The Logic Behind The Magic. “Emergency Shutdown System (ESD) Operating Philosophy 3"

Start-up and Maintenance inhibit/ Override

In order to be able to start-up equipment or sections process, it is necessary to inhibit some inputs to the ESD system, as sensor signal may be in abnormal state prior to start-up and could cause a shutdown. Such inhibits are designated as "start-up inhibits".

While Maintenance inhibits are set from the  maintenance / inhibit console for maintenance purpose

Each inhibit function is reset automatically either by the sensor signal reverting to normal state or after a predetermined time delay. The automatic resets are achieved by the Safety Shutdown System.

Start up / maintenance inhibits are set manually by the operator from the Operation Console through a touch-target on process control displays which appear only when the signal to be inhibited is in alarm and not in technical fault.

Start –up / Maintenance inhibit statuses are indicated on the process control displays and ESD displays of the Operation Console and printed on the events printer.

This type of logic usually have 1 signals output and 3 input status that are shown in the control room,
 Input:

1. keyswitch Maintenance Override "Enable"
2. Maintenance Override Command "Changed"

Output:

1. Maintenance Override Status "Accepted"

The logic are:

1. Maintenance Override Status "Accepted" activated if Keyswitch maintenance override and Override Command is activated
2. Maintenance Override Status "Accepted"is deactivated. If Keyswitch maintenance override is deactivated and Master Reset is activated

Sample of the logic is shown below: