Liquid level control proportional

Figure 4.3 Liquid level control proportional mode...

For all commercial devices, the proportional gain is a positive quantity. Because we use negative feedback (see Fig. 5.2), the controller output moves in the reverse direction of the controlled variable.1 In the liquid level control example, if the inlet flow is disturbed such that h rises above hs, then e < 0, and that leads to p < ps, i.e., the controller output is decreased. In this case, we of course will have to select or purchase a valve such that a lowered signal means opening the valve (decreasing flow resistance). Mathematically, this valve has a negative steady state gain (-Kv)2... 

Processes having the term 1/5 in their transfer function, when they are controlled with proportional controller, do not exhibit offset for set point changes but they do for sustained load changes (e.g., step changes). Let us demonstrate this important feature for the liquid-level control system shown in Figure 14.6a. The output... 

Liquid-level control Consider the two liquid-level control systems for the bottom of a distillation column and its condenser s accumulation drum (Figure 13.2d and e). Our control objective is to keep each liquid level within a certain range around the desired set point. Consequently, proportional control alone is satisfactory. 

If possible, use simple proportional controller. Simple proportional controller can be used if (a) we can achieve acceptable offset with moderate values of Kc or (b) the process has an integrating action (i.e., a term 1/5 in its transfer function) for which the P control does not exhibit offset. Therefore, for gas pressure or liquid-level control we can use only P controller. 

IV.2 Consider the liquid level control loop of Figure 13.2d. The differential pressure transducer cell exhibits second-order dynamics, the controller is proportional, and the control valve is linear with flow characteristic curve given by... 

The feedforward system imposes an external material balance as well as a an internal material balance on the process. The internal balance is maintained by liquid level control on the discharge of each effect. Analysis of a level loop indicates that a narrow proportional band (less than 10%) can achieve stable control. However, because of the resonant nature of the level loop which causes the process to oscillate at its natural frequency, a much lower controller gain must be used (proportional bands 50-100%). A valve positioner is recommended to overcome the nonlinear nature of valve hysteresis. 

Varying the flow of a compressible fluid controls the pressure in a large volume. This process is dominated by a single large capacitance with no dead time. The measurement is normally noise free and, owing to its capacitive nature, is characterized by a slow response and a small process gain. As shown for liquid level control, a proportional controller is more than adequate for gas pressure control. 

The maintenance of constant liquid level in the reflux drum can be expressed by the following proportional control equation... 

LEVEL LOOPS. Most liquid levels represent material inventory used as surge capacity. In these cases it is relatively unimportant where the level is, as long as it is between some maximum and minimum levels. Therefore, proportional controllers arc often used on level loops to give smooth changes in flow rates and to filter out fluctuations in flow rates to downstream units. 

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. 

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. 

Oil and water are mixed together and then decanted. Oil flow rate is ratioed to water flow rate Ff. Interface is controlled by oil flow Fj, from the decanter with a proportional level controller. Water flow F ) from the decanter, which is liquid full, is on pressure control (PI). Steadystate flow rates are ... 

The liquid flow rate from a vertical cylindrical tank, 10 feet in diameter, is flow-controlled. The liquid flow into the tank is manipulated to control liquid level in the tank. The control valve on the inQow stream has linear installed characteristics and can pass 1000 gpm when wide open. The level transmitter has a span of 6 feet of liquid. A proportional controller is uaed with a gain of 2. Liquid density is constant. 

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. 

The statement that the mass, or weight flow of vapor through the trays, increases as the refluxed rate is raised is based on the reboiler being on automatic temperature control. If the reboiler were on manual control, then the flow of steam and the reboiler heat duty would remain constant as the reflux rate was increased, and the weight flow of vapor up the tower would remain constant as the top reflux rate was increased. But the liquid level in the reflux drum would begin to drop. The reflux drum level recorder controller (LRC) would close off to catch to falling level, and the overhead product rate would drop, in proportion to the increase in reflux rate. We can now draw some conclusions from the foregoing discussion ... 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

In recycle systems, the design of the chemical reactor and the control of the reactants inventory are interrelated [8]. Figure 4.2 shows two different ways of controlling the inventory in a simple system. The first strategy consists of setting the feed on flow control. Consider the simple example presented in Figure 4.2(a). When more fluid is fed to the vessel, the level increases. The outlet flow rate, which is proportional to the square root of the liquid level, will also increase. After some time, the feed and outlet flows are equal, and a state of equilibrium is reached. 

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-... 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

The critical product-quality and safety-constraint loops were tuned by using a relay -feedback test to determine ultimate gains and periods. The Tyreus-Luyben PI controller tuning constants were then implemented. Table 11.12 summarizes transmitter and valve spans and gives controller tuning constants for the important loops. Proportional control was used for all liquid levels and pressure loops. 

In the multiloop controller strategy each manipulated variable controls one variable in a feedback proportional integral derivative (PID) control loop. Taking a single-feed, two-product distillation column with a total condenser and a reboiler as an example, a basic list of possible controlled variables includes the distillate and bottoms compositions, the liquid levels in the reflux accumulator and the column bottom, and the column pressure. The main manipulated variables are the reflux, distillate, and bottoms flow rates and the condenser and reboiler heat duties. 

The Lever oscillator [39], Fig. 16, allows the application of series resonance configurations with one-side quartz electrode grounding. Since the effect of parasitic capacitance is minimized and simple shielding is possible, this circuit configuration is especially suited for under-liquid QCM. Besides the series resonance frequency, the series resonance resistance Rs can be measured. For this purpose the Lever oscillator allows a largely transistor current gain-independent measurement of the resistance. An automatic level control provides a signal proportional to Rs. 

Liquid level can be controlled effectively with proportional control. 

IV.18 Consider the two interacting tanks of system 2 in Figure PII. 1. We want to control the liquid level h2 of tank 2 by manipulating flow rate F through a proportional controller. Assume that tank 1 has a cross-sectional area of 5 fit2, while for tank 2 the cross-sectional area is 2 ft2. Initially, the system is at steady state with Fx = 1 ft3/min, h i = 4 ft, and hi = 3 ft. Find the values of the controller gain that produce... 

III.35 Consider the storage tank of Figure PHI.la. Suppose that we want to control the liquid level in the tank at the height of 5 ft, by manipulating the effluent flow rate F2, according to the following proportional control law ... 

If a simple P controller is unacceptable, use a PI. A PI controller should be used when proportional control alone cannot provide sufficiently small steady-state errors (offsets). Therefore, PI will seldom be used in liquid-level or gas presure control systems but very often (almost always) for flow control. The response of a flow system is rather fast. Consequently, the speed of the closed-loop system remains satisfactory despite the slowdown caused by the integral control mode. 

---