Hydrogen leak detection systems may be required by the Authority Having Jurisdiction (AHJ) or may be installed as a means for enhancing safety. Leak detection can be achieved by providing hydrogen (or flammable gas) detectors in a room or enclosure, or by monitoring the internal piping pressures and/or flow rates for changes that would suggest a leak is present in the system. Other methods include providing detectors in close proximity to the exterior piping or locating hydrogen piping within another pipe and monitoring the annulus for leaks.
Regardless of the method used, leak detection systems should, at a minimum, incorporate automatic shutoff of the hydrogen source when hydrogen is detected. For systems designed to monitor hydrogen concentrations in rooms or areas, the leak detection system should also warn personnel with visible and audible alarms when the environment is becoming unsafe.
The number and distribution of detection points and time required to shut off the hydrogen source should be based on factors such as leak rates, ventilation rates, and the volume of the space being monitored. Locations to consider include:
- Permanent installations in indoor storage facilities
- Critical locations where leaks may occur (typically immediately above or where flow is concentrated) during experiments or the operation of processes involving hydrogen
- Areas where hydrogen may accumulate.
Performance criteria for an area hydrogen leak detection system should be established for the specific application intended. Parameter values for consideration include the following:
- The accuracy of the detector should be 0.25% by volume throughout the sensor measurement range.
- For area monitors: response times of 10-30 seconds at a concentration of 1% by volume; for sensors deployed within enclosures: response time of 1 second at a concentration of 1% by volume.
- Sensors should have a minimum range of 0 to 4 vol% with a lower detection limit of 0.4 vol%. However, it is recommended that the sensor have a range of 0 to 10 vol% with a lower detection limit of 0.1vol%.
The design of a leak detection system must ensure that any leaking hydrogen would pass by the detector. The sensitivity of the detector to other gases and vapors should be considered in the selection of the detector and should be explained to personnel.
A good practice is to set the detectors to alarm at 1 vol% hydrogen in air, which is 25% of the lower flammability limit (LFL). If automatic shutdown is incorporated into the system, manual reset should be required to restart the system.
Portable gas detectors are valuable for local leak detection. Portable detectors should be used for entry or re-entry into rooms in which an alarm has occurred to ensure that the hydrogen has dissipated.
Maintenance and recalibration of leak detectors should be performed every 3-6 months and recorded in facility records or manufacturer's instructions. Best practices for system checkout include:
- Always allow enough time for troubleshooting/debugging a monitoring system before it's used.
- Use inert gas and bubble indicators (soap in water) to identify leaks during system/vessel checkout. Use of helium is recommended, if available, as its atomic size is similar to hydrogen and will help identify small leaks.
- Piping and equipment leak checks with both soap solution and helium should be done before allowing any hydrogen to enter the system.
The following safety events occurred in a variety of settings, but each of them illustrates some key lessons learned about the critical need for hydrogen detection equipment: