Gas detectors to reduce refinery explosion risk

01 September 2008

To reduce explosion risk, refineries need to be monitored for toxic emissions that can be produced when processing gases and petrochemicals.
Gas installations can be designed to eliminate risk in areas where dangerous substances may be found, whilst others detect combustible gas leaks and prevent fires and explosions.

Both natural gas and crude oil contain sulphur compounds and nitrogen which, during processing, can lead to the emission of sulphur dioxide, hydrogen sulphide, mercaptans and nitrogen dioxide. In such applications, where a known hazard exists on a permanent basis, fixed gas detection systems can be used to provide continuous monitoring or, in the event of an emergency, to shut down all potential ignition sources. However, refineries also need regular maintenance, which can necessitate part or the whole of the plant to be shut down.

During repair or maintenance, workers may be required to enter poorly ventilated confined spaces such as tanks or pipelines. In this instance and to ensure their safety, portable gas detection equipment should be used to check for the presence of oxygen, hydrogen sulphide and carbon monoxide, as well as flammable gases, before entry.

The area to be monitored, the type of sensor, the control methods and ongoing maintenance requirements will all have a bearing on which is the right system for the particular application.

Different sensors can be used in different applications. Broadly speaking, electrochemical sensors are used to detect lower concentrations of toxic gases, whereas catalytic or infrared (IR) sensors are used to monitor combustible substances in concentrations below the LEL.

Short term tubes provide on the spot gas measurement and are suitable for monitoring personal exposure, spot check measurements, leak checks and confined space investigation. Ideal for use as part of the risk assessment process, the Draeger-Tube range enables fast, accurate measurement of over 500 different types of gases and vapours and can be used with the Draeger Chip Measurement System, a portable, multi gas detection system that requires minimal user training and which provides an immediate, true digital readout without the need for further evaluation. Short term pumps are also used in conjunction with the tubes and enable rapid measurements to be taken whilst providing optimum volume and flow specifications. Incorporating automatic stroke counters and a clear end of stroke indicator, pumps require no special tools and give accurate and reproducible results.

A wide variety of personal, single gas monitors monitor a broad range of gases including carbon monoxide, hydrogen sulphide or oxygen. The Draeger Pac 7000, incorporates a data event logger so that, as well as being assured of constant personal monitoring, the user can also record the results.

For optimal use, when selecting these type of monitors ensure that the sensors have a short reaction time and find out how often maintenance is required. The latest electrochemical sensors respond immediately to any gas hazard and can offer completely maintenance free operation for two years. The better units will combine vibrational with visual and two-tone audible alarms, which are activated as soon as the threshold levels are reached. These functions can all be easily and quickly carried out via an infra-red interface.

Portable multi-gas detectors are able to measure a number of gases simultaneously. Available with individually adjustable visual and acoustic alarms, the Draeger X-am 2000 is ergonomically designed, lightweight and compact and offers reliable measurement of combustible gases. With a low cost of ownership and dust and water-resistant to IP67, this rugged instrument remains fully functional and ready for use even after being dropped in water. The integrated rubber protection and shockproof sensors of the X-am 2000 also provide additional resistance to impact and vibration and the unit is also able to withstand electromagnetic interference.

For maximum portability units are small in size. However, they are often accidentally placed in a jacket pocket. The X-am 2000 has gas inlets on both the top and front to ensure that, even if this were the case, it will still provide a reliable warning against gas hazards. Miniaturised sensors such as the XXS generation from Draeger, gas detectors can retain their powerful functionality and still incorporate two-button control panels, a large liquid crystal display and easy menu guidance. Explosive hazards are also likely to be a problem in refinery applications. For improved safety when facing unknown hazards, some units incorporate a catalytic Ex sensor which, when calibrated to methane, responds quickly to explosive gases and gives an immediate warning to the user.

The Draeger Polytron Pulsar Open Path Gas Detector is able to detect hydrocarbon leaks within a line of sight of up to 200m. This robust unit has built-in directional guidance and can be both aligned and commissioned without the need for special training.

The SIL 2 rated Polytron Pulsar is explosion proof and ideal for the detection of the alkane series, as well as propylene, methanol, ethanol and ethylene, with a response time of less than two seconds. The integral calibration requires no manual adjustment or standard test gas and the alignment and signal strength parameters can be logged and used to determine optimum operating conditions.

A continuous signal between the receiver and transmitter allows the system to adapt to difficult environmental conditions, and high power xenon lamps, combined with a sophisticated algorithm which varies their intensity and frequency, makes the unit immune to solar radiation, stack flares, arc-welding and resonance effects from rotating machinery. Impervious to the effects of fog, mist and snow across the beam, it also features heated optics to eliminate snow and icing and prevent condensation.

Meeting ATEX, IECEx UL, CSA and GOST approvals, the Polytron Pulsar can be supplied with a variety of operating distances from 4 to 60m, 30 to 120m or 100 to 200m and is designed for use in temperatures ranging from –40 to +60°C.

To ensure maximum performance, different sensor positioning strategies can be implemented to suit different workplace environments. Such strategies are generally used to provide spot, area or perimeter monitoring.

Spot monitoring is used where the potential source of the leak is known and the sensors can be positioned to ensure that leaks are detected quickly. Area monitoring requires an increased number of sensors to cover an entire area and is generally used where the source of the leak is not known. Perimeter monitoring is used in applications where the outer limits of the installation need to be checked and where it is important that potential hazardous gases do not reach neighbouring areas.

European Standard BSEN 50073:2000 lists a number of factors that should be taken into account when determining suitable locations. These include the location, i.e. indoor or outdoor site, potential sources such as the location and nature of the potential vapour/gas sources as well as the chemical and physical data of the potential gases/vapours present.

Other factors include leak control, the nature and concentrations of possible gas releases, the presence of cavities and jets and the general topography of the site. Air movements should also be taken into consideration as well as temperature effects, the local environment of the plant, the location and number of personnel in the plant and the location of potential sources of ignition. Any structural arrangements such as walls, troughs or partitions, which could allow gas to accumulate, should also be considered.
The Standard also states that the placement of the sensors and sampling points should be determined following the advice of experts. It also advises that the agreement reached on the locations of sensors and sampling points should be recorded.

Used extensively in the North Sea, the Draeger Polytron can also be found in the Middle East, North and South America, Asia, West Africa and the former Soviet Union. Amongst other keynote users, Shell UK Oil Products has recently entered into a 3-year agreement with Draeger for the supply of fixed gas monitors for use at the Stanlow Manufacturing Complex.