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Pipe Fittings

This chapter presents various types of pipe fittings. Of all the fittings, the elbow is the one most often used. Simply put, the elbow, or ell, is used when a pipe changes direction. Elbows can turn up, down, left, right, or any angle in between. When one finds it necessary to draw a 90° elbow or calculate how much space it will occupy in a routing configuration, knowing its length becomes essential. An elbow's length is commonly referred to as the center-to-end dimension and is measured from the centerpoint of its radius to the end of either opening. Dimensional sizes of fittings are typically provided by the manufacturer of the fitting. Manufacturers issue dimensioning charts containing lengths for a particular fitting. Another elbow that may be used under certain circumstances and with permission from the customer is the 90° short-radius elbow. The 90° short-radius ell makes a much sharper turn than does the long-radius ell.

Emissions from Pipe Fittings and Gaskets

Threaded pipe fittings in the seal flush line can be significant leak sources, with readings above 1,000 ppm.4,17 Similar emission levels may be measured near the gasket region on the seal chamber face. Any leakage from these areas may drift into the emission measurement area for the mechanical seal. The mechanical seal may then be erroneously implicated as a leaker. It should be standard practice to sniff nearby hydraulic fittings and the flange gasket area if excessive VOC concentrations are detected adjacent to the mechanical seal.

Leak-tight threaded pipe fittings can be more easily attained using anaerobic paste-type sealants rather than PTFE tape. The seal chamber face must be smooth to be emission tight. Gaskets and O-rings must be free of nicks and scratches.

32.16.2 Thermoplastic Fittings Manufacturing

Thermoplastic pipe fittings may be injection-molded, fabricated, rotomolded, or thermoformed. Injection-molded fittings are generally made in sizes through 12-in. nominal diameter. Typical molded fittings are tees, 45-degree and 90-degree elbows, reducers, couplings, caps, flange adapters, stub ends, branch saddles, service saddles, and self-tapping saddle tees. Electrofusion couplings and fittings are either made by injection molding or machined from pipe stock. Electrofusion fittings and couplings are made with a coil-like integral heating element incorporated into the fitting. Joining with other fittings uses an electrical fusion device that provides electricity into the heating element, which melts the adjacent thermoplastic material and creates a fusion-welded joint.

Larger-diameter fittings exceed the capabilities of injection molding and are typically fabricated. Rotomolding is used for the manufacture of polyethylene large-diameter (up to 60 in.) and custom fittings for polyethylene corrugated drainage piping applications.

Thermoformed fittings are made by heating a section of pipe and then using a forming tool to reshape the heated area. Examples of thermoformed fittings are sweep elbows, swaged reducers, and forged stub ends. Some polyethylene corrugated pipe fittings and appurtenances are also thermoformed.

All proprietary joints shall be made in accordance with the manufacturer’s instructions. Care shall be taken to establish satisfactory jointing techniques for all water service pipework. When making joints by welding, brazing, or soldering, precautions shall be taken to avoid the risk of fire. All burrs shall be removed from the ends of pipes and any jointing materials used shall be prevented from entering the waterways. All piping and fittings shall be cleaned internally and free from particles of sand, soil, metal filings, and chips, etc.

8.19.3 Cast iron pipes

Flexible mechanical joints shall be made in accordance with the manufacturer’s instructions.

For molten lead joints, the spigot and socket shall be centered with rings of dry yarn caulked tightly into the bottom of the spigot to prevent the entry of lead into the bore of the pipe and to prevent contact of lead with the water.

Synthetic yarns that do not promote the growth of bacteria shall be used to prevent contamination of the water. The remainder of the joint space shall be filled with molten lead (taking care that no dross enters the joint), cold wire, strip, or spun lead (lead wool). The joint shall be caulked to a smooth finish with pneumatic tools or a hand hammer of mass not less than 1.5 kg. When working with spun lead, caulking tools shall be of a thickness to fill the joint space, ensuring thorough consolidation of the material to the full depth of the socket.

Lead joints shall be finished about 3 mm inside the face of the socket.

Flange joints shall be made with screwed or cast on flanges.

8.19.4 Steel pipes

Welded joints shall not be used where a protective lining would be damaged by heat, or where the pipework is employed as a primary circulation to an indirect hot water heating system.

Screwed joints in steel piping shall be made with screwed socket joints using wrought iron, steel, or malleable double crimping fitting. A thread filler shall be used. Exposed threads left after jointing shall be painted or, where installed underground, thickly coated with bituminous or other suitable corrosion preventative agent.

Flange joints shall be made with screwed or welded flanges of steel or cast iron using jointing rings and, if necessary, a suitable jointing paste. The nuts shall be carefully tightened, in opposite pairs, until the jointing ring is sufficiently compressed between the flanges for a watertight joint.

8.19.5 Unplasticized PVC pipes

8.19.5.1 Mechanical joints

Mechanical joints in unplasticized PVC piping of sizes 2 and upwards shall be made in accordance with BS4346: Part 2, by the use of push-fit integral elastomeric sealing rings which are compressed when the plain ended pipes are inserted into the adjoining sockets. The plain pipe ends shall be chamfered and the surfaces cleaned and lubricated.

The chamfered pipe end shall be inserted fully into the adjoining socket (except where provision is to be made for expansion), or as far as any locating mark put on the spigot end by the manufacturer. The sealing rings shall comply with BS2494.

8.19.5.2 Compression joints

Compression joints shall only be used with unplasticized PVC piping of size 2 and smaller. The joints shall be of the nonmanipulative type. Care shall be taken to avoid overtightening.

8.19.5.3 Solvent cement welded joints

Solvent cement welded joints in unplasticized PVC piping shall be made using solvent cement complying with BS4346: Part 3 recommended by the manufacturer of the pipe. The dimensions of the spigots and sockets shall comply with BSEN1452: Part 1–5.

Joints may also be made using integral sockets formed in the pipes and solvent cemented.

8.19.5.4 Flanged joints

Flanged joints used for connections to valves and fittings shall use full-face flanges or stub flanges, both with corrosion resistant or immune backing rings and bolting.

8.19.5.5 Polyethylene pipes

Mechanical joints shall be either plastics or metal proprietary compression fittings, for example, brass, gunmetal, or malleable iron. These shall include insert liners to support the bore of the pipe except where the manufacturer of the fitting instructs otherwise.

To ensure satisfactory jointing of the materials from which the pipe and transition elbow are made compatibility shall be established. The manufacturer’s instructions shall be carefully followed.

No attempt shall be made to joint polyethylene piping by solvent cement welding.

Large pipe fittings and valve components must be press forged and will require extensive machining. Whereas small parts such as the flange previously described can be quickly heated and cooled, and given optimum process conditions, should exhibit microstructure and properties similar to pipe and tube, the properties of large forgings will be location and thickness dependent. While no large forged part has yet been made from 740H, the properties of a solution-annealed, water-quenched and aged 343-mm-diameter bar shown in Table 14.2 are informative. Yield strength near the surface is comparable to that of thin wall tube, but yield strength at the bar center, while meeting ASME minimum, is significantly lower. Ductility and toughness were good. A hardness traverse taken on the as quenched bar showed VHN 170 at the surface and VHN 290 at the center. This is indicative of strong auto-aging in the bar center. Because the γ′ that forms on slow cooling is relatively coarse, after the final aging treatment, the bar center will have lower strength than the surface. The microstructure and creep strength at the center of the bar has not been evaluated.

A calculated continuous cooling transformation diagram for alloy 740H is shown in Fig. 14.26. This diagram supports the notion that significant γ′ hardening will occur even during water quenching of a large forging. A cooling simulation was conducted for the bar heat treatment using DEFORM software [49]. The cooling rate at a depth of 25 mm was 315°C/min and at the bar center was 30°C/min. Based on the calculated CCT diagram, there should be about 10% γ′ in the center and no γ′ at the surface. That is consistent with the experimental results.

Filament-wound pipe fittings, such as elbows and tees have been used in the chemical, and oil industry since the 1980s.9 Traditionally, composite pipe fittings were produced manually or semi-manually, but the development of CNC winders with six or more axes has allowed automated production of pipe fittings since the 1990s. The efficiency of these advanced machines depends on methods and software to determine winding patterns and perform fabrication of the complex shape within manufacturing specifications. Winding pattern generation is particularly challenging since a substantial amount of data storage/processing is required to meet manufacturing requirements (e.g., fiber tension and full-coverage) of non-axisymmetric patterns, which are required for filament-wound elbows or tees.72 On the other hand, it is worth noting that CAM software capability, rather than hardware, is considered the limiting factor for improving the performance of automated winders of non-axisymmetric parts. Consequently, general-purpose filament winding systems for pipe fittings are currently deemed impractical due to the lack of universal mathematical models and design software for CAM.9,73 Although some progress has been made to determine closed-form solutions for efficient winding patterns on specific shapes, such as elbows,74,75 most CAM systems still implement approximate methods to design and produce specific pipe fitting geometries.73 An illustration of a software-generated winding pattern, and the resulting wound elbow, is included in Fig. 11.75

Leaking valves and pipe fittings are the next concern when pressure is dropping during a test. Test sections should be isolated at pipeline block valves by using slip blinds to insure no leakage. If the test section cannot be blinded but the valves are double blocked instead, the operator must measure pressure increase in the adjacent section between the double-blocked valves to insure a tight seal exists. You need to be careful when using a thin “fire blind” at an isolation valve because under pressure the thin blind will deform and the blind cannot be removed without removing the entire valve. This often requires calling in vacuum trucks to remove product on the opposite side of the test valve being removed.

So, leakage through valves and fittings jeopardizes the chances for a successful test and may lead to data that cannot be correlated, and in that situation, the pipeline must be retested.

Tree piping is defined as all pipe, fittings, or pressure conduits, excluding valves and chokes, from the vertical bores of the tree to the flowline connections. The piping may be used for production, pigging, monitoring, injection, servicing, or testing of the subsea tree. Inboard tree piping is upstream of the first tree wing valves. Outboard tree piping is downstream of the first tree wing valve and upstream of the flowline connector.

Tree piping is normally designed in accordance with ASME B31.3. The guidelines in the API specifications are general and, in many cases, open to interpretation. It is up to the manufacturer to apply his engineering judgment.