Don't Over Look PID in APC

That Old Stalwart PID Can Do a Lot of Heavy Lifting in Your Process Control Efforts

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By Greg McMillan

There is a confluence of capability and profitability for advanced control. Valves with smart digital positioners and new sensor technologies with smart transmitters have a resolution and threshold sensitivity better than 0.1%. Smart analyzers are more reliable and maintainable. An enhanced PID can deal inherently with analysis sample, cycle and multiplex time delays. A whole host of PID features available today can make optimization easier and faster to implement. At the same time, energy costs have more than doubled.

Simplifying Optimization

The full capability of the PID is not used. The May-August 2011 series of "Control Talk" columns (May, p. 64; June, p. 63; July, p. 47; August, p. 63, or on extraordinary PID innovations highlighted some of the many possibilities for using more of the power of the PID. The PID is capable of achieving a safe, clean, efficient, profitable and compliant production unit—the key characteristics of a sustainable plant.

By using ever-expanding features of the PID, advanced control can be implemented with a simple configuration change. PID valve position controllers (VPC) and override controllers can be added to optimize existing loops without the need for new measurements or valves. Options are simply turned on to help the PID fulfill new roles. Additional software, tools and graphics are not needed. PID faceplate controllers are just added to existing displays. Training is minimal and can focus on functionality, since the operators are comfortable and proficient with the standard PID faceplate interface.

Table 1. Examples of optimization by valve position control.

Minimizing Energy Use

Wherever there is an adjustable utility source, there is an opportunity to use a VPC to increase its efficiency. The largest of the valve positions of the process loops that are users of the utility is selected as the process variable (PV) for the VPC. The setpoint is a maximum desirable throttle (optimum) position of the process PID valve, which is a constraint to less energy use. The output of the VPC cascades to the setpoint of a utility PID.

The most frequent configuration involves selecting the furthest open valve of process loops, setting the utility flow to a unit operation. The VPC maximizes the valve positions of process loops, throttling utility flows to minimize the pressure from boilers, compressors and pumps; maximizing the temperature from cooling towers and chillers; and minimizing the temperature from heaters.

Table 2. Key PID features for valve position control.

To reduce compressor energy, use the widest open valve of gas feed loops for a parallel train of reactors as the PV for a VPC whose output adjusts the pressure setpoint of the compressor, resulting in a more optimum speed. The VPC lowers the compressor pressure setpoint until the one of the feed valves reaches the maximum throttle position. For liquid feeds, the VPC lowers pump speed to reduce energy use. For a boiler, the VPC lowers the boiler pressure setpoint to force the furthest open steam control valves in a parallel train of columns or vessels to a maximum throttle position.

The control valve could be the output of a steam flow controller providing boil-up via a reboiler or a reactor temperature controller providing heat-up via a jacket. For a chiller or cooling tower, the VPC raises the supply temperature to force the furthest open the valve of a user loop to a maximum throttle position. For heaters, the VPC lowers the supply temperature.

For improving the efficiency of nearly identical parallel trains and loops, use a high signal selector to choose the furthest open valve as the input to a single VPC. While there is no such thing as identical equipment, the dynamics can be similar enough regardless of which valve is selected to enable the use of a single VPC. If the unit operations or process loops are quite different, a VPC is dedicated to each process loop valve. A low and high signal selector of VPC outputs is used to lower supply pressure and raise supply temperature, respectively.

When the situation involves utilities with different costs, a VPC can maximize the use of the least expensive utility, such as gas or liquid wastes. The VPC increases the waste fuel setpoint until the purchased fuel (e.g., natural gas or fuel oil) valve reaches a minimum throttle position. A low conventional fuel setpoint limit ensures flame stability for boilers.

Maximizing Production

To maximize the production rate of a unit operation, use a VPC to monitor each valve throttled by process PID to increase the feed rate. Since the process loops are quite different, a VPC is dedicated to each, and the lowest output of the override VPC is used to set feed rate.

To maximize the production rate of a reactor with process loops for pressure control and jacket and condenser temperature, use the lowest output of a VPC to prevent the vent valve or cooling water valve to the condenser or jacket from opening too far. To prevent carry-over into the vent system, add an override PID that uses radar to measure foam level. Alternatively, the VPC controllers can be used to raise the reactor temperature setpoint to a permissible limit, increasing an exothermic reaction rate until a VPC says enough is enough. The high limit prevents undesirable reactions or excessive heat release.

For a column, the feed setpoint is the lowest output of the individual VPC responsible for preventing flow, level, temperature and pressure control loops from running out of valve, that is, having it go too far open and losing sensitivity from the installed valve characteristic being too flat at the upper end of the valve stroke.

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