A list is given below, for the abnormal conditions, which must be measured by function qualification instruments. State display or alarm signals from such instruments are for the purpose of alerting the process operator, thus enabling corrective action to be taken.
In cases of emergency or to safeguard vital equipment, the instruments automatically initiate trip or shutdown actions.
- High level (H) initiates an alarm.
- Extreme high level (HH) trips the inlet valve shut.
- Low level (L) initiates an alarm.
- Extreme low level (LL) trips the outlet valve shut.
Low flow (L) initiates an alarm and may also open a minimum flow spill-back or recycle valve to prevent the pump from overheating.
Extreme low flow (LL) trips the pump motor to prevent damage. High-pressure (H) increases the overhead condenser coolant flow.
Extreme high pressure (HH) initiates an alarm and opens a vent valve to flare. High temperature (H) initiates an alarm.
Extreme high temperature (HH) trips the fuel inlet valves to protect the furnace coil from overheating.