Andy Avenell

Product Manager – Fixed

Systems

Crowcon Detection

Instruments Ltd.

andy.avenell@crowcon.com

The job of any gas detector is to sense gas in concentrations low enough to provide an alarm before a hazardous concentration has accumulated. To this end, flammable gas detectors are scaled in the ‘% LEL’ range (100 % LEL equating to the concentration at which the gas may become ignitable), toxic gas sensors are scaled in the ‘ppm’ range (parts per million), and oxygen sensors measure from 0–25 % volume (with 20.9 % representing the normal oxygen concentration in air).

Explosive Risk

In order for gas to ignite there must be an ignition source, typically a spark (or flame or hot surface) and oxygen. For ignition to take place, the concentration of gas or vapour in air must be at a level such that the ‘fuel’ and oxygen can react chemically. The power of the explosion depends on the ‘fuel’ and its concentration in the atmosphere. The relationship between fuel/air/ignition is illustrated in the ‘fire triangle’.

The ‘fire tetrahedron’ concept has been introduced more recently to illustrate the risk of fires being sustained due to chemical reaction. With most types of fire the original fire triangle model works well – removing one element of the triangle (fuel, oxygen or ignition source) will prevent a fire occurring. However, when the fire involves burning metals like lithium or magnesium, using water to extinguish the fire could result in it getting hotter or even exploding. This is because such metals can react with water in an exothermic reaction to produce flammable hydrogen gas.

Not all concentrations of flammable gas or vapour in air will burn or explode. The Lower Explosive Limit (LEL) is the lowest concentration of ‘fuel’ in air that will burn and for most flammable gases this is less than 5 % by volume. So there is a high risk of explosion even when relatively small concentrations of gas or vapour escape into the atmosphere.

There may be as many as 700 gas detectors on a single offshore platform.

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Example of Lower Explosive Limits (LEL) and Upper Explosive Limits (UEL) of methane and hydrogen.

Removing one element of the triangle will prevent a fire occurring.

LEL levels are defined in standards: ISO 10156, and IEC 60079. The ‘original’ ISO standard lists LELs obtained when the gas is in a static state. LELs listed in the EN and IEC standards were obtained with a stirred gas mixture; in some cases this resulted in lower LELs (i.e. some gases proved to be more volatile when in motion).

Toxic Risk

Gases and vapours produced by offshore activities can, under many circumstances, have harmful effects on workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Many toxic substances are dangerous to health in concentrations as little as 1 ppm (parts per million). Given that 10,000 ppm is equivalent to 1 % volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health.

Gaseous toxic substances are especially dangerous because they are often invisible and/or odourless. Their physical behaviour is not always predictable: ambient temperature, pressure and ventilation patterns significantly influence the behaviour of a gas leak. Hydrogen sulphide for example is particularly hazardous; although it has a very distinctive ‘bad egg’ odour at concentrations above 0.1 ppm, exposure to concentrations of 50 ppm or higher will lead to paralysis of the olfactory glands rendering the sense of smell inactive. This in turn may result in the assumption that the danger has cleared. Prolonged exposure to concentrations above 50 ppm will result in paralysis and death.

Definitions for maximum exposure concentrations of toxic gases vary according to country. Limits are generally timeweighted as exposure effects are cumulative: the limits stipulate the maximum exposure during a normal working day.

Oxygen – Too High or Too Low?

The normal concentration of oxygen in fresh air is 20.9 %. An atmosphere is hazardous if the concentration of oxygen drops below 19.5 % or goes above 23.5 %. If the concentration falls to 17 %, mental and physical agility are noticeably impaired; death comes very quickly if it drops only a few percent more. At these levels unconsciousness takes hold so rapidly that the victim will be unaware of what is happening.

Without adequate ventilation, the simple act of breathing in a confined space will cause the oxygen level to fall surprisingly quickly. Combustion also uses up oxygen, which means that engine-driven plant and naked flames such as welding torches are potential hazards. Steel vessels and chambers that have been closed for some time are similarly dangerous because corrosion may have occurred, using up vital oxygen in the process.

Oxygen can also be displaced. Nitrogen, for example, when used to purge hydrocarbon storage vessels prior to re-use, drives oxygen out of the container and leaves it highly dangerous until thoroughly ventilated.

High oxygen levels are also dangerous. As with too little, too much will impair the victim’s ability to think clearly and act sensibly. Moreover, oxygen- enriched atmospheres represent a severe fire hazard. From clothing to grease, materials will burn much more vigorously under these conditions. Common causes of oxygen enrichment include leaks from welding cylinders and even from breathing apparatus.

Communication Technology on Offshore Rigs

On a rig, gas detectors are connected to a centralised control system, which is responsible for indicating the current gas level and triggering alarms when pre-defined gas thresholds are exceeded. Gas detectors will typically be located within the hazardous area, with the control system mounted in the ‘safe area’ potentially hundreds of metres from the detectors.

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Gas detectors are essential to provide dependable early warning of gas hazards.

Detectors are typically connected to the control system via cables, using ‘point-topoint’ topologies where each detector connects to a discreet input on the control system via an independent cable (or separate cores within a multi-core cable from a ‘marshalling cabinet’).

This conventional ‘point-to-point’ method of operation has been the preferred technique for decades for signal reliability and system security reasons. With a dedicated cable and controller input for each detector, a single failure will affect that specific detector only: the rest of the system can remain operational.

Point-to-point systems typically utilise analogue signals from the detector to indicate the gas level. Analogue signals however can only communicate a limited amount of information from the detector, typically: gas value (4–20 mA), fault (<3 mA), gas reading over-scale (>21.5 mA).

The emergence of digital and communications technologies has enabled a far greater range of information to be communicated to a control system, as well as providing opportunities to reduce the amount of cables needed to connect detectors.

The HART Communications Protocol (www.hartcomm.org) complements conventional 4–20 mA systems by super-imposing additional diagnostic information onto the 4–20 mA signal. This data can be read using a HART enabled hand-held device or Asset Management System (AMS) to diagnose faults and manage system calibration and maintenance.

Whilst the safety function of the detector is still performed by the analogue 4–20 mA signal and conventional controller, the HART data enables access to additional information such as device temperature, serial number, calibration, last calibrated date, fault status, supply voltage and signal current.

Developed by Modicon Inc., the Modbus protocol has been in existence since the early-1990s and is an address-based protocol whereby each ‘node’ or gas detector in this case is communicated using a unique address. Information such as gas level, alarm and fault status is stored in registers within the detector, and the ‘Modbus Master’ control system routinely addresses individual detector ‘nodes’ to retrieve data. A full guide to Modbus can be downloaded from www. modbus.org/docs/PI_MBUS_300.pdf.

Like the HART Communications Protocol, Modbus can be used in conjunction with the analogue signal to provide additional detector information, or can be used as the primary means of communication with a control system such as a PLC (Programmable Logic Controller) or SCADA (Supervisory Control and Data Acquisition) system.

Foundation Fieldbus (FF) is a well-established solution widely used for process instruments, but as ever the safety industry is slower to change from established and trusted systems and practices. FF provides the opportunity to use alternative cabling interfaces and data can be transferred via conventional copper cables or fibre-optic cables. Data can also be transferred via Intrinsically Safe (I.S.) interfaces. For more information visit www.fieldbus.org .

All of the communications technologies listed above enable detectors to be installed in an addressable cable configuration rather than the conventional point-to-point topology. Thus cable installation costs can be significantly reduced by connecting multiple detectors onto a single cable where the cable loops from one detector to the next. The cost of control equipment can also be significantly reduced as a single controller can communicate with a fleet of detectors (addressing each detector ‘node’ individually on a sequential basis).

Wireless communications are a very attractive proposition where running additional cables to new or additional detectors is impractical. Detectors may be powered locally (via cables, batteries or solar panels), and transmit the gas levels and status information to a control system via a radio signal. Although wireless products are available on the market, a global ‘standard’ for the protocol and frequency deployed has yet to be established, and therefore suitability will depend on local regulations in the region in which the device is to be used.

Installing wireless devices also requires very careful consideration to the characteristics of each detector location to ensure guaranteed signal integrity and security. In practice, at present wireless devices are viable mainly where they are needed for temporary area monitoring or where gas detection is needed in a location where installing a conventional cabled detector is impractical.

The majority of gas detection systems on offshore installations continue to use conventional 4–20 mA point-to-point systems. For established platforms where safety systems are already installed there are no installation savings to be made by using addressable systems as the assets are already in-place. There is however an option to replace older gas detectors with HART enabled devices to realise the benefits of access to additional diagnostic data for asset management purposes.

For new installations, communications technologies such as Foundation Fieldbus may provide a better overall solution to conventional analogue systems, but as always the individual safety case assessment is the critical determining factor.

Most gas detectors should be calibrated every six months to ensure optimum operation. However, a new range of IR (infrared) detectors allow users to extend maintenance checks to once every 12 months – and this only requires a ‘gas test’, not full re-calibration, which is more time consuming. ‘Bump-test’ stations and intelligent instrument management hubs also enable simple day-to-day testing of portable gas detectors and easy management of maintenance cycles.