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Gas Detection - Latest developments in portable and fixed gas detection

Published: 10th Nov 2010

ARTICLE CONTINUES BELOW

The purpose of this article is to inform readers of a number of the latest developments in gas detection across a range of different disciplines.

New Photo Ionisation Technology for monitoring VOCs

Entry into confined spaces or areas where explosive or toxic gases may be present demands that a gas monitor is worn. Many areas present the same predictable issues regarding those gases that need to be measured, usually explosive gas including LEL (Lower Explosive Limits - Methane, Ethane etc), Oxygen depletion, Carbon Monoxide and Hydrogen Sulphide.

However, frequently solvents or Volatile Organic Compounds (VOCs) are present and a mixture of standard sensor technology coupled with PID technology can accommodate a much wider range of gas detection, and produce higher levels of safety.

The latest innovation in measuring VOCs is the development of miniature PID sensors. A Photo Ionisation Detector in the shell of a Series 4 / ‘A’ Class gas sensor enabling PID technology to be deployed across a wide range of gas detection technology, both portable and fixed. Incorporating the patented Fence Electrode Technology to increase resistance to humidity and contamination, this sensor is now increasingly being used in portable confined space gas detectors.

Mini PID technology also offers a more accurate and wider detection range when incorporated into standard PID instruments. Many users need a very sensitive instrument with parts per billion measurement. Others want to identify high levels of VOCs, sometimes up to 20,000 ppm (parts per million) or 2% volume. The Mini PID’s increased range means that it is now possible to consolidate all this in one instrument to meet more demanding total VOC monitoring requirements.

Ground gas monitoring innovations

The traditional methodology for ground gas monitoring has been ‘spot monitoring’ - taking event readings at various set times to predict the sub surface gas regime  on both Brownfield and Landfill sites.

This methodology is in general current practice under both the EU Landfill Directive 1999/31/EC, which is supported by guidance publications from the Environment Agency, and the procedures for Environmental Compliance Gas Monitoring on Land Development Sites, which have come about by a different route - that is, via the publication of the DEFRA / Environment Agency Contamination Land Report 11 (CLR 11) - ‘Model Procedures for the management of Land Contamination’; a joint NHBC/RSK document entitled ‘Guidance on evaluation of development proposals on sites where methane and carbon dioxide are present’; and the CIRIA Report C665 ‘Assessing risks posed by hazardous ground gases to buildings’, which have culminated in the development of BS8485 ‘Code of Practice for the characterisation of ground gas in Brownfield development’.

All this guidance was based on the technology available at the time. Now, with the introduction of a new generation of continuous monitoring instruments, a key breach in the current approach has been filled allowing for the monitoring of gas concentrations over time in both the pre and post development of these sites.

These instruments can be installed in an existing or new borehole on a site and left to continuously monitor gas levels over several months, and can monitor Methane (CH4), Carbon Dioxide (CO2), Oxygen (O2), Borehole Pressure (Barometric) and Atmospheric Pressure for use on conventional risk sites such as ex-landfills.

However, there are also options to include Hydrogen Sulphide (H2S), Carbon Monoxide (CO), Volatile Organic Compounds (VOCs), using Photo Ionisation Detector (PID) technology and even a water level transducer to measure water table movement. This allows the user to monitor all of these parameters in one unit and most importantly at regular intervals, usually once an hour, over a specific monitoring period.

Using time series data it is possible to more accurately determine the true subsurface gas regime and therefore predict with a much higher level of accuracy how this may change in the future. Using existing methodology it has always been difficult to accurately gauge the risk of sites and has in the past lead to developments occurring on sites which still posed some risk.

One local authority in southern England is using this type of instrumentation on a housing development built in the mid 1990s over a former landfill site.

A continual monitoring programme had been in place using spot sampling technology and a level of gas had been monitored on one borehole, suggesting there were some residual gas issues on the site, and that local properties could be at risk.

In a joint approach between the Environment Agency, the local authority and a specialist consultancy it was decided this site needed some further investigation as a matter of urgency. Key monitoring points were identified and the instruments were deployed over a six month period, with data retrieved at regular intervals. The quality of the time series data provided enabled the local authority to make key decisions about the site and also revealed that, in fact, gas levels were at safe, sustainable levels which would not pose any risk to residents.

This technology is also being used by developers before construction, allowing them to create extremely high quality plans and predict with accuracy the point at which existing Brownfield sites can be developed. The existing methodology requires up to 24 months of monitoring using spot sampling, on multiple occasions in a month depending on the risk level of the receptor and the site. However, time series data has been used to reduce this required monitoring period to as little as one month.

As the instrumentation is continuously in position and monitoring at regular intervals, it is possible to establish with little doubt if there is any risk. With the conventional spot monitoring approach it is possible to completely miss gas fluxes, so time series data becomes invaluable as it is removing the lacking variable in the current approach - time.

Using this technique it is also possible to characterise boreholes. This is an approach which had been suggested in older guidance, but due to the lack of workable technology became less used in the industry.

Purging a borehole using Nitrogen and then fitting the instrumentation on a high resolution time history (e.g. as little as every three minutes), it is possible to chart the recovery of the borehole (the time in which the borehole refills with gas).

This allows developers and consultants to establish if gas levels previously found on site were due to a site truly gassing, or due to a build-up of pressure over time due to water table/pressure movements and no venting on the borehole. This means a developer can easily chart if gas levels present on site could be dealt with using the floor void beneath properties.

In other applications the technology is being used internationally in both indoor air quality applications (monitoring gas levels in the floor void in properties) and also near petrol stations using the Photo Ionisation Detector (PID) to monitor potential leaks in both petrol and diesel storage tanks. Such leaks can cost millions to filling stations, as well as the effect on the environment, so this type of monitoring can be invaluable.

This new technology does not exclude all ‘spot monitoring’ requirements and can be used in conjunction with existing instrumentation when measuring things like flow rates. Latest hand held ground gas portable measuring instruments now incorporate additional features which meet the latest, most stringent, MCERTs equipment standard. Latest requirements demand cross sensitivity tests to Hydrogen Sulphide, as well as greater linearity, greater repeatability and less temperature variation.

It is also known that the presence of Hydrocarbons is potentially going to give erroneous or over range Methane readings, so the latest instrumentation now incorporates a Hexane measurement channel. If the Methane level is measured as open range, or abnormally high, the Hexane indicator can confirm Hydrocarbon interference. This reading is a simple crude indicator and is also useful for checking if any remedial work undertaken has been successful.

A separate instrument is usually used to measure VOCs, (Volatile Organic Compounds), normally a PID (Photo Ionisation Detector). Small quantities of Methane are not detected by a PID but because methane attenuates or ‘soaks up’ a VOC signal in larger quantities (typically over 10% Methane) found in boreholes, it is possible to measure grossly erroneous readings on your PID.

To help counter this, the latest portable ground gas instrumentation have a PID compensation value which can be used to accurately correct your PID reading when up to 10% methane is present in a?borehole.

‘Plug and play’ fixed gas detection

Traditionally, fixed gas detection systems offer a single gas sensor which can be deployed in multiples over a specified area, and are hard wired to send a signal back to a control panel/display system - which in turn can control an array of audible or visual warning devices, as well as communicating with a building management system to alert users to a specific problem.

These tend to be in larger installations and when a smaller user wants just one or two points of fixed gas detection, this can suddenly become a costly process, with initial installation and commissioning costs, and ongoing on-site calibration and maintenance.

New developments in fixed gas detection technology have now enabled a more low cost, but equally robust solution for the smaller user wishing to install a fixed gas detector. An IP66 rated stand alone, plug in detector which incorporates its own audible/visual alarms and a built in display with event logging capabilities, is now available for single gas monitoring. Just plug into a power source and go.

This can significantly reduce the cost of ownership of fixed gas detection for both the smaller user and in instances where gas detection needs to be semi-portable, perhaps being deployed in different areas for a number of months at a time. The unit can even be unplugged and returned for calibration, giving further cost savings.

A cost effective way to swap out flammable pellistor sensors for infrared

In another fixed gas detection breakthrough, a new infrared sensor has been specifically designed to enable pellistor based gas detection systems to be upgraded to infrared technology without changing the original control system, junction boxes or cables. These new sensors produce a mV Wheatstone Bridge output, as used on conventional pellistor based systems, and can replace old pellistor heads by simply mounting on the original junction box, and connecting to the original cable.

This concept enables an upgrade to dual-wavelength IR gas detector technology without incurring the very significant costs associated with replacing the control system and re-installation.

Pellistors (or Catalytic Beads) have been the flammable gas sensor of choice for oil and gas applications and general industry. They do, however, have several technical limitations:

• They do not fail safe. Pellistors are easily poisoned by substances such as silicones, sulphur and lead, rendering them insensitive to gas • Pellistors must be operated behind a sinter (flame arrestor) which may become blocked, thus preventing gas from reaching the sensor • Pellistors are high maintenance. The sensors must be regularly tested with gas to ensure they are operational. Pellistors typically last 3-5 years only • Pellistors may burn-out if exposed to flammable gas concentrations above 100% LEL (Lower Explosive Limit) • Pellistors need oxygen to be present to work. Their efficiency reduces in oxygen deficient environments

This new technology overcomes all of these issues, and delivers highly dependable and rapid gas detection with no un-revealed failures.

More accurate flue gas analysis

The latest designs of hand held flue gas analysers incorporate hydrogen-compensated CO sensors to comply with the more stringent requirements of the full European Directive EN 50379-Parts 1, 2 and 3 on emissions from condensing boilers, and can have infrared sensors fitted for the direct measurement of carbon dioxide, rather than this being derived by calculation.

With the strict health and safety guidelines the use of Hydrogen compensated sensor technology with condensing boilers can be critical to ensure your boiler is both energy efficient and safe. A ‘hydrogen compensated’ CO sensor provides more accurate CO readings in flue gas samples. Most of the popular analysers on the market have non compensated CO sensors, which are cross sensitive to hydrogen. The amount of hydrogen produced in the combustion process is dependent on flame conditions and is extremely variable, particularly on commercial burners where higher flame temperatures may result in  hydrogen production.

When accuracy matters - and it always does - these hand-held combustion analysers are the market leaders. With accuracy levels of +/- 0.2% for CO2 /O2 and +/-5ppm CO, they are the instruments of choice for all Health and Safety focused countries.

Direct measurement of O2, CO/H2, flue gas temperature, combustion air temperature, draught and pressure with calculated values for CO undiluted, Lambda, CO2, efficiency (Eta), temperature difference, pressure difference Eta-BW, flue gas losses (qA) and dew point. With the added value of having optional Bluetooth, micro SD memory card and IR printer, this type of instrument will cover any eventuality.

Author Details:

The author, Julian Butler, is National Sales Manager of Shawcity Limited, a multi disciplined gas detection company based in Faringdon, Oxfordshire. Julian has been in the Gas Detection industry for the last seven years after many years in the electronics industry, and now specialises in PID Technology and Ground Gas monitoring.

As well as their large range of Gas Safety and Environmental Monitoring equipment, Shawcity also provide equipment to the Occupational Hygiene industry including, Noise, Vibration, Dust, Air Quality and Heat Stress instrumentation.

Shawcity is a member of CoGDEM, the Council of Gas Detection and Environmental Monitoring, a UK-based trade association which supports the gas detection industry.

www.shawcity.co.uk

www.cogdem.org.uk www.osedirectory.com/health-and-safety.php

Published: 10th Nov 2010 in Health and Safety International

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