Table 3–4 Examples of Sheet Gage Metal Thickness
Gage 16 14 12 11 10
SMACNA (in) .0538 .0667 .0966 .1116 .1265
AWS rolled (in) .0678 .0747 .1046 .1196 .1345
AWS galvanized (in) .0710 .0785 .1084 .1233 .1382
3.4.4 Pipe Specifications
A piping system is made from a number of pipes, fittings and components. Having selected the pipe material and grade, the selection of the matching fitting or component, forging or casting specification and grade is not self evident. Because of the multitude of pipe materials, engineering firms or projects will develop pipe specifications (“pipe specs” or “pipe codes”) for different types of services. For example, the pipe specification for a stainless steel system may list the pipe as ASTM A 312 TP304; small bore fittings (2” and smaller) as ASTM A182 F304; large bore fittings (larger than 2”) as ASTM A403 WP-304; and flanges as ASTM A 182 F304. An example of pipe specification is given in Chapter 4.
Pipe fittings are fabricated to conform to the dimensional requirements of standards ASME B16 (Chapter 1). Each piping and pipeline engineer should take the time to carefully read at least ASME B16.5 (flanges), ASME B16.9 (butt welding fittings), and ASME B 16.11 (threaded and socket welding fittings). Often times, engineers and mechanics are surprised to find out that these standards do not control every dimension of the fitting; they do specify end-to-end lengths and end bevels, but they will not necessarily specify maximum wall thickness, and much of the fitting quality is left to the competence of the fabricator, with no enforcement authority to verify that a fitting marked ASME B16 does indeed comply with the standard’s materials, fabrication, heat treatment and dimensional requirements. The same applies to proof testing. Fittings are qualified by proof testing of a production unit. A good fabricator will repeat this test when changing fabrication methods or parameters. The owner must therefore exercise good judgment in selecting a reliable supplier. Best value (quality/price) is more important than lowest price.
3.4.5 Machining and Finishing
Pipe and fittings can be formed, machined or finished in many ways: cutting by hand tools (for example cutting soft tubing, or preparing pipe bevels for welding), drilling using a rotating drill bit (such as drilling of bolt holes in a flange forging), bending (cold bending of tubing or small diameter pipe or hot induction bending of large diameter line pipe). Pipe bending may be accomplished with a mandrel (a solid mandrel with articulated end disks inserted inside the pipe and rotate as the pipe is bent) or with packed sand, or without mandrel or packed sand. Finishing the pipe surface is achieved by grinding, sandblasting, brushing or polishing. The surface finish is typically defined in average surface roughness (arithmetic average of deviations of the actual surface from the mean line) or as the root mean square “rms” (root mean square of deviations of the actual surface from the mean line) [ASME B46.1]. The surface finish is typically expressed in microinches (µin). For example, the typical average surface roughness of a flange face for a metal ring is very smooth at 60 µin or less, while it is serrated at 500 µin for a soft sheet gasket.
3.4.6 Base Metal Imperfections
It is clear from the wide range of fabrication techniques that pipes, fittings (such as tees or elbows) and components (such as valves or strainers) can contain imperfections introduced in the foundry or during the fabrication process. If these imperfections are large they would constitute flaws or defects and the pipe or component would be rejected or repaired if permitted in the material specification.
Flaws, if they do exist, may consist of shrinkage porosities or cracks, inclusions, laminations, laps, seams, holes, or hot tears. The welds of welded pipes or fittings may also include weld flaws such as cracks, lack of fusion or lack of penetration, undercuts, inclusions, cavities, protrusions or misalignments (Chapter 16). But with today’s technologies and quality assurance, significant mill flaws that will cause failures or leaks during hydrotest or in-service are very rare.
Non-metallic inclusions (for example oxides, sulfur or manganese) are an example of base material imperfections. They appear as stringers in the skelp. During the making of the plate, these stringers are forced along the grain direction, parallel to the skelp’s faces.
They become circumferential imperfections when the skelp is bent to form the pipe. If the stringer is at the edge of a plate, it can be pushed radially inward or outward as the two ends are welded together to form the pipe. In this case, the stringer acts as a crack, in the form of a hook, part radial and part circumferential. The skelp edge defect has now become a weld defect and the radial portion of the crack can be the source of failure by crack propagation under large hoop stress or a source of accelerated corrosion called
“grooving corrosion” [Kiefner].
While significant flaws (the kind of flaws that must be repaired or could cause failure in service) area a rare occurrence in reputable pipe mills, shallow surface scratches and marring as a result of machining, handling and transport are inevitable. These shallow surface marks are generally acceptable, provided (a) they are not a source of corrosion (for example scratching a stainless steel surface with a carbon steel tool or brush would leave a carbon steel residue on the surface that would form rust), (b) the remaining wall is above the minimum required by specification, (c) the pipe is not intended for high pressure service (hoop stress larger than 50% yield), and (d) they do not affect the flow or process. A gouge (knife-like cut on the pipe surface) can cause rupture in large hoop stress service, as discussed in Chapter 21, and therefore deserves special attention.
3.5 MECHANICAL PROPERTIES
Mechanical properties of pipe and pipe component materials consist of strength, hardness, toughness and fatigue strength.
3.5.1 Strength
Yield stress, ultimate strength and elongation at rupture are fundamental mechanical properties of pipe and fitting materials. They reflect the ability of the material to be fabricated and to resist applied loads in service. All three properties are essential for piping systems.
Minimum strength properties are typically required in standard material specifications.
For ASTM pipe, a standard size tensile test specimen of pipe material is cut out from the pipe wall or, for small pipe and tubing, made out of a section of pipe or tube [ASTM A 370]. The specimen is placed in a tensile test machine and a steadily increasing tensile force is applied to the specimen, at a rate between 10,000 psi/min and 100,000 psi/min.
Passed yield, the maximum strain rate is 0.5/min [ASTM E 8].
For carbon steel, a plot of the engineering stress (force applied to the specimen divided by the initial specimen cross sectional area) versus the corresponding strain (elongation of the specimen divided by its initial length) will have the general shape shown in Figure 3-8. Compilations of stress-strain curves for a wide range of materials are available through ASM International [ASM Atlas]. Such a curve is referred to as the engineering stress-strain curve, to differentiate it from the true stress-strain curve in which the true stress is the applied force divided by the concurrent necked down area of the specimen, and the true strain is the elongation divided by the concurrent length. In the case of Figure 3-8, between 0 psi and approximately 37,000 psi the stress-strain relationship is linear and steep. At around 37,000 psi the material’s stress-strain relationship exhibits a marked departure from linearity. The stress at which this occurs is the yield stress, denoted SY.
For steel, the yield stress is typically determined as the stress at which removing the applied load would result in a permanent elongation of the specimen of 0.2%. For line pipe, yield is defined on the basis of 0.5% permanent elongation [API 5L]. For a uni-axial tensile test, in the elastic region the relationship between engineering stress and engineering strain is
σe=E εe
σe=engineering stress, psi εe=engineering strain E=Young’s modulus, psi
Young’s modulus, named after British doctor, physicist and Egyptologist Thomas Young (1773–1829), is a measure of the elasticity of a material. It varies with temperature, the higher the temperature the softer the material and the lower its Young’s modulus, as shown in Table 3-5.