Methane

General

The principal danger of methane is explosion. Therefore, where methane is present,precautions should be taken to prevent ignition. Presence of methane should be dispersed by using forced ventilation.

All mechanical and electrical equipment should be explosion-protected.

Occurrence

A thorough design specifications and safe construction practices should be considered on the occurrence of methane.

• • Methane (CH4) is a potentially explosive gas, naturally produced by the decomposition of organic matter. It is commonly found in, or near carboniferous rocks, coal seams, shales, and oil-bearing strata. It can be also found in peat, organic silts, and the place where organic matter decays in wet conditions. It can be influenced by groundwater movement and can travel laterally for a considerable distance along joints and fissures.

  • Gas can also accumulate under river or water bodies if the gas is trapped below the impermeable strata. In urban areas, it can be found under landfills, in sewer, in sewer sludge or leaking from gas mains.

  • Methane can appear in the excavation as a steady infiltration, a heavy emission, or in a sudden influx when a pocket of gas under pressure is penetrated. Methane dissolved in groundwater can enter tunnel works and the gas can subsequently be released into the tunnel atmosphere.

  • Methane can form layers in the crown of the tunnel and can migrate from the source of entry to a point some distance away. Therefore, an explosion can also occur some distance away from the point of entry.

  • Methane can occur with carbon dioxide and other gases and is denser than air. Therefore, methane could be found near the base of the excavation and migrate down inclines.

  • The methane layers can be dispersed by using high-velocity airflow in the tunnel or by directing a flow of air to the crown of the tunnel by means of local air movement.

Explosion Characteristics

The danger from methane is that it is potentially explosive when mixed with air. The lower and upper explosive limits (LEL and UEL) were updated recently in Europe as 4.4%  and 17 % respectively by volume. The methane density is 0.6 times that of air, and the source of accumulation is the tunnel crown. When mixed thoroughly, it does notseparate again, and can therefore be safely handled in a ventilation system if diluted.

Tunneling through coal measures should be handled carefully, and any accumulation of the coal dust should be prevented as the methane explosion could spark coal dust explosion (BS 6164: 12.3)

Detection and Monitoring

Where the presence of methane is expected, the tunnel should be monitored by using fixed monitoring equipment, supplemented by the use of portable monitoring equipment. The system should be provided with an alarm system with a predetermined alarm level. On alarm activation, an emergency procedure should be implemented.

The presence of methane must be captured and addressed to the designer and client promptly. The nature and the source of the gas should be tracked and the inflow intensity should be measured.

Effective gas measurement equipment should be installed, maintained and calibrated on set interval.

Monitoring should be carried out using fixed monitoring equipment in the pit bottom, along the crown of the tunnel, on the excavation machinery at the tunnel face and in extraction ventilation ducts. The system should be supplemented by monitoring, possibly using portable equipment, at the following locations:

• • along the invert;

  • in the general body of the air within the tunnel;

  • at low-level points, such as sumps and pits;

  • where methane layering is suspected;

  • in voids above crown level;

  • upwind of tunnel machinery, electrical switchgear, and transformer.

Monitored data should be available to site managers on the surface and the tunnel face. The written, printed and electronic records should be kept showing the following details:

• • calibration of the equipment;

  • the result of routine monitoring;

  • an abnormally high concentration of methane;

  • atmospheric pressure;

  • position of sampling;

  • time of the day;

  • date;

  • air quantity flowing at each sampling point.

Atmospheric monitoring equipment should conform to the approved source and with the given specifications (BS 6164: 12.4

)

MUST KNOW

As according to the Alberta Occupational Health and Safety Code (current as of January 1, 2019) an employer must follow the following:

As According to Part 4, Section 52(1)

If the hazard assessment identifies a potential atmospheric hazard and a worker is required or authorized by an employer to enter the confined space, the employer must ensure that a competent worker performs a pre-entry atmospheric test of the confined space to

(a) verify that the oxygen content is between 19.5 percent and 23.0 percent by volume, and

(b) identify the amount of toxic, flammable or explosive substance that may be present.

As According to Part 4, Section 52(2)

The employer must ensure that the testing required by AOHS Part 4: 52(1) is performed using calibrated test instruments appropriate for the atmosphere being tested and the instruments are used in accordance with the manufacturer’s specifications.

As According to Part 4, Section 52(3)

The employer must ensure that as often as necessary after the first time a worker enters the confined space, a competent worker

(a) performs the tests specified in AOHS Part 4: 52(1), and

(b) identifies and records any additional hazards.

As According to Part 4, Section 52(3.1)

The employer must ensure that if there is a potential for the atmosphere to change unpredictably after a worker enters the confined space, the atmosphere is continuously monitored in accordance with AOHS Part 4: 52(2).

As According to Part 4, Section 52(4)

If tests identify additional hazards, the employer must deal with the identified hazards in accordance with this Code.

As According to Part 4, Section 52(5)

The employer must ensure that the procedures and practices put in place under AOHS Part 4: 52(4) are included in the code of practice.

As According to Part 4, Section 52(6)

The employer must ensure that the results of tests required by AOHS Part 4: 52 are recorded.

Danger level

Table 1 - Danger Level

Source of Ignition

Sparks can be ignited by different means. An electrical spark or a hot spot can be generated by friction in machinery, by overloaded or inadequately cooled electric cables or lamps, or by faulty earthing or metal cutting and grinding.

Spark can be produced by violent contact between metals and rocks. Cutting machines should be operated at low speed and cooled with water sprays or jets.

Electric arcs arise in making and breaking contacts in a live circuit, and from loose contacts. Therefore, if the methane concentration is anticipated above 12.5, explosion-protected equipment should be used under such an environment (BS 6164: 12.6).

Explosion protection and working under a potentially explosive environment

While working under explosion-prone environment due to methane, advice should be taken from a specialist in explosion protection on the use in compressed-air working (BS 6164: 12.7).

Where a potentially explosive contaminant is kept below 100% LEL by means of ventilation, an “explosive atmosphere: is not present. Consequently, guidance and work procedure should strictly adhere to BS 6164: 7.2.

MUST KNOW

Alberta Occupational Health and Safety Code Relevant to the Topic

As according to the Alberta Occupational Health and Safety Code (current as of January 1, 2019) an employer must follow the following:

According to Part 4, Section 21(1)

If a worker may be exposed to a harmful substance at a work site, an employer must

identify the health hazards associated with the exposure and assess the worker’s exposure.

According to Part 4, Section 21(2)

The employer must ensure that a worker who may be exposed to a harmful substance at a work site

(a) is informed of the health hazards associated with exposure to that substance,

(b) is informed of measurements made of airborne concentrations of harmful substances at the work site, and

(c) is trained in procedures developed by the employer to minimize the worker’s exposure to harmful substances, and understands the procedures.

According to Part 4, Section 21(3)

A worker who is provided with training under AOHS Part 4: 21(2) must use the procedures appropriately and apply the training.