to the control of liquid level in the sense that some application use averaging control while other require tight control around a set point. /owever, high and low limits are usually more serious concern for pressure control than for level control, because of safety and operational issue. For self regulating process, pressure is relatively easy to control, ecept when the gas is in equilibrium with a liquid. Gas pressure is selfregulating when the vessel (or pipeline) admits more feed when the pressure is low and reduces the inta-e when the pressure becomes high. Integrating processes occur when the eit pressure is determined by a compressor, in analog to liquid when there is a pump for the eit stream. For pressure control, PI controllers are normally used with only a small amount of integral control action. sually the pressure vessel is not large, leading to relatively small residence times and time constant. *erivative action is normally not needed because the process response times are usually quite small compared to those of other process operations.
%&+P&#$%#& General guidelines for temperature control loops are difficult to state because of the wide variety of processes and equipment involving heat transfer and their different scale. For eample, the temperature control problems are quite different for heat echangers, distillation column, chemical reactors and evaporators. %he presence of time delays and0or multiple capacitances will usually place a stability limit on the controller gain. PI* controllers are commonly employed to provide quic-er responses than can be obtained with PI controllers. 1!+P!'I%I!2 1omposition control loops generally have characteristic similar to temperature loops but with certain differences3 a) +easurement (instrument) noise is a more significant problem in composition loops b) %he time delay associated with the analyer and its sampling system may be a significant factor %hese two factors can limit the effectiveness of derivatives action. 4ecause of their importance and the difficulty of control, composition and temperature loops often are prime candidates for the advanced control strategies.
