Life safety: Toxic gas detection

Walking through a drill-site area, a worker smells rotten eggs and stops for a few minutes to assess where a gas leak might exist. Rubbing his itchy eyes as he investigates, he notices he no longer smells the hydrogen sulphide odour. He does not realize the gas has deadened his olfactory senses and unless he leaves the area within seconds, he could lose consciousness and possibly his life. A well-researched gas-detection strategy can help reduce the likelihood of situations such as this.

“NTMOS sensors can provide very fast speed of response to hydrogen sulphide gas. T50 can be as quick as five seconds.”
-Charlie Hoff, Product Line Manager, Detector Electronics Corp

Risks from gas leaks come in many forms. Broadly defined, the categories are toxic gases, combustible gases, and gases that are both toxic and combustible.

• The risks of toxic gas leaks include death by asphyxiation or poisoning of the body.
• Combustible gas leaks pose risks such as fire or explosion after exposure to an ignition source, which could be merely the friction associated with gas escaping from a pipe fissure.
• An example of a gas that is both toxic and combustible is the deadly hydrogen sulphide gas, present at oil drill, oil production, and refining sites. 

An effective gas-detection strategy often includes both portable and fixed detection devices specifically for life safety. Portable gas detectors can be carried on a pocket or a belt for personal protection. Fixed gas detection systems are installed at fixed locations to provide long-term service life and detection for that area. The fixed detector monitors a specific location all day.

Addressed in this article are technologies and approaches that concern only fixed detection of toxic gases. 

Technologies to Detect Toxic Gases

Two main fixed-detector families are available to detect toxic gases: electrochemical cells and Metal Oxide Semiconductor (MOS)-type sensors.

Relatively stable and repeatable, electrochemical sensors can be used to detect a wide range of toxic gases in a variety of applications. The sensors do, however, have limitations that include restrictions in very hot and very cold environments. The limits arise because the sensors use an electrolyte that can evaporate in hot arid conditions. In addition, electrochemical sensors generally have a slower speed of response and shorter life span than current MOS-type sensors.

MOS-type sensors can live a long life while operating in a wide operating temperature range with good performance in low humidity environments. However, MOS sensor accuracy historically has not been ideal in regions prone to major changes in ambient relative humidity.

Recently, nanotechnology (NT) has been added to MOS sensors. NTMOS sensors can provide very fast speed of response to hydrogen sulphide gas. T50 (in a 0-100ppm application) can be five seconds. The speed benefits arise, in part, because each nanotube's total surface area is many times the surface area of its footprint.

In addition, NTMOS sensors do not go to sleep, as do many conventional MOS sensors. The term "sleep" means that after long periods of time, typical MOS sensors without periodic H₂S exposure cannot sense and react to H₂S. This undisclosed condition results in a deadly combination of unknown response time and loss of mitigation. Sleeping is overcome by the material design of NTMOS sensors.

The NTMOS advance has been proven to improve performance and accuracy significantly beyond MOS technology in arid, humid, hot, cold, and highly changeable environments. The new design has also improved longevity and accuracy of MOS sensors.

Be aware that if a device is called a monitor rather than a detector, it is not tested to meet required life-safety standards such as ISA or FM.

Detector Characteristics

When considering an H₂S gas detector, review the important detector characteristics: performance, availability, self-diagnostics, response speed, accuracy, repeatability, and maintenance.

• Performance: The detector should be tested to a known standard, such as ISA 92.0.01.
• Availability: Detectors must operate correctly, without faults.
• Self-diagnostics: If a detector does experience a fault or break down, it should annunciate the abnormal condition. By avoiding such undisclosed detection failures, the person responsible for gas monitoring can take appropriate measures.
• Speed of response: The more quickly the leak can be detected, the more likely that the risk for potential injury and property loss can be mitigated.
• Accuracy and repeatability: People's lives depend on the detectors' ability to annunciate gas leaks.
• Maintenance: By their nature, gas detectors require routine maintenance: functional tests, calibration, and periodic replacement of the sensor, filter, and electronics.

Third-party certifications to standards and focus on CIS requirements  

Some gas-detector manufacturers rely solely on their own internal test to self-certify to standards. Others add to their own testing by calling upon unbiased third-party testing agencies or insurance companies. These third-party testers can provide additional assurances and document development procedures, testing processes, and results.

Although safety-device manufacturers know their devices and are experts in their field, third-party testing provides an important third-eye into the design. In addition, the testing agencies maintain a high level of expertise in the most current standards and best testing procedures.

To the user's benefit, each third-party certified product carries with it a certification report from multiple certifying bodies. This report contains important information ranging from restrictions of use, to diagnostics coverage within the certified device, to reliability statistics. Additionally, ongoing periodic testing requirements of the device are clearly outlined and include:

• Restrictions of use
• Diagnostics coverage within the certified device
• Reliability statistics
• Ongoing device testing requirements

For example, all of Det-Tronics main product lines are fully Russian and Kazakh approved, with the Russian approvals being valid for use as low as -55 deg C. Det-Tronics' NTMOS gas detector is currently certified to -40 deg C.

Final considerations

Gas detection is as complex as the process being protected and careful analysis of the hazards is necessary to clearly identify the nature of the risk. Safety manufacturers with partners in CIS, Russia, Azerbaijan, and Kazakhstan should be authorized system designers and should be familiar with local requirements. Gas detectors make a vital contribution to the hazard management process. When they are effectively applied, they can save lives and property in the event of a potentially catastrophic leak.