Welding Inspection Technology PDF

WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS gas, and a greater variety of materials are usually involved. Protection from excess exposure is usually accomplished by ventilation. Where exposure would exceed permissible limits with available ventilation, suitable re...

WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS gas, and a greater variety of materials are usually involved. Protection from excess exposure is usually accomplished by ventilation. Where exposure would exceed permissible limits with available ventilation, suitable respiratory protection must be used. Protection must be provided for welding, cutting, and other personnel in the area. Because welding helmets and dark filter plates restrict the visibility of welders, these people may be even more susceptible than ordinary workers to injury from unseen, unguarded machinery. Therefore, special attention to this hazard is required. When repairing machinery by welding or brazing, the power to the machinery must be disconnected, locked out, tried, and tagged to prevent inadvertent operation and injury. Welders assigned to work on equipment with safety devices removed should fully understand the hazards involved, and the steps to be taken to avoid injury. When the work is completed, the safety devices must be replaced. Rotating and automatic welding machines, fixtures, and welding robots must be equipped with appropriate guards or sensing devices to prevent operation when someone is in the danger area. Exposure Factors Position of the Head The single most important factor influencing exposure to fumes is the position of the welder’s head with respect to the fumes plume. When the head is in such a position that the fumes envelop the face or helmet, exposure levels can be very high. Therefore, welders must be trained to keep their head to one side of the fume plume. Sometimes, the work can be positioned so the fume plume rises to one side. Pinch points on welding and other mechanical equipment can also result in serious injury. Examples include resistance welding machines, robots, automatic arc welding machines, jigs, and fixtures. To avoid injury with such equipment, the machine should be equipped so that both of the operator’s hands must be at safe locations when the machine is actuated. Otherwise, the pinch points must be suitably guarded mechanically. Metalworking equipment should not be located where a welder could accidentally fall into or against it while welding. During maintenance of the equipment, pinch points should be blocked to prevent them from closing in case of equipment failure. In very hazardous situations, an observer should be stationed to prevent someone from turning the power on until the repair is completed. Types of Ventilation Ventilation has a significant influence on fume amounts in the work area, and the welder’s exposure to them. Ventilation may be local, where the fumes are extracted near the point of welding (see Figure 2.8), or general, where the shop air is changed or filtered. The appropriate type will depend on the welding process, the material being welded, and other shop conditions. Adequate ventilation is necessary to keep the welder’s exposure to fumes and gases within safe limits. Fumes and Gases Work Area Welders, welding operators, and other persons in the area must be protected from over-exposure to fumes and gases produced during welding, brazing, soldering, and cutting. Overexposure is exposure that is hazardous to health, or exceeds the permissible limits specified by a government agency. The U.S. Department of Labor, Occupational Safety and Health Administration (OSHA), Regulations 29 CFR 1910.1000, covers this topic. Also, the American Conference of Governmental Industrial Hygienists (ACGIH) lists guidelines in their publication, Threshold Limit Values for Chemical Substances and Physical Agents in the Workroom Environment. Persons with special health problems may have unusual sensitivity that requires even more stringent protection. The size of the welding or cutting enclosure is important. It affects the background fume level. Fume exposure inside a tank, pressure vessel, or other confined space will be higher than in a high-bay fabrication area. Background Fume Level Background fume levels depend on the number and type of welding stations and the duty cycle for each power source. Design of Welding Helmet Fumes and gases are usually a greater concern in arc welding than in oxyfuel gas welding, cutting, or brazing. A welding arc may generate a larger volume of fume and The extent a helmet curves under the chin toward the chest affects the amount of fume exposure. Close-fitting helmets can be effective in reducing exposure. 2-9 CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS WELDING INSPECTION TECHNOLOGY Figure 2.8—Movable Fume Extractor Positioned Near the Welding Arc Base Metal and Surface Condition rate of the welding or cutting processes. Therefore, fume concentration must be controlled by ventilation. The type of base metal being welded influences fume components and the amount generated. Surface contaminants or coatings may contribute significantly to the hazard potential of the fume. Paints containing lead or cadmium generate dangerous fumes during welding and cutting. Galvanized material creates zinc fumes which are harmful. Adequate ventilation is the key to control of fumes and gases in the welding environment. Natural, mechanical, or respirator ventilation must be provided for all welding, cutting, brazing, and related operations. The ventilation must ensure that concentrations of hazardous airborne contaminants are maintained below recommended levels. Many ventilation methods are available. They range from natural drafts to localized devices, such as air-ventilated welding helmets. Examples of ventilation include: Ventilation The bulk of fumes generated during welding and cutting consists of small particles that remain suspended in the atmosphere for a considerable time. As a result, fume concentration in a closed area can build up over time, as can the concentration of any gases evolved or used in the process. Particles eventually settle on the walls and floor, but the settling rate is low compared to the generation 1. Natural 2. General area mechanical ventilation 3. Overhead exhaust hoods 4. Portable local exhaust devices 2-10 WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS 5. Downdraft tables 6. Crossdraft tables 7. Extractors built into the welding equipment 8. Air-ventilated helmets Welding in Confined Spaces Special consideration must be given to the safety and health of welders and other workers in confined spaces. See ANSI publication Z117.1, Safety Requirements for Working in Tanks and Other Confined Spaces, latest edition, for further precautions. Gas cylinders must be located outside the confined space to avoid possible contamination of the space with leaking gases or volatiles. Welding power sources should also be located outside to reduce danger of engine exhaust and electric shock. Lighting inside the work area should be low voltage, 12 V, or if 110 V is required, the circuit must be protected by an approved Ground-Fault Circuit-Interrupter (GFCI). A means for removing persons quickly in case of emergency has to be provided. Safety belts and lifelines, when used, should be attached to the worker’s body in a way that avoids the possibility of the person becoming jammed in the exit. A trained helper, a “standby,” should be stationed outside the confined space with a preplanned rescue procedure in case of an emergency (including not entering the confined space to aid the first worker without proper breathing apparatus). Figure 2.9—Welding in Confined Spaces Besides keeping airborne contaminants in breathing atmospheres at or below recommended limits, ventilation in confined spaces must also (1) assure adequate oxygen for life support (at least 19.5% by volume); (2) prevent accumulation of an oxygen-enriched atmosphere, (i.e., not over 23.5% by volume); and (3) prevent accumulation of flammable mixtures (see Figure 2.9). Asphyxiation can quickly result in unconsciousness and death without warning if oxygen is not present in sufficient concentration to support life. Air contains approximately 21% oxygen by volume. Confined spaces must not be entered unless well ventilated, or the inspector is wearing an approved air supplied breathing apparatus and has proper training to work in such spaces. A similarly equipped second person must be present as a standby. Lighter-than-air gases, such as helium and hydrogen, may accumulate in tank tops, high areas, and near ceilings. The precautions for confined spaces also apply to those areas. If practical, a continuous monitoring system with audible alarms should be used for work in a confined space. Oxygen-enriched atmospheres pose great danger to occupants of confined areas. They are especially hazardous at oxygen concentrations above 25%. Materials that burn normally in air may flare up violently in an oxygenenriched atmosphere. Clothing may burn fiercely; oil or grease soaked clothing or rags may catch fire spontaneously; paper may flare into flame. Very severe and fatal burns can result. Before entering confined spaces, the space should be tested for toxic or flammable gases and vapors, and adequate or excess oxygen. The tests should be made with instruments approved by the U.S. Bureau of Mines. Protection in confined spaces must be provided welders and other personnel in the enclosure. Only clean, respirable air must be used for ventilation. Oxygen, other gases, or mixtures of gases must never be used for ventilation. Heavier-than-air gases, such as argon, methylacetylenepropadiene, propane, and carbon dioxide, may accumulate in pits, tank bottoms, low areas, and near floors. Positive pressure self-contained breathing apparatus must be used when welding or cutting related processes 2-11 CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS are done in confined areas where proper ventilation cannot be provided and there is immediate danger to life and health. It must have an emergency air supply of at least five minutes duration in the event that the main source fails. WELDING INSPECTION TECHNOLOGY Table 2.2 Toxic Metals 1. 2. 3. 4. 5. 6. 7. 8. Welding of Containers Welding or cutting outside or inside containers and vessels that have held dangerous substances presents special hazards. Flammable or toxic vapors may be present, or may be generated by the applied heat. The immediate area outside and inside the container should be cleared of all obstacles and hazardous materials. If repairing a container in place, entry of hazardous substances into the container from the outside must be avoided. The required personal and fire protection equipment must be available, serviceable, and in position for immediate use. Antimony Arsenic Barium Beryllium Cadmium Chromium Cobalt Copper 9. 10. 11. 12. 13. 14. 15. Lead Manganese Mercury Nickel Selenium Silver Vanadium Commission jurisdiction require special considerations and may also require compliance with state and local regulations. These materials also include X-ray machines and radiographic isotopes. When welding or cutting inside vessels that have held dangerous materials, the precautions for confined spaces must also be observed. Gases generated during welding should be discharged in a safe and environmentally acceptable manner according to government rules and regulations. Provisions must be made to prevent pressure buildup inside containers. Testing for gases, fumes, and vapors should be conducted periodically to ensure that recommended limits are maintained during welding. When toxic materials are encountered as designated constituents in welding, brazing, or cutting operations, special ventilation precautions must be taken. The precautions assure that the levels of these contaminants in the atmosphere are at or below the limits allowed for human exposure. All persons in the immediate vicinity of welding or cutting operations involving these materials must be similarly protected. An alternative method of providing safe welding of containers is to fill them with an inert medium such as water, inert gas, or sand. When using water, the level should be kept to within a few inches of the point where the welding is to be done. The space above the water should be vented to allow the heated air to escape. With inert gas, the percentage of inert gas that must be present in the tank to prevent fire or explosion must be known. How to safely produce and maintain a safe atmosphere during welding must also be known. Handling of Compressed Gases Gases used in welding and cutting operations are packaged in containers called cylinders. Only cylinders designed and maintained in accordance with U.S. Department of Transportation (DOT) specifications may be used in the United States. The use of other cylinders may be extremely dangerous and is illegal. Cylinders requiring periodic retest under DOT regulations may not be filled unless the retest is current. Highly Toxic Materials Cylinders may be filled only with the permission of the owner, and should be filled only by recognized gas suppliers or those with the proper training and facilities to do so. Filling one cylinder from another is dangerous and should not be attempted by anyone not qualified to do so. Combustible or incompatible combinations of gases must never be mixed in cylinders. Certain materials, which are sometimes present in consumables, base metals, coatings, or atmospheres for welding or cutting operations, have permissible exposure limits of 1.0 mg/m3 or less. Among these materials are the metals noted in Table 2.2. Manufacturer’s Material Safety Data Sheets should be consulted to find out if any of these materials are present in welding filler metals and fluxes being used. Material Safety Data Sheets should be requested from suppliers. However, welding filler metals and fluxes are not the only source of these materials. They may also be present in base metals, coatings, or other sources in the work area. Radioactive materials under Nuclear Regulatory Welding must not be performed on gas cylinders. Cylinders must not be allowed to become part of an electrical circuit because arcing may result. Cylinders containing shielding gases used in conjunction with arc welding must not be grounded. Electrode holders, welding torches, cables, hoses, and tools should not be stored on gas cylinders to avoid arcing or interference with valve 2-12 WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS operation. Arc-damaged gas cylinders may rupture and result in injury or death. Cylinders must not be used as work rests or rollers. They should be protected from bumps, falling objects, weather, and should not be dropped. Cylinders should not be kept in passageways where they might be struck by vehicles. They should be kept in areas where temperatures do not fall below –20°F or exceed 130°F. Any of these exposures, misuses, or abuses could damage them to the extent that they might fail with serious consequences. Cylinders must not be hoisted using ordinary slings or chains. A proper cradle or cradle sling that securely retains the cylinder should be used. Electromagnets should not be used to handle cylinders. Cylinders must always be secured by the user against falling during either use or storage (see Figure 2.10). Acetylene and liquefied gas cylinders (dewars) should always be stored and used in the upright position. Other cylinders are preferably stored and used in the upright position, but this is not essential in all circumstances. Before using gas from a cylinder, the contents should be identified by the label thereon. Contents should not be identified by any other means such as cylinder color, banding, or shape. These may vary among manufacturers, geographical area, or product line and could be completely misleading. The label on the cylinder is the only proper notice of the contents. If a label is not on a cylinder, the contents should not be used and the cylinder should be returned to the supplier. Figure 2.10—Inert Gas Cylinders Attached to Manifold System A valve protection cap is provided on many cylinders to protect the safety device and the cylinder valve. This cap should always be in place unless the cylinder is in use. The cylinder should never be lifted manually or hoisted by the valve protection cap. The threads that secure these valve protection caps are intended only for that purpose, and may not support full cylinder weight. The caps should always be threaded completely onto the cylinders and hand tightened. relief or safety valve, rated to function at less than the maximum allowable pressure of the welding equipment, should also be employed. Valve functions prevent equipment failure at pressures over working limits if the regulator should fail in service. Gas cylinders and other containers must be stored in accordance with all state and local regulations and the appropriate standards of OSHA and the National Fire Protection Association. Safe handling and storage procedures are discussed in the Handbook of Compressed Gases, published by the Compressed Gas Association. Valves on cylinders containing high pressure gas, particularly oxygen, should always be opened slowly to avoid the high temperature of adiabatic recompression. Adiabatic recompression can occur if the valves are opened rapidly. With oxygen, the heat can ignite the valve seat that, in turn, may cause the metal to melt or burn. The cylinder valve outlet should point away from the operator and other persons when opening the valve to avoid injury should a fire occur. The operator should never stand in front of the regulator when opening a cylinder to avoid injury from high pressure release if the regulator fails. Many gases in high-pressure cylinders are filled to pressures of 2000 psi or more. Unless the equipment to be used with a gas is designed to operate at full cylinder pressure, an approved pressure-reducing regulator must be used to withdraw gas from a cylinder or manifold. Simple needle valves should never be used. A pressure- 2-13 CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS WELDING INSPECTION TECHNOLOGY cylinder and regulator, or between the regulator and hose suggests an improper combination of devices being used. Before connecting a gas cylinder to a pressure regulator or a manifold, the valve outlet should be cleaned. The valve outlet should be wiped clean with a clean, oil-free cloth to remove dirt, moisture, and other foreign matter. Then the valve should be opened momentarily and closed immediately. This is known as “cracking the cylinder valve.” Fuel gas cylinders must never be cracked near sources of ignition (i.e., sparks and flames), while smoking, nor in confined spaces. Use of adapters to change the cylinder connection thread is not recommended because of the danger of using an incorrect regulator or of contaminating the regulator. For example, gases that are oil-contaminated can deposit an oily film on the internal parts of the regulator. This film can contaminate oil-free gas and result in fire or explosion when exposed to pure oxygen. A regulator should be relieved of gas pressure before connecting it to a cylinder, and also after closing the cylinder valve upon shutdown of operation. The outlet threads on cylinder valves are standardized for specific gases so that only regulators or manifolds with similar threads can be attached (e.g., flammable gas cylinders typically have a left-hand thread while nonflammable gas cylinders have a right-hand thread). The threads and connection glands of regulators should be inspected before use for dirt and damage. If a hose or cylinder connection leaks, it should not be forced with excessive torque. Damaged regulators and components should be repaired by properly trained mechanics or returned to the manufacturer for repair. A suitable valve or flowmeter should be used to control gas flow from a regulator (see Figure 2.11). The internal pressure in a regulator should be released before it is connected to or removed from a gas cylinder or manifold. It is preferable not to open valves on low pressure, fuel gas cylinders more than one turn. This usually provides adequate flow and allows rapid closure of the cylinder valve in an emergency. High pressure cylinder valves, on the other hand, usually must be opened fully to backseat (seal) the valve to prevent leaks during use. Manifolds A manifold is used when gas is needed without interruption or at a higher delivery rate than can be supplied from a single cylinder. A manifold must be designed for the specific gas and operating pressure, and be leak tight (see Figures 2.12 and 2.13). The manifold components should be approved for such purpose, and should be used only for the gas and pressure for which they are approved. Oxygen and fuel gas manifolds must meet specific design and safety requirements. The cylinder valve should be closed after each use of a cylinder and when an empty cylinder is to be returned to the supplier. This prevents loss of product through leaks that might develop and go undetected while the cylinder is unattended, and avoids hazards that might be caused by leaks. It also prevents backflow of contaminants into the cylinder. It is advisable to return cylinders to the supplier with about 25 psi of contents remaining. This prevents possible contamination by the atmosphere during shipment. Piping and fittings for acetylene and methylacetylenepropadiene (MPS) manifolds must not be unalloyed copper or alloys containing 70% or more copper. These fuel gases react with copper under certain conditions to form unstable copper acetylide. This compound may detonate under shock or heat. Pressure Relief Devices Only trained personnel should be allowed to adjust pressure relief devices on cylinders. These devices are intended to provide protection in the event the cylinder is subjected to a hostile environment, usually fire or other source of heat. Such environments may raise the pressure within cylinders. To prevent cylinder pressures from exceeding safe limits, the safety devices are designed to relieve the contents. Manifold piping systems must contain an appropriate overpressure relief valve. Each fuel gas cylinder branch line should incorporate a backflow check valve and flash arrester. Backflow check valves must also be installed in each line at each station outlet where both fuel gas and oxygen are provided for a welding, cutting, or preheating torch. These check valves must be checked periodically for safe operation. A pressure reducing regulator should always be used when withdrawing gas from gas cylinders for welding or cutting operations. Pressure reducing regulators must be used only for the gas and pressure given on the label. They should not be used with other gases or at other pressures although the cylinder valve outlet threads may be the same. The threaded connections to the regulator must not be forced. Improper fit of threads between a gas Unless it is known that a piping system is specifically designed and constructed to withstand full cylinder pressure or tank pressure of the compressed gas source supplying it, the piping system must be protected with safety pressure relief devices. The devices must be sufficient to 2-14 WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS Figure 2.11—Acetylene and Oxygen Regulators and Inert Gas Flowmeters Figure 2.12—Acetylene Manifold System prevent development of pressure in the system beyond the capacity of the weakest element. can increase dramatically. Pressure relief devices protecting fuel gas piping systems or other hazardous gas systems should be vented to safe locations. Such pressure relief devices may be relief valves or bursting discs. A pressure reducing regulator must never be solely relied upon to prevent over pressurization of the system. A pressure relief device must be located in every section of the system that could be exposed to the full source supply pressure while isolated from other protective relief devices (such as by a closed valve). Some pressure regulators have integral safety relief valves. These valves are designed for the protection of the regulator only, and should not be relied upon to protect the downstream system. Gases Oxygen Oxygen is nonflammable but it supports the combustion of flammable materials. It can initiate combustion and vigorously accelerate it. Therefore, oxygen cylinders and liquid oxygen containers should not be stored near combustibles or with cylinders of fuel gas. Oxygen should never be used as a substitute for compressed air. Pure oxygen supports combustion more vigorously than air, which contains only 21% oxygen. Therefore, the identification of the oxygen and air should be differentiated. In cryogenic piping systems, relief devices should be located in every section of the system where liquefied gas may become trapped. Upon warming, such liquids vaporize to gas, and in a confined space, the gas pressure 2-15 CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS WELDING INSPECTION TECHNOLOGY must not be used to blow dirt from work and clothing because they are often contaminated with oil, or grease, or combustible dust. Only clean clothing should be worn when working with oxygen systems. Oxygen must not be used to ventilate confined spaces. Severe burns may result from ignition of clothing or hair in an oxygen-rich atmosphere. Fuel Gases Fuel gases commonly used in oxyfuel gas welding (OFW) and cutting (OFC) are acetylene, methylacetylene-propadiene (MPS), natural gas, propane, and propylene. Hydrogen is used in a few applications. Gasoline is sometimes used as fuel for oxygen cutting (it vaporizes in the torch). These gases should always be referred to by name. Acetylene in cylinders is dissolved in a solvent (such as acetone) so that it can be safely stored under pressure. In the free state, acetylene should never be used at pressures over 15 psi [103 kPa] because it can dissociate with explosive violence at higher pressures. Acetylene and MPS should never be used in contact with silver, mercury, or alloys containing 70% or more copper. These gases react with these metals to form unstable compounds that may detonate under shock or heat. Valves on fuel gas cylinders should never be opened to clean the valve outlet near possible sources of flame ignition, or in confined spaces. When fuel gases are used for a brazing furnace atmosphere, they must be vented to a safe location. Before filling a furnace with fuel gas, the equipment must first be purged with a nonflammable gas. Nitrogen or argon can be used to prevent formation of an explosive air-fuel mixture. Figure 2.13—Oxygen Manifold System Oil, grease, and combustible dusts may spontaneously ignite on contact with oxygen. All systems and apparatus for oxygen service must be kept free of any combustibles. Valves, piping, or system components that have not been expressly manufactured for oxygen service must be cleaned and approved for this service before use. Special attention must be given when using hydrogen. Flames of hydrogen may be difficult to see and parts of the body, clothes, or combustibles may, therefore, unknowingly come in contact with hydrogen flames. Fuel Gas Fires Apparatus expressly manufactured for oxygen service, and so labeled, must be kept in the clean condition as originally received. Oxygen valves, regulators, and apparatus should never be lubricated with oil. If lubrication is required, the type of lubricant and the method of applying the lubricant should be specified in the manufacturer’s literature. If it is not, then the device should be returned to the manufacturer or authorized representative for service. The best procedure for avoiding fire from a fuel gas or liquid is to keep it contained within the system, that is, prevent leaks. All fuel systems should be checked carefully for leaks upon assembly and at frequent intervals after that. Fuel gas cylinders should be examined for leaks, especially at fuse plugs, safety devices, and valve packing. One common source of fire in welding and cutting is ignition of leaking fuel by flying sparks or spatter. Oxygen must never be used to power compressed air tools. These are usually oil lubricated. Similarly, oxygen In case of a fuel fire, an effective means for controlling the fire is to shut off the fuel valve, if accessible. A fuel 2-16