In the majority of process systems, there are one or more process parameters that must be maintained. The purpose of a process control scheme is to maintain a desired parameter regardless of any external influence. Process parameters in typical water systems include temperature, pressure and level. Since water systems generally utilize one or more plant utilities, it is often fluctuations in the parameters of these utilities that a process system must address. Three general schemes may be utilized to address the utility fluctuations - isolation, regulation and accommodation. The application of each scheme it described below.

    > Isolation

    > Regulation

    > Accommodation

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One possible application of the isolation scheme is for feed water flow control in a still or steam generator. In this application, fluctuation in feed water pressure is negated by the use of a break tank. Once the feed water is in the break tank, the pressure is atmospheric regardless of the line pressure feeding the tank. The water in the tank is then supplied to the unit by a feed pump pumping against a manual feed control valve or an orifice.

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The regulate scheme may be used to regulate the pressure of feed water or plant steam to a still or steam generator. The regulator can be set to maintain the pressure of either of these utilities. Regulators may be a part of the utility piping external to the still or generator, or they may be integral to the unit piping. Since regulators do not offer a "closed" position, an additional automated valve is required in the downstream line. The regulate scheme only regulates the utility parameter which is not necessarily the critical parameter that must be maintained.

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The accommodate scheme is similar to regulator scheme with one major exception - the utility is regulated, as necessary, to maintain the desired critical parameter.

For this scheme three primary components are required:

• Sensor to measure and transmit a signal representative of the critical parameter



• Controller to compare the actual values of the parameter to the desired value uses the difference between the two to produce an output signal to eliminate the difference



• Final control element to control the utility parameter on the output signal from the controller

These elements are usually integrated into the still or generator with the unit control system handling the controller function. This scheme provides the most accurate control of the measured parameter under all conditions.

The Proportional, Integral, and Derivative constants in the controller logic are used to calculate what the output should be relative to the measured error.

Proportional
The controllers output is proportional to the error or a change in measurement.

Integral
The controllers output is proportional to the amount of time the error is present.

Derivative
The controllers output is calculated by the rate of change of the measurement with time.