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Detecting Hydrogen Gas and Flames

05 July 2011

Author: Cliff Anderson, Director of Marketing Detector Electronics Corporation (Det-Tronics)

Hydrogen is the first element on the periodic table and is an essential element in the manufacturing of many of our everyday products. Although the general public might view hydrogen as an alternative fuel for cars, the main uses of hydrogen are found in hydrocarbon processing and other important manufacturing processes. Hydrogen rarely occurs in elemental form in nature, but is highly reactive. We must have respect for its highly reactive and explosive properties, and understand how to safely store, transport, and use hydrogen. We also must know how to detect and respond when it escapes its confines. 

Hydrogen Gas and Flame Characteristics
In average conditions, people cannot see, smell, or taste the presence of hydrogen gas. Hydrogen gas, however, is very flammable and only a small amount of energy will ignite it. In fact, if leaking from a pipe at a high enough pressure, hydrogen gas can self ignite without an external energy source. 
A hydrogen flame poses special dangers beyond those posed by hydrocarbon flames because human senses cannot easily detect it. For example, if you come upon a hydrogen flame, 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. 
In addition, as you approach the flame, you won’t feel the heat because very little infrared (IR) radiation is present — it is IR radiation that gives us the sensation of heat when we stand next to a flame. 
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. 

Team Strategy to Detect Hydrogen 
The safest protection strategy against a hydrogen flame is to prevent hydrogen gas from escaping, i.e., follow good process-maintenance practices. If, however, a leak does occur, the area should be well ventilated to prevent hydrogen build up. A gas detection system should be in place 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 as a team, gas detectors and flame detectors can quickly identify a gas leak or a resulting flame. For example, an enclosed battery room might 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 the true answer 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 flame or explosion. Two of the common technologies for combustible gas detection are infrared and catalytic bead. Only one of those technologies is appropriate for hydrogen.
An infrared gas detector responds to gases that absorb IR radiation – such as methane and propane (hydrocarbons). But hydrogen cannot absorb IR radiation, thus IR gas detectors will not detect it and should not be used. 
This leaves 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 silicones – 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 and disperses 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. 

Flame Detection Technologies 
Figure 1: Det-Tronics x3302 Multispectrum IR flame detector. 
The partner to gas detection is flame detection. Hydrogen presents several flame-detection challenges. Hydrogen burns a very pale blue to nearly invisible flame. Technologies to detect hydrogen flames include flame detectors that sense the non-visible spectrum of electromagnetic radiation, which includes ultraviolet (UV) and IR radiation. 
But in the beginning was the broom. Picture this. A worker walks along a catwalk adjacent to hydrogen lines. As he walks, he holds a dry grass broom in front of him sweeping the air. He walks slowly. If the broom catches fire, he stops and knows a hydrogen flame is ahead. 
Fortunately, safety system manufacturers offer tools beyond the broom. They’ve developed technologies such as thermal detectors, UV flame detectors, and multispectrum IR flame detectors. 

Flame Detection Technology: Thermal Detectors 
Because a thermal detector will not alarm unless it feels the heat, positioning a thermal detector directly above the possible site of a hydrogen flame is logical. However, the source of the hydrogen leak might create a flame that is directed away from the detector. The hydrogen flame’s low IR radiation may not be enough to set the radiant heat detector into alarm. Thermal detectors are helpful, but proper positioning is the biggest challenge. 

Flame Detection Technology: UV Detectors 
UV detectors use basic anode/cathode Geiger-Muller-type vacuum tubes to sense UV radiation emitted by a flame. UV radiation enters the vacuum tube through a quartz window and strikes the cathode. The energy from the UV photon releases a photo electron and creates an electrical impulse as it travels to the anode. This is a basic technology that dates back to the beginning of the 20th century. 
Hydrogen flames, compared to hydrocarbon flames, emit little visible light and little IR radiant heat. Instead, energy is radiated primarily in the UV band. Therefore, without doubt, UV detectors excel at detecting hydrogen flames. In addition, they have a good detection range and can see a 24-inch plume flame up to 50 feet away (see figure 2). 

Figure 2: General hydrogen flame detection ranges for UV and IR flame detectors. 
UV detectors, however, are sensitive to arcs, sparks, welding, lightning, and other UV-rich sources. When those relatively safe conditions are present, UV detectors could go into alarm condition. False alarms can be expensive and can reduce people’s sensitivity to potential hazards. Therefore, users should match the appropriate technologies to the applications they face. 
UV detectors, with their very fast response time and good detection range, are best suited for applications where the false-alarm sources can be controlled, such as in enclosed rooms. But keep in mind that most enclosed rooms have ventilation ducts that can reflect UV from lightning and welding – thus causing UV detectors to alarm. 

Flame Detection Technology: Multispectrum IR detectors 
Multispectrum IR flame detectors use a combination of IR sensor filters and software analysis to both see the flames and reduce false alarms. Some multispectrum IR detectors have been designed specifically to detect the low-level IR radiation of hydrogen flames using a unique set of IR filters. 
These special multispectrum IR flame detectors have very good detection range with good response time to the low levels of IR from hydrogen flames, but do not incur false alarms for arcs, sparks, welding, and lightning. In addition, the multispectrum IR detector has complete solar resistance and is insensitive to artificial lights and most “blackbody” radiation, which plague other detection technologies. 
By selecting the optimum IR filter set, some available detectors have doubled the UV range and can detect a 24-inch plume flame at 100 feet (see figure 2). The result is increased flame sensitivity with discrimination of non-flame sources in situations where traditional flame detectors are unsuitable. 
The multispectrum IR detectors do have limits, however. For example, their detection range is reduced with the presence of water or ice on the lens. To mitigate the problem, some detectors are manufactured with lens heaters that melt ice and accelerate the evaporation of water. 
For most applications, indoors and out, multispectrum IR flame detection has become the preferred choice for detecting hydrogen flames. 

Understanding Is the Key
Hydrogen is a valuable element with a growing list of uses. Like all combustible products, hydrogen can be a threat to people and property. But by understanding its gas and flame characteristics, we are able to formulate reasonable strategies to continuously monitor and quickly detect leaks and mitigate flames. 

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