The Quick Answer
A 20 seamless steel pipe is a pipe made from a solid steel billet that is pierced to form a hollow tube, with a nominal pipe size of 20 inches. The 20 is a nominal designation rather than a direct measurement of every dimension: for NPS 14 and larger, the nominal size does equal the outside diameter, so a 20 inch pipe has an outside diameter of 20 inches, or 508.0 mm. The wall thickness, and therefore the bore and the pressure rating, depends entirely on the schedule number that is ordered with the size.
What Seamless Manufacture Gives You
In seamless production the billet is heated and pierced by a mandrel, so the finished pipe has no longitudinal weld and no weld heat-affected zone. The practical consequences matter in pressure service. There is no seam to act as a preferential initiation site for corrosion or creep damage, wall thickness is uniform around the circumference, and the pipe performs consistently under thermal cycling because there is no weld metal with different properties along its length. These characteristics are why codes for high-pressure and high-temperature service treat seamless pipe as the default choice, and why a 20 inch line carrying steam, gas or hydrocarbons is normally specified as seamless rather than welded.
Wall Thickness and Schedule
The schedule number is the most important variable in a 20 inch pipe enquiry, because it determines both the pressure capacity and the weight of the line. The table below lists the common schedules for NPS 20 with the specified wall thickness, and the corresponding theoretical mass calculated from the standard formula for steel pipe using an outside diameter of 508.0 mm.
| Schedule | Wall thickness, mm | Theoretical mass, kg per m | Typical duty |
|---|---|---|---|
| SCH 10 | 6.35 | 78.6 | Low pressure, lightweight systems |
| SCH 20 | 7.92 | 97.7 | General low pressure service |
| STD | 9.53 | 117.1 | General purpose, moderate pressure |
| SCH 40 | 12.70 | 155.1 | General purpose, higher pressure |
| SCH 60 | 15.88 | 192.7 | Higher pressure process lines |
| SCH 80 (XS) | 19.05 | 229.7 | High pressure service |
| SCH 100 | 22.23 | 266.3 | Heavy wall process piping |
| SCH 120 | 25.40 | 302.3 | Heavy wall process piping |
| SCH 140 | 28.58 | 337.8 | Severe service |
| SCH 160 (XXS) | 31.75 | 372.9 | Very high pressure, severe service |
Two conventions are worth remembering. Extra strong, abbreviated XS, is identical to schedule 80 at this size, and double extra strong, abbreviated XXS, is identical to schedule 160. Theoretical mass is a calculated figure based on nominal wall thickness; the actual mass of a delivery will differ slightly because mill tolerances allow small variations in wall and length.
Standards and Materials
Seamless pipe in this size is ordered against the specification that matches its service. ASTM A106 covers seamless carbon steel pipe for high-temperature service and is the usual choice for steam and process lines, with Grade B the most widely stocked. ASTM A53 covers seamless and welded pipe for general use in mechanical and low-pressure applications, with Grade B common in structural and utility duty. ASTM A333 covers pipe for low-temperature service where notch toughness at sub-zero temperature is required. ASTM A335 covers seamless ferritic alloy steel pipe for high-temperature service and applies when the line needs chromium or chromium-molybdenum steel rather than carbon steel. API 5L is used for line pipe in oil and gas transmission, and its grades are selected according to the pipeline design code rather than by schedule alone.
How to Specify a 20 Seamless Steel Pipe Correctly
To order or specify this pipe accurately, more than the phrase 20 seamless is needed. A complete specification includes the nominal pipe size, the manufacturing route, the schedule, the material or ASTM standard, the length basis and any additional testing. A worked example makes the point: 20 inch NPS seamless pipe, schedule 40, ASTM A106 Grade B, random length, with a mill test certificate to EN 10204 3.1.
Nominal pipe size: 20 inches, outside diameter 508.0 mm
Manufacturing route: seamless
Schedule: for example schedule 40 or schedule 80
Material standard: for example ASTM A106 Grade B, ASTM A53 Grade B or ASTM A333
Length: random length, or fixed lengths such as 6 m, 12 m or 40 ft
Ends: plain, bevelled or threaded, with or without caps
Inspection: mill test certificate, dimensional check, and ultrasonic or hydrostatic testing where required
Specifying the length basis and the end finish at the enquiry stage avoids arguments later, because cutting, bevelling and end protection all affect the delivered cost of a heavy wall 20 inch line.
FAQ
Q: What is the outside diameter of a 20 inch pipe?
For nominal pipe sizes of 14 inches and above the nominal size equals the outside diameter, so a 20 inch pipe has an outside diameter of 20 inches, or 508.0 mm.
Q: What is the difference between schedule 40 and schedule 80?
Both share the same outside diameter, but schedule 80 has a heavier wall, so it carries higher pressure and has a smaller bore than schedule 40.
Q: Which standard is used for a 20 inch steam line?
ASTM A106 Grade B is the usual carbon steel specification for high-temperature service, unless the design calls for an alloy grade under ASTM A335.
Q: Is the pipe available in alloy steel?
Yes. Seamless chrome and chrome-molybdenum pipe in this size is supplied to the relevant ASTM specification when carbon steel cannot meet the design temperature.
Q: How is the mass of the pipe calculated?
The theoretical mass follows the standard relationship between wall thickness, outside diameter and the density of steel, and is quoted per metre or per foot of length.
Q: What lengths are supplied?
Random lengths as standard, and fixed lengths such as 6 m, 12 m or 40 ft when specified at the ordering stage with the required end finish.
