SPC/Enforcement/88
This SPC provides further guidance on the reporting and investigation of certain gas related incidents, and interpretation of the Gas Safety (Management) Regulations 1996 (GSMR) and the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 1995 (RIDDOR).
It supplements and should be read in conjunction with SPC/Admin/40, which gives guidance on the procedures for reporting and processing of gas incidents under RIDDOR and GSMR, and SPC/Enforcement/89, which gives guidance on the content and format of GSMR investigation reports submitted by gas conveyors.
1 Gas safety incidents have a high public profile and it is important that they are reported and investigated properly. This guidance has been produced to assist inspectors and the gas industry in determining whether particular incidents involving gas escapes from a network meet the relevant RIDDOR and GSMR requirements for reporting, notification, investigation and submission of investigation reports to HSE. Gas escapes from gas fittings in domestic premises which result in a fire or explosion automatically require investigation by the gas conveyor in accordance with Regulation 7(12) of GSMR and also require notification under Regulation 6(1) of RIDDOR if they result in a death or major injury.
2 Gas conveyors have a duty under Regulation 7(13) of GSMR to investigate certain gas escapes that occur on their networks, i.e. those escapes that have, or are likely to have, resulted in a fire or explosion. The investigation should be carried out to establish the source of the escape and, so far as is reasonably practicable, the reason for it.
3 HSE's guidance to GSMR (L80) makes it clear that small escapes from weeping joints do not require investigation, but that investigation is required where a gas escape is of a sufficient scale to have made a fire or explosion likely. The guidance indicates that a gas escape of sufficient scale is one where there is an accumulation of a 'sufficiently large gas/air mixture above the lower explosive limit (LEL)'.
4 Although the Regulations and supporting guidance do not specify what constitutes a 'sufficiently large' gas/air mixture, conveyors should investigate gas escapes where the quantity of flammable mixture would, if ignited, present a risk to safety (whether or not harm actually occurs). This will depend on the circumstances of the leak and should be judged on a case-by-case basis.
5 Regulation 7(13) of GSMR and the relevant 'Dangerous Occurrence' criteria of RIDDOR Schedule 2, Part 1 provide a useful reference point for determining whether the amount of escaping gas constitutes a risk to safety. For example, a gas escape of 10 kg or more from a main or service into a building (paragraph 20(1)(a)(iii) of RIDDOR Schedule 2, Part 1) should be regarded as meeting the 'sufficiently large' test and so be within the scope of GSMR Regulation 7(13). However there might be rare instances where escapes of less than 10 kg (and where the LEL is exceeded) present a risk to safety.
6 In addition, the guidance to GSMR states 'an investigation would be expected of leaks where gas transporters, or their emergency service providers, judge it prudent in the interests of safety to evacuate the property'. The Distribution Network Operators’ criteria for evacuation of buildings include the measurement of gas at 20% of the LEL, on the basis that there may be pockets of gas elsewhere within the flammable range. They now use this as a benchmark for determining the requirement for a GSMR investigation, HSE considers this to be acceptable.
7 For gas escapes in the open air, paragraph 20(b) of RIDDOR Schedule 2, Part 1 (Escape of Flammable Substances) is relevant. In this case, a rapid gas release of 500kg or more (e.g. due to failure of a fusion joint, fracture of a cast iron main or interference damage) resulting in a flammable gas/air mixture would also be within the scope of Regulation 7(13) of GSMR.
8 The criteria that should be considered and which require a degree of judgment include the amount of gas and the (short) time period over which the escape occurs before the threshold quantity is released. In all cases, potential ignition of the flammable mixture should be assumed.
9 Appendix 1 to this SPC gives examples of the amount of gas likely to be released from damaged mains and service pipework of various diameters operating over a range of pressures. This may be of use in determining whether a gas escape is RIDDOR reportable and meets the above criteria for investigation under Regulation 7(13) of GSMR.
10 For domestic and network gas escapes causing or likely to cause fire and explosion, Regulation 7(16) of GSMR requires conveyors to notify HSE of their intent to carry out an investigation before the investigation begins. The purpose of the notification is to ensure that evidence is not disturbed until HSE has been consulted and has decided either to carry out its own investigation or to allow the conveyor to investigate. Notifications should be made by telephone to the relevant HID SI3 Gas and Pipelines team in Sheffield, Norwich or Aberdeen.
11 The conveyor may have to investigate the source of the escape in order to make the situation safe. In doing so the conveyor should avoid, so far as is practicable, disturbing potential evidence relating to the cause of the escape. Once the situation has been made safe, further investigation should not begin until HSE has been notified and agreement has been reached with an inspector.
12 Where deaths, major injuries and/or severe damage to property are involved, HSE should be notified forthwith, even out of hours (via the duty officer system). In other cases, providing there is no need to take urgent action, the notification can be made on the next working day. HSE's Field Operations Directorate (FOD) will routinely be the first responders to incidents and that reporting should therefore be via the standard RIDDOR reporting arrangements. Where there are unusual circumstances or if a major incident has taken place, gas conveyors can consult the relevant HID SI3 Gas and Pipelines team for advice.
1 The rate of gas release from damaged pipework is dependant on many factors, including the gas pressure, dimensions of the pipework, characteristics of the hole/crack, length of service to damage and characteristics of the material surrounding the main/service.
2 As a guide to the amount of gas that maybe released from damaged pipework, the following tables have been derived from calculators included within British Gas Research & Technology Report No ERS R5503, June 1995. Table 1 shows the flow rate of gas that could be expected from a damaged pipe where the pipe diameter is more than twice the diameter of the hole, and Tables 2 and 3 indicate the times taken under various conditions to release sufficient gas to require a formal report to be made. If the time of leakage is known, an estimate of the amount of gas likely to have been released can be made.
3 Previously, estimates of the time to release specific amounts of gas from open-ended pipe systems have been given. However, under any particular circumstances the amount of gas released is dependant on the system, relative main/service dimensions, length of pipe from main to open end, pressure, pipe material, cover etc. The tables below give an indication of the maximum amount of gas that would be released from an open end by assuming the hole diameter is the diameter of the pipe.
| Hole Dia." | Hole Area | LP 10" wg | LP 15" wg | MP 5psi | MP 20 psi | |
|---|---|---|---|---|---|---|
0.5 |
0.2 |
21 |
25 |
87 |
207 |
m3/hr |
1 |
0.8 |
89 |
110 |
378 |
871 |
|
2 |
3 |
356 |
442 |
1,512 |
3,485 |
|
4 |
12 |
1.42 |
1,766 |
6,049 |
13,940 |
|
10 |
75 |
8.900 |
11,040 |
37,800 |
87,130 |
|
| Hole Dia." | Hole Area | LP 10" wg | LP 15" wg | MP 5psi | MP 20 psi | |
|---|---|---|---|---|---|---|
0.5 |
0.2 |
2,151 |
1,736 |
506 |
220 |
mins |
1 |
0.8 |
496 |
401 |
117 |
51 |
|
2 |
3 |
124 |
100 |
29 |
13 |
|
4 |
12 |
31 |
25 |
7 |
3 |
|
10 |
75 |
5 |
4 |
1 |
0.5 |
|
| Hole Dia." | Hole Area | LP 10" wg | LP 15" wg | MP 5psi | MP 20 psi | |
|---|---|---|---|---|---|---|
0.5 |
0.2 |
43.02 |
34.72 |
10.13 |
4.39 |
mins |
1 |
0.8 |
9.91 |
8.02 |
2.33 |
1.01 |
|
2 |
3 |
2.48 |
2 |
0.58 |
0.25 |
|
4 |
12 |
0.62 |
0.5 |
0.15 |
0.06 |
|
10 |
75 |
0.1 |
0.08 |
0.02 |
0.01 |
|
Note: In all cases if the gas main is cracked then divide the gas main diameter by 4 and use this figure as hole diameter in the above tables.
Conversions:
Gas is transported to domestic premises in Great Britain via eight Distribution Networks (DNs). The geographical operation and ownership of the DNs is shown below. The identification of the operational team within the HID SI3 Gas & Pipelines Unit with enforcement responsibility for the DNs is also given.
