Gas in Arc welding

Gases used in arc welding processes are the shielding gases. Shielding gases used in arc welding are argon, helium, and carbon dioxide. The gases have a remarkable effect on the overall performance of the welding system. The main function of these gases is to protect the weld pool from adverse reactions with atmospheric gases. Oxygen, nitrogen and water vapour present in ambient air can cause weld contamination. Weld shielding, always involves removal of potentially reactive gases from the vicinity of the weld, preventing the detrimental effects on the molten metal of the surrounding atmosphere. Shielding gases also stabilizes the arc and enhances the metal transfer mode in arc welding processes. The shielding gas interacts with the base and filler metal and changes basic mechanical properties of the weld area, such as strength, toughness, hardness and corrosion resistance. Shielding gases moreover have important effects on the formation of the weld bead and the penetration pattern. The usage of shielding gases can lead to different penetration and weld bead profiles. However, apart from all these important effects, the gases have to be handled with care. These gases that stored in compressed gas cylinders are potentially hazardous because of the possibility of a sudden release of gas by removal or breaking off of the valve. High-pressure gas escaping from such a cylinder causes it to be like a rocket which may smash into people and properties. In storage, transport and operation of compressed gas cylinders it is imperative to observe the following rules: Whether in use or stored, the cylinders should be kept vertical and secured so as to avoid falling by means of chains and clamps. To open cylinder valves hammers and wrenches must not be used. The proper trolley should be used for moving cylinders from one point to another in the workshop. The cylinder should never be carried on shoulders because in case it falls it can not only injure the person but may also explode. Compressed gas should not be exposed to sunlight or heat as this may lead to an increase in the pressure leading to an explosion. The temperature of the gas cylinder should not be allowed to exceed 54 oC. Cylinder valve must be opened gradually with proper care otherwise it may damage the regulator diaphragm. Cylinders must have caps during storage and transport.

Radiographic Testing

RT is a volumetric examination method used for examining the entire specimen rather than just the
surface. It is the historical approach to examine completed welds for surface and subsurface
discontinuities. The change in absorption of radiation by solid metal and in areas of a discontinuity is used in this method. The radiation transmitted reacts with the film, a latent image is captured, and when the film is processed (developed) creates a permanent image (radiograph) of the weld. Some methods also use electronics to create a digital image and are referred to as “filmless.” Due to the hazard of radiation, and the licensing requirements, the cost can be higher and at the same time, the number of trained personnel is limited, than with other NDE methods. An NDT examiner interprets and evaluates the radiographs for differences in absorption and transmission results. Radiographic results display is different as compared with the normal background image of the weld or part being inspected. The radiographer also makes sure that the film is exposed by the primary source of the radiation and not backscatter radiation. The NDT examiner that performs the film interpretation, evaluation and reporting should be certified as a minimum to ASNT Level II requirements. However, all personnel performing radiography are required to attend radiation safety training and comply with the applicable regulatory requirements. There are very specific requirements with regard to the quality of the produced radiograph, including the sharpness of the image, the ability to prove adequate film density in the area of interest and sensitivity to the size and type of expected flaws. Requirements listed in Article 2 include:

a. Method to determine if backscatter is present. 

b. Permanent identification, traceable to the component. 

c. Film selection in accordance with SE-1815. 

d. Designations for a hole or wire-type image quality indicators. 

e. Suggested radiographic techniques.

f. Facilities for viewing radiographs

g. Calibration (certification of source size).

The exposure and processing of a radiograph are considered acceptable when it meets the required quality features in terms of sensitivity and density. These factors are designed to ensure that imperfections of a dimension relative to section the thickness will be revealed.

Mechanical Joints

Threaded joints are the oldest method of joining piping systems. Thread cutting should be regarded as a precision machining operation. Typical threading die. For steel pipe, the lip angle should be about, but for brass, it should be much smaller. Improper lip angle results in rough or torn threads. Since pipe threads are not perfect, joint compounds are used to provide leak tightness. The compounds selected, of course, should be compatible with the fluid carried and should be evaluated for possible detrimental effects on system components. Manufacturers’ recommendations should be followed. Where the presence of a joint compound is undesirable, dry seal pipe threads in accordance with ASME B1.20.346 may be employed. These are primarily found in hydraulic and pneumatic control lines and instruments. Flanged joints are most often used where disassembly for maintenance is desired. A great deal of information regarding the selection of flange types, flange tolerances, facings and gaskets, and bolting is found in B16.5. The limitations regarding cast iron-to-steel flanges, as well as gasket and bolting selection, should be carefully observed. The governing code will usually have further requirements. Gasket surfaces should be carefully cleaned and inspected prior to making up the joint. Damaged or pitted surfaces may leak. Appropriate gaskets and bolting must be used. The flange contact surfaces should be aligned perfectly parallel to each other. Attempting to correct any angular deviation perpendicular to the flange faces while making up the joint may result in overstressing a portion of the bolts and subsequent leakage. The proper gasket should be inserted making sure that it is centred properly on the contact surfaces. Bolts should be tightened hand-tight. If necessary for alignment elsewhere, the advantage may be taken of the bolt hole tolerances to translate or rotate in the plane of the flanges. In no case should rotation perpendicular to the flange faces be attempted? When the assembly is in its final location, bolts should be made up wrench-tight in a staggered sequence. The bolt loading should exert a compressive force of about twice that generated by the internal pressure to compensate not only for internal pressure but for any bending loads which may be imposed on the flange pair during operation. For a greater guarantee against leakage, torque wrenches may be employed to load each bolt or stud to some predetermined value. Care should be exercised to preclude loading beyond the yield point of the bolting. In other cases, special studs that have had the ground of the end to permit micrometre measurement of stud elongation may be used. Flange pairs which are to be insulated should be carefully selected since the effective length of the stud or bolt will expand to a greater degree than the flange thicknesses, and leakage will occur. Thread lubricants should be used, particularly in high-temperature service to permit easier assembly and disassembly for maintenance.

Construction of Pipeline

Designing and constructing a pipeline is a major undertaking, requiring a wide variety of engineering and construction skills. A large pipeline the operator would have the internal resources (both trained and experienced manpower and equipment) to undertake all phases of pipeline construction, it is more likely that virtually all of the major phases of construction will be contracted out to companies possessing the necessary expertise and capacities to complete the task. While that guarantees the critical requirements of the pipeline construction will be met, it also introduces the need to control logistics to ensure that all contractor activities are coordinated and not mutually exclusive of one another. Construction can take place because pipeline construction equipment is distributed along the pipeline route in a moving assembly line in which only one major item of construction equipment is normally needed at any one point of time. The distance along the pipeline over which this equipment is deployed is relatively shorter and less than a mile, but there may be several sets of construction equipment operational along the pipeline route at any given time. The complete set of equipment — for ditching, welding, coating, lowering in, and backfilling are called spreads. A single pipeline may be built using several spreads, reducing the overall construction period, but also increasing the number of people and secondary resources required to support them. Large pipeline projects can also be divided into two or more segments, and different construction contractors may be used to install each segment. Various construction activities also take place simultaneously on a number of segments. Each of these contractors may field several spreads to build a segment. The actual installation of the pipeline includes these major steps: 

1. clearing the ROW as needed.

2. Ditching.

3. Stringing pipe joints along the ROW.

4. Welding the pipe joints together.

5. Applying a coating and wrapping the exterior of the pipe (except for the portions of the pipe at each end, which is sometimes coated before being delivered to the job site).

6. Lowering the pipeline into the ditch.

7. Backfilling the ditch.

8. Testing the line for leaks.

9. Cleanup and drying the pipeline after testing to prepare it for operation.

10. Reclaiming impacted environmental areas.

Visual Testing

Visual inspection (VT) refers to the detection of surface imperfections using the eye. Usually being applied without any kind of additional equipment, VT can be improved by using aids such as a magnifying glass to improve its effectiveness and scope. VT is one of the primary NDT methods. Since it relies on an evaluation made using the eye, VT is generally considered to be the primary and oldest method of NDT. Due to the relative simplicity and as it does not require sophisticated apparatus, it is a very inexpensive method thus provides an advantage over other NDT methods. VT is an ongoing inspection that can be applied at various stages of construction. The primary limitation of VT is it is only capable of evaluating discontinuities, which can be seen on the surface of the material or part. On several occasions, there are some visual indications of a subsurface imperfection that may need an additional NDT method to provide verification of the subsurface discontinuity. VT is often taken to be effective when it is performed at all stages of any new fabrication and is the main method used during the inspection of pressure equipment. If applied after welding has been completed, it is possible that subsurface flaws may not be detected. Thus it can be said that VT will only be fully effective if it is applied throughout any fabrication or inspection. An effective VT that is applied at the correct time will detect most defects or discontinuities that may later be found by some other costly and time-consuming NDT method. A flaw, such as incomplete fusion at the weld root, can be repaired easily and quickly right after it is produced, saving on expense and time required repairing it after the weld has been inspected using some other NDT technique. VT provides immediate information on the condition of pressure equipment regarding such things as corrosion, bulging, distortion, correct parts, failures, etc. VT requires three basic conditions to be in place. Good vision: to be able to see what we are looking for, good lighting: the correct type of light is important & experience: to be able to recognize problems. As mentioned previously, one of the advantages of VT is that there is little or no equipment required, which improves its economy or portability. Equipment so as to improve the accuracy, repeatability, reliability, and efficiency of VT, include various devices. Magnifying glasses can also be used for a more detailed look at some visual feature. As such proper care must be taken to avoid making erroneous decisions regarding the size or extent of some discontinuity when its image is magnified.

Demagnetization

Demagnetization is possible in different ways. One of the most common is to subject the magnetized part to a magnetizing force that continually reverses its direction while it is gradually decreasing in strength. As the decreasing magnetizing force is applied, first in one direction and then in the opposite direction, the residual magnetization of the part is decreased. Generally, a high-intensity demagnetizer is used. The demagnetization is most common but does not demagnetize as deep or complete as a DC step down unit. This decreasing magnetization is accomplished by smaller and smaller hysteresis loops created by the application of decreasing current. A smaller and narrower loop shows lower residual magnetism. All steels have a certain amount of coercive force, making it extremely difficult if not impossible to demagnetize them completely. The only way to completely demagnetize some materials is to heat them to their Curie point or above. Under normal conditions, a part is considered to be satisfactorily demagnetized if, when checked with a field indicator, the magnetic field is below minimum limits. The Code requires demagnetization when the residual field in the part:

• Could interfere with subsequent processing or usage such as machining operations where chips will adhere to the surface of the part of the tip of a tool may become magnetized from contact with the magnetized part. Such chips involve in smooth cutting by the tool adversely affecting both finish and tool life. Other reasons to demagnetize would be in cases where residual magnetism:

• May interfere with electric arc welding operations. Residual magnetic fields may deflect the arc away from the point at which it should be applied.

• May interfere with the functioning of the part itself, after it is placed into service. Magnetized tools, such as milling cutters, hobs, etc., may hold chips and cause rough surfaces, and may even be broken by adherent chips at the cutting edge.

• Moving parts, especially in the oil, may hold particles; for instance, on balls or races of ball bearings, or gear teeth causing wear.

•capable of holding particles that interfere with later applied coatings such as plating or paint. Demagnetization may not be required where:

• Part material is low carbon steel and has low retentivity.

• The material consists of structural parts such as weldments, large castings, boilers, etc., where the presence of a residual field would have little or no effect on the proper performance of the part.

• The the part is to be subsequently processed or heat-treated and in the process will become heated above its Curie point or about 770 °C (1390°F) for steel.

• Apart is to be subsequently re-magnetized in another direction to the same or higher level at which it was originally magnetized as, for example, between the steps of circular and longitudinal magnetizing, for MT purposes.

Plasma Spray

A plasma spray torch includes nitrogen, hydrogen, or helium in some cases, is permitted to stream between a water-cooled copper anode and a tungsten cathode. An electric arc is started between the two anodes through a high recurrence release and is then managed to utilize a powder. The arc ionizes the gas, making high-pressure plasma. The subsequent increment in gas temperature, which may surpass 30,000°C, thus expands the gas volume and consequently its pressure and speed as it leaves the nozzle. Gas speed, which may be supersonic, must not be taken as molecule speed. In plasma splash torch the power level range from 30 to 80 kW, reaching as big as 120 kW. Argon is generally picked as the foundation gas because it is chemically inactive and has great ionization qualities. Including the diatomic gasses, hydrogen or nitrogen can build the gas enthalpy. The powder is generally brought into the gas flow either simply outside the light or in a separating way out locale of the nozzle (anode). The powder is warmed and quickened by the high-temperature, high-speed gas plasma flow. Torchworking parameters and design are vital in deciding the speed and temperature achieved by the powder molecules. The working items incorporate not just gas stream, power level, powder feed rate, and bearer gas flow, additionally the separation from the substrate (standoff) to the torch and the deposition angle. The standoff is of significant importance because satisfactory separation must be accommodated warming and quickening the powder, yet excessive separation will permit the powder to cool and lose speed as the gas stream is quite chilling and moderating off. The size and morphology of powder particles affects the rate of warming and speeding up and thus, the effectiveness of testimony and covering quality. As often as possible, a to some degree higher cost for powder with more tightly size appropriation is more than adjusted for by the enhanced deposition effectiveness. Powder speeds as plasma splash deposition range from around 300 to 550 m/s. Temperatures are frequently at the melting point or marginally above. By and large, higher temperatures and molecule speeds over the melting point however without extreme super-heating, yield coverings with the most astounding densities and bond qualities.