Special Challenges with Gas Detection

What Mark Twain wrote about words can apply to a gas monitoring system. 'The difference between what's right and what's almost right is like the difference between lightning and a lightning bug.'

In other words, when it comes to protecting your facility from hazardous gases, it pays to know the difference between gas monitoring strictly for compliance and gas monitoring for true life safety as well as operational efficiency.

Industrial processes increasingly involve the use or manufacture of highly dangerous substances, particularly toxic and combustible gases. Inevitably, occasional escapes of gas occur, which create a potential hazard to the industrial plant, its employees and people living nearby. Worldwide incidents involving asphyxiation, explosions and loss of life are a constant reminder of this problem.

While life safety is a major benefit of gas detection, don't forget that gas monitors also contribute to worker health, property protection and operational productivity, all of which impact the bottom line.

The industrial hygienist, as a proactive safety leader of the workplace, needs timely, accurate warnings of a gas leak so that building occupants can be evacuated in time, if necessary, to a safe place, and so that gas leaks can be mitigated quickly to prevent the overtaking or destruction of property.

Choosing the right sensing technology, the right gas instrumentation installation scheme and the right mix of fixed and portable monitors will do the job.

INSTALLATION BASICS

”How many detectors do I need?” and “Where should I locate them?” are two of the most common questions about installing gas detectors, and probably two of the most difficult to answer. Unlike other types of safety-related detectors, such as smoke detectors, the location and quantity of detectors required in different applications is not clearly defined.

The placement of detectors should be determined through collaboration. This meeting of the minds includes the advice of experts with specialized knowledge of gas dispersion; experts with knowledge of the process plant system and equipment involved; and safety and engineering personnel. The agreement reached on the location of detectors also should be recorded.

Detectors should be mounted where the gas is most likely to be present. Locations requiring the most protection in an industrial plant or commercial building would be around gas boilers, compressors, pressurized storage tanks, cylinders or pipelines. Areas where leaks are most likely to occur include valves, gauges, flanges, T-joints and filling or draining connections.

THE DANGER OF GOING IT ALONE

It is all too easy to make mistakes when the industrial hygienist chooses to configure a gas monitoring system alone, says Troy Baker, a gas monitoring consultant with Honeywell Analytics. “Too many don't pay attention to rooms adjacent to the room which is being monitored. One should have an idea of what industrial processes are performed or what equipment is used in these adjacent areas: arc welding? Propane-driven forklifts? The gases generated here can drift into adjacent areas, causing a toxic-combustible cocktail.”

In his 23 years of experience installing gas monitors, Baker has seen his share of mistakes with gas monitoring installations, such as not calibrating or commissioning instruments properly, improperly positioning the equipment, underestimating the amount of sensors needed, using the wrong sensor or not fully understanding a sensor's limits.

“I have observed that some otherwise very knowledgeable fire marshals, for example, mistakenly believe that the LEL sensor on their portable gas detector can apply to all flammable gases, including ammonia. In fact, the chemical composition of different hydrocarbons can vary tremendously. The explosive threshold between methane, or natural gas, and jet fuel, for example, can differ by 300 percent,” says Baker.

BEWARE THE GAS COCKTAIL

“Under-monitoring is a frequent mistake,” Baker continues. “An insidious problem to deal with is the dangerous blending of toxic and combustible gases.” Ammonia leaks, for example, are seldom purely ammonia; often, they are mixed with other liquids such as those from lubricants to cleaning solvents. These substances effectively can change the lower explosive threshold from, say, 40,000 parts per million to 37,000 parts, he says. These toxic and combustible threats are common at wastewater plants, chemical plants, rubber and other specialty manufacturing facilities.

Baker cites many other examples: A mechanical room that uses flammable gases may give off carbon monoxide; a battery room that uses forklifts may give off carbon monoxide that potentially can mix with off-gassing elements from hydrogen charging stations; propellants used in heating elements; and more.

GUIDANCE FOR GAS MONITORING

Considerable guidance is available from standards such as EN50073, a guide for selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or oxygen. Similar international codes of practice, e.g. National Electrical Code (NEC) or Canadian Electrical Code (CEC), may be used where applicable. In addition, certain regulatory bodies publish specifications giving minimum gas detection requirements for specific applications. These references are useful, but tend to be either very generic and therefore too general, or application-specific and therefore irrelevant in most applications.

There are, however, a number of simple considerations to keep in mind when designing a gas monitoring system:

  • To detect gases that are lighter than air (e.g., methane and ammonia), detectors should be mounted at a high level and preferably use a collecting cone.

  • To detect heavier-than-air gases (e.g., butane and sulfur dioxide), detectors should be mounted at a low level.

  • Consider how escaping gas may behave due to natural or forced air currents; mount in ventilation ducts if appropriate.

  • When locating detectors, consider the possible damage caused by natural events, such as rain or flooding. For detectors mounted outdoors, use the weather protection assembly.

  • Use a detector sunshade if locating a detector in a hot climate and in direct sun.

  • Consider the process conditions. Butane and ammonia, for instance, normally are heavier than air, but if released from a process line that is at an elevated temperature and/or under pressure, the gas may rise rather than fall.

  • Detectors should be positioned a little way back from high pressure parts to allow gas clouds to form. Otherwise, any leak of gas is likely to pass by in a high speed jet and not be detected.

  • Consider ease of access for functional testing and servicing.

  • Detectors should be installed at the designated location with the detector pointing downwards to ensure that dust or water will not collect on the front of the sensor.

  • When siting open path infrared devices, ensure that there is no permanent obscuration or blocking of the IR beam. Some types of short-term blockage may be accommodated.

  • Ensure the open path devices are mounted to sturdy structures that are not susceptible to vibration.

SERVICING A GAS MONITORING SYSTEM

A vital part of ensuring that fixed and portable gas detection equipment correctly operates is periodic servicing, maintenance and calibration. Gas detection applications vary widely, and so do the factors that affect the frequency of servicing required for proper operation. A suitable service period that takes account of each application's unique set of factors must be established.

Traditionally, gas detection users had their own in-house service departments that were responsible for servicing, maintaining and calibrating their gas detection equipment and other safety-related equipment. Increasingly, many users now choose to outsource part or all of this function in order to reduce fixed costs and at the same time ensure that people with specialist knowledge of the equipment are responsible for it. It also is becoming more common for leading gas detection companies to also offer service of third party gas detection equipment as well as their own. As users continue to demand better efficiencies from outsourced suppliers, the trend in the future likely is to require gas detection companies to offer a “one-stop shop” for the service and maintenance of complete safety systems.

Gas detection company service departments also offer site surveys, installation, commissioning and training. Advice from experts in gas detection helps ensure the selection of the most suitable detection technologies and most appropriate detector locations.

Properly commissioning a system ensures that it fully is functioning as designed and accurately detecting gas hazards. Many companies require that employees who use personal gas detection equipment, or work in areas that have fixed systems installed, are formally trained on the use and routine maintenance of the equipment. Service training departments may offer certified training courses designed to suit all levels of ability from basic gas detection principles through to advanced, custom-designed technical courses.

Some gas detectors now offer ‘smart’ sensors that are pre-calibrated and can be fitted and used without the need for additional calibration and setup in the field. The additional use of intrinsically safe design also can allow the “hot swap” of these sensors without the need for removing power from the detector. Other recent innovations include the use of “auto-cal” routines where the user is taken through a sequence of on-screen calibration steps to ensure correct set up. These innovations help keep service times to a minimum while ensuring accurate calibration.

The modular design of modern gas detectors enables more efficient servicing. The replacement of modules rather than component level service/repair greatly reduces turnaround time and therefore system down time.

APPLIED KNOWLEDGE IS POWER

Choosing the right instrumentation, installation scheme and service plan for monitoring hazardous toxic or flammable gases can go a long way to avert threats to people, property and your company's bottom line.

The industrial hygienist who is proactive about gas monitoring, works with management to anticipate problems and takes action to prevent problems before they happen contributes to increased productivity, reduced insurance and medical costs, reduced operating costs and safe, healthy workers.


Donald Galman is an editor and writer for Honeywell Analytics, Honeywell's gas monitoring instrumentation business. Contact him at 847-955-8389 or [email protected].

5 Tips for Choosing a Gas Detector

  • Does it fit the application?
  • Is it easy to install?
  • Does the product work as it's supposed to?
  • Is service readily available, and is it the best of its kind?
  • Does it provide the best information to make smart, fast decisions in an emergency?
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