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Causes of Dead Time in a Control Loop

I always cover process characteristics as part of the process control training classes I present. It’s necessary for understanding process behavior and controller tuning.

The picture below shows the typical response of a self-regulating process after a step change in controller output. The process dead time (td) follows the change in controller output (CO). The process time constant is indicated with the Greek symbol τ (tau).

Dynamic Process Response to a Step Test

Dynamic Process Response to a Step Test

During the discussion on process characteristics, I show students how dead time affects the minimum settling time of a control loop. Even with the best possible tuning, a loop will still need a minimum of four times the dead time to settle out after a set point change or a disturbance. (Some people say a loop needs the equivalent of 10 dead times to settle, but appropriate tuning can normally do better than that if speed is the objective.)

Loop Response after a Disturbance

Loop Response after a Disturbance

During a training class on controller tuning that I recently presented, one of the students pondered the relationship between dead time and minimum settling time for a while and then asked me how one can decrease the dead time of a process. I answered that the length of dead time is mostly determined by the process design, but if you consider all the contributors to dead time, there might be some of them you can reduce or eliminate.

Here is a list of contributors to dead time:

  • Actual process transportation lag. This is the time it takes your control action to progress through the process equipment and reach the sensor. There is seldom something you can do about transportation lag, but in some cases you may be able to move the sensor closer to the control action to shorten the time delay.
  • Small lags in control loop. Although these are technically not true dead time, small lags increase the apparent dead time of a loop, and has the same effect on tuning and settling time as true dead time. Small lags creep in all along the control loop, and can be a significant contributor to overall dead time:
    • Thermowell thickness. Use the thinnest allowable thermowell for the fastest response.
    • Thermocouple or RTD response time. Use fast-responding devices to reduce dead time. For example, grounded thermocouples respond significantly faster than ungrounded ones.
    • Tightness of fit of thermocouple or RTD. A less-than-tight fit of a temperature sensor inside a thermowell can add an enormous lag to the control loop.  Consider using heat transfer compound to improve temperature response if conditions allow.
    • Instrument dampening or filtering. Unless you have a good reason for using instrument dampening or filtering, turn this feature off or set it to zero.
    • Pneumatic tubing. 500 feet of ¼” tubing has a lag of about 4 seconds. This is very long, considering that a 4-20 mA signal will have no delay along the same length. There is very little reason to still have long runs of pneumatic tubing in plants today.
    • Old positioner. Positioners used to be so slow that they were not recommended for use on valves in flow control loops. Nowadays they respond very fast. If you have old positioners on a loop you want to tune faster, consider replacing it with a new, fast positioner.
    • Slew rate of valve. A control valve can take a considerable time to slew to a new position. The larger the position change, the longer it takes to get there. Installing high-volume positioners can dramatically shorten the slew time on slow valves.
    • Velocity limiting of controller output. Some controllers are set up to limit the rates at which their outputs change. This may be necessary to protect process equipment, but consider setting this as fast as allowed by the equipment.
  • Controller scan interval. The periodic-execution nature of a digital controller will add an average dead time of one half of the scan interval to the dead time of a loop.
  • Analyzer sampling time. Similar to controller scan interval, but normally much longer in duration. If an analyzer samples the process every 5 minutes, the periodic sampling adds an average of 2.5 minutes to the loop dead time.

Some of these contributors to dead time may seem small or even trivial, but if you consider that 5 seconds of additional dead time increase the minimum loop settling time by 20 seconds or more, the value in finding and eliminating the small lags in a loop is more obvious.

Contact me if you have any questions.

 

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners

 

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