GAS DETECTION: A PERMANENT FEATURE
By: Alan Skinner and Andy Avenell

Introduction: fixed versus portable
To protect individual workers against gas hazards, portable detectors have become the tool of choice. They are lightweight, relatively inexpensive, and able to go wherever the user takes them. However, they are designed primarily to monitor only a small area around the operator. Moreover, as warning devices they are fundamentally passive. If remedial measures are to be taken, if the emergency services are to be called, or if personnel on another site are to be alerted, it will fall to human operators to do these things. In certain situations, then, a fixed detection system will be more appropriate. Not only will it offer automated, round-the-clock coverage over a wide area: it can be configured to perform a range of automatic actions in the event of an emergency.




Vortex is a sophisticated digital control system from Crowcon. Here, the control unit is supplied in a custom-built cabinet which contains detectors for carbon monoxide, hydrogen sulphide, methane and oxygen, as well as a gas sampling system. Gas samples are continuously pumped in from a network of remote underground positions.


Identifying risks
Gas hazards can be divided into three categories – toxic, flammable and Oxygen-related (too high or too low). These may be faced either singly or in combination by workers in a number of industries. For example, the common toxic gas carbon monoxide is encountered in power generation, steel production and wherever generators or motor vehicles are run. Hydrogen sulfide, which is created by bacterial action on decaying organic matter, is another widespread toxic hazard that affects the sewage treatment, oil production and refining, tunnelling and excavation industries.

These and other toxic gases have a cumulative effect: the longer one is exposed to them, the greater the risk. Consequently, safe exposure limits are often expressed as average figures over time. In the USA, these are known as occupational health Guidelines for Chemical Hazards from National Institute for Occupational safety and health, organizations known as (NIOSH, OSHA and ACGIH) and typically include a short-term exposure limit (STEL) over 15 minutes and a long-term exposure limit (LTEL) over 8 hours. Either figure may be referred to as a time-weighted average (TWA). For some particularly toxic substances, maximum exposure limits (MEL’s) are used to stipulate peak (as opposed to time-weighted) levels.

In the case of flammable gases and vapours it is instantaneous, not averaged, levels that matter. For an explosion to take place, the air must contain a certain minimum concentration, dependent on the substance in question, which is known as the lower explosive limit (LEL). Once this level is reached – and for many hydrocarbon gases it is as low as five percent – only a spark, flame or even a hot surface is needed to create a fireball. Flammable hazards are easy to find throughout industry. Methane is prevalent in oil production and sewage treatment, while solvents represent a hazard in paint stores and printing plants. Leaks and spillages of hydrocarbon fuel, whether from pipelines, tanks, cylinders, pumps or engine-driven plant, are an obvious hazard. Furthermore, everyday places like fuel stations and bus depots can be as much at risk as oil platforms, refineries and other specialist environments.

Oxygen depletion is a risk wherever gases such as nitrogen and carbon dioxide are stored – for example in laboratories, hospitals and wine cellars – as leaks from cylinders can displace oxygen from an enclosed space. Oxygen can also be absorbed by chemical or biological processes, such as metal corrosion inside a storage vessel or the rotting of vegetable matter in a silo. Conversely, too much oxygen, perhaps from leaking breathing apparatus or a welder’s cylinder, will create an enriched atmosphere in which materials will burn dangerously quickly.

Not all fixed gas detection systems are used to protect personnel and plant: long-term monitoring may be a regulatory or legislative requirement for environmental reasons. For instance, turbine exhaust gases from power stations may contain nitrogen oxides, sulphur dioxide and carbon monoxide. Emissions of all of these must be constantly monitored and kept within prescribed limits. Landfill operators are similarly obliged to log the carbon dioxide, hydrogen sulphide and methane which are produced by their sites.

Detector placement Permanently installed gas detector units, or “heads”, have two principal parts: the sensor itself, which is easily removed for replacement when necessary, and a wall- or ceiling-mounted junction box. This contains the components necessary to process the output from the sensor before it is sent to a central control unit. If a sensor “sees” a dangerous gas level at any time, the control unit raises the alarm.

The careful positioning of detectors is crucial. Naturally, the entire area at risk should be covered. While small-scale systems might involve only one detector, more sophisticated wide-area networks may include many heads, all connected to the same central controller. Ideally, detectors will be placed not only at the likely source of a hazard but also around the plant perimeter.




The simplest gas detector control units are compact, wall-mounted devices with basic on/off/reset controls. Sold to swimming pool operators, this package combines Crowcon’s single-channel Gasflag controller with a chlorine detector and a sounder/strobe unit.


The height of installation will be determined to some extent by the density of the gas to be detected: low if heavier than air, high if lighter. However, this is something of an oversimplification as, in practice; gases do not separate into discrete layers, but exist in generally homogeneous mixtures. Other factors affecting gas movement, such as temperature, pressure, and wind direction (where appropriate) and the impact of any forced ventilation must be considered.

For installations where explosive atmospheres may be encountered, certified flameproof (explosion-proof) or intrinsically safe detectors must be used. In Europe, all equipment to be used in hazardous areas must comply with the recently introduced ATEX directive, with similar country-specific standards operating elsewhere. A comprehensive discussion of the international standards governing hazardous area equipment is beyond the scope of this article. However, for obvious reasons, the importance of correct product labelling and selection cannot be overstated.

Decisions regarding detector positioning and installation should be made only by experienced personnel familiar both with the process concerned and with the gases to be detected. A British and European Standard BS EN 50073:1999 (Guide for selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or oxygen) provides further information on this topic.

Sampling systems
Accessories are available to ensure accuracy in certain conditions. For example, on ceiling-mounted detectors used for low-density gases such as methane, collector cones can be fitted to direct gas towards the sensor. In hot climates, sensors exposed to direct sunlight should be fitted with sun shades. However, sometimes it is simply not possible to take readings with a gas detector at the source, and a remote sampling system is needed. Simple single-detector units can be used to sample gas levels in ducts which are too small for direct detector installation, or in which the air speed is too fast for meaningful measurements. More advanced products are available to draw samples from inaccessible or aggressive environments – for example, air from extremely hot turbine enclosures is checked remotely for flammable gas leaks – or from multiple points, such as a network of boreholes across a landfill site.




The multi-channel digital controller Vortex provides up to 12 gas detection channels, including up to three for fire, as well as 24 user-configurable relay outputs to drive external alarms and safety equipment.


Sensor choices
A range of sensor technologies are available to cover the majority of gases likely to be encountered. The “catalytic bead or pellistor” is a widely used sensor containing two tiny wire coils, one of which has been treated to create a measurable temperature rise in the presence of most flammable gases and vapours. “Poison-resistant” versions which are immune to the desensitising effects of silicones and other compounds are now available.

Where extreme reliability is required, or where very high flammable gas levels are expected – for example in the offshore and petrochemical industries – infrared sensors are used instead. These optical devices measure the absorption of infrared (IR) energy by molecular bonds. Although unsuitable for molecules containing only one type of atom, such as hydrogen and oxygen, IR sensors provide precise measurement of methane, carbon dioxide, sulphur dioxide and other gases and vapors whose molecules contain dissimilar atoms.

To detect toxic gases and oxygen, electrochemical sensors are generally used. This broad category includes a number of sensor types, some of which operate like tiny fuel cells. Other more specialist sensors include the “sulphistor”, a patented device able to measure hydrogen sulphide at higher temperatures than its electrochemical equivalent, and thermal conductivity sensors for measuring binary mixtures of flammable and inert gases.

PID (Photo-Ionisation Detector) sensors are now becoming commonly available for detecting VOC’s (Volatile Organic Compounds) in exposure limit concentrations (i.e. parts per million). An Ultraviolet light source is used to break down chemicals to positive and negative ions that are monitored by a detector. PID sensors are highly sensitive and offer detection of hundreds of chemicals including Benzene, Toluene and Xylene in ppm levels. Although initially an expensive sensing technology, PID detectors are now becoming much more affordable and are being used in many industries.

‘Smart’ sensor technology is now widely available, and adds many advantages to traditional sensors. Smart sensors such as those based on Crowcon’s ‘I-module’ technology can be supplied pre-calibrated, saving time and expense in the field. Smart sensors also provide comprehensive temperature compensation to the sensor, can be inter-changed easily (i.e. one gas type swapped for another) allow simple range-changes, and can provide sensor lifetime data.

The complete system
To monitor and act upon the signal from a sensor, control equipment is needed. The simplest gas detector control units are compact, wall-mounted devices offering single-channel operation (that is, able to monitor a single detector). Designed for use by personnel who may not necessarily be familiar with gas monitoring equipment, they have fascias with no complex displays and only basic on/off and reset controls. Internal sounders and flashing signals alert anyone in the vicinity to a detected hazard.

More sophisticated controllers have multiple input channels to monitor several detectors. A numerical display on the control panel will allow the user to check the exact concentration of gas that each detector "sees", while extra relay connections can be used to trigger external equipment. Complete systems can incorporate sounders, strobes, fans, data loggers, modems – in fact, whatever is necessary to raise the alarm, remedy the situation and report what has happened.

Further flexibility is offered by control units with analogue outputs. Typically producing industry standard 4-20 mA signals – the same standard used by many detector heads – these will allow measured gas data to be recorded by an external data logger or, if required, sent to a remote "repeater" panel, allowing personnel elsewhere to monitor gas levels as they change. However, dedicated gas detector control units are not always needed. Where 4-20 mA gas detectors are used, they can instead be connected directly to DCS (distributed control system) or SCADA (supervisory control and data acquisition) equipment, which logs and acts upon inputs from a wide range of sensors and devices.

The most advanced control units are fully modular systems designed to monitor perhaps dozens of gas detectors. Digital outputs allow the equipment to interface with sophisticated data management hardware such as programmable logic controllers (PLC’s) or building management systems (BMS’s). Installations of this type are widely found in the offshore, petrochemical and processing industries, and may incorporate a range of additional sensors, such as smoke and heat detectors, pressure transducers and door switches to provide a comprehensive picture of site safety at all times.

Author Alan Skinner Business Development Manager Fixed Systems USA and Andy Avenell Product Manager UK for Crowcon’s range of permanently installed gas detectors and control equipment.

Crowcon Detection Instruments,
www.crowcon.com
Company contact: Alan Skinner
Tel 800 527 6926
Fax 859 957 1044
crowcon@crowconusa.com

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