Sunday 9 September 2012

Site Welding : Problems And Solutions -Part2

Tools, Equipment and facilities

A compromise between equipment and production costs must be reached. Specialism welding equipment often requires experienced operators, who are not likely to be found or cannot be retained on the permanent basis, owing to non-continuous operations. Often even though a field training program for the use of such equipment is provided, the site welders resent change and do not readily accept new ways of doing things. The delay in overcoming this attitude, and in acquiring sufficient experience for proficient use of the new equipment, mean that its potential economic advantages are not realized. A possible solution to this problem is that operators of such specialized equipments are hired on a permanent basis and are available either directly to run the machines or to supervise their operator. Once a decision has been reached regarding the type and quantity of tools, equipments and facilities required at a given site, the details of the field welding shop and arrangement of the equipment can be based on sound industrial engineering practices.

Environmental effects on site welding

The basis difference between site welding and manufacturing shop welding is the uncontrolled environment at the site. At the site, often only crude, temporary shelters can be justified, usually taking the form of tarpaulins attached to a simple wooden framework to provide a minimum of shelter. Unless welders are provided with a safe and reasonably comfortable working platforms or scaffold, the quality and quantity of work will suffer. Conventional welders uniforms are unsuited to extremes of climate. The helmet, leather jackets, and aprons, are unsuited for the dissipation of body heat in tropical climate. The necessary freedom of movement is impeded by the usual heavy clothing provided for arctic climates. Possible improvement is the use of light weight fireproof fabrics for tropical climates, and electrically heated suits, as used in aircraft practice, for arctic conditions. The need for the acclimatization of welders is another factor emphasizing the advantages of employing and training local labour for site welding. Extremes in environment have their effect on the weld deposit as well as on the operator. In tropical conditions the problem of maintenance of preheat may be nicely solved whereas in arctic climates, it will be accentuated. In frigid locations, the need for preheat can be minimized by selection of base material and electrodes with low transition temperatures to reduce the hazards of brittle fractures. High wind velocities, if there at site, will considerably effect the welding arcs and hence the weld deposits.

Site welding quality control

A discussion is necessary between the contractor and the client before work is started, to agree upon the interpretation of the applicable code or specification as to weld quality and the manner and extend to which the various inspection tools are to be employed. The need for this mutual understanding is specially evident in regard to pipe welding quality, where it is fairly common practice to apply pressure vessel code criteria of acceptability to pipe which can only be welded from the outside. Here difficulties frequently arises out of the welding operators inability to cope with the irregularities to be expected in mill end pipe. Difference in internal diameter, out of roundness and inability to rework the route of the weld, frequently lead to indications on radiographs that tax the ability of an extremely skilled interpreter to render a satisfactory estimate of weld quality. Once the welding quality or qualities have been agreed upon, the problem of producing the welds to these requirements becomes paramount. The first, vitally important, prerequisite is the accuracy of fit-up of the parts to be welded. A welder of just adequate skill can generally make an acceptable weld if he starts with good fit-up, whereas the most skilled welder will frequently find himself unable to cope with the situation if the fit-up is poor. The welding supervisor and inspectors can most profitably pay their way by insuring that the welders start with well set-up jobs. The depositing of the root weld pass is a critical factor, whether it be pipe or plate that is being welded. Where skilled welders are in short supply, they may deposit only the root passes whereas the remainder of the weld can generally be completed without difficulty by welders of lesser ability. The maximum emphases on fit up and root welding applies to all qualities of welding, but varies in degree with the economics of the quality aimed at. For example on the lowest quality of welding, accessed only by visual examination, accurate fit up by experienced pipe fitter and extensive supervision, inspection, etc cannot be provided at the cost allowed for this quality of welding. Usually non-destructive tools like radiography, ultrasonics, magnetic particle inspection, penetrant and fluorescent dyes, and visual aids can be employed to the extend required to provide assurance that a given weld quality is being furnished.