Using PPM flammable gas detectors to increase safety

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Figure 1: Selective ppm H2 response in the presence in interference gases.

One of the most often used flammable gas detection technologies is the LEL sensor.

LEL, short for "Lower Explosive Limit," is defined as the lowest concentration (by percentage) of a gas or vapor in air that can propagate an explosion. For example, methane (CH4) and hydrogen (H2), two of the most common flammable industrial gases, exhibit LEL values of five percent and four percent by volume in air, respectively. The focus on LEL detectors as the go to technology for flammables is natural due to the clear danger of explosions, mining history, and the confined space entry laws that specifically call out LEL detection.

LEL flammable gas detectors are clearly an important part of a gas detection program, but due to their underlying technology, they are more effective for measuring relatively high flammable gas concentrations. The 100 percent LEL (the minimum concentration of a flammable gas present to support combustion) concentrations mentioned above for CH4 and H2 correspond to 50,000 ppm and 40,000 ppm, respectively. Typical fire regulations call for gas alarms at 10 percent LEL and 20 percent LEL. The usual technology in these 100 percent LEL sensors is a Wheatstone bridge circuit with an active (with catalyst) pellistor and a passive (without catalyst) pellistor that interact with the flammable gas being measured. In practice, this technology is often not very reliable at five percent LEL and below, despite often being deployed as the sole detection technology for flammable measurements. Although LEL flammable gas detectors are often used in industry, they are not very effective for measuring H2 below 2000 ppm and CH4 below 2500 ppm.

Figure 2: Typical gas cabinet for holding cylinders and controlling the delivery of the gas to processes.

Although less frequently utilized, high-quality flammable gas sensors optimized for the ppm concentration range have been developed and are available in the marketplace. Such sensors are ideally suited to gas detection scenarios where the concentrations that must be measured are significantly below the normal LEL range of a typical flammable gas detector. Some of the higher-quality sensors of this type utilize so-called "hot wire" semiconductor technology in combination with molecular sieve technology, they even have to ability to specifically measure the flammable gas of interest. See the graph of the response curve (Figure 1) utilizing a hydrogen specific H2 ppm sensor below. The sensor not only responds at a flammable gas concentration below the capabilities of a normal LEL sensor, a mixture of H2 and ethanol (Et-OH) and isopropyl alcohol (IPA) reacts almost exclusively to the H2 not the other substances.

To illustrate the effectiveness of this technology in practice, we outline a specific example of gas cabinets where the ppm flammable gas sensors is particularly effective. A common situation in semiconductor and gas plants, among other industrial settings, is the use of dopant gas bottles with a small amount of one substance in a high concentration of flammable gas; typically, over 90 percent flammable gas. H2 is commonly utilized this way with a smaller amount of a doping substance such as phosphine (PH3). Keep in mind, PH3 is very toxic and has a TLV of 50 ppb. These gas bottles are often used in a gas cabinet (see Figure 2).

In Table 1 below, we list various hypothetical concentrations you might see in such a bottle of dopant gas.

Table 1: Model scenario for phosphine PH3 dopant in a balance of hydrogen H2.

Even with the PH3 (dopant) gas concentrations of 10 percent, the bottle contains 90 percent hydrogen. That is well above 100 percent LEL for hydrogen (four percent by volume) above. If you had a catastrophic failure of the system and the gas was freely flowing into ambient areas or exhaust, you could have explosive mixtures that a normal high-concentration LEL sensor would pick it up. However, gas releases in gas cabinets or other gas delivery systems are rarely catastrophic. Usually, they are very small leaks that develop over time. In our example, in Table I, we have imagined a realistic small leak that delivers only 250 ppm of H2 to the detectors and calculates various ambient concentrations of the PH3 (dopant) gas that ensues. It's important to point out that a typical a standard 100 percent LEL sensor will not detect this small leak event at all. Further, at very often utilized dopant PH3 concentrations, even this low-level leak scenario is producing > LDL levels of PH3 in the environment. It is also true that big leaks almost always start as small leaks. From the perspective of increasing the safety factor for such systems and hopefully preventing a catastrophic leak, there are clearly some advantages. Further, in this scenario you can warn users of toxic hazards by setting your flammable gas detection ranges around TLV values of the dopant gas.

Figure 3: DOD EC20 System with PPM Hydrogen Sensors for a Dopant Application

A real-world example of a gas detection setup utilizing ppm flammable gas detectors is shown in Figure 3 where 20 points of gas detection are monitoring 20 points in ambient and exhausting areas where ppm gas leaks might commonly occur. There are many other commonly encountered scenarios where measuring flammable gas concentrations at low levels will result in a higher level of safety. We argue that it's always best to have as early an indication of a potentially flammable/explosive gas mixture leaks as possible and that ppm gas detection technology should be utilized as part of the gas safety system. Additionally, via detection of the flammable carrier gas, you may be able to also increase safety from the dangers of highly toxic dopant gases sometimes introduced into gas mixtures with high concentrations of flammable gases.

Contact your local DOD Technologies representative for more information or visit DODtec.com.