Whether you can see them or not, the oscillations from control valves are everywhere in your process. It's just a matter of how large and how important. When the amplitude is less than the exception reporting setting of controllers, data highways and historians, oscillations do a disappearing act on operator displays and trends. This does not mean they are insignificant because the exception reporting (percent change in a process variable that triggers an update) is often set by systems support to minimize traffic and data storage requirements, rather than to show change. Besides, management is much happier when they see straight lines. However, such bliss is short-lived if the hidden cycling translates to product quality or process efficiency issues. So what is the real deal with the source, diagnosis, impact and possible solutions for these oscillations?
Flow Control Troublemakers
The true measure of an automation system is its ability to control change. If there were no load upsets in feed conditions (flow or composition), equipment performance (fouling and efficiency), ambient conditions (temperature and humidity), utilities (temperature and pressure), or changes in set points (due to changes in product demand), we could all retire.
The measurement is the window into the process and the final element is the method of affecting the process. The measurement should provide a fast, undistorted and reproducible view of small changes, and the control valve must be able to make small, rapid changes and not itself be the source of unwanted upsets. In order for a control valve to accomplish its goal, dead band, stick-slip (Figure 1), and, in some cases, stroking time must be minimized.
Dead Band Discussion
Dead band occurs only when the valve needs to change the direction of its stroke. It is measured by how much the signal must change direction to reverse the direction of the stroke. The official test is done for a full-scale stroke in both directions but dead band occurs for any stroke whenever the direction is reversed. It is caused by lost motion and is due mostly to backlash from linkages and actuator shaft and stem connections.
It is worse for rotary valves because of the gaps in rack and pinion gear teeth, the slots in scotch yoke actuators, the key locks in shaft-stem connections, and the linkage that transfers vertical actuator shaft motion to disc, plug or rotary ball movement. A dead band of 8% can be common for such valves even though they are outfitted with digital positioners. More actuator torque does not solve the problem either. The time it takes for the controller output to work through the dead band is dead time that increases the errors from load disturbances.
The problem is not seen for setpoint changes or step changes in the controller output that are much larger than the dead band. Thus, loop analysis or tuning based on large setpoint changes, open-loop step tests or relay methods, will not reveal the additional dead time. For pressure control, it can mean the possible rupture of discs or vessels.
If you consider that the peak error and integrated error for load upsets are proportional to the dead time and dead time squared, respectively, dead band is a hidden menace. There is some consolation, though. For pure dead band, once the valve does move, it then can respond to small changes in signal in the same direction and dead band can cause a limit cycle only in a loop with an integrating response (e.g., level) or a runaway response (an exothermic reactor). A limit cycle is a sustained oscillation of nearly equal amplitude caused by a nonlinear response such as dead band.
Stick and slip occurs whenever the valve needs to move, even in the same direction. After it moves, it cannot move again unless the change in signal is greater than the stick. When it does move, the valve jumps or slips by an amount that usually is larger than the change in signal. Stick and slip generally occur together and have the common cause of friction in the actuator design, stem packing and seating surfaces. Piston actuators, high temperature packing, and tight shutoff in rotary valves (the so-called high-performance valve) can lead to the worst cases of stick-slip. It also can initiate shaft windup, where the actuator shaft moves but the ball, disc or plug does not. It is much worse at positions less than 20% where the ball, disc or plug starts to seat.
Here's the rub: If there is stick-slip, the controller will never get to setpoint, and there will always be a limit cycle. The big squeeze from graphite, environmental packing — particularly when they are tightened — and low-leakage classes are the biggest culprits. A bigger actuator may help, but will not eliminate the problem. An undersized actuator can cause a huge amount of additional slip. Stick-slip of 20% often occurs at breakaway from the seat of high-performance valves and for any valve with Graphoil packing and no positioner. Even with a positioner, a stick-slip of 4% is common with high-friction packing.
Once a valve moves it responds as a velocity-limited exponential. For small changes, the response looks like two time constants in series. For large changes, the response looks like a ramp. Stroking time is a consideration for large actuators and compressor surge or pressure control. The addition of a booster will reduce stroking time, but will introduce a relatively large dead band (e.g., 2%), particularly when designed for piston actuators. The use of a booster without a positioner can cause positive feedback and butterfly disc instability from high outlet-port sensitivity. A booster in series with a positioner must have a bypass around the booster adjusted to prevent a limit cycle.