Boiler Interlock and Permissive Calibrations
A Boiler Safety Interlock is a device that monitors the safe running state of a critical boiler function and if the state becomes unsafe the interlock will trip the boiler to prevent unburnt fuel from entering the furnace.
Interlocks monitor pressure, differential pressure, vibration and device integrity. Examples are high gas pressure, low gas pressure, low combustion air pressure, high furnace pressure, high steam pressure, low drum level, auxiliary low drum level, stack damper open, FD Fan running ID Fan running and more.
Interlock calibrations are performed by removing each
boiler safety interlock and then applying the process pressure or differential pressure to verify that the interlocks dry contact will open to trip the boiler. If the interlock monitors a low pressure the interlock will open when the applied pressure drops down to an unsafe state. Inversely, if the interlock monitors a high pressure the interlock will open when the applied pressure rises to an unsafe state. Integrity interlocks are mechanical devices that will open dry contacts when a device stops running or reaches an unsafe speed or state like fan running or vibration monitors. Other integrity interlocks can be PLC Programed relays that open in situations like fuel or air set point deviation, critical transmitter bad quality or fuel and air deviation. Some integrity interlocks can trip a boiler from process related safety devices like ammonia vapor present sensors.
Permissives are devices that monitor the boiler light off sequence and verify that the controlling devices are in a proper position to safely ignite the boiler burner. Examples are fuel gas flow control valve is at low positon, gas flow control safety shutoff valves are closed, proof of purge air flow present and more.
Interlock and Permissive calibrations should be performed annually during scheduled shutdowns or turnarounds as termed in some industries. Performing these calibrations is a tried and true way of insuring that a boiler will be operating in a reliable and safe manner. There is a lot to be gained from comparing the previous year settings to any drift that may occur with the current settings measured. For this reason a professional boiler field service engineer should always provide paperwork showing as found and as left settings for the customer. Using a comparison may help find any failure trips that are about to happen and save costly down time during production.
Interlock and Permissive calibrations performed annually can also insure that every safety device has been tested by removal, pressurizing to the process level and verified contacts open when raised or lowered to the trip point. Likewise running trip tests before performing combustion tuning will insure that the wiring and control system are doing their job. Having detailed paperwork is good information to have handy for an insurance inspector or state inspector.
Collins Combustion routinely performs these checks and provides a detailed set of paperwork showing as found and as left settings as well as verified trips with comments. Collins Combustion has certified calibration tools that are used to perform all Interlock and Permissive calibrations.
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