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Maurice Stewart, in Surface Production Operations, 2016
Carbon steel pipe is the most commonly used material for process piping. It has the advantage of wide availability, high strength, and a large array of connection possibilities, for example, screwed, socket-welded, and butt-welded. Steel pipe should be selected for the required strength and durability required for the application and the ductility and machinability required to join it and form it into piping spools. The pipe must withstand the pressure, temperature, and corrosion conditions of the application. These requirements are met by selecting pipe made to an appropriate ASTM or API standard.
Carbon steel pipe is used for liquid, gas, and steam services both above- and belowground services. It is not recommended for use in corrosive services but may be used in caustic services.
There are many grades or strengths of carbon steel pipe and they are available in a number of wall thicknesses. We have seen that the allowable stress is used to determine what wall thickness is required. The allowable stress is a function of both the metallurgy of the material and the method of manufacturer.
The various piping specifications provided by ASTM and API provide guidelines for both the metallurgy and the method of manufacture. The most widely piping specifications for process lines are ASTM Specifications A-53 and A-106 and API Standard 5L. The principal wall thicknesses used are defined by schedules, for example, Schedule 40, Schedule 80, and weights, for example, STD, XS, and XXS. Both ASTM A53 and ASTM A106 pipe are fabricated SMLS or seamed, by electric resistance welding, in Grades A and B. Grades B have higher tensile strength. Three grades of ASTM A106 are available—Grades A, B, and C, in order of increasing tensile strength.
Table 3.2 provides the specifications for a given temperature range. For years, most pipes were made from Grade B steel, which has a minimum yield strength of 35,000 psi. Construction of high-pressure, large-diameter, cross-country transmission lines created a need for high-strength field-weldable steel that would allow a substantial savings in steel tonnage. API Grades X-42 through X-70 were developed with strengths of 42,000-70,000 psi.
Table 3.2. Metal design temperature for piping
| Design temperature | Specification |
|---|---|
| 650 to 60 °F 59 to − 20 °F − 21 to − 50 °F − 51 to − 150 °F − 151 to − 325 °F | API 5L A-53 A-106 API 5L A-106 A-333 Grade 1 or 6 A-333 Grade 304 A-312 Grade 304 A-358 Grade 304 A-312 Grade 304 A-358 Grade 308 |
Note: Metal design temperature shall be the design operating temperature plus 50 F or 10%, whichever is greater, for services 60 °F and above. For services 59 °F and below, subtract 5 °F or 10% whichever or greater from the design operating temperature where applicable; allowance should also be included for temperature effects due to process variation, especially at low temperature.
Figure 3.2 illustrates a typical stress-strain diagram for steel pipe. Table 3.3 illustrates the savings that can be realized by using Grade X pipe. Care must be taken to balance savings against corrosion allowances, special welding techniques required, minimum wall thickness criteria of the codes, and reduction in safety for hot-tapping operations. Some high-strength or alloyed pipe may not be suitable for certain corrosive environments. Table 3.4 compares the relative cost of steel pipe versus common alloys.
Figure 3.2. Stress-strain diagram for API 5LX X-46 pipe.
Table 3.3. Comparative cost of Grades B and X—grade pipe for a 4.5 in. OD line having a maximum allowable working pressure of 3100 psi and a design factor of 0.72 (Reference: ASME B31.8 and B31.4)
| Pipe grade and yield strength | Pipe wall thickness required (in.) | MAWP (psi) | Cost ($/ft.) | Cost saving over grade B ($/mile) |
|---|---|---|---|---|
| Grade B (35,000 psi) | 0.337 | 3774 | 38.26 | |
| Grade X-42 (42,000 psi) | 0.237 | 3185 | 30.02 | 43,507 |
| Grade X-46 (46,000 psi) | 0.219 | 3219 | 25.40 | 67,900 |
| Grade X-52 (52,000 psi) | 0.188 | 3120 | 22.70 | 82,156 |
Note: Cost based on FOB mill price January 2015 for electric resistance-welded pipe.
Table 3.4. Comparison of steel pipe versus other alloys
| Carbon steel | 1 |
| Stainless steel 304L | 3 to 5 |
| Stainless steel 316L | 4 to 6 |
| Nickel 200 | 19 to 38 |
| Monel-Inconel-Incoloy | 12 to 20 |
| Hastelloy | 25 to 38 |
| AL alloys | 4 |
| Copper | 3 |
| Lead | 1 |
| Gold | 14,000 |
| Platinum | 17,000 |
Steel specifications in other countries may correspond with US specifications. Some corresponding international standards for carbon steels and stainless steels are shown in Table 3.5.
Table 3.5. Comparison: US and international specifications for steel pipe
| USA | UK | W. Germany | Sweden | |
|---|---|---|---|---|
| Carbon steel pipe | ASTM A53 | BS 3601 | DIN 1629 | |
| Grade ASMLS | HFS 22 and CDS 22 | Si 35 | SIS 1233-05 | |
| Grade B SMLS | HFS 27 and CDS 27 | St 45 | SIS 1434-05 | |
| ASTM A53 | BS 3601 | DIN 1626 | ||
| Grade A E RW | ERW 22 | Blatt 3St 34–2 ERW | ||
| Grade B E RW | ERW27 | Blatt 3 St 37–2 ERW | ||
| ASTM A53 | BS 3601 | DIN 1626 | ||
| FBW | BW 22 | Blatt 3St 34–2 FBW | ||
| ASTM A106 | BS 3602 | DIN 17175a | ||
| Grade A | HFS 23 | St 35-8 | SIS 1234-05 | |
| Grade B | HFS 27 | St 45-8 | SIS 1435-05 | |
| Grade C | HFS 35 | |||
| ASTM A134 | BS 3601 | DIN 1626 | ||
| EFW | Blatt 2 EFW | |||
| ASTM A135 | BS 3601 | DIN 1626 | ||
| Grade A | ERW 22 | Blatt 3 St 34–2 ERW | SIS 1233-06 | |
| Grade B | ERW 27 | Blatt 3St 37–2 ERW | SIS 1434-06 | |
| ASTM A139 | BS 3601 | DIN 1626 | ||
| Grade A | EFW 22 | Blatt 2 St 37 | ||
| Grade B | EFW 27 | Blatt 2 St 42 | ||
| ASTM A155 | BS 3602 | DIN 1626, Blatt 3 with certification C | ||
| Class 2 | ||||
| C 45 | St 34-2 | |||
| C 50 | St 37-2 | |||
| C 55 | EFW 28 | St 42-2 | ||
| KC 55 | St 42–2 • | |||
| KC 60 | EFW28S | St 42–2 # | ||
| KC65 | St 52 3 | |||
| KC 70 | St 52-3 | |||
| API 5L | BS 3601 | DIN 1629 | ||
| Grade A SMLS | HFS 22 and CDS 22 | St 35 | SIS 1233-05 | |
| Grade B SMLS | HFS 27 and CDS 27 | St 45 | SIS 1434-05 | |
| API 5L | BS 3601 | DIN 1625 | ||
| Grade A E RW | ERW 22 | Blatt 3 St 34–2 ERW | SIS 1233-06 | |
| Grade BERW | ERW 27b | Blatt 4 St 37–2 ERW | SIS 1434-06b | |
| API 51 | BS 3601 Double-welded | DIN 1626 | ||
| Grade A EFW | EFW 22 | Blatt 3 St 34–2 FW | ||
| Grade BEFW | EFW 27b | Blatt 4 St 37–2 FW | ||
| API 5L | BS 3601 | DIN 1626 | ||
| FBW | BW 22 | Blatt 3 St 34 2 FBW | ||
| Stainless steel pipe | ASTM A312 | BS 3605 | WSN Designation | |
| TP 304 | Grade 801 | 4301 × 5 CrNi 18 9 | SIS 2333-02 | |
| TP 304H | Grade 811 | |||
| TP 304 L | Grade 801 L | 4306 × 2 CrNi 18 9 | SIS 2352-02 | |
| TP 310 | 4841 × 15 CrNiSi 25 20 | SIS 2361-02 | ||
| TP 316 | Grade 845 | 4401 × 5 CrNiMo 18 10 4436 | SIS 2343-02 | |
| TP316H | Grade 855 | |||
| TP316L | Grade 845L | 4404 × 2 CrNiMo 18 10 | SIS 2353-02 | |
| TP 317 | Grade 846 | |||
| TP 321 | Grade 822 Ti | 4541 × 10 CrNiTi 18 9 | SIS 2337-02 | |
| TP 321H | Grade 832 Ti | |||
| TP 347 | Grade 822 Nb | 4550 × 10 CrNiNb 18 9 | SIS 2338-02 | |
| TP347H | Grade 832 Nb |
a
Specify “Si-killed.”
b
Specify API Si Crede B letting procedures for these steels.
The manufacturing process of pipe is determined by the material, diameter, wall thickness, and quality for a specific service. Carbon steel piping is classified according to the manufacturing methods as follows:
•
Steel and malleable iron
○
SMLS
○
Electric resistance weld (ERW)
○
Submerged arc weld (SAW)
○
Double submerged arc weld (DSAW)
○
Furnace weld, butt-welded or continuous weld
○
SMLS pipe is produced by heating a round billet of steel and then piercing it with a bullet-shaped piercer, over which the steel is stretched. This is followed by rolling and drawing to produce the desired dimensions. The final product is hydrostatically tested, inspected, coated if required, and stenciled with the specification. SMLS pipe is used in high-pressure, most critical locations and under most severe operating conditions. SMLS pipe is supplied according to ASTM Specifications A53, A106, A333, A312, A358, etc., and API 5L pipe
Sizes: 1/8″ (3.175 mm) nominal to 26″ (660.4 mm) OD. Less than 2 3/8″ (60.325 mm) OD is known as pressure tubing that has different dimensional standards (wall thickness and diameter). SMLS pipe, where available, is used in oil and gas production facilities both onshore and offshore (other than transmission lines) less than 26″ (660.4 mm) OD.
ERW pipe is made from coils that are cupped longitudinally by forming rolls and a thin-pass section of rolls that brings the ends of the coil together to form a cylinder. The ends pass through a high-frequency welder that heats the steel to 2600 °F and squeezes the ends together to form a fusion weld. The weld is then heat-treated to remove welding stresses and the pipe is cooled, sized to the proper OD, and straightened. ERW pipe is produced either in individual lengths or in continuous lengths that are then cut into individual lengths. ERW is supplied according to ASTM A53 and A135 and API Specification 5L. It is supplied is sizes 2 3/8″ (60.325 mm) to 30″ (762 mm) OD.
ERW is the most common type of manufacturing process due to its low initial investment for manufacturing equipment and the process' adaptability in welding different wall thicknesses. The pipe is not fully normalized after welding, thus producing a heat-affected zone on each side of the weld that results in nonuniformity of hardness and grain structure, thus making the pipe more susceptible to corrosion. Therefore, ERW pipe is not as desirable as SMLS pipe for handling corrosive fluids. However, it is used in oil and gas production facilities and transmission lines, after normalized or cold expanded, for 26″ (660.4 mm) OD and larger lines.
SAW and DSAW pipes are produced from plate (skelp's), which are either formed into a “U” and then an “O” and then welded along the straight seam (SS) or twisted into a helix and then welded along the spiral seam (SW). DSAW longitudinal butt joint uses two or more passes (one inside) shielded by granular fusible materials where pressure is not used. DSAW is used for pipe greater than 4″ (508 mm) nominal. SAW and DSAW are mechanically or hydraulically cold expanded and are supplied according to ASTN Specifications A53 and A135 and API Specification 5L. It is supplied in sizes 20″ (508 mm) OD to 30″ (762 mm) OD.
This refers to the same process and is called furnace-butt weld. The pipe is produced by the continuous-welding butt-welded process. Butt-welded pipe is made from Bessemer steel, with a high phosphorus content, which offers superior welding characteristics. The longitudinal seam is joined by mechanical pressure after the entire steel strip from which the tube is formed has been heated to proper welding temperature. Furnace butt-welded pipe is normally used for domestic (United States) and firewater service only. In the butt-welded process (Figure 3.3), the skelp, whose edges have been slightly beveled for joining, is hot-rolled and heated in a furnace to a welding heat. The pipe is then pulled through a ring die or bell and the pipe is welded by pressing the edges together at a high temperature. The pipe OD is reduced and the wall thickness is achieved in a stretching mill. A saw cuts the pipe to length and the pipe enters a sizing mill that reduces the pipe to the final OD. The pipe is straightened, end-finished, hydro-tested, coated if required, stenciled, and inspected.
Figure 3.3. Butt-welded manufacturing process.
(Courtesy of Kawasaki Steel Corporation)Butt-welded pipe is supplied according to ASTM Specifications A53 and A120 and API Specification 5L for line pipe. It is supplied in sizes from 1/8 to 4 in. in diameter. This is a low-cost manufacturing method, and because of the quality of the welding, it has a joint efficiency of not more than 60%. Furnace-butt weld pipe is not recommended for use where internal corrosion is anticipated.
Spiral-welded pipe is produced from coils of steel that are unwound and flattened. The flattened strip is formed by angled rollers into a cylinder of the desired diameter. Interior and exterior SAW seal the spiral seam. At the end of the coil, a new coil is butt-welded to the trailing edge of the pipe, forming a cross seam. The pipe is cut to length and the ends are beveled if required. Spiral-welded pipe is primarily used for water distribution service. Spiral-welded pipe is available in sizes from 24 in. (60 cm) to 144 in. (365 cm).
