Seamless vs Welded Pipe
The difference is a single number in the wall thickness equation. Here is what the joint quality factor E does to the metal you have to buy.
- Seamless joint factor E
- 1.00
- ERW joint factor E
- 0.85
- Furnace butt weld E
- 0.60
- Effect on wall
- Lower E ⇒ thicker wall required
Short answer. Seamless pipe is pierced and drawn from a solid billet and has no longitudinal weld; welded pipe is rolled from plate or strip and seam-welded. The practical difference is the joint quality factor E in the wall thickness equation: seamless takes E = 1.00, ordinary ERW takes 0.85, and furnace butt welded takes 0.60. A lower E means a thicker wall for the same pressure — so welded pipe is cheaper per foot but not always cheaper per installed line.
E is where the difference becomes arithmetic
ASME B31.3 sizes a pressure-retaining wall with the same equation whatever the pipe is made of, and the manufacturing method enters only through E:
t = P·D / [ 2(S·E·W + P·Y) ]
Drop E from 1.00 to 0.85 and the required wall rises by about 18%. Whether that costs money depends on whether the next schedule up is needed — if the calculated wall still fits inside the schedule you were going to buy anyway, the lower E is free.
| Manufacturing method | Joint factor E | Why |
|---|---|---|
| Seamless | 1.00 | No longitudinal weld exists, so nothing derates. |
| Electric resistance welded (ERW) | 0.85 | Longitudinal weld, no filler metal, not normally radiographed. |
| Electric fusion welded, double butt, 100% radiographed | 1.00 | Full radiography restores the seamless factor. |
| Electric fusion welded, double butt, spot radiographed | 0.90 | Partial examination, partial credit. |
| Electric fusion welded, single butt | 0.80 | Welded from one side only. |
| Furnace butt welded, continuous weld | 0.60 | The lowest factor in the table; A53 Type F. |
E multiplies the allowable stress. It applies to the longitudinal seam only — it does not derate girth welds.
Worked example: NPS 6 at 1000 psig
Take NPS 6 pipe, outside diameter 6.625 in (168.3 mm), design pressure 1000 psig, allowable stress 20,000 psi, Y = 0.4. The required pressure-design wall for each manufacturing method is:
| Manufacturing method | E | Required wall | Required wall (mm) | vs seamless |
|---|---|---|---|---|
| Seamless | 1.00 | 0.1624 in | 4.12 mm | 0% |
| Electric resistance welded (ERW) | 0.85 | 0.1904 in | 4.84 mm | +17% |
| Electric fusion welded, double butt, 100% radiographed | 1.00 | 0.1624 in | 4.12 mm | 0% |
| Electric fusion welded, double butt, spot radiographed | 0.90 | 0.1800 in | 4.57 mm | +11% |
| Electric fusion welded, single butt | 0.80 | 0.2020 in | 5.13 mm | +24% |
| Furnace butt welded, continuous weld | 0.60 | 0.2671 in | 6.79 mm | +65% |
Pressure design thickness only. Corrosion allowance, threading and grooving depth, and mill tolerance (commonly 12.5%) are added on top.
Seamless pipe
- No longitudinal weld to inspect, derate or fail
- E = 1.00, so the thinnest calculated wall
- Required by many specs for hydrogen, lethal and high-temperature service
- Available in heavy walls and small bore where welded is not
- More expensive per foot, and lead times can be long
- Wall thickness varies more around the circumference
Welded pipe (ERW/EFW)
- Cheaper per foot, especially in large diameter
- More uniform wall thickness and better dimensional control
- Widely available from stock in common sizes
- Carries a joint factor below 1.00 unless fully radiographed
- The seam is a defined feature — it must be located away from branch connections and supports
- Excluded by some specs from severe cyclic and sour service
Large diameter changes the answer
Above about NPS 16, seamless pipe becomes scarce and expensive — the piercing process has practical limits. Large-diameter lines are normally welded pipe, and where the service is demanding the answer is not seamless but fully radiographed welded pipe, which recovers E = 1.00 through examination rather than through the absence of a seam.
See also: A106 vs A53 · wall thickness calculation · material grades · material selection.
Common questions
Is seamless pipe stronger than welded pipe?
The base metal is the same strength. What differs is the longitudinal seam: welded pipe carries a joint quality factor E below 1.00 unless it is fully radiographed, which means a thicker wall is required for the same pressure.
What is the joint efficiency of ERW pipe?
E = 0.85 for ordinary electric resistance welded pipe under ASME B31.3. Fully radiographed double-butt electric fusion welded pipe takes E = 1.00.
Is A53 seamless or welded?
Both. ASTM A53 covers Type S (seamless), Type E (electric resistance welded) and Type F (furnace butt welded). The type must be stated on the purchase order because each carries a different joint factor.
Why is seamless pipe specified for high-temperature service?
Partly for the joint factor, and partly because the seam is a metallurgical discontinuity that behaves differently from the base metal under creep and thermal cycling.
Does the weld seam need to be oriented in a particular way?
Most specs require the longitudinal seam to be positioned away from branch connections, from support contact points, and generally in the upper quadrant so it can be inspected.