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THE TIME it takes to bring a slow process variable from its minimum or maximum to set point is a significant portion of a batch or startup phase. The more important analytical loops, pressure, and temperature loops often have a response that ramps most of the way to set point.
In batch processes, the ramp is most likely due to the process actually having an integrating response because the discharge valve and possibly the vent valve are closed at the start of the batch. The outlet valves may also be closed in continuous processes when a vessel is being filled or pressurized.
Even when the startup of a continuous loop is delayed until after an outlet flow is established, the small dead time and large time constant of a well-mixed volume results in a loop response that looks like an integrator. Even more problematic is the runaway response, where the process variable actually starts to accelerate. All of these responses benefit from special considerations, such as using a Lambda factor of less than 1.0 and pre-positioning of valves, as discussed in “Life's a Batch,” CONTROL, June ’05, and in a dozen Control Talk columns on process control techniques and tuning for batch processes.
Most of the articles and books on tuning loops concentrate on set point and load changes around operating point. For integrating processes, the focus is on surge tank level loops because these are the major source of variability in a plant because they jerk feeds around or cause a rolling action primarily from too much reset action (too little reset time). Bringing a composition, pressure, or temperature rapidly from either end of the scale to a set point is a different story.
Nothing Says Forever Like Tradition
There are four major practices for starting up a loop with a large process time constant or slow ramp time compared to the dead time. These loop practices for fast batch and startup response are:
Figure 1 shows the batch or startup response of a pressure loop with an integrating response for practices 1, 2, and 4. Practice 3 isn’t shown because it’s not viable for integrating processes. There are other practices, such as ramping the set point, for unit operations, where it’s desirable that the approach of the process variable to set point and the output to its final resting value is moderated or that a profile be enforced.
FIGURE 1: BATCH AND STARTUP
This graph shows the batch or startup response of a pressure loop with an integrating response for practices 1, 2, and 4.
In practices 1 and 2, the controller output is at its initial value at one end or the other of the output scale (often zero). All methods assume the pump and block valves have already been started and opened, respectively.
In Practice 1, if the loop is tuned to minimize variability in the controller output, which is the case for surge-volume level control, the batch phase may time out before the process reaches set point. For example, if the process time constant is 50 min, and a Lambda factor of 5 is used, then the closed-loop time constant is 250 minutes, and the time to reach 98% of set point is 1,000 min (or four closed-loop time constants). A similar situation exists for slow ramp rates. For integrating processes, the Lambda factor is the ratio of the closed-loop arrest time to the open-loop arrest time. The arrest time is the time it takes for a PV to make a designated allowable excursion for a given upset. If the response is a runaway, then vessel relief devices or interlocks might activate if the Lambda factor is greater than 1. The vessel could get off the ground before the loop gets off the ground.
A dead time much faster than the process time constant or ramp rate means that usually a Lambda factor of less than 1 (closed-loop time constant or arrest time less than the open-loop time constant or arrest time) is permissible for stability, and desirable for fast control of these process variables. This is particularly important for Practice 1 because you’re relying on reset action to get you to set point. All of the batch or startup responses in Figure 1 use a Lambda factor less than 1.
In Practice 2, the set point is changed from its initial to final value at one execution or more after the controller is switched to auto. Note that, if you switch the set point within the same execution of the module as the switch of the mode, then you’ll probably end up with the same response as Practice 1. In the second batch or startup response, the set point change kicks the output, which gives the process variable a boost on its way to set point. The time to reach set point (rise time) is nearly cut in half, but the settling time is about the same. Since, the overshoot is minimal, the rise time might be more important. Also, the controller tuning could be tweaked to reduce settling time.
Many astute automation engineers will preposition the controller output by what is called a “head start” or “process action.” For self-regulating loops, the valve position might set at what was considered to be a normal throttle position or final resting value (FRV) seen from previous trends when the process variable had settled out at set point. This corresponds to a Lambda factor of 1 because, if held at this position, it will drive the process variable with a time constant equal to the process time constant.
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