Which precautions against fire and explosion to be followed while welding?
25 Sep '18, 06:03
Before welding, flame cutting or other hot work, a check should be made that there are no combustible solids, liquids or gases present or adjacent to the area of hot work. Welding or other hot work should never be undertaken on items covered with grease, oil or other flammable materials.
The vessel should have a permit to work system in force, this will include a “Hot Work Permit”, prior to starting any hot work on board, a responsible person is to complete and sign a “Hot Work Permit”, inspected and signed by the Master of the vessel. The permit is to be kept at hand at the place of work.
When welding is to be carried out in the vicinity of open hatches, suitable screens should be erected to prevent sparks dropping down the holds. Where necessary combustible materials and dunnage should be moved to a safe distance before commencing operations. Portholes and other openings through which sparks may fall should be closed where practicable.
Where work is being carried out close to or at bulkheads, decks or deckheads, the remote sides of the divisions should be checked for materials and substances which may ignite and for cables, pipelines or other services which may be affected by the heat. Particular attention should be taken when proposing to weld onto bulkheads or tank tops for temporary securing purposes. Familiarisation of the structure of the ship and the exact location of all fuel and ballast tanks is important in order that the aforementioned welding does not take place on fuel tank bulkheads. This is particularly relevant in respect of steel tank tops where there will inevitably be vapor on the lower side of the tank top.
Cargo, fuel tanks, cargo holds or other tanks should be certified as gas free before any repair work is commenced. The testing should include the testing of adjacent spaces, double bottoms, cofferdams, etc. Further tests should be carried out at regular intervals and before hot work is recommenced following any suspension of the work. When preparing tankers or similar ships, all tanks, cargo pumps and pipelines should be thoroughly cleaned with particular attention paid to the draining and cleaning of pipelines that cannot be directly flushed using the ship's pumps.
Welding and burning operations should be properly supervised and kept under regular observation. Suitable fire extinguishers should be kept at hand ready for use during the welding operation. A second person should be standing by with a fire extinguisher in order to sight any potential fire hazard.
Due to the risk of delayed fires due to the use of welding operations, appropriate and frequent checks should be made for at least two hours after the cessation of a major welding operation.
Relevant codes of safe working practice and company regulations should be adhered to.
Protective clothing and equipment complying with the relevant British Standard Specifications should be worn by the operator and as appropriate by those working with him to protect them from particles of hot metal and slag and from accidental burns and their eyes and skin from ultra-violet and heat radiation.
The operator should normally wear:
- welding helmet with the correct grade of transparent eyepiece for the current being used. Eye goggles or a hand-held shield may be suitable alternatives in appropriate circumstances
- leather welding gauntlets
- leather apron (in appropriate circumstances)
- long-sleeved natural fibre boiler suit or other approved protective clothing
- Clothing should be free of grease and oil and other flammable substances.
Using electric welding equipment
In order to minimize risk from electric shock, electric welding power sources for shipboard use should have a direct current (DC) output not exceeding 70V.
A 'go and return' system utilizing two cables from the welding set should be adopted; the welding 'return' cable should be firmly clamped to the workpiece.
The 'return' cable of the welding set and the workpiece or workpieces should be separately earthed to the ship's structure. The use of a single cable with hull return is not recommended.
To avoid voltage drop in transmission, the lead and return cables should be of the minimum length practicable for the job and of an appropriate cross-section.
Cables should be inspected before use; if the insulation is impaired or conductivity is reduced, they should not be used.
Cable connectors should be fully insulated when connected, and so designed and installed, that current carrying parts are adequately recessed when disconnected.
Electrode holders should be fully insulated so that no live part of the holder is exposed to touch, and, where practicable, should be fitted with guards to prevent accidental contact with live electrodes and as protection from sparks and splashes of weld metal.
A local switching arrangement or other suitable means should be provided for rapidly cutting off current from the electrode should the operator get into difficulties and also for isolating the holder when electrodes are changed.
Specific precautions during electric arc-welding
The welding operator should wear the protective clothing specified but should additionally wear non-conducting safety footwear. Clothing should be kept as dry as possible as some protection against electric shock; it is particularly important that gloves should be dry because wet leather is a good conductor.
An assistant should be in continuous attendance during welding operations. He should be alert to the risk of accidental shock to the welder, ready to cut off power instantly, raise the alarm and apply artificial respiration without delay. The desirability of a second assistant should be considered if the work is to be carried out in difficult conditions.
Where persons other than the operator are likely to be exposed to harmful radiation or sparks from electric arc welding, they should be protected by screens or other effective means.
In restricted spaces, where the operator may be in close contact with the ship's structure or is likely to make contact in the course of ordinary movements, protection should be provided by dry insulating mats or boards.
These are increased risks of electric shock to the operator if welding is done in hot or humid conditions; body sweat and damp clothing greatly reduce body resistance. Under such conditions, the operation should be deferred until such time as an adequate level of safety can be achieved.
In no circumstances should a welder work while standing in water or with any part of his body immersed.
The electrode holder should be isolated from the current supply before a used electrode is removed and before a new electrode is inserted. The precaution is necessary because some electrode coatings have extremely low resistance. Even a flux coating which is normally insulating can become damp from sweating hands and thus potentially dangerous.
When the welding operation is completed or temporarily suspended, the electrode should be removed from the holder.
Hot electrode ends should be ejected into a suitable container; they should not be handled with bare hands.
Spare electrodes should be kept dry in their container until required for use.
Specific precaution for gas welding and cutting
The oxygen and acetylene reducing station and bottles are to be situated outside the machinery spaces, installed in their individual fireproof lockers with a remotely operated water sprinkler system.
The pressure of oxygen used for welding should always be high enough to prevent acetylene flowing back into the oxygen line.
Acetylene should not be used for welding at a pressure exceeding 1 atmosphere gauge as it is liable to explode, even in the absence of air, when under excessive pressure.
Back pressure valves should be fitted adjacent to the torch in the oxygen and acetylene supply lines.
Flame arrestors should be provided in the oxygen and acetylene supply lines and will usually be fitted at the low pressure side of regulators although they may be duplicated at the torch.
25 Sep '18, 06:05