Level control tuning

Insufficient surge volume between columns. This usually compels level controller tuning for a fast response (in the bottom sump or reflux drum), causing flow fluctuations. 

Figure 7.15 (a) Initial controller faceplate and initialize, (b) Proportional level control tuning. 

Determining settings for the reflux drum level controller is, in this case, difficult unless a large reflux drum holdup is available. Preferably one should make 5 minutes level controller tuning will require a dynamic analysis of overall column material balance such as discussed in Chapter 14. If steam flow is metered by an orifice, it should be linearized with a square root extractor. 

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

The tuning of proportional level controllers is a trivial job. For example, we could set the bias value at 50 percent of full scale, the setpoint at 50 percent of full scale, and the proportional band at 50. This means that the control valve will be half open when the tank is half full, wide open when the tank is 75 percent full, and completely shut when the tank is 25 percent full. Changing the proportional band to 100 would mean that the tank would be completely full to have the valve wide open and completely empty to have the valve shut. 

PID controller tunings for this model have been given by a number of researchers [9-13], Chen and Fruehauf [9] have given an industrial example of the level control in a distillation column where the open loop dynamics follows the IPTD model with parameters kp = 0.2 and d = 1A min. 

Figures 7. Simulated start-up of vinyl acetate polymerization at low emulsifier level (0.01 mol/L H20) under closed-loop control with arbitrarily selected controller tuning constants and manipulation of initiator flow rate at 50°C conversion in R1—STD feedback (--------------------------) vs. DTC (----)...

Rigorous nonlinear models must be used in analyzing batch reactors because of the changing process parameters. Continuous reactors operate around some steady-state level, so linear models are sometime adequate for establishing controller tuning constants. 

There are two controllers. The proportional reactor level control has a gain of 5. The reactor temperature controller is tuned by running a relay-feedback test. The manipulated variable is the cooling water flowrate in the condenser. With a 50-K temperature transmitter span and the cooling water control valve half open at design conditions, the resulting tuning constants are Kc = 4.23 and = 25 min. 

If the tuning of the reactor level controller in the conventional struc-... 

A word needs to be said about controller type and controller tuning. Controller algorithm selection and tuning are important to the success of any control system. Two features should be recognized about the Eastman process. First, it is an integrating process with little selfregulation in terms of pressure, liquid levels, and chemical components. Second, there are no tight specifications on any variables. 

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 addition to providing sufficient surge capacity, care must be taken in tuning the separator level controller. The gain of this controller must be chosen such that the change in reactor temperature provides a reasonable change in the production of condensable material from the reactor. As with any cascade scheme, the primary (level) controller gain depends on the span of the secondary (temperature) controller transmitter. 

Initially use proportional-only controllers in all loops except flow7 controllers, where the normal tight tuning can be used K = 0.5 and T = 0.3 minutes). Set the gains in all level controllers (except reactors) equal to 2. Adjust the temperature, pressure, and composition controller gains by trial and error to see if you can line out the system with the proposed control structure. If P-only control cannot be made to work, PI will not w7ork either. When stable operation is achieved, add a little reset action to each PI controller (one at a time) to pull the process into the setpoint values. 

This scheme is recommended when the bottom flow is one of the smaller flows in the column, particularly when the bottom flow is less than 20% of the vapor boilup. This scheme has little interaction with the energy balance, as it provides a good range of control with only small changes in the bottom flow. However, the tuning of the sump level loop usually makes this scheme slower than the others. An inverse response is also possible with this sump level control loop. This type of response occurs when an increase in steam flow temporarily causes the sump level to increase before it begins to decrease. If this occurs, the level loop must be detuned even more. 

---