In the industrial world, the term piping is usually understood to cover pipe; tubing; fittings such as tees, elbows, flanges and reducers; valves and hearders used in oil refineries, power stations, nuclear plants, chemical and petrochemical plants and other industrial plants.

The term pipelines usually applies to long transmission pipelines designed to conduct liquids such as water, crude oil and petrol, and gases such as natural gas.

Today, piping systems and pipelines in industry are almost fully welded. Threaded joints are rarely used. Flanged joints are used only where sections have to be opened for internal inspection or replacement.

Piping and pipelines are dealt separately in this section. Penstocks are also considered to be transmission pipelines, but for convenience they are dealt with in the section on power generating plant.

11.1 PIPING

Industrial pipings are critical items in a production plant and they frequently operate under high pressures, high temperatures and in corrosive atmospheres. The efficiency, productivity and safe operation of plants depend to some extent on how effectively, piping systems withstand the rigours of service. Serious consideration has to be given to the selection of grades and sizes of materials, design, fabrication, erection, testing and inspection. Guidance is provided by various codes and standards applicable to weld piping systems prepared by technical societies, trade associations and standardisation bodies. For example, the American National Standards Institute (ANSI) has issued Code for Pressure Piping, which covers Power Piping, Industrial Gas and Air Piping, Pertoleum Refinery Piping, Oil Transportation Piping, Refrigeration Piping, Chemical Industry Process Piping, Nuclear Power Piping, Gas Transmission and Distribution Piping Systems.

Piping connected to boilers are covered in several sections of the ASME Boiler and Pres­sure Vessel Code. The American Petrol Institute (API) has issued a standard for Field Welding of Pipe-Lines. ASME Guide for Gas Transmission and Distribution Piping Systems is another useful publication. The American Welding Society has published the following recommended welding practices :

(a) Welding of Austenitic Chromium-Nickel Steel Piping and Tubing, D10.4 (1966).

(b) Welding of Chromium-Molybdenum Steel Piping, D 10.8 (1961).

(c) Recommended Practices for Gas Shield-Arc Welding of Aluminium and Aluminium Alloy Pipe, D10.7 (1960).

(d) Welding Ferrous Materials for Nuclear Power Piping, D10.5 (1959).

(e) Gas Tungsten-Arc Welding of Titanium Piping and Tubing, D10.6 ( 1959).

To ensure satisfactory welding of piping installation, it is first necessary to establish and qualify the welding procedure covering base metal specifications, filler metals, edge preparation and joint fit-up, pipe position, welding process, process parameters, welding techniques, preheat, interpass and postheat schedules, and final inspection and testing. It is also necessary to qualify the welders for the welding procedure adopted. Standard procedures for the qualification of welders and welding machine operators are given in relevant codes, for example, in section IX of the ASME Boiler and Pressure Vessel Code.

Pipe materials and fittings are available in standardised specifications, sizes and with standard tolerances. Pipes are available in long lengths as seamless or welded pipes. Pipings are longitudinally welded in a tube mill from strips by using the electric resistance butt or high-frequency resistance welding process, while pipes for pipelines are welded along their long seams in a pipe mill by the automatic submerged-arc or MIG/CO2 process. In the erection of pipings and pipelines, welding is restricted to girth joints or to joints between pipes and their attachments. Hence in the following sections, only girth welding techniques are described.

The metals used for piping are : carbon steel, wrought iron, C-Mo steels, Cr-Mo alloy steels, cryogenic steels, stainless steels, Al and its alloys, Ni and its alloys, Cu and its alloys and Ti and its alloys.

Carbon steel. Carbon steel piping is mostly welded by the manual metal-arc process using E6010 or E7018 class of electrodes. For critical applications which demand full penetration welds, split or solid backing rings are provided on the inside, or the well-penetrated root pass is made with the TIG process as described in Chapter 5. This technique applies to all metals. MIG/CO2 process using gas mixture of CO2 and argon is used on less critical piping, where full root penetration and fusion are not essential. In shop fabrication of thick-walled pipe having

O. D. of more than 200 mm, automatic submerged arc welding is used for the filling passes, after the root pass has been completed with the manual metal-arc or TIG process. If backing rings are used and the fit-up is good, the entire joint can be made by the SA process. Generally preheating is not necessary if the carbon content of the steel is below 0.30%. If the wall thickness exceeds 19 mm, postweld heat treatment is usually recommended. It consists of heating to 600- 650°C and holding for one hour per 25 mm of wall thickness, with a minimum holding time of 30 min, and then cooling in still air. For further details, relevant codes must be consulted.

During manufacture of boiler units large number of tube butt welds have to be made with the tubes positioned at any angle from horizontal to vertical, and being often in positions of restricted access. Automated orbital TIG welding machines with automatic cold wire feed have been developed for this purpose. A typical orbital TIG welder has a weldhead, covering tube sizes in the 25-50 mm O. D. range and requires only 44.4 mm clearance between adjacent tubes. It features an integral wire-feed system, i. e., the wire-feed facility is mounted on the head and rotated with the electrode block. Arc-voltage control provides a means of maintain­ing a constant preset distance between electrode and workpiece. These facilities allow for a number of continuous orbits (i. e., multiple weld pass) to be made around the tube joint. Such a machine can be applied on pipings of all industrial metals. Lately welding heads capable of joining tubes 18.2 mm O. D. with a clearance of only 16.8 mm have been produced.

Wrought iron. Wrought iron piping has low carbon content (0.12% maximum). It is usually welded by the manual metal-arc process. It is advisable to use low welding currents and speeds. Preheating and postheating are generally not required.

C-Mo steel. The welding processes used for these steels are the same as those used for carbon steels. For manual welding, electrodes of E7010-A1, E7016-A1 or E7018-A1 are used. For SA welding, the Mo alloy of the weld-metal is derived either from the wire or the flux. Preheat and postheat data are given in Chapter 10 while discussing the weldability of these steels.

When used in service temperatures exceeding 425°C, C-Mo steels have been known to undergo graphitisation, i. e., the carbon transforms to nodules of graphite, which substantially reduces the toughness of the steel. Though such unfavourable phenomenon can be suppressed by stress-relieving the welded joints at 720°C for four hours, use of C-Mo steel pipings for high temperature applications is being discouraged.

Cr Mo steels. These grades are mostly used for service in the 400-593oC temperature range. They are usually welded by the manual metal-arc process, using low-hydrogen type low-alloy steel electrodes of matching alloy contents.

For submerged-arc welding, it is advisable to use neutral flux and alloyed wire in preference to alloyed flux and neutral wire, because in the latter case, the alloy balance in the weld deposit gets upset during multi-pass welding at high interpass temperatures.

Low-temperature steels. The types of steel used for various low-temperature service pipings are given in Table 11.1. They are usually welded by the MMA process. The suitable AWS classes of electrodes are indicated in the Table. Preheating is a must for Ni steels, because nickel renders the steel to get air-hardened. Preheat and postheat data are given in Chapter 5.

Martensitic stainless steels. These are hardenable steels and are susceptible to cracking during welding. Preheat and postheat operations are necessary. The postweld heat treatment must immediately follow the completion of welding without withdrawing the preheat.

Welding data are given in Table 11.2. If for some reasons postheating is not possible, type 310 or 309 stainless steel filler wire must be used.

Ferritic stainless steels. These steels are less susceptible to cracking during welding than the martensitic types, but they may become embrittled due to the high temperatures attained during welding and consequent grain growth. To remove embrittlement, the steel is annealed for one hour between 705 and 790°C, and then quenched or air-cooled. The welding data are given in Table 11.3.

Al and its Alloys. These alloys are commonly welded by the TIG process and in some cases by the MIG process. Before attempting to weld pipings, welders must undergo training and gain some experience. In welding horizontally positioned fixed piping, the molten metal sinks due to its high fluidity. Aluminium backing rings and consumable insert rings are some­times used to obtain good root penetration. Preheating is generally not necessary, but may be used with advantage when the diameter exceeds 60 mm. Preheat temperature ranges between 280 and 3000C. Some Al alloys are unfavourably affected when preheated above 2000C. Hence, high preheat temperatures must be used with care.

Ni and its alloys. These alloys are commonly used in piping because of strength properties, good corrosion resistance to many acids, and easy weldability. They can also be readily welded to ferritic and austenitic steels. The welding processes commonly used are : MMA, TIG and MIG. Backing rings should not be used, because they promote crevices, root cracks and corrosion. Consumable insert rings should be preferred. During root pass welding, the inside of piping must be purged with inert gas, which can be helium, argon, hydrogen or their mixtures.

It is important to remember that Ni and its alloys are susceptible to embrittlement by accidental presence of lead, sulphur, phosphorus and some low-melting metals.

Copper and its alloys. They are commonly welded by oxyacetylene, MMA, TIG and MIG processes. It is advisable to use backing rings whenever possible, because of the high fluidity of molten copper. Because of the high heat conductivity of copper, preheating with a gas torch is necessary when large diameter or heavy-walled pipes are being welded. Red brass and yellow brass are preferably welded by the oxyacetylene process to minimise vaporisation of zinc. Cupronickel 30 (i. e., 70:30 alloy) is extensively welded and used for water pipe and condenser tubing on ships, because of its superior resistance to sea water corrosion. The most suitable welding processes for this alloy are MMA and TIG.

Ti and its alloys. Welding of these materials demands special techniques and specialised skill on the part of the welder. Pipes of wall thickness 1.6 mm and below are normally welded by the TIG process without filler wires. For heavier pipes, filler metals are used. Unless the filler wire is thoroughly cleaned and handled with care, it can contaminate the weld. Contamination also occurs if the hot end of the wire is withdrawn from the gas shield and exposed to atmosphere during intermittent deposition. Special care must be taken that there is 100% root penetration all over the joint. A small root defect can develop into a crack during service and lead to serious failure.

Dissimilar metals. Pipings of dissimilar metals often welded in power plants, oil refineries, nuclear plants, etc. The metals commonly involved are carbon steels, low-alloy steels, stainless steels and nickel and its alloys. Normal welding procedures can be used in these cases, because the melting points of these metals are fairly close. The main considerations are filler metal compositions and preheat/postheat temperatures. For dissimilar joints involving non-ferrous alloys, the filler metal and welding procedure must be carefully determined after studying the metallurgical aspects of the joint in question.

Fig. 11.1 Edge preparations of pipe end for MMA welding

Sometimes, it helps to butter the joint edge metal having the higher melting point before final welding. For example, when carbon steel is to be joined to silicon-bronze, the carbon steel is buttered with silicon-bronze weld deposit. When the metals to be joined have widely different melting points, brazing, braze welding or soldering should be resorted to.