Many oil fields, especially mature ones,can produce high levels of Hydrogen Sulfide(H2S), which is deadly at even low concen-trations.As drilling is often in remote loca-tions, getting to the nearest hospital in timeto respond to an exposure event could beimpossible. It is always crucial to be ableto detect the gas as soon as possible when a

leak occurs, in even the most challengingconditions.

It is important to remember that detec-tion coverage for H2S in facilities where itis known to be present is an ongoing pro-gram that needs to be constantly evaluatedand updated. Many facilities have con-stantly changing conditions with connec-

tions that are affected by the highly corro-sive properties of H2S. Some facilities sim-ply do not have adequate coverage or don’tcompletely understand the options availableto protect their plant and the people whowork there.

The toxicity of H2S gas is extremelyhigh. Its flammability level is actually

FSM | May 2013 www.fsmresponse.com

Detection ASAP is CrucialUnderstanding H2S and its Danger in the Field

BY JONATHAN SAINT

much lower than its toxic level for humans.Fortunately, having the distinct odor of rot-ten eggs allows plant workers to detectH2S gas in the relatively safe range of partsper billion.

The primary goal of any fixed point H2Sdetection system is to safeguard workers bywarning them of the presence of hazardoustoxic levels of H2S in their workplace. Asmanufacturers continue to improve toxicgas detection technology, it is important tounderstand how these may be applicable inyour installation.

Electrochemical and metal oxide semi-conductor (MOS) cells have for many yearsbeen the only field-proven toxic sensingtechnologies, but more advanced opticalsensing technologies are now emerging andhave widened the scope. The use of per-sonal gas detectors, or hand-held portablegas detectors, in combination with a com-prehensive fixed gas detection system isexcellent practice for complete site safety.In many countries standards exist that re-quire every worker in a potentially haz-ardous area to be equipped with a personalgas detector.

It is important to note that H2S beingheavier than air, sinks to the lowest lyingarea. Sensing elements must be positionedaccordingly to optimize performance.

MOS Sensors The typical construction of an MOS

(metal oxide semiconductor) sensing ele-ment includes a platinum heater element,an insulation medium and the sensing ele-ment itself, which is a gas-sensitive resis-tive film. This film will employ traditionalmaterials or materials that are enhanced atthe nano-level to improve performance.When H2S comes into contact with thisfilm there are measurable changes in theelectrical conductivity. These changes aretypically amplified in a transmitter device.

MOS detectors have a long life comparedto electrochemical sensors and continue tooperate in wide ranging temperatures, par-ticularly high temperatures, as well as in ex-tremely dry conditions. However, similarto electrochemical detectors, MOS detec-tors are not fail-safe and a change in oxygenlevels may affect their output.

Recently, advances in nano-enhancedmaterial construction have been able to ef-fectively deal with the above challenges.While the appearance and operating princi-ple of an NE-MOS sensor is identical tothat of a traditional MOS sensor, NE-MOSbenefits from a mechanically conformed ar-ray of sensing components known as “nan-

otubes” being applied to the resistive film ina manner in which they are perfectlyaligned, symmetric, and extremely concen-trated during the manufacturing process.

Traditional materials are produced usinga process that leaves gaps and creates irreg-ularities, resulting in the performance chal-lenges outlined above. Nano-enhancedmaterials equate to increased overall sens-ing capability, faster response, and muchhigher stability.

Electrochemical DetectorsIn an electrochemical sensor the cells

combine enclosed electrodes and elec-trolyte. H2S diffuses through a permeablemembrane, the volume of H2S increases inthe air, an oxidation or reduction reactionoccurs at one of the electrodes, and as a re-sult, a linear current change occurs. This en-ables a display or an amplifier device togenerate an indication of the H2Slevel.These detectors also have high sensitivityand repeatability, which has established thisas the toxic detection technology of choicein a wide variety of applications.

The use of electrochemical sensors indesert and arctic regions is not the ideal so-lution though. The detector’s lack of re-silience in high and low temperatures andthe effect of humidity on the detector’sperformance are serious considerations.Normally one can expect a T90 responsein around about 30 seconds in temperateconditions.

When you start to approach -20°C thedetector’s speed of response reduces signif-icantly, as temperatures continue to lowerthe output will decrease even further.

Other considerations with electrochemi-cal sensors are that they do require routinecalibration; every six months is typicallyrecommended. It may be required more fre-quently depending on the site conditions.The sensors are not inherently fail-safe butsome manufacturers have employed elec-tronic sensor health monitoring which willgenerate a fault output. And electrochemicaldetectors are all cross sensitive to other spe-cific types of gases which need to be con-sidered during installation to avoid exposureto these contaminants.

Optical Sensing TechnologyOptical hydrocarbon detectors are a field-

proven detection solution in many indus-trial applications and optical H2S detectorsuse the same basic operating principle. Inan optical H2S sensor, the signal is ab-sorbed by the H2S gas as it passes throughthe detector’s optical path. The sensor will

record the signal reduction and a micro-processor will calculate a correspondinggas value.

Because the sensor uses positive feed-back at its zero gas level, this technology isinherently fail safe. Any internal damage tothe detector results in an impact on pro-cessing so they must continuously performvaluable fault diagnostics to ensure opti-mum performance, which also compensateseffectively for temperature and humidity.

Optical sensors are deployed as either afixed point-type detectors or an open pathinstallation. Fixed point-type gas detectorsmust be strategically located to effectivelyprovide coverage for a targeted area. Regu-lar routine inspections are good practice butfull calibrations are typically only requiredevery 12 months.

Open path or line-of-sight (LOS) detec-tors use employ an optical transmitter and apaired receiver located at a maximum spec-ified distance apart to monitor the space be-tween them. Any volume of the H2S gaspassing through the beam of light will pro-vide a reading. But it is not currently pos-sible to accurately determine the ppm levelwith this technology. They also cannothave any obstructions to their path, whichmakes their effective application somewhatlimited in areas where workers are con-stantly present. They are well suited formonitoring gas migration into perimeterareas of an installation.

Manufacturers of H2S sensing solutionsface several unique challenges in a wide va-riety of applications. As the current oil andgas fields located in hospitable regions arecompleting their life cycle, there is going tobe a fundamental shift to the more extremeoperating locations of the world. In this con-text, it is critical that the safety devices per-form at a high level in these conditions inorder to protect plant and personnel as wellas maximize uptime in these locations thatrequire more capital to operate.

Operators need to consider specific ap-plication and environmental conditions tobe able to select the best available technol-ogy that will provide the highest degree ofsafety and performance. As new technolo-gies emerge it may become possible tocombine the point type devices, open-pathoptical detectors, with new and emergingtechnologies like ultrasonic gas leak detec-tion and laser-based sensing for providingenhanced safety to support even higherstandards. FSM

Jonathan Saint is with Emerson ProcessManagement. He can be reached atJonathan.Saint@emerson.com.

FSM | May 2013 www.fsmresponse.com