Site Welding -Part 1

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.

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.

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.

Here we will discuss about the qualification tests for welders, economic choice of tools, the environmental contingencies of open air welding and quality control. There are innumerable factors which can be considered in regard to site welding so no single operational policy can be established to cover all site conditions. Therefore some of the factors affecting site welding in general will be discussed.

One of the major decisions in pressure equipment construction work like vessels, exchangers, piping, etc concern the division of work between sub-contracts to permanent fabricating shops and the site forces. Generally, where the freedom of choice exists, the decision would favor the permanent fabricating shops rather than the site, but this is not always the case. Exceptions are large projects of such duration as to justify field shops, with facilities which may surpass those of competitive manufacturers, and which may be limited to welding operations or may include forming, bending, heat treatment, inspection, etc. In the absence of such field facilities, site assembly is limited to constructions whose dimensions exceed shipping clearances or acceptable shipping costs, and to let items which are switched to field fabrication as a result of vendor or mill delays, changes, additions, etc. Often such site welding will reach an impressive percentage of the overall project. From the unit cost stand point, shop fabrication is usually lower, although the difference is minimized and not infrequently reversed where the site has a favorable work load and attendant higher utilization of manpower or manufacturer handling cost are excessive owing to dimensions or weight or where delivery requirements involve appreciable shop overtime or bonus payment or where careful timing of delivery is necessary owing to space limitation and freight demurrage. As regards the relative quality of works performed, the site fabrication may surpass that of competitive manufacturers, because, the major factor is acceptance of the need for more adequate supervision and inspection to compensate for the disadvantages of less favorable conditions, in addition there is the greater availability at the site of the customer's inspection staff and engineers who have a direct concern as to adequacy of the work, whereas the manufacturer's shops are less conveniently located and his organization is much less directly concerned.

Lack of successive project continuity and attendant economies militate against a permanent welding operator force that can be moved from job site to job site. The assembling of an adequate number of welders of the various degrees of required skill at a given location can be frequently be a major problem. The most that can be done progressively to improve the general level of skill and versatility is to maintain a nucleus of specialist for operations demanding maximum skill, who also act as instructors to the numerous project-hired operators, most of whom are new with the company and often unacquainted with the type of work involved. The remaining training burden must be carried by the supervisors, foremen and engineers. Standard qualification test for welding operators may be inadequate as a measure of ability to perform production work at a specific level of quality. For improved assessment after the qualification test, the operators are employed on non-critical work, such as temporary structures, supports, etc. which enables the welding supervisors and inspectors to evaluate individual skills and establish various degrees of proficiency so that tasks may be assigned in accordance with ability. Radio-graphic examination of qualification test plates would be helpful. However a test plate represents the best quality of which an individual operator is capable, rather than the average which he will attain in production. If at the job site, sufficient number of skilled workers are not available, two alternatives are available- to train welders at the site or to recruit skilled workers where available and transport them to the site. The first alternative, at many places, has been especially successful, particularly because of the usual willingness and enthusiasm displayed by unskilled or native labour when learning a trade. It is only natural that the importation of skilled labour is constructed as a reflection on their latent capabilities. A factor that usually militates against the importing of skilled labour is the psychological environmental effect. Men have been selected for foreign work on the basis of skill and productivity in their local environments and frequently at the destination become dissatisfied so that their skill and productivity deteriorated. This is particularly applicable to welding as contrasted to other crafts, probable owing to the less continuous and strenuous effort involved, and the consequent opportunity for the mind to wander. Therefore in selecting welders for overseas work, one would be well-advised to base selection on emotional stability more than on skill and productivity.

Site Welding - Part 2

Figure shows a cross-section of an air cooled IC engine depicting the principal parts. Generally, for stand-by plant, water cooled engine is preferred but where there is scarcity of water or in mobile power plants, air cooled engine is preferred. The principal parts are cylinder, cylinder head, piston, inlet valve, Inlet port, Exhaust valve, Valve spring, cooling fins, wrist pin, connecting rod, crankcase, crankpin, crank, crankshaft.

The function of air intake system is to convey fresh air through louvres and air filter to the cylinder via intake manifold. In order to augment the power, supercharger is fitted in between the filter and engine and the super charger is driven by the engine itself.

The purpose of exhaust system is to discharge the engine exhaust to the atmosphere with minimum noise. Figure shows the exhaust system. The exhaust manifold connects the engine cylinder exhaust outlet to the exhaust pipe which is provided with a muffler or silencer to dampen the fluctuating pressure of the exhaust line which in turn reduces most of the noise which may result if gases are discharged directly to the atmosphere. It is advisable to use flexible tubing system for exhaust pipe to take up the facts of expansion due to high temperature and to isolate the exhaust system from the engine vibration.

Appreciable amount of heat from the engine exhaust goes as a waste. In order to utilize this, a heat recovery steam generator (HRSG) may be used to generate low pressure steam for process work.

It is supposed to be the heart of diesel engine and its failure means stopping of the engine. The fuel injection system performs the following functions:

1. It filter the fuel insuring oil free from dirt.
2. It meters the correct quantity of fuel to be injected in each cylinder.
3. It times the injection process in relation to the crankshaft revolution.
4. It regulates the fuel supply.
5. It atomizes finely the fuel oil for better mixing with the hot air leading to efficient combustion.
6. It distributes, the atomized fuel properly in the combustion chamber.

There are two ways to atomize the fuel. In one case, air injection is used while in other methods, pressure or mechanical or solid injection is used. Nowadays the air injection is obsolete and mechanical injection is invariably used. In mechanical or solid injection system, the fuel oil is forced to flow through spray nozzles at pressure above 100 bar. There are three types of solid injection system namely:

1. Common rail injection system
2. Individual pump injection system.
3. Distributed system.

1. Common rail injection system: As the name implies, a single pump supplies fuel under high pressure to a fuel header or common rail as shown in figure from where the fuel goes to each of the nozzle located in the cylinder. The timing of injection is maintained by mechanically operated valve and the amount of fuel is controlled by the push rod stroke.

2. Individual pump injection system: As the name implies, the system has an independent high pressure pump for each cylinder which meters, pumps and controls the timing of fuel injection as shown in figure. Each cylinder is provided with one injector and the pump and injector may be integrated as one unit. The fuel is brought to the individual pump from storage tank through course filter, low pressure pump and fine filter. The high pressure pump is equipped with a control mechanism and at the proper time, a rocker arm actuates the plunger and thus injects the fuel into the cylinder. The amount of fuel injected is regulated by the effective stroke of the plunger. It is the most popular fuel injection system in practice.

3. Distributed system: Figure shows arrangement of distributed system. In this system a metering and high pressure pump is used to pump the metered quantity of fuel on the rotating distributor which distributes the fuel to the individual cylinder at the correct timing. The number of injection stroke per cycle for the pump is equal to the number of cylinders. The fuel is fed to the high pressure pump from storage tank through course filter, IP pump and fine filter. Since the metering and timing of injection is accomplished by one plunger, equal amount of fuel is supplied to each cylinder at the same point in the cycle.

The liquid fuel in the injection system filters into the combustion chamber through the injector. Fuel injector employed in CI engine is of automatic type. It is mounted on the cylinder body at such a location which yields better performance. Quick and complete combustion is insured by a well designed fuel injector. The fuel injector assembly consists of the following:

1. Needle or nozzle valve
2. A compression ring
3. A nozzle
4. Injector body

Figure shows a cross-sectional view of a typical bosch fuel injector. The fuel from the fuel pump is fed down to the nozzle mouth through long drilling passage. The fuel pressure acts on the differential area of the nozzle valve which lifts against the spring force, and thus allows the fuel to enter into the combustion chamber via small orifice (holes) in the form of finally atomized spray. Once the fuel from the delivery pump gets exhausted, the spring pressure pushes the nozzle valve back on its seat.

The design of nozzle is mainly based on the types of combustion chamber used insuring proper and efficient combustion of fuel. The type of nozzles used in diesel engines are:

1. Single hole
2. Multi hole
3. Pintle type
4. Pintaux type

Force circulation cooling system

• Open cooling system: This system is applicable only where plenty of water is available. The water from the storage tank is directly supplied through an inlet valve to the engine cooling water jacket. The hot water coming out of the engine is not cooled for reuse but it is discharged.
• Natural circulation system: The system is closed one and designed so that the water may circulate naturally because of the difference in density of water at different temperatures.It consists of water jacket, radiator and fan. When the water is heated, its density decreases and it tends to rise, while the colder molecules tend to sink. Circulation of water then is obtained as the water heated in the water jacket tends to rise and the water cooled in the radiator with the help of air over the radiator either by ram effect or by fan or jointly tends to sink. The direction of natural circulation which is slow is shown by arrows.
• Force circulation cooling system: Figure shows force circulation cooling system which is closed one. The system consists of pump, water jacket in the cylinder, radiator, fan and a thermostat. The coolant is circulated through the cylinder jacket with the help of a pump which is usually a centrifugal type, and driven by the engine. A function of thermostat which is fitted in the upper hose connection initially prevents the circulation of water below a certain temperature, usually upto 85C through the radiation so that water gets heated up quickly. Stand-by diesel power plant upto 200 kVA use this type of cooling.

Lubrication system:

The following are the applications of diesel electric power plants:

1. Pick load plant: Diesel plants are suitable as pick load plants in combination with  thermal or hydal plant. The pick load unit needs easy, starting and stopping and diesel plants serve this purpose.
2. Stand-by unit: There are many situations in which stand-by units are needed such as the main unit fails or cannot cope with the demand. As an example due to less rainfall in a particular year, the hydro plant cannot meet the demand. Thus diesel units are installed as stand-by unit to supply power in parallel to generate the short fall of power.
3. Central stations: Due to ease in installation starting and stopping, diesel electric plants can be used as central station where the capacity requirement is more.
4. Starting station: For starting large power plant, auxiliaries such as FD and ID fans, BFP, circulating water pump, etc can be run by installing a diesel electric power plant.
5. Energy unit: In many developing countries like India, power failure/interruption is a common feature for hours. Under this circumstances, the power to vital units such as hospitals or industrial plants can be supplied by installing diesel electric power plant.