Control Valve Position

Thus the VPC control structure is able to achieve good temperature control while adjusting feedflow to close to the maximum. 

However, the steam flow set point signal coming from the TC is also sent as the process variable signal to a valve-position controller (VPC) whose set point signal is the minimum steam flow rate. Valve-position control is a type of optimizing control suggested over three decades ago by Shinskey who used it to achieve floating pressure control in distillation columns. Many other practical applications have been described.  

this control structure will always control temperature with steam and provide good temperature control. However, there is no absolute guarantee that the steam flow rate will never drop below its minimum limitation. 

Shinskey [Chem. Eng. Prog., Vol. 72(5), 1976, p. 73, and Chem. Eng. Prog., Vol. 74(5), 1978, p. 43] proposed the use of a type of control configuration that he called valve position control. This strategy provides a very simple and effective method for achieving optimizing control. The basic idea is illustrated by several important applications. 

Since relative volatilities increase in most distillation systems as pressure decreases, the optimum operation would be to minimize the pressure at all times. One way to do this is to just completely open the control valve on the cooling water. The pressure would then float up and down as cooling water temperatures changed. 

However, if there is a sudden drop in cooling water temperature (as can occur during a thunder shower or blue norther ), the pressure in the column can fall rapidly. This can cause flashing of the liquid on the trays, will upset the composition and level controls on the column, and could even cause the column to flood. 

To prevent this rapid drop, Shinskey developed a floating-pressure control system, sketched in Fig. 8.S. A conventional PI pressure controller is used. The output of the pressure controller goes to the cooling water valve, which is AC so that it will fail open. The pressure controller output is also scot to another controller, the valve-position controller (VPC). This controller looks at the signal to the valve, compares it with the VPC setpoint signal, and sends out a signal which is the setpoint of the pressure controller. Since the valve is AC, the setpoint of the VPC is about 5 percent of scale so as to keep the cooling water valve almost wide open. 

The VPC scheme is a different type of cascade control system. The primary control is the position of the valve. The secondary control is the column pressure. The pressure controller is PI and tuned fairly tightly so that it can prevent the sudden drops in pressure. Its setpoint is slowly changed by the VPC to drive the cooling water valve nearly wide open. A slow-acting, integral-only controller should be used in the VPC. 

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Valve Positioners The valve positioner, when combined with an appropriate actuator, forms a complete closed-loop valve-position control system. This system makes the valve stem conform to the input signal coming from the process controller in spite of force loads that the actuator may encounter while moving the control valve. Usually, the valve positioner is contained in its own enclosure and is mounted on the control valve. 

A valve position controller is used to reset the setpoint of the furnace exit temperature controller such that the control valve that is the most open of the four control valves on the hot oil streams flowing through the process heat exchangers is 80 percent open. 

A valve position controller is used to minimize operating pressure in a distillation column. Assume that the openloop process transfer function between column pressure and cooling-water flow Gju = P/F is known. 

However, since Mi is more expensive than Mj, we wish to minimize the long-term steadystate use of Mi- Therefore, a valve position controller" is used to control Mi at M . What is the closedloop characteristic equation of the system ... 

We want to analyze the floating pressure valve position control system proposed by Shinskcy. Let the transfer function between cooling water flow and column pressure be... 

An integral-only valve position controller is used. 

The state variables that provide valve position control are used to diagnose the health of the final control element. In addition, some digital valve controller designs integrate additional sensors into their construction to provide increased diagnostic capability. For example, pressure sensors are provided to detect supply pressure, actuator pressure (upper and lower cylinder pressures in the case of a springless piston actuator), and internal pilot pressure. Also, the position of the pneumatic relay valve is available in some designs to provide quiescent flow data used for leak detection in the actuator. 

Greg Shinskey1 presents a practical discussion of some of the advantages and problems of this control structure. The use of a valve position control (VPC) structure is recommended. A reactor temperature controller sets the coolant valve position. Then a VPC looks at the position of the coolant valve and adjusts the flowrate of the reactor feed to keep the coolant valve near its wide open position. 

When a compressor is supplying gas to several parallel users, the goal of optimization is to satisfy all users with the minimum investment of energy. The minimum required header pressure is found by the valve position controller (VPC) in Figure 2.20. It compares the highest user valve opening (the... 

TDIC-2 is the range controller, which is set by the optimized set point of SP-2. This is the range value corresponding to the optimum approach. TDIC-2 throttles the water circulation rate and sets the speed of the CT water circulating pumps in such a way that none of the process user valves (CV-4) will be fully open. This is guaranteed by the cascade loop of PDIC-3 and valve position controller (VPC-4). 

The distribution of the return water among several cooling tower cells can satisfy the dual goals of sending a preset percentage to each and to do that at a minimum pumping cost. The second goal is served by the valve position controller (VPC), which increases the measurement of all flow ratio controllers (FFICs) until the most open valve opens to 90%. 

Minimum pressure operation can be achieved by adjusting the set point of the pressure controller so as to keep the condenser fully loaded at all times. This can be done by using valve position control (VPC), which keeps the condenser control valve in a nearly fully open position ([a] in Figure 2.87) or by refrigerant level control ([b] in Figure 2.87). The column pressure can also be minimized on the basis of condensate temperature ([c] in Figure 2.87). 

In cases where both the demand for the product and the availability of feedstock are unlimited, increasing the throughput of the column maximizes profitability. In such installations a valve position controller can be... 

In order to make sure that the pressure controller (PC-01) set point is changed slowly and in a stable manner, the valve position controller (VPC-02) is provided with integral action only, and its integral time is set to be about 10 times that of PC-01. In order to prevent reset windup when PC-01 is switched to manual or local control from cascade, the valve position controller is also provided with an external feedback signal off the pump speed. 

If the liquid electrolyte design is selected for the electrolyzer, the optimization controls in Figure 4.1 (gatefold) include the electrolyte balancing controls based on the valve position control (VPC-32) of the variable-speed pumping station (VP-6). These controls are the same as those described for VP-1 and elaborated on in Chapter 2, Section 2.17.2. The power distribution controller (PoC-15) serves to control the electric power sent to the electrodes of the electrolyzer, and the pressure controller PC-14 serves to maintain the H2 pressure in the distribution header at around 3 bar (45 psig). 

When charging gas into several tanks or serving other users, the workload on the compressors is minimized by minimizing the pressure drop across the distribution valves. Therefore, the valve position controller (VPC-41) keeps opening all distribution valves by lowering the discharge pressure (PC-40) until the most open valve reaches nearly full opening (90%). 

Control engineers know that it takes one manipulated variable for each measured variable we wish to control to setpoint. When the number of controlled variables equals the number of manipulated variables we pair up the different variables and use PI controllers for regulation. Sometimes we are fortunate to have more manipulated variables than control specifications. We can then optimize the use of the manipulators while controlling to setpoint (e.g., valve position control). Sometimes, however, the number of control objectives exceeds the number of available manipulators and we cannot control all variables to setpoint. This is when the concept of partial control is useful. 

Shinskey [Chem. Eng. Prog. 72 5) 13, 1976 Chem.Eng. Prog. 74(5) 43,1978] proposed the use of a type of control configuration that he called valve position control. 

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