Gas detection – A potentially explosive subject

Author : David Fenton, head of marketing, Draeger Safety UK and Steve Robinson, business development manager

09 April 2010

Health and safety is a serious concern throughout the petrochemicals industry and both portable and fixed gas detection systems play an important role in keeping plant and personnel safe. Used to eliminate risks in areas where toxic substances might be found, gas detection systems are also able to detect combustible gas leaks to prevent fire and explosions as well as environmental pollution.

Different systems will be required for different hazards and different applications. An oil refinery, for instance, is an organised and co-ordinated collection of manufacturing processes designed to produce physical and chemical changes in crude oil and convert it into everyday products such as petrol, diesel, lubricating oil, fuel oil and bitumen. Virtually every process results in the creation of dangerous gases.

Explosive hazards will need to be monitored around non-hermetic compressors and pumps and at LPG storage and filling sites. Toxic and explosive hazards might be found around flanges, whereas hydrogen sulphide could be a problem in desulphurisation plants, and chlorine could pose a threat in water treatment areas. Dangerous gases can also occur in "odd" areas where leaks have been channelled by wind direction or by gravity where substances such as LPG, which moves likes water, find their way into trenches.

Toxic gases obviously need to be detected before they reach sensitive locations such as nearby living quarters or roads, the air intake of a control room, the power plant, and hot spots such as hydrocrackers and ovens.

Petrochemical plants and refineries also need regular maintenance and may require part or the whole of the plant to be shut down. During repair or maintenance, for example, workers may be required to enter poorly ventilated confined spaces such as tanks or pipelines. In this case 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 purpose of the monitoring will help to define which type of equipment is the most appropriate. Is it purely a safety measure for an individual or are there other, more wide-reaching requirements? What alarm thresholds are appropriate and how will the alarm information be processed? When working with potentially explosive gases, for instance, a gas detection system can be designed to trigger a countermeasure as soon as a predetermined gas concentration level has been reached. If this measure proves to be inadequate and the gas concentration continues to rise, then a second alarm threshold will be crossed and all potential ignition sources can be automatically switched off.

A workplace assessment will show where, how often, and in what concentrations and quantities the hazards are likely to be released. This, in turn, can be used to determine the most appropriate type of gas detection equipment.

Portable gas detection
The correct selection of portable equipment will be determined by a host of factors such as whether single or multiple gas detection is required, which type of sensor is most appropriate, if it is to be used single-handedly or worn around the neck and, of course, the speed of the reaction time.

In terms of investment, other issues might include the availability of a total care contract where the equipment is owned and cared for under a lifetime product guarantee including calibration, servicing and spare parts costs. It is also possible to acquire product without actual ownership (i.e. without capital expenditure) for either short or long term usage, again with the benefits of an ongoing service and support contract to include calibration, service, repair and access to specialist personnel for advice and even on site support during shut down periods.

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, for example, 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 (hand bellows) 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.

Personal, Single Gas Monitors can be used to monitor a broad range of gases including carbon monoxide, hydrogen sulphide or oxygen. Some of these, such as the Draeger Pac 7000, for instance, also incorporate a data event logger so that, as well as being assured of constant personal monitoring, the user can also record the results.

When selecting these types 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. Also, check which alarms are incorporated. The better units will combine vibrational with visual and two-tone audible alarms, which are activated as soon as the threshold levels are reached. Finally, are they easy to use? Exactly how easy is configuration, calibration and downloading of the data event logger? These functions can all be easily and quickly carried out via an infra-red interface. If records need to be kept, this is an important feature.

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 a prime example in that it is ergonomically designed, lightweight and compact and offers reliable measurement of combustible gases and vapours as well as oxygen, carbon monoxide and hydrogen sulphide. 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. Integrated rubber protection and shockproof sensors also provide additional resistance to impact and vibration and the unit is also able to withstand electromagnetic interference.

For maximum portability these types of unit should be small in size and easy to carry. As a result, however, they are often accidentally placed in a jacket pocket – but do they still work when used like this? Units such as the X-am 2000 have 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. Also, does the compact design mean that the performance is affected? Some units can be as small as a mobile phone. By using the latest, 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.

Fixed systems
Installed on a permanent basis and used in the toughest environmental conditions, fixed gas detection systems are available in a wide variety of forms. 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.

For optimum safety, sensors and sampling points should be positioned so that gas accumulations are detected before they create a significant hazard. To ensure maximum performance, different sensor positioning strategies can be implemented to suit different workplace environments. Whilst these can, of course, be combined or modified, they 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.

For instance, a number of long beam (open path) detectors such as the Draeger Polytron Pulsar, spaced at five metre intervals and at least two different heights, can provide an effective gas detection system on offshore installations. They can be combined with infrared point detectors located close to the most likely release points and/or in areas highly congested with equipment to quickly and reliably detect gas leaks.

Advanced sensor technology
Fast response, stable signals, good measurement performance, low drift, long extended lifetime, extended temperature ranges, poison resistance and wide measuring ranges form just part of the sensor selection criteria.

Where the detection of toxic gases or oxygen depletion are concerned, traditional small and mass produced electrochemical sensors have been eclipsed by larger, intelligent sensors. These types of sensor are able to communicate with the transmitter and feature an EPROM memory which stores calibration information as well as gas data, such as target gas and range. Incorporating temperature and pressure compensation, this built-in intelligence not only leads to higher accuracy but it also extends calibration intervals to just once a year. In addition, they offer improved longevity which, in turn, means that fewer replacement sensors are required. As a result, the lifetime costs of the system are significantly reduced. Available with a variety of mounting capabilities, they also offer better flexibility in terms of selectable response times and ppb (parts per billion) test rates.

These sensors are also easy to use. As the gas data is stored within the sensor itself and the sensors arrive pre-calibrated from the manufacturer, they no longer have to be calibrated in the transmitter. Furthermore, the plug-in construction offers true "plug-and-play" functionality and, once installed in the transmitter, the EPROM will communicate with the transmitter without further intervention or calibration by the operator.

Better manufacturing techniques and advances in technology have also improved the detection of flammable gases and vapours with traditional pellistor or catalytic bead detectors being replaced by superior technologies such as infra red (IR). Even in the harshest of conditions, the DraegerSensor IR, for example, is easy to install – simply unscrew the pellistor and screw in the IR replacement. The inclusion of Wheatstone Bridge emulation, which is able to duplicate a standard pellistor output, also means that existing cables and control devices from most manufacturers can still be used.

IR technology is also more robust and stable and is immune to poisons such as H₂S, for example – a very important benefit in petrochem applications. As a further benefit, the increased accuracy of IR is able to reduce maintenance and calibration intervals. Instead of the need for six-monthly or, in some cases, the more frequent calibration intervals required by catalytic devices, IR systems can extend calibration intervals to one year. Together with an overall life expectancy of more than 15 years, this technology dramatically reduces lifetime ownership costs.

The highly sensitive Draeger Polytron IR Ex can reliably monitor a wide range of flammable gases as well as vapours such as alcohols, ethers, cyclo and glycol-ethers, alkanes and alkyl compounds as well as ketones, esters, fuel hydrocarbons, alkyl benzenes (xylene and toluene) and amines. It can also be set to issue alarms at different stages. By way of example, the first alarm threshold (e.g. 5 or 10% LEL and requiring no operator acknowledgement) could be used to switch ventilation fans into high speed mode. The second alarm threshold (at 30 or 40% LEL) could be used to switch off power to the storage facility and to warn the operator.