Meeting the challenge of H2S monitoring

Hydrogen sulphide (H2S) is a hazardous gas that must be monitored around upstream oil and gas facilities. Detectors are often required to work in challenging environments, such as the extreme climate of the Middle East. This presents a problem for traditional H2S detectors, which rely on wet chemistry, with the target gas inducing changes in an electrolyte. High temperatures and low humidity tend to dry the sensors out, impairing performance and leading to frequent replacements and higher costs.

One possible alternative is metal oxide semiconductor (MOS) technology, which doesn’t suffer from the same problem. However, MOS sensors have several other significant drawbacks. Most worryingly, some are prone to “going to sleep” if they don’t encounter gas for a period, presenting a real safety issue.

In addition, MOS sensors must be heated to produce consistent results. They take time to warm up, resulting in a significant delay between switch-on and the sensor correctly responding to gas. Manufacturers commonly recommend that MOS sensors be allowed 24-48 hours to equilibrate before calibration. This extends the time for servicing and maintenance and hinders production.

Heaters are also power-hungry and can result in dramatic changes of temperature in the DC power cable. This can result in significant changes in voltage at the detector head and corresponding inaccuracies in the gas level reading.

MOS sensors are based around semiconductors, which are prone to drift with changes in humidity. The semiconductors in computer chips are encased in epoxy resin to prevent such problems, but this would obstruct the ability of a gas sensor to do its job. The exposed sensing element is also vulnerable to drift when it is in an acidic atmosphere, which is typical in the sandy environment of the Middle East.

Drift can result in false alarms at near-zero levels of H2S. This is sometimes managed using “zero suppression” at the control panel, but this has significant safety implications. The control panel may continue to show a zero readout for some time after H2S levels have started to rise. This late registering of low-levels of H2S can delay warning of a significant gas leak, in turn delaying an evacuation and risking lives.

These issues can be compounded by any changes in voltage at the detector head and inaccuracies in the gas level reading caused by the heating element, as mentioned previously.

On the plus side, MOS sensors react very rapidly to H2S. However, the need for a sinter counteracts this benefit. H2S is a “sticky” gas that adsorbs onto surfaces, including sinters, slowing down the rate at which gas reaches the detection surface.

A fresh solution

There is a way to overcome all these challenges by adapting the electrochemical approach to H2S detection to make it less vulnerable to drying out. The High Temperature (HT) H2S sensor for XgardIQ, from Crowcon uses a combination of two adaptations to prevent evaporation, even in the harshest climate.

First, the sensor is based on a hygroscopic (water-loving) electrolytic gel that is designed to maintain moisture levels. Second, the size of the pore through which gas enters the sensor has been reduced, making it even harder for moisture to escape.

When stored at 55°C or 65°C for over a year, the HT H2S loses just 3% of its weight, which correlates very low moisture loss. A standard H2S electrochemical sensor would typically lose 50% of its weight in 100 days under these conditions. This means that unlike traditional models, the new sensor offers a life expectancy of over 24 months, even under desert conditions.

Crowcon’s HT H2S sensor works happily in an operating environment of up to 70°C at 0-95%rh. At temperatures above -25°C, this 0-200ppm sensor has a T90 response time of less than 30 seconds, which is better than most other electrochemical sensors for H2S.