Statistical Process Control (SPC) is a method for Indirect fault detection in normal operation.

Alarms are used to trigger the operators', to attract their attention, and to inform about operational risks. If the operators are not aware of the new risks, they might not be able to prevent the failure. This chart demonstrates the following alarming features:

• A parameter used for hazard identification, based on measurements. For example, the temperature of an liquid in a container may be used to indicate of the potential incidence.
• Forbidden range represents values of the  parameters which might result in an incidence. They are defined by the safety limits.
• The values of the parameters typical of routine operation are defined by warning limits.
• The alerting point is a point where the measurements cross the warning limits.
• The grace time may be estimated by extrapolation of the measurements, between the warning limits and the safety limits.

This chart demonstrates a situation in which the machine should alert the operators, and the effect of the operator's intervention.

The chart illustrates two ranges of the state variables:

• In routine operation, deviation from the range results in an alarm
• In emergency operation, deviation from the safety range to the forbidden range results in automatic shut down.

In the example, the machine resumes routine operation following a corrective action by the operators.

Updated on 19 Mar 2017.