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Gas Detection Myth

The Myth of Combustible Gas Detection %LEL & PPM

Detecting combustible gases in % Lowest Explosion Limit (%LEL) has been a common practice for many years ever since the market realised the importance. Since then there are various sensing technologies available in the market with Infrared and catalytic sensors the most common. Infrared sensing technology is preferred over catalytic mainly because of fast response time and less susceptible to silicone poisoning. The catalytic sensor is selected for the cost effectiveness.

Many people are confused over the ultimate objectives of installing a combustible gas detector. In most cases, the objective is to detect gas leakage at the early stage so that planned maintenance can be scheduled to rectify the leakage before the leakage lead to major catastrophe.

Gas detection in %LEL in most cases will not be sufficed to provide early indication or alarm in the event of minor gas leakage. For example, the atmosphere with 4 %Vol of H2 is enough to trigger combustion/explosion because the condition of 100 %LEL is achieved. 4% vol of H2 is equivalent to 40,000 ppm. In the event of minor leakage with anything less than 2,000 ppm the %LEL gas detector will indicate zero reading. If the gas detector is configured to detect gas leakage in ppm level, the early indication of H2 leakage could have been achieved. Infrared sensor will not be able to provide early indication because the ppm level of H2 is not sufficient to trigger any indication on %LEL scale.

In the open air environment such as chemical refinery, the air dilution and crossed wind will create more challenges for detecting gas leakage in the early stage unless the gas detectors are installed very close to the leakage spot.

The leakage zone might have already leaking 5000 ppm or 10% LEL equivalent of combustible gas but the %LEL gas detector still indicate 0 %LEL because when the combustible gas travel to the gas detector, it has been so much diluted or the crossed wind has blown the combustible gas away from the gas detector. The situation can become worst if the environment is stale and the combustible gas is quick to build up in the area. Any spark near the leakage zone could have been triggered an explosion if the leakage has been left unattended.

The situation is similar to the leakage of kerosine, diesel or any similar fuel where the %LEL gas detector cannot register any reading even though the vapour of these liquid has been spreading around the leakage zone.

It does appear detecting combustible gas in ppm could solve the problem of early detection but detecting combustible gases in %LEL has been the industry practise. It is difficult to associate ppm reading with the danger level of combustible gas because most people have long been accustomed to 10% LEL, 50% LEL and 100% LEL.

In order to detect combustible gas leakage at the early stage and also provide warning when the atmosphere has achieved combustive situation, it is important to detect combustible gases in ppm and %LEL. However, from the economic perspective of installing 2 units of gas detectors, one for ppm and one for %LEL this approach is almost not practical. 2 units of gas detectors also required 2 nos of 4-20 mA I/O count at the PLC/DCS which incur further cost.

Riken Keiki have made the dual range detection possible by introducing ppm/%LEL gas detector SD-1NCW, SD D58 dual range and GD-A80NC