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.