Code of Practice - Safety for Gas Welding and Flame Cutting by LD
Table of Contents
Introduction
This article would introduce the content of “Code of Practice – Safety for Gas Welding and Flame Cutting” (Total 41 pages).
As some contents are so corny, such as the responsibilities of persons concerned or the procedure of risk assessment, this article would put more focus the below topics:
The difference between gas weld, arc weld and flame cut.
Type of gas weld
Safety measures and practices for gas weld and flame cut
Emergency preparedness
Control of plant of equipment (especially under Cap. 295 Dangerous Goods Ordinance 危險品條例)
Gas weld v.s. Arc weld v.s. Flame cut
Gas weld and flame cut
The most common gas welding process is oxyfuel welding. It is one of the oldest welding processes, but in recent years it has become less popular in industrial applications.
A welding torch is used to weld metals. Welding metal results when two pieces are heated to a temperature that produces a shared pool of molten metal.
Compared to Arc Weld
Gas welding processes can be applied to electrically conductive and non-conductive metals. And no electrical connection is desired for the process.
The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene 乙炔 in oxygen to produce a welding flame temperature of about 3100 °C..
A similar process, generally called oxyfuel cutting (flame cut), is used to cut metals.
The gas weld can be divided into three categories – homogeneous, heterogeneous, and autogenous (no need filler rod) as introduced later.
Arc weld
This welding method uses a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point.
The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas.
Compared to gas weld
An electrical power supply is prerequisite for every arc welding process.
Arc temperature is very high (the core temperature can exceed 6,000°C).
Arc welding processes are applicable to electrically conductive metals
High temperature together with narrow arc increases heat density, which, in turn, helps quickly fusing the base metals during welding. Thus higher welding speed can be employed that makes arc welding processes more economic and productive.
High heat density also improves fluidity of the molten weld metal. As a result, the molten metal can penetrate deep into the root gap leading to deeper penetration.
Arc welding is preferred for joining comparatively thicker components (typically thickness > 2.5 mm). Excessive distortion can be observed if applied for thin plates.
Type of gas weld
homogeneous, heterogeneous, and autogenous gas weld
The gas weld can be divided into three categories – homogeneous, heterogeneous, and autogenous weld.
Homogeneous : The filler material is the same as the base material
+VE: Tendency of crack formation during solidification reduces
-VE: Not possible for joining of very thin sheets.
Heterogeneous: The filler material is different from the base material,
+VE: Stronger joint strength
-VE: Compatibility between filler material and base material must be checked
Autogenous: No filler material is used. Instead, the base material is fused together to create the weld.
+VE: (1)Best method of fusing thin sheets. (2) weld appearance is good.
-VE: Thickness of the plates is also limited to only 2 – 3mm
-VE: Strength of the joint is not good—it is less than the strength of base plates -<< Not comply with CoP #.
Requirement for welding consumable unfer Code of Practice for the Structural Use of Steel 2011 (2021)
The welding consumable shall conform to the acceptable standards given in Annex A1.4 with chemical contents matching the parent metal and mechanical properties not inferior to the parent metal.
Safety measures and practices for gas weld and flame cut
1. General
The primary consideration in selecting appropriate safety measures is to control hazards at source by control measures, whereas protective measures should be considered only as supplementary means in protecting staff against the hazards.
Proper design, construction and installation of plant and Equipment.
Establishing safe work systems and procedure
Provision of appropriate information, instruction and training to the staff;
Measures to ensure that equipment are properly used and maintained
Regularly reviewing the effectiveness of the safety measures, safety rules and safe working procedures
2. Ventilation
Function
Removing gas from leakage, if any, so as to prevent accumulation of flammable/explosive mixtures in the atmosphere.
Removing heat as well as hazardous fumes and gases arising from the welding or cutting operation
Supplying fresh air to meet the respiratory needs of persons inside the workplace.
Type of ventilation
General dilution ventilation 稀釋性通風 – fresh air is introduced into the workplace by mechanical means
Local exhaust ventilation 局部抽氣通風 – air contaminants are captured and removed by forced air current through near the point of emission
3. Personal protective equipment
Eye protection
Protects the eye from injuries by radiation and foreign objects
Welding or cutting of metals by means of oxy-acetylene flame is a specified process under the Factories and Industrial
Undertakings (Protection of Eyes) Regulations (Cap. 59 sub.leg.)
Equipment:
approved shield such as hand shield
approved eye protectors such as goggles
Skin and body protection
Protect workers against burns by the flame of the blowpipe, hot slag 熔渣 or workpiece 工件
Equipment:
facemasks, gauntlets長手套
Respiratory protection
Provide additional protection to workers from toxic fumes.
When welding or cutting is performed in a confined space, workers may be required to wear supplied-air type respirator.
Equipment:
Filtering facepiece respirators such as N95 masks
Powered air purifying respirators#
Supplied-air type respirator (for confined space)
#Powered air purifying respirators
PAPRs consist of a headgear-and-fan assembly that takes ambient air contaminated with one or more type of pollutant or pathogen, actively removes (filters) a sufficient proportion of these hazards, and then delivers the clean air to the user’s face or mouth and nose.
PAPRs have low breathing resistance, unlike filtering facepiece respirators such as N95 masks (also one of the respirator). A PAPR may have adjustable air flow rates for added comfort.
Powered air purifying respirators
4. Good practice during and after welding or cutting
Blowpipes should be handled carefully and should not be hung on a gas cylinder shroud or left unattended even for a short period of time.
It is necessary to ensure that slag and workpiece are completely cooled down before leaving the work area.
Care should be taken in lighting up and shutting down the blowpipe to prevent flashback# and backfire# (recommended procedure refers to CoP)
# Flashback 回火
Refers to the return of the flame from the nozzle of the blowpipe upstream along the gas supply line back to the gas cylinder, with possible subsequent explosion.
# Backfire 後燃
refers to the return of the flame into the blowpipe neck or body, the flame remaining alight causing heating up of the blowpipe
5. Fire prevention and protection
Appropriate ventilation to prevent gas accumulation in the workplace.
Regular equipment checks for gas leakage.
Move all flammable or combustible materials to a safe place away from the working area. Use suitable means to protect combustible materials that cannot be moved from close contact with flame.
After the work, it is necessary to ensure that the blowpipe has been properly shut down, and sparks and slag from the work are extinguished and the workpiece cooled down before leaving the work area.
Suitable fire-fighting equipment such as fire extinguishers should be provided nearby the work area.
6. Welding or cutting on containers with combustible fluids (particular for dismantling of plant)
Obtain full information about the plant and equipment to see whether combustible fluid is involved.
Drain off combustible fluids, including flammable ones.
Emergency preparedness (chapter 7 of CoP)
The proprietor of an industrial undertaking should establish emergency procedures and ensure that the staff familiarize themselves with the arrangements in case of emergencies.
Gas leakage
If gas leakage is found in the gas supply line,
(a) Isolate the gas supplies by closing the valves of the gas cylinders or the gas supply systems;
(b) Inform all persons in the workplace;
(c) Isolate all ignition sources, such as distinguishing flames;
(d) Evacuate the workplace and call the Fire Services Department.
Fire involving acetylene cylinder
Any fire involving acetylene cylinder always poses a risk of cylinder explosion, and should be dealt with by the Fire Services Department. However, under safe circumstances, the following initial measures may be suitably adopted:
(a) Cool the cylinder by spraying with water only if it is safe to do so;
(b) Close the cylinder valve to control the fire only if it is safe to do so;
(c) Evacuate the building by activating the fire alarm or by any other means;
(d) Never attempt to move or remove an acetylene cylinder that has been involved in a fire
Control of Plants and Equipments
Rationale for gas weld and flame cut system
Basically, gas from the cylinder is supplied through a gas hose to the blowpipe where the fuel and oxidant gases are mixed in an appropriate proportion and combusted to form the required flame. The gas supply should be adjusted and monitored by a pressure regulator with pressure gauge connected to each gas cylinder.
The general setup is as below:
Non-return valve and Flashback arrester
Non-return valve 止回閥 at each gas inlet of the blowpipe to prevent back-feeding of oxidant gas to the fuel gas line and vice versa
Flashback arrester 防止回火安全掣 incorporating pressure or temperature cut-off valve to prevent propagation of flashback from the blowpipe and
gas supply lines into the cylinders.
Safe storage of gas cylinders and equipment check
Store gas cylinders in a cool and well-ventilated place, away from direct sunlight, or any heat and comply with requirements under Dangerous Goods Ordinance (Chapter 295).
Always keep gas cylinders in an upright position and securely anchored to strong supports
All plant and equipment should be regularly examined for defects and malfunctions.
Equipment check includes (for example)-
Checking for defective or damaged pressure gauges;
Ensuring that there is no gas leakage in the gas supply lines by checking hose 軟氣喉 connections for leakage using soapy water 肥皂液
Requirements for gas cylinders under Dangerous Goods Ordinance (Chapter 295)
Keep the quantity of gas cylinders in storage to a practical minimum and in compliance with the requirements under the Dangerous Goods Ordinance (Chapter 295).
Below are some brief introduction only, details would be discussed separately.
Application for Dangerious Goods Store Licence(s) 危險品倉牌照
Under Dangerous Goods Ordinance, Chapter 295, no person shall store any such substances in excess of their respective exempted quantities in any premises or place without a licence issued by the Director of Fire Services [the Authority]. 超過豁免量→申請牌照]
Exempted Quantity for Category 2 and Category 5 Dangerous Goods
Category 2: Compressed Gases
Category 5: Substances giving off inflammable vapours
Application procedure for DG license to store DG in construction site
An initial inspection would be conducted within 14 working days upon receipt of an application of DG license with application documents.
Upon completion of works of DG store, applicant should notify the Authority for the follow-up inspection which would be conducted within 7 working day on receipt of notification.
Applicant will be informed to collect the DG Store licence within 6 working days upon confirmation of full compliance with the safety requirements.
DG license should be displayed in a conspicuous location inside the DG store for inspection by the Authority.
Licences so issued will be renewed annually.
The licensee is required to notify the Authority on the occasion of relocation of an approved DG store through submitting new site plans.
Application documents for DG license to store DG in construction site
(A) Two sets of plans: (1). Site Plans with locations of DG stores and the whole site, (2). Layout plans of the proposed DG store as shown below for reference:
Site Plan showing location of DG Stores
Layout plans of the proposed DG store
(B) A statement: Licensee’s information, details of the Dangerous Goods Store, type of DG and the maximum quantities to be stored
…Requirement for BD stores would be discussed under separated article.
THIS DOCUMENT IS FOR REFERENCE PURPOSES ONLY.