Integral Time

Integral time should be proportional to the time it takes for the process to respond to control action. When the process responds quickly, the integral time can be shorter. If the integral time is too short, the control valve reaches its limit before the measurement has time to respond. When the measurement does respond, it will overshoot the setpoint, causing the integral to drive the valve to its opposite limit.

The time lag built into the gradual response of integral action lengthens the period of oscillation of a loop. For a loop with proportional-plus-integral control, the period of oscillation after a load change is longer than for proportional alone.

For loops where the exact value of the controlled variable is not critical, the shorter period of the proportional-only controller can be an advantage. For example, a vessel may operate within a wide range of liquid level without adversely affecting pressure or gas quality. Therefore, the system level does not have to be accurately controlled, and proportional control is often sufficient.

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