Seamless steel pipe and spiral-welded steel pipe sit on opposite ends of the diameter-and-pressure map, and the 2026 sourcing window from Chinese mills confirms the split is still governed by O.D. range, pressure class, and end use rather than price alone [S1][S3].
The two products are routinely mis-bid against each other by buyers who see only the word "steel pipe." The shorthand working rule among EPC piping engineers: specify seamless steel pipe when the run is hot, high-pressure, or sour-service; specify spiral-welded pipe when the run is large-diameter, low- to mid-pressure, and cost-per-meter dominates the BOM [S3][S8].
What Each Product Actually Is
Seamless steel pipe is a hollow tube produced by piercing a solid billet — hot-rolled or cold-drawn — with no longitudinal or spiral weld seam, which is why it is the default for [high-pressure boiler tubes, line pipe, and OCTG casing] under standards such as ASTM A106, ASTM A335, API 5L, and API 5CT [S3].
Spiral steel pipe (also called SSAW, or spiral-welded duct) is made by helically forming a steel strip into a cylinder and welding the single helical seam, either submerged-arc (SAWH) or double-side submerged-arc. The 2026 product listings from Okorder describe spiral welded steel pipes in material grades Q195–Q235, lengths 1–12 m, wall thickness 1.0–14 mm, and O.D. 20–273 mm in the standard range, with 52″ API large-diameter spiral pipe cited as a separate SKU [S8].
Manufacturing Route and Why It Matters for Spec
The piercing-based seamless route forces the steel to flow uniformly around the circumference, which yields isotropic mechanical properties and eliminates the seam as a fatigue or corrosion initiation point — the reason seamless pipe is the default for refinery, nuclear, and high-temperature service [S3].
The spiral-welded route allows the mill to build very large diameters from narrow coil stock, which is mechanically and economically impossible to achieve with a piercing mill; the helical seam redistributes hoop stress across many oblique weld beads, but introduces one long, continuous HAZ that must be 100% radiographed for line-pipe duty [S1][S8]. For low-pressure water, piling, ventilation ducting, and structural casings, that seam is acceptable and the cost delta is decisive [S3][S8].
Diameter, Wall, and Pressure Envelope
On the 2026 sourcing sheets, ASTM A106 Grade B seamless pipe is quoted at US$480–520 per ton / piece on Made-in-China, with hot-dipped galvanized square hollow section at US$530–580 — essentially a flat band, indicating the mill cost driver is grade and wall, not diameter, in the small-bore range [S5].
Spiral pipe on Okorder in 2026 carries a minimum order of 5 metric tons and a supply capability of 3,000 m.t./month from Tianjin, with lead time 10–30 days, and the spec sheet locks O.D. 20–273 mm at wall 1.0–14 mm in standard Q195–Q235 grades [S8]. The 52″ API large-diameter spiral SKU is a separate, project-typical listing rather than a stock item [S8]. For comparison, the Sunco seamless catalogue covers 2-inch small-diameter tube, API 5L X42 line pipe, and SCH40/SCH80 hot-rolled pipe — the diameter range that seamless mills can pierce economically [S3].
Standards and Grades Buyers Should Quote
For seamless line pipe and process pipe, the dominant 2026 spec set is API 5L (PSL1/PSL2 grades A, B, X42–X70) for line pipe, API 5CT (K55, N80) for casing, ASTM A106 Grade B for high-temperature carbon, and ASTM A335 for alloy [S3].
For spiral-welded pipe, the reference standards are typically API 5L PSL1 for line-pipe grade spiral, AWWA C200 for water, ASTM A252 for piling, and EN 10219 for structural hollow sections — though the mill stock on Okorder defaults to GB/T grade Q195–Q235 for general/structural duty [S8]. Note that the same mill (e.g., Hebei Zhongkuang Steepipe) lists both ERW, seamless, and spiral under one roof, which is normal: the equipment lines are separate, the grade range overlaps, and the buyer's spec dictates which line runs [S1].
Selection Criteria: A Side-by-Side Map
On diameter: seamless mills economically cover roughly 1/8″ to 26″ (≈6–660 mm) with peaks in the 2″–16″ band, while spiral mills economically cover 20″–120″ (≈500–3,000 mm) and are the only practical route above ~24″ for many buyers [S3][S8].
On pressure and temperature: seamless is the default for high-pressure steam, refinery hydrocracker loops, and boiler tubes because there is no seam to fail; spiral is acceptable for low- to mid-pressure water transmission, dredge lines, piling, and structural ductwork where the design factor absorbs the seam penalty [S3][S8]. On cost: per-ton seamless sits higher because of the piercing mill, hot-reduction, and higher rejection rate; per-meter spiral is lower in the large-diameter band because coil-fed forming is cheap [S5][S8]. On lead time: stock spiral Q195–Q235 ships in 10–30 days from Tianjin [S8]; seamless in API 5L X-grade is typically make-to-order with longer mill lead times.
Where Each Is the Wrong Choice
Do not specify seamless for ventilation ductwork, large-diameter water main less than 10 bar, or concrete-pile casing — the tonnage will blow the BOM and the mill may refuse the diameter outright [S8]. For broader context on [steel pipe selection across pressure, media, and code boundaries], the Seamless Steel Pipe Buying Guide 2026 lays out the same envelope against ERW and LSAW alternatives.
Do not specify spiral for high-temperature steam, refinery reactor effluent, or any service that ASME B31.3 / B31.1 flags as requiring seamless — the helical seam cannot be justified against the code case. Nor use spiral for small-bore instrument or hydraulic lines: minimum commercial spiral O.D. sits at 20 mm and the wall tolerance band is too loose for hydraulic service [S3][S8]. For complementary downstream decisions on [how to choose between actuation and isolation hardware on these same pipe runs], see the Solenoid vs Diaphragm Valve: Actuation Speed Decision Map and the Pinch Valve vs Plug Valve reference.
2026 Sourcing Signals Worth Tracking
Hebei Zhongkuang's July 2026 product page lists a 5-ton minimum order with 2,000 t/month supply across ERW, spiral, and carbon seamless — a useful live benchmark that the Chinese merchant mill still treats all three as interchangeable SKUs under one commercial roof [S1]. Nansteel and Sunco both run ISO 9001-certified production with seamless + spiral lines and export direct from Cangzhou and Hunan respectively, with active project references including offshore platforms in South Africa, Saudi shipyard work, and Bangladesh TITAS gas transmission [S2][S3][S7]. Broadway Steel's project list adds SOCAR Azerbaijan and confirms seamless is the workhorse for international oil & gas EPC packages [S7].
For the relevant spec sheets and selection criteria, see spiral duct, and wiring duct.