Control Level with a Cascade

This requires different thinking about master and slave loops.

By Cecil L. Smith, Cecil L. Smith, Inc.

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The previous articles in this series on level control focused on proportional-integral control ("Neglect Level Control at Your Peril" and "Be Levelheaded About Surge-Tank Control") and use of nonlinear control equations ("Avoid Vessel Level Trips"). In this final installment, we'll look at cascade control.

In digital systems, proceeding from a simple feedback to a cascade configuration requires only one hardware component — a device to measure the inner loop variable. In level control schemes, the sensor monitors flow through the final control element. This permits configuring a flow controller and implementing a level-to-flow cascade. The level controller (in the outer or master loop) provides the set point to this flow controller (in the inner or slave loop).

In most cascade applications the objective is to improve control of the outer loop variable. Level control differs in this regard. Especially when used with surge vessels, the aim is to allow level to vary as much as possible "within reason" (meaning that no high or low level trips occur during routine process operations). Rarely can you justify purchase and installation of a flow meter on the basis of improved control of vessel level.

 Alternating Filters Figure 1. Switching to a clean filter causes disturbance to discharge flow.
Alternating Filters
Figure 1. Switching to a clean filter causes disturbance to discharge flow.

Particularly with surge vessels, the level controller intentionally is tuned to respond slowly — with the objective of maintaining as smooth a discharge flow as vessel capacity permits. Any disturbance to the discharge flow will affect vessel level but not very quickly, thus exposing the downstream processing unit to upsets from the altered flow.

FILTER ON DISCHARGE FLOW
In the process shown in Figure 1, the discharge flow goes through one of two cartridge filters, with only one in service at a given time. As fluid passes through, pressure drop across the filter slowly increases. When this pressure drop attains a specified value, the flow is switched to the other filter.

On switching to the clean cartridge, the pressure drop across the filters abruptly decreases. The major consequence is a sharp increase in discharge flow. This affects vessel level but the impact, except for possibly raising the probability of a process trip on low level, is of no concern. What is of concern is the potential repercussion of the higher flow on the downstream unit.

 Constant Feed Rate Figure 2. Switching filters increases discharge flow, spurring decrease in level.
Constant Feed Rate
Figure 2. Switching filters increases discharge flow, spurring decrease in level.

Figure 2 illustrates the effect of switching the filters (at an interval of approximately 8 hours) for a constant feed rate of 200 liters/min and no noise on the level measurement. The discharge flow abruptly increases, which causes the level to decrease. The subsequent drop in discharge flow stems partly from the response of the level controller and partly from the increased resistance to flow within the filter. When the level controller is conservatively tuned, equilibrium conditions aren't established prior to the next switch.

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