Proportional-Only Control

All base level controllers in e train should have proportional-only control. In some plants, these controllers are difficult to find because they have been considered inferior. Proportional control is the best, if you can permit the measurement to move around the set point. Surge capacity is frequently provided in the design of accumulators, but then negated by specifying proportional-plut-reset controllers. Proportional-only control cannot be achieved with most instruments by turning the reset adjustment to the maximum reset time. A slow cycle with high amplitude will result. 

Integral mode This improves on the proportional-only control by repeating the proportional action within a unit time while a deviation from set point exists. The regulating unit is only allowed to be at rest when set point and... 

Derivative mode This improves on the proportional-only control by responding solely to the rate of change of the deviation but not in any way to the actual value of the deviation. Derivative action is always used with proportional control. 

For MTHW and HTFIW systems, heat emitters may be as for LTFIW systems, except that, for safety reasons, units with accessible surfaces at water temperature would not normally be employed. Embedded panel coils may be used in conjunction with a MTHW or HTHW distribution system, with insulating sleeves around the coil piping to reduce the heat flow. Alternatively, the coils can be operated as reduced temperature secondary systems by allowing only a small, carefully controlled proportion of flow temperature water to be mixed with the water circulating in the coils. 

As illustrated in Figure 29, the proportional only control mode responds to the decrease in demand, but because of the inherent characteristics of proportional control, a residual offset error remains. Adding the derivative action affects the response by allowing only one small overshoot and a rapid stabilization to the new control point. Thus, derivative action provides increased stability to the system, but does not eliminate offset error. 

Proportionality constant (k), in high pressure chemistry, 13 406 Proportional-only controller, 20 693 Proportional plus integral feedback controllers, 20 692... 

The level controller is a proportional-only feedback controller. 

A. PROPORTIONAL ACTION. A proportional-only feedback controller changes its output signal, CO, in direct proportion to the error signal, E, which is the difference between the setpoint, SP, and the process measurement signal, PM,... 

B. PROPORTIONAL CONTROLLER. The output of a proportional controller changes only if the error signal changes. Since a load change requires a new control-valve position, the controller must end up with a new error signal. This means that a proportional controller usually gives a steadystate error or offset. This is an inherent limitation of P controllers and why integral action is usually added. 

Tray 4 temperature on the Lehigh distillation column i controlled by a pneumatic Pf controller with a 2-mipute reset time and a 50 percent proportional band. Temperature controller output (COr) adjusts the Ktpoint of a steam flow controller (reset time 0.1 min and proportional band 100 percent). Column base level is controlled by a pneumatic proportional-only controller setting bottoms product withdrawal rate. 

Three vertical cylindrical tanka (10 feet high, 10 feet diameter) are used in a process. Two tanks are process tanks and are level controlled by manipulating outflows using proportional-only level controllers (PB 100). Level transmitter Spans are 10 feet. Control valves are linear, 50 percent open at the normal liquid rate of 1000 gpm, air-to-open, constant pressure drop. These two process tanks are 50 percent full at the normal liquid rale of 1000 gpm. 

Use proportional-only level controls in surge tanks and column bases to smooth out disturbances. 

Show that a proportional-only level controller on a tank will give zero sieadystate error for a step change in level setpoint. 

J1. Two tOO-barrel tanks are available to use as surge volume to filter liquid flow rate disturbances in a petroleum refinery. Average throughput is 14,400 barrels per day. Should these tanks be piped up for parallel operation or for series operation Assume proportional-only level controllers. 

Figure 10.St>,c gives tbe root locus plots for two values of the reset lime constant z,. With Zj = 4.5 in Fig. lO.gt), the ultimate gain is reduced to 5i (compared to the ultimate gain of 64 for a proportional only controller), and the gain that gives a closedloop damping coelTicient of 0.316 is 15. There is a first-order root on the real axis near the origin.

The openloop transfer function relating steam flow rate to temperature in a feed preheater has been found to consist of a steadystate gain K, and a flrst-order lag with the time constant T. The lag associated with temperature measurement is t . A proportional-only temperature controller is used. 

Example 11.1. Consider the process with a series cascade control system sketched in Fig 11.le. The secondary controller B, and the primary controller Bj are both proportional only. 

A proportional-only controller is used to control the liquid level in a tank by manipulating the outflow. It has been proposed that the sfeadystate offset of the proportional-only controller could be eliminated by using the combined feedforward-feedback system sketched below. 

When a proportional-only feedback controller is used, the ultimate gain is 2. Outline your procedure for finding the optimum value of tj, if a proportional-derivative controller is used. The optimum tj> wilt give the maximum value for the ultimate gain. 

The control valve on the steam has linear installed characteristics and passes 500 Ibi min when wide open. An electronic temperature transmitter (range 50-250°F) is used, A lemp>efature measurement lag of 10 seconds and a heat trans fer lag of 20 seconds can be assumed. A proportional-only temperature contioller is used. 

If a proportional-only controller is used, what value of controller gain will give a maximum closedioop log modulus of - -2 dB ... 

J6. The frequency response data given below were obtained by pulse-testing a closed-loop system that contained a proportional-only controller with a proportional band of 25. Controller setpoint was pulsed and the process measurement signal was recorded as the output signal. 

J0. A process has an openloop transfer function that contains a positive pole at -H 1/t, a negative pole at — 10/t and a gain of unity. If a proportional-only controller is used, find the two values of controller gain that give a maximum closedloop log modulus of -1-2 decibels. 

Exarngde 15.15. Determine the dosedloop characteristic equation for the system whose openloop transfer function matrix was derived in Example 15.14. Use a diagonal controller structure (two SI SO.controllers) that are proportional only. 

Selectivity is more important than efficiency (AO in determining resolution because Rs is directly proportional to selectivity, but is proportional only to the square root of efficiency (see Fig. B4.2.5 along with Equation B4.2.1 and Equation B4.2.2. Hence, a four-fold increase in efficiency is required to double resolution, as compared with a two-fold increase in selectivity. In practice, selectivity depends partly on the chromatographic technique employed but can usually be controlled by manipulating experimental conditions, such as the pH and ionic strength of the mobile phase. Because this can be done easily and predictably, selectivity is the factor that is exploited to achieve maximum resolution in column chromatography rather than efficiency, which is fixed by the particle size and uniformity of the medium selected. 

Two temperature controllers are used. The first manipulates the flowrate of the A feed. A 45 min ramp in this reactor temperature controller is used with Kci = 0.5 and Tj2 = 20 min. Two 30-s lags are included in the loop. The span of the temperature transmitter is 50 K, and the maximum flowrate FA0 of the reactant A is 0.004 m3/s. The second temperature controller setting the flowrates of the hot and cold streams to the jacket is proportional-only with a 330 K setpoint and a gain of 0.05. The maximum cold water and hot water flowrates are 0.005 and 0.002 m3/s, respectively. 

The positioner is a high-gain (0.5 to 10% proportional band), proportional-only, valve stroke position controller. Its set point is the controller output signal. The main purpose of having a positioner is to guarantee that the valve does in fact move to the position that the controller requires. 

The set point of a regulator is provided integrally, and remote control is usually not possible. It is a single-mode (proportional only) controller,... 

Systems that are fully automated also include several proportional plus integral plus derivative (PID) controllers. The PID controllers control individual set-point functions and can monitor alarm conditions. Independent PID controllers can control their specific function without a PLC. Should an independent PID controller fail, only the specific function it controlled cannot be adjusted. 

Then the inventory loops are revisited. The liquid holdups in surge volumes are calculated so that the time constants of the liquid level loops (using proportional-only controllers) are a factor of 10 larger than the product-quality time constants. This separation in time constants permits independent tuning of the material-balance loops and the prod-... 

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