Hydrogen usage is growing. The general public sees and reads about hydrogen as an alternative fuel for cars. However, the big use for hydrogen is found in hydrocarbon processing and other important manufacturing processes. Hydrogen is the first element on the periodic table and is an essential element in the manufacturing of many of our everyday products. We must have respect for its explosive properties, but not be afraid of it. We must understand how to safely store, transport, and use hydrogen. We also must know how to detect and to respond when it escapes its confines.
The Nature of Hydrogen
Under day-to-day conditions, people cannot see, smell, or taste the presence of hydrogen gas. Hydrogen, however, is very flammable and requires only a small amount of energy to ignite. In fact, if leaking from a pipe at a high enough pressure, hydrogen gas can self ignite without the aid of an external energy source.
A hydrogen flame poses special dangers beyond those posed by hydrocarbon flames because human senses cannot easily detect it. If you come upon a hydrogen fire, you will not see it – even up close. You might see an area ahead of you shimmer as you would see a mirage. You might see sparkles, dust particles briefly burning. You might think your eyes are playing tricks on you.
In addition, you will not feel the heat as you approach the flame. You won’t feel the heat of the hydrogen fire because very little infrared (IR) radiation is present; the IR radiation gives us the sensation of heat when we stand next to a fire.
Because there is little radiant heat emitted to the environment and nothing to see, your senses won’t warn you to stop. You might walk directly into the flame.
The physical flame has more “punch” than a hydrocarbon flame, i.e., the temperature is higher. The result? If the flame impinges on another piece of equipment, the heat of a hydrogen flame will have a stronger effect than a hydrocarbon flame of the same size. Objects in the flame path will heat faster, which could cause a second event to occur – creating a possible chain reaction of serious events.
The Strategy of Detecting Hydrogen
The safest protection strategy against a hydrogen fire is to prevent the hydrogen from escaping by following good process maintenance practices. If, however, a leak does occur, the area should be well ventilated to prevent hydrogen build up. Also in place should be a gas detection system to alert operators to the leak before it ignites. But if the leak does ignite, you need to detect the flame quickly and accurately.
Working together, gas detectors and flame detectors can quickly identify a gas leak or the resulting flame. Gas and flame detectors should work as partners to monitor the same area.
For example, an enclosed battery room can contain hydrogen generated from the batteries. Sitting in the control room, an operator might be alerted to a burp of hydrogen gas. If the alarm generated by the gas detector stops, will the operator think the burp was truly a short-duration small hydrogen leak? Or has the hydrogen ignited and turned into a flame? The operator will not know unless the flame is detected.
Gas Detection Technologies
Gas detection represents the first line of defense in the case of a hydrogen release. Ideally, actions can be taken to stop the hydrogen release before a fire or explosion. Two of the common technologies for combustible gas detection are infrared and catlytic bead.
An infrared gas detector responds to gases that absorb IR radiation – such as methane and propane (hydrocarbons). But because hydrogen is not a hydrocarbon, IR gas detectors do not detect it and should not be used.
This leaves only catalytic bead type detectors for detecting hydrogen at lower flammable limit (LFL) levels. In fact, a catalytic bead sensor detects any combustible gas that combines with oxygen to make heat. If the gas can burn in air, this detector will sense it.
The catalytic gas sensor (or Pellistor) usually consists of a matched pair of platinum wirewound resistors, one of which is encased by a bead of ceramic. The active catalytic bead is coated with a catalyst; the reference catalytic bead remains untreated. This matched pair is then enclosed behind a flameproof sinter, or porous filter. In operation, the beads are resistively heated. When a combustible gas comes in contact with the catalytic surface, it is oxidized. Heat is released, causing the resistance of the wire to change. The reference bead, or passive bead, maintains the same electrical resistance in clean air as the active bead, but does not catalyze the combustible gas. The sensor compares the currents. If the current is different, the detector can alarm. If there is no gas cloud, both beads will have the same current.
The catalytic bead sensors do have shortcomings, however. For example, they don’t annunciate when they fail. Also, they are susceptible to poisoning and dying from chemicals such as silicon hydrides – common chemicals in industrial environments. In these cases, the porous filter gets clogged so that the active bead cannot sense gas and becomes the same as the reference bead. If the active bead cannot sense gas, the operator back in the control room won’t know. Periodic testing is required to ensure proper sensor operation.
In placing the gas detectors, consider that hydrogen is the lightest gas and floats up quickly while dispersing easily. Make sure the gas sensor is close to and above where a leak might occur. For example, a gas detector could be located above a valve stem.