Balanced sequencing of final elements

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Overview:
Balanced sequencing of on/off final control elements is a common process control problem with broad-reaching applications. Examples include cooling water pump arrays and wet well level control. These devices typically need to be started and stopped in response to system demand or uncontrolled upsets, and in order to keep the pumps working on their performance curves nearest the best efficiency point (BEP). Doing so ensures system demand is met and process upsets are handled, as well as minimizing power consumption and wear on these expensive electromechanical assets.

Obviously, the minimum number of devices to which this control scheme can be applied is two, with three to five (or more) being typical scenarios. In some cases, the pumps are sized (or the demand constant enough) such that a fixed number of devices are required to run at all times to maintain proper control. In this situation, fixed sequencing works well, where each device is started and stopped on a schedule, ensuring that the required number of devices is operating at any time, and that each is being given an equal amount of runtime. In many cases, however, the downstream system demand or uncontrolled upsets are so variable that any number of pumps may be required at any time. Two scenarios of variable demand pump sequencing will be examined — arrays comprised entirely of fixed speed pumps and those arranged entirely with variable speed pumps.