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Dec
04

Deadly Deadband

A control valve isn’t doing much to help a control loop deal with the minute by minute onslaught of disturbances if it does not respond to the controller’s output. Yet there is normally nothing in a control valve’s specification form to insure the control valve actually moves. A step forward has been the ANSI/ISA standard 75.25.01 for a control valve step response testing procedure but I wonder if any where near as much effort is put on making sure the valve movement is smooth and sensitive as is spent on the valve size and leakage spec?

I was sensitized to the sensitivity of the control valve because my first area of expertise was pH when I moved from E & I Construction to Engineering Technology. The high process gain for strong acids and bases makes pH loops ideal for identifying valve response limitations. A jump in valve position of just 0.1% can cause a several pH swing. Putting a pH loop in automatic may initiate large amplitude oscillations even though there are no load upsets. In the end I realized great control valve sensitivity could reduce the number of stages of neutralization and save big bucks in process equipment required (see the 3rd edition of the ISA book titled Advanced pH Measurement and Control).

There is a growing awareness that a resolution limit from stick-slip in a control valve can cause a limit cycle in a control loop because the valve position is never exactly were it needs to be. Even if there are no disturbances, integral action in the controller drives the output until it moves, but then it steps right past the right valve position. Besides the limit cycle, there is also a dead time that is the resolution limit divided by the rate of change of the valve signal (controller output). To make things worse a slower rate of change of the controller output increases the resolution limit in some positioner designs. Consequently as the controller tuning is slowed down (Lambda is increased), the dead time and possibly the resolution limit is increased.

Deadband can be just as deadly. Whenever the controller output has to reverse direction, the change has to be greater than the deadband before the valve moves. The result is a dead time that is proportional to the deadband divided by the rate of change of the valve signal (controller output). If the are two integrators in the loop, deadband also creates a limit cycle. The two integrators can be the result of a controller with the integral action on an integrating process (e.g. level) or a cascade loop where the secondary and primary loops both have the integral mode (e.g. PI or PID controller) as discussed in the article “Life is a Batch” in the June 2005 issue of Control magazine.

Stick-slip normally originates from friction in stem packing or from sealing surfaces on the trim. Excessive tightening of the packing, high temperature packing (e.g. graphoil), older types of environmental packing, tight shutoff ball and disc seals, and low gain or spool positioner designs create more stick-slip. The friction is generally worse near the closure position, so most tests results are cited at higher valve positions (e.g. > 20%).

Ever since I started my career almost 40 years ago, inexpensive actuators and positioners have been added to tight shutoff rotary valves original designed for on-off or isolation service. The package is attractively priced and pitched as a control valve that meets or more unfortunately exceeds the valve’s capacity and leakage spec. If the process, mechanical, and instrument design engineer each add extra capacity in the piping, pump, and valve, the result is the extreme sport of a control valve riding the seat. If engineers attempt to make the control valve serve the additional purpose of isolation besides throttling, the problem of popping on and off the seat is magnified. In general, an isolation valve does not make a good throttling valve and vice versa.

In rotary valves, shaft windup can occur, where the actuator shaft twists but the ball or disc does not move because of high friction of the sealing surfaces. Eventually, the ball or disc breaks free and jumps to a new position. If the positioner, no matter how smart it think it is, measures actuator shaft position rather than ball or disc travel, it may report everything is relatively OK. I have seen a whole series of fancy plots from a smart digital positioner with vertical travel actuator shaft position feedback consistently show the stick-slip was less than 0.5% for a butterfly valve designed for tight shutoff (not too bad for the particular application). A travel gage added to the disc in the shop test setup gave the reality check that the stick-slip was actually 9% (lousy for any application).

Deadband is also known as backlash and is often larger in rotary valves because of rotary actuator and shaft coupling design or the need to translate from vertical to rotary motion. Be careful about the use of the term deadband. Purists will argue that deadband is the offset in the plot between an increasing and decreasing valve position for a full scale change in valve signal. In practical terms we think of deadband as the reversal in valve signal necessary to reverse valve position anywhere in the signal range. In the following plot of actual ball travel versus controller output, the stick-slip is evident for changes in the same direction and the deadband shows up for a change in direction of the valve signal. This plot is for the controller in automatic and shows that with a bit of understanding and practice, the dead band and resolution limit can be identified from trend charts. For rotary valves, this presumes there is a measurement of the actual ball or disc position or flow thorough the valve. For sliding stem valves, actuator shaft position read back is normally sufficient because there is a more direct connection of the shaft to the trim stem and no translation of motion.

Valve Deadband and Resolution

For the use of a model predictive control to achieve better valve sensitivity and rangeability see the article “A Fine Time to Break Away from Old Valve Problems” in the October 2005 issue of Control magazine. For equations on how to estimate the amplitude and period of limit cycles from a resolution limit or deadband see the article “What is Your Flow Control Valve Telling?” in the May 2004 issue of Control Design magazine.

To end on a lighter note, here is list to identify with:

Top Ten Exceptional Valves

(10) A measurement with 0.1% repeatability

(9 A control valve with 0.1% dead band

(8) A control valve with 0.1% resolution

(7) A controller that is tuned

(6) A process that is simulated

(5) Any computer picked out by your son

(4) Any canceled all week team building exercise

(3) Any afternoon meeting at the Oasis in Austin

(2) Any conference in Park City

(1) Any writing expedition in Naples

Next week’s blog discusses the merits of a block added to the PID controller output to compensate for valve resolution and deadband.