OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 10 Fire and Explosion Hazards

Section 171 Compressed and liquefied gas

Subsections 171(1) through 171(3) and subsections 171(5) through 171(8)

Compressed and liquefied gas containers and systems must be protected against damage and dislodgment that could result in a fire or explosion. The manufacturer’s specifications must always be followed.

Cylinders that have their valve stem break off can become rocket-like projectiles. Oxygen cylinders, for example, can explode if grease or oil is permitted to enter the cylinder or its regulator.

Acetylene cylinders contain acetone (a flammable liquid) in the bottom of the cylinder to help hold the acetylene. If the cylinder is on its side, the acetone may escape, causing the cylinder to explode. Acetylene cylinders must always be secured in their upright position.

Subsection 171(1)(b)

A cylinder of compressed flammable gas cannot be stored in the same room as a cylinder of compressed oxygen unless specific requirements of Part 3 of the Alberta Fire Code (1977) are met. This subsection is understood to apply to the indoor storage of compressed gas within a building. In this case, section 3.2.8.2 of the Alberta Fire Code applies.

Section 3.2.8.2 of the Alberta Fire Code requires that the storage room meets the following requirements:

(a) the room must be separated from the remainder of the building by a gas-tight fire separation having a fire-resistance rating of at least 2 hours;

(b) the room must be located on an exterior wall of the building;

(c) a person must be able to enter the room from the exterior;

(d) any doors from the room into the interior of the building must be

(i) equipped with self-closing devices, and
(ii) constructed in such a way that gases from the storage room cannot enter other parts of the building;

(e) the room must be designed using good engineering practice to prevent critical structural and mechanical damage resulting from an internal explosion;

(f) the room must be provided with natural or mechanical ventilation as required by subsection 4.1.7 of the Alberta Fire Code;

(g) the room must not contain fuel-fired appliances or high temperature heating elements; and

(h) the room must not be used for any purpose other than the storage of Class 2 gases. Class 2 gases are defined in the federal Transportation of Dangerous Goods Regulations.

Subsection 171(4)

Each hose of an oxygen-fuel system e.g. oxyacetylene torch system that uses acetylene and oxygen gases, must have

(a) a flashback device installed at either the torch end or the regulator end, and
(b) a back-flow prevention device installed at the torch end.

Flashbacks are the unintentional and uncontrolled burning of gas back into an oxygen-fuel system, resulting in possible damage to the equipment. This can range from carbon being deposited within the torch tip, valves and hose, which affects their operation, to substantial and expensive damage to the regulator and possibly the cylinder. A flashback may cause the torch and hoses to explode.

A flashback arrestor is a device designed to prevent the backflash of a flame through the torch into the hoses and regulator by quenching the flame. Most flashback arrestors available today also contain check valves intended to prevent the backflow of gases in addition to providing protection against flashbacks.

A backflow preventer is sometimes called a reverse flow valve or check valve. It is designed to prevent gases coming from the torch from mixing and flowing back into the hose lines. A backflow preventer will not always stop a flashback from reaching the hoses, regulator and cylinders.

Since flashback arrestors and backflow preventers serve different safety functions, a combination of both devices is required. Flashbacks can occur due to:

(a) excessive or incorrect pressures. The gas at the higher pressure flows into the lower pressure line. This can occur if incorrect pressures are used or if incompatible equipment is connected together;
(b) a leak from a regulator, hose or connection that results in a drop in pressure, and gas from the higher pressure line back-feeds into the other line;
(c) leaking valves that allow gas to mix when the equipment is not in use;
(d) lighting up with both torch valves open, but one cylinder closed; and
(e) nozzle blockage or faulty equipment.

Flashback arrestors and backflow prevention devices are intended to enhance safety on oxygen-fuel systems where there is a potential for the unwanted and hazardous creation of flammable or explosive mixtures within hose lines.

Such mixtures can inadvertently be created through improper operating procedures or equipment malfunction. Oxygen, at higher pressure than the fuel gas, can back up into the fuel gas line due to a plugged tip orifice, or fuel gas can back up into the oxygen line if, for example, the oxygen cylinder goes empty while cutting.

Where there is no oxygen being supplied, and there is only one line supplying the fuel gas to the torch and nozzle, there is no possibility of reverse flow that could produce a hazardous gas mixture. It is therefore not necessary to install flashback arrestors in such systems e.g. such torches and nozzles are commonly used in the plumbing and HVAC industries, as well as for heating and brazing applications in industrial settings.