In the process industry, what a control loop eventually manipulates in nearly all applications is a flow via a final control element such as a control valve, damper, or variable speed prime mover (pump, fan, or compressor). Dampers and variable speed prime movers are commonly found in utility systems. Peristaltic pumps are used in labs and positive displacement pumps are used for extremely low additive flows in plants. In instances, mass flow controllers (thermal mass flow meters with an integrated PI controller and valve) and remotely set pressure regulators are used. However, in production units, control valves are used as the final element in 95% or more of the loops.
Do we know for a change in controller output, did the valve actually move and if so when? Do we know when the control valve is the source of process variability? Do we know what makes a valve "Good" or Bad" in terms of its ability to do its job?
In valve selection and specification, a lot of effort is put into making sure the valve passes the required flows, has minimal leakage, no plugging, and has materials of construction and packing that withstands process composition and conditions. The dynamic response is often neglected possibly because response criteria and requirements are not well understood. Since most loops are digital, the question comes down to whether the change in controller output in a given scan results in a change in position of the internal trim (closure component such as a plug, ball, or disc). Of course most valves will eventually re-position, but the internal trim may not move until the total accumulated change in the controller output is large enough to
(1) Exceed the sensitivity of the positioner and actuator
(2) Change the pressure in the actuator enough to move its shaft
(3) Work through the play in shaft/stem linkages or connections (backlash)
(4) Break free the internal trim from packing, seating, and sealing friction (sticktion).
The result is a delay and a jump followed by a slow transition to a new position. The jump from sticktion causes a limit cycle in any PI or PID control loop. The deadband from backlash causes a limit cycle in any PI or PID control loop on an integrating process (e.g. level or batch temperature). The delayed and slow response adds pure and effective deadtime, respectively, to the loop.
The ultimate question is what should a user specify in terms of valve response? The table ControlValveResponseCriteria.pdf provides a summary of the parameters that makes a valve rated "Great", "Good", "Fair", "Poor", and "Bad". For most loops where process variable deviations of 0.5% are tolerable, a "Fair" valve will suffice. For loops where tighter control is needed (e.g. column, crystallizer, evaporator, or reactor temperature), a "Good" valve is needed. For loops with high process gains (e.g. pH), a "Great" valve is required to prevent self-inflicted oscillations from limit cycles being larger than the allowable deviation around set point (pHControlValveSizeandResolution.pdf). For tight control in loops with extremely fast dynamics (e.g. polymer pressure and incinerator pressure) a "Great" valve or a special variable speed drive may be needed (see "Analog Control Holdouts" on this website).
The ISA-75.25.01-2000 (R2006) draft standard "The Test Procedure for Control Valve Response Measurement from Step Inputs" and ISA-TR75.25.02-2000 (R2006) draft technical report "Control Valve Response Measurement from Step Inputs", use the time to reach 86% of the final response as a major criteria. This assumes the step input size is larger than the valve resolution and deadband for steps in the same direction and reverse direction, respectively. This 86% response time for small steps can be estimated as the sum of the pre-stroke deadtime and secondary lag time plus twice the primary lag time. For example, the 86% response time of a "Good" valve would be about 1.3 seconds for a 0.5% step (0.2 sec + 0.1 sec + 2*0.5 sec). For large step sizes encountered in surge and vessel pressure control systems, the 86% response time can be estimated as the sum of the pre-stroke deadtime and secondary lag time plus the stroking time to reach 86% of the step size. For example, the 86% response time of a "Good" valve would be about 2.45 seconds for a 50% step (0.2 sec + 0.1 sec + 0.86*0.5*5 sec). Note that the actuator size, pneumatic connections, and accessory (e.g. booster, positioner, and solenoid valve) flow coefficient determines the pres-stroke deadtime and stroking time, The pre-stroke and stroking values are based solely on actuator shaft movement and are determined by the manufacturer for tests of an actuator not connected to a valve. The sensitivity of the actuator and positioner is the minimum change in signal that causes a change in shaft position within a reasonable time frame (e.g. 10 seconds). Diaphragm actuators and digital positioners have the best sensitivity. Rack and pinion actuators and spool positioners have the worst sensitivity. Pneumatic positioners and scotch-yoke actuators are also bad news. The deadband from backlash in stem and shaft connections and the resolution from friction in packing, seats, and seals are determined after the actuator shaft moves. For practical purposes, the sensitivity of the actuator and positioner can be combined with the resolution limit of the valve for a total resolution of the package.
I have been particularly sensitized to valve response due to working on pH, furnace pressure, and compressor control. To add insult to injury, a proliferation of piping valves with piston actuators and spool positioners developed as a result of the emphasis on tight shutoff and low cost rather than response. These on-off valves posing as throttling valves created a problem for all types of loops. The idea was if the on-off valve worked well for sequencing and safety systems and was already in the piping spec, why not slap on a positioner and make it a throttling valve. Often the process variability from valve limit cycles was attributed to unknown process disturbances since there was no readback of actual closure component position.
This blog is getting long so I will just close with some figures on valve dynamics (ControlValveDynamics.pdf) from my new book The Essentials of Modern Measurements and Final Elements - A Guide to Design, Configuration, Installation, and Maintenance.
In upcoming entries we will seek to sort fact from fiction and hopefully provide some insight on valve rangeability and variable speed drive dynamics and rangeability.
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