Evaporator Base Sections and Accumulators

Evaporator base sections and accumulators are generally too large for good dynamic control. Evaporator and accumulator liquid levels should be proportional-only control. This permits the volumes to be used as surge capacity. Valve positioners should be used on the control valves in level control loops to avoid cycling due to hysteresis in the valve. 

All base level controllers in e train should have proportional-only control. In some plants, these controllers are difficult to find because they have been considered inferior. Proportional control is the best, if you can permit the measurement to move around the set point. Surge capacity is frequently provided in the design of accumulators, but then negated by specifying proportional-plut-reset controllers. Proportional-only control cannot be achieved with most instruments by turning the reset adjustment to the maximum reset time. A slow cycle with high amplitude will result. 

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