15.12 Steel Pipe Manufacture and Materials
BS 534 covers carbon steel pipes, joints and specials (bends and other fittings) but is partly replaced by BS EN 10224 (pipe ranging from 26.9 to 2743 mm outside diameter using steel of yield strengths 235, 275 and 355 N/mm2) and by BS EN 10311 for joints. BS EN 10312 covers stainless steel pipe. EN standards have been published and others are under development for polyethylene, galvanized, liquid epoxy and polyurethane coatings, mortar linings and external concrete and insulating coatings, but reference can be made to BS 534 for coatings and linings. BS EN 10224 uses pipe consistent with BS EN 10216-1, 10217-1 and 10220, but pipe to ISO 3183 (API 5L) and other standards can be used. CP 2010 Part 2 for design and construction of steel pipes on land remains current. BS EN 1295-1 covers structural design of buried pipelines for the water industry. Eurocode 3: BS EN 1993-4-3, issued in 2007, applies to design of steel pipelines which are not treated by other European standards covering particular applications; it can be used as soon as its national annex is published. This code requires consideration of 5 ultimate limit states, including fatigue, and three serviceability limit states including vibration. PD 8010-1 and -2 are intended primarily for oil and gas pipelines but apply to and provide useful design information for the water industry.
BS EN 10224 covers four principal welding methods for manufacture: butt (BW)—outside diameter up to 114.3 mm; electric (resistance) welded (EW)—outside diameter up to 610 mm; seamless (S)—outside diameter up to 711 mm and submerged arc welded (SAW)—outside diameter 168.3 to 2743 mm. In ISO 3183 the designations EW and SAW are recognized but seamless pipe is designated SMLS and LW means laser weld.
Steel pipes are fabricated from steel plate bent to a circular form or they may be continuously produced from a coil of steel strip bent to a spiral and butt welded along the spiral seam. Joints between coil ends of spiral welded pipes are known as skelp end welds. Butt welded pipes are made from rolled strip with a longitudinal seam furnace butt welded by a continuous process. Lengths of pipe are usually in the range of 9 to 12 m dependent on manufacture, transport and project requirements. Weld beads must be machined flush with the pipe surface at pipe ends to make them suitable for joint couplings. Spigot and socket ends, where shaped, are formed by die. Weld bead height needs to be limited for coating and lining. Electric (resistance) welding is done by passing electric current (by induction or direct contact) across the edges which are joined under pressure, without filler metal. Heat treatment at least of the weld zone is usual in sizes larger than DN 200. EW pipes now tend to be known as HFI (high frequency induction) pipes. Inspection typically includes chemical and mechanical material tests, ultrasonic inspection of plate and welds, radiography of welds and hydraulic pressure tests.
There are no standard classes for steel pipes: wall thickness above about DN 750 is designed for handling; internal pressure; buckling under external pressure and internal sub-atmospheric pressure; and to limit deflection when buried. External load carrying capacity in trunk mains is mostly a function of the backfill and compaction design. BS 534 sets out nominal wall thicknesses considered to be the minimum for handling and typical buried installations.
Steel grades as designated in ISO 3183 and, as from 2008, the American Petroleum Institute standard API 5L are designated by grade and by yield stress in thousands of psi, as Table 15.3. Grades less than grade B would not normally be used. Grades up to about X60 can normally be welded without special heat treatment. Their price is only marginally above that for grade B and provide good economy where high pressure or (typically for pipes above ground or installed underwater) significant longitudinal bending resistance is required.
Table 15.3. Steel grades to API 5L / ISO 3183
AWWA M11 gives a range of thicknesses and pressures and steels for diameters up to 4000 mm. Sizes in M11 are designated by outside diameter below 30 inches (762 mm), otherwise by inside diameter.
Pipe wall thickness, t (mm) for internal pressure is determined by hoop stress, as follows:
where P is the internal pressure (N/mm2); D is the external diameter (mm); a is the design or safety factor; σ is the minimum yield stress (N/mm2); and e is the joint factor. The design factor, joint factor and definition of wall thickness depend on the design code. Design factors for hoop stress typically range from 0.4 to 0.8; the joint factor is 1.0 for SAW pipes and certain codes require the negative tolerance to be deducted from wall thickness. ASME codes B31.4 and B31.8 quote a basic design factor of 0.72 and state that this includes for thickness tolerance. For water supply under normal conditions, it is suggested here that the design factor of 0.5 (as given in AWWA M11 and the WRc pipes selection manual) is overly conservative and that, for high pressure long distance pipelines, a factor of 0.72 is realistic (after deducting thickness tolerance and any corrosion allowance) and up to 0.83 may be considered in some circumstances (PD 8010, BS EN 14183). For many water supply pipelines wall thickness is determined by handling and installation and the need to control deflection.
Further consideration can be given where particular conditions warrant: for example the American Society of Mechanical Engineers (ASME) code B31.8 quotes design factors for a variety of laying conditions. Where necessary the analysis can be elaborated to include ring bending, longitudinal bending, longitudinal stress from temperature changes, Poisson's ratio effects on buried (and thus restrained) pipe under hoop tension, combined (equivalent) stresses and where appropriate, for example for underwater pipes, can include strain based design.
BS EN 10224 and BS 534 give dimensions for common fittings, for example bends and branches. However, fittings can be made to any dimensions required, bends being made by cutting and welding together sections of pipe. For outside diameters up to 1016 mm, bends can be made by forming. Design of fittings and of any reinforcement needed is described in AWWA M11.