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By Cecil L. Smith, Cecil L. Smith, Inc.
The level in a process vessel often isn't viewed as a critical variable to control. Especially in surge vessels, controller performance typically is considered satisfactory if no high- or low-level trips occur. Vessel level's effect on the corporate bottom line usually is nil. So, it's easy to see why vessel level controls receive little attention.
However, neglecting level control carries definite risks. Indeed, loss of level control has contributed to three major industrial accidents (see: "Don't Underestimate Overfilling's Risks").
Processes consist of a collection of unit operations. The various flow streams that interconnect these unit operations propagate variance from one to the next. This variance ideally should diminish as it spreads but the nature of some unit operations actually amplifies it.
Control loops also can amplify variance, either through improper operating objectives or poor tuning. The flow between some unit operations depends upon a level controller's output to a final control element on a flow stream. Two factors make level loops a potential source of variance:
Figure 1 shows a vessel with four feed streams and one product stream. The level controller influences just the discharge stream. The only difference between controlling using a feed stream instead of a discharge stream is directionality (increase-increase versus increase-decrease). In Figure 1 the final control element is a valve but a pump with a variable frequency drive often is a viable, and possibly preferable, alternative.
The controller in Figure 1 translates variations in vessel level to variations in discharge valve opening and, hence, discharge flow. Maintaining level within a given proximity to its set point requires certain changes in discharge flow. Generally, the tighter the level is controlled, the larger the necessary variations in discharge flow.
Now, let's consider some characteristics of a feedback controller:
Although instrument technicians often are responsible for tuning level controllers in a plant, propagation of variance is a process issue that's most appropriately addressed by process engineers.
Most level processes are integrating/ramp/non-self-regulated, the primary exception being gravity flow applications. When the level controller is on manual, with an integrating process:
When the level doesn't directly impact any flow in or out, the dynamic characteristics of the process act as an integrator. The integrator in the reset mode of a controller coupled with an integrator in the process can have adverse consequences.
Starting from an equilibrium state (total flow in equals total flow out), any upset results in a ramp change in level, hence the term "ramp process." If the upset conditions persist, the ramp continues until the level reaches a limiting condition, usually in the form of a high- or low-level process trip. When no control actions are taken, such processes don't seek an equilibrium, hence the term "non-self-regulated process."
Figure 2 illustrates the response in level to an upset to the material balance. When the material balance is closed (imbalance is zero), vessel level is constant. In Figure 2, this is the case prior to time 0. At that point the discharge valve opening is reduced by 10%, which decreases discharge flow and causes level to increase.
All examples we'll discuss pertain to a straight-walled vessel containing a constant density liquid, hence the ramp has a constant slope as in Figure 2. We'll express the level as a percentage of the level measurement span. The response in Figure 2 is for a 12,000-L vessel. The average flow through the vessel is 200 L/min, giving a residence time of 60 min or 1 hr.
A simple characterization of a level process relies on two parameters whose value can be readily obtained from the response in Figure 2: