Alarms, Safety Trips and Interlocks

Alarms are used in a power plant for several reasons :

  • To guarantee the safety of operating personnel;
  • To prevent the destruction of the capital investment;
  • To minimize unit downtime;
  • To comply with local, state, national and other regulations;
  • To avoid civil suits resulting from property or personal damage external to the plant.

Where delay or lack of response by the operator is likely to lead to the rapid development of a hazardous situation, instruments should be fitted with a trip system which would activate automatically to avert a hazard.

The basic components of an automatic trip system are :

  • A sensor to monitor the control variable;
  • A link to transfer the signal to the actuator;
  • An actuator to carry out the required action.

An example of this: in the steam collector, there is a valve connected to drainage in a local level
control with electrodes acting as an alarm and as the set point.

Where it is necessary to follow a fixed sequence of operations, interlocks are included to prevent operators from departing from the required sequence.

Care should be taken to test all of the interlocks in a plant’s automation during commissioning or whenever changes are made to the plant’s control and automation systems.

An example of this: The turbine needs the low level alarm in the cooling tower and the low vacuum alarm cleared before opening inlet steam valves.