The problems with Pellistors

Flammable gas sensors are used extensively in hydrocarbon plants to detect and give warning of the build-up of explosive levels of combustible gases, most typically methane. A typical plant can have from 50 to 2000 of these sensors, which cost £150-200 each.

Conventionally, this detection has been carried out using Pellistor or catalytic sensors. These detect flammable gases by burning the target gas on a heated filament, surrounded by a reaction enhancing catalyst, which is contained within an explosion-proof Exd enclosure that also contains a reference filament, identical to the sensor but lacking the catalytic enhancement. The burning gas increases the resistance of the heater filament. This change is sensed remotely in a Wheatstone bridge circuit which compares the signal from the heater and reference filaments.

These sensors are installed at strategic points in the hazardous installation area, and are wired back to remote control and monitoring equipment with three-core armoured cables connected to the Pellistor through a junction-box.

The problem is that Pellistor sensors have been found to be unreliable. They are prone to calibration drift, and have a tendency to give incorrect readings in high gas concentrations. More critically, Pellistors can be 'poisoned' by contaminants such as silicon, chlorine compounds, heavy metals and sulphur compounds, which react with the catalyst during the burning process and cause an inert layer to build up on the filament surface. This severely degrades the Pellistor's ability to detect gas, but is not at all evident during normal operation.

As a result, Pellistors need regular maintenance, typically every three months. This high maintenance regime, when coupled with regular sensor replacements and multiple sensor points, all adds up to a high cost of ownership.

Alternative flammable gas sensors are now available from most major gas sensor manufacturers. These are based on optical absorption technology, which is universally acknowledged as highly reliable. However, they require significant cabling and modifications to control equipment, so they are really only financially viable as a part of new builds or total installation refits. Pellistors are, therefore, still used in existing installations - giving their manufacturers a lucrative replacement market.

A sensor which combines the reliability of infrared detection with Wheatstone bridge field control technology would therefore be extremely useful, and AMGas has developed just such a unit with the LEDPoint PRS. Designed to retrofit with Pellistor-basted sensors, it fits the field-mounted terminal boxes and also features a three-wire connection which replicates a Pellistor circuit, allowing the sensor to be driven and monitored using Pellistor control cards. The output from the drive electronics is a temperature-corrected, linear signal which mimics a Pellistor resistive output.

The PRS (Pellistor replacement sensor) consists of a dual beam, dual wavelength infrared LED-based gas sensor. It has three main modules: the power supply/converter, the target gas cell and the main electronics/active optical equipment enclosure. A focusing mirror is mounted at the power supply end of the target gas cell, and light from the electronics/active optical component enters the target cell via an infrared transmitting window. A mirror heater is mounted inside the power supply case to prevent condensation.

The use of a dual-beam configuration gas cell with LED light sources provide long-lifetime operation, free from catastrophic failures and elimination of calibration drift due to optics contamination. The basic design target for the sensors is a unit life well in excess of five years. This will result in cost savings to systems operators, because their point detectors will not need to be replaced at their current frequency.In use, the target gas enters and exits the PRS gas cell by diffusion. However, because the sensor is designed for use in the harshest environments, the IR window and focusing mirror of the gas cell must be protected from environmental contamination. This is achieved using a removable 1mm+ pore sized particulate filter and a stainless steel splashguard.

The PRS is totally poison-resistant and features temperature correction with optical monitoring and fault diagnostics. And, of course, it responds extremely fast. Each PRS is connected back to a central control and monitoring station, enabling remote monitoring of the environment status. The unit can be used in the harshest environments and can be operated both horizontally and vertically.

Dr Allen Mabbitt is the cofounder of AMGas.