Press | Published on 2nd Jan 2017
Gas entering the UK transmission and distribution grid has to meet certain specifications under Gas Safety Management Regulations (GSMR). Under these regulations, a variety of different parameters and limits are used to describe what may be generally referred to as “gas quality”. These GSMR limits have been established over many years in order to ensure that gas combustion in all types of domestic appliance, and in commercial and industrial applications, is safe and efficient.
The Wobbe Index is generally regarded as the key combustion parameter for fuel gases. For a given gaseous fuel, it can be defined as the gross calorific value over the square root of the relative density. Its primary use is to serve as an indicator of the interchangeability of different gases. For standardised injector and supply-pressure conditions, gases of the same Wobbe Index (or Wobbe number) should provide the same thermal output when fully combusted, generally expressed in MJ/m3.
The Oban trial
A trial in Oban – a resort town on the South West coast of Scotland, which has its own isolated gas network – was undertaken by Scotia Gas Networks (SGN) to investigate the supply of gas outside the current UK specification legislated for by GSMR. Funded by the Network Innovation Competition, it sought to provide evidence to support a cost-effective new gas supply solution for the Scottish Independent Undertakings (SIU’s) – isolated local distribution networks – following the closure of the LNG storage facility at Avonmouth. It was also foreseen as providing a potential roadmap for the UK adoption of a widened gas-quality (Wobbe Index) specification, subject to the project findings.
Oban has a number of special qualities which make it an ideal choice for this ground-breaking work.
Requiring a specific exemption from GSMR to be granted by the HSE, the project involved a study of approximately 1100 properties and 2500 appliances fed by the SIU, including identifying and replacing any appliances unable to cope with varying gas quality. This initially involved the use of “test gases”, defined in EN 437, and providing the accepted threshold upper and lower Wobbe Index limits for H-gases in the EU, for short-term safe appliance operation. Then, LNG supplies were sourced which exceeded the upper Wobbe Index permitted by GSMR, before regasification and injection into the SIU, to be combusted by connected appliances. Spot-checks were carried out over the project duration to check appliance combustion was acceptable to industry norms, and that there were no other issues apparent.
Clearly, safe appliance operation is something HHIC take very seriously, and we engaged with those leading the project throughout. In particular, we were able to liaise between appliance manufacturers and the technical research team to help ensure specific queries were examined fully.
The final report has now been published it has 14 “main conclusions”. One of which is that: Domestic and small commercial appliances correctly installed, serviced and operated can safely burn gas with WI up to 54.76 MJ/m3, based on appliance operation on G21 test gas. This figure is below the project’s proposed upper limit value for WI of 53.25 MJ/m3, but exceeds the current upper UK limit values for WI of 51.41 MJ/m3 in “normal” conditions and 52.85 MJ/m3 in emergency supply conditions.
HHIC will be taking the time to dissect the information provided in the report in detail, and will of course be working with its members to fully review and digest the project findings and recommendations.
5th Jun 2017
1st Jun 2017
17th May 2017
15th May 2017
2nd May 2017
1st May 2017
10th Apr 2017
3rd Apr 2017
29th Mar 2017