Seamless vs Welded Steel Pipe: Differences, Standards & How to Choose
Seamless or welded? This is the first decision on any pipe requisition — and it directly affects your project cost, delivery time, inspection scope, and pressure rating. Get it wrong and you either overspend on seamless pipe where welded would do, or you risk a failure by specifying welded pipe in a service that demands seamless.
The short answer: seamless pipe is stronger, has no weld seam to fail, and is required for high-pressure and critical service. Welded pipe costs less, is available in larger diameters, ships faster, and performs perfectly well for the majority of industrial applications.
The real answer depends on your operating pressure, pipe size, applicable code, and budget. This guide gives you the technical comparison and a clear decision framework.
How Each Type Is Made
Seamless Pipe Manufacturing
Seamless pipe starts as a solid cylindrical steel billet. The billet is heated to forging temperature (approximately 1,200°C) and pierced through the center using a mandrel to create a hollow shell. This shell is then rolled, stretched, and sized to achieve the specified outside diameter, wall thickness, and length.
The result is a pipe with no weld seam — a continuous, uniform tube of steel. The grain structure flows around the entire circumference without interruption.
Key standards for seamless pipe: ASTM A106 (high-temperature service), ASTM A53 Type S (general service), API 5L (pipeline), ASTM A333 (low-temperature), ASTM A335 (alloy steel for high-temperature).
Welded Pipe Manufacturing
Welded pipe is made by forming flat steel (coil or plate) into a tubular shape and welding the edges together. The welding method and the orientation of the seam define the pipe type:
ERW (Electric Resistance Welded) — steel coil is unrolled, formed into a tube, and the longitudinal seam is welded using high-frequency electric current. No filler metal is added. Modern high-frequency ERW produces a very narrow heat-affected zone and the weld seam is virtually invisible after manufacturing. This is the most common welded pipe type for sizes up to NPS 24".
LSAW (Longitudinal Submerged Arc Welded) — a steel plate is formed into a tube shape (using UOE or JCOE press methods) and welded longitudinally by submerged arc welding on both the inside and outside. LSAW produces a high-quality double-welded seam with excellent mechanical properties. Standard for large-diameter pipeline pipe from NPS 16" to NPS 60"+.
SSAW / HSAW (Spiral Submerged Arc Welded) — steel coil is fed at an angle and formed into a tube with a helical (spiral) weld seam. The spiral seam is welded by submerged arc welding on both sides. SSAW is cost-effective for large diameters and is commonly used for water transmission, structural piling, and lower-pressure applications. Available from NPS 16" to over NPS 100".
Key standards for welded pipe: ASTM A53 Type E (ERW), API 5L (all welded types for pipeline), ASTM A672 (LSAW for high-pressure), ASTM A252 (piling), AWWA C200 (water pipe).
Head-to-Head Comparison
Factor | Seamless Pipe | Welded Pipe (ERW/LSAW/SSAW) |
|---|---|---|
Weld seam | None — uniform strength throughout | Present — potential weak point (though modern ERW/SAW welds approach base metal strength) |
Pressure capacity | Higher — no seam to fail under pressure | Lower than seamless (~80–95% depending on type), though LSAW approaches seamless performance |
Wall thickness uniformity | Less uniform — rolling process creates slight eccentricity (typically ±12.5%) | More uniform — starts from precision-rolled plate or coil |
Size range | NPS 1/8" to ~NPS 26" (limited above 26") | ERW: NPS 2"–24". LSAW: NPS 16"–60"+. SSAW: NPS 16"–100"+ |
Cost | 30–50% more expensive than equivalent welded pipe | Lower cost per meter, especially in larger diameters |
Lead time | Longer — 8 to 16 weeks typical for non-stock sizes | Shorter — ERW often from stock. LSAW: 6–12 weeks |
Surface quality | Rougher internal surface from piercing process | Smoother internal surface (especially ERW from precision coil) |
NDT requirements | Ultrasonic testing for wall thickness verification | Weld seam NDT required (UT or RT) plus pipe body inspection |
Corrosion resistance | No weld HAZ — uniform corrosion behavior | Weld seam and heat-affected zone (HAZ) may corrode preferentially in some environments |
Maximum temperature | Limited only by material grade | Same — limited by material grade, not manufacturing method |
Weldability (field joining) | Excellent — uniform composition and microstructure | Good — but pre-heat requirements may differ at weld seam locations |
When You Must Use Seamless
Certain applications and codes require seamless pipe — no substitution allowed:
High-pressure process piping (ASME B31.3 critical service) — when design pressure exceeds what the weld joint efficiency factor allows for welded pipe, seamless is required. ASME B31.3 assigns a weld joint efficiency factor (E) to welded pipe: ERW = 0.85, LSAW/DSAW = 0.95. Seamless pipe has E = 1.00, meaning the full allowable stress can be used in pressure calculations.
Boiler tubes and high-temperature steam lines (ASME B31.1) — boiler superheater and reheater tubes are almost universally seamless per ASTM A106, A210, or A213 specifications.
Sour service (H2S environments) — seamless pipe is preferred because there is no weld HAZ that could be susceptible to sulfide stress cracking (SSC) or hydrogen-induced cracking (HIC). Many operators and national oil companies mandate seamless for sour pipelines.
Nuclear piping — nuclear power plant piping codes typically mandate seamless pipe for primary coolant and safety-critical systems.
Small-diameter, high-pressure instrumentation and hydraulic tubing — always seamless.
When the project specification says "seamless" — many EPC contractor specifications and national oil company standards (Saudi Aramco SAES, ADNOC specifications, Shell DEPs) mandate seamless for specific services regardless of whether welded pipe could technically meet the pressure requirements.
When Welded Pipe Is the Right Choice
Welded pipe is not a compromise — it is the correct specification for the majority of industrial piping applications:
Large-diameter pipelines (NPS 16" and above) — seamless pipe above NPS 26" is either unavailable or prohibitively expensive. LSAW and SSAW are the standard manufacturing methods for large-diameter oil, gas, and water transmission pipelines worldwide. API 5L covers both seamless and welded pipe equally.
Structural and piling applications — structural pipe, casing, piling, and general fabrication do not require seamless. Welded pipe (ERW or SSAW) meets all structural requirements at significantly lower cost.
Water transmission and distribution — AWWA standards for potable water pipe specify welded (LSAW or SSAW) construction. These pipelines operate at low-to-moderate pressure where the cost advantage of welded pipe is substantial.
Fire sprinkler and low-pressure utility piping — ASTM A53 Type E (ERW) is the standard for fire protection and general plumbing applications.
When budget drives the decision — on large infrastructure projects in Africa and the Middle East involving hundreds or thousands of meters of pipe, the 30–50% cost premium for seamless pipe can represent hundreds of thousands of dollars. If the service conditions and code allow welded pipe, specifying seamless is simply throwing money away.
When large quantities and fast delivery matter — ERW pipe is typically available from stock in common sizes and schedules. Seamless pipe in non-standard sizes often requires 8–16 weeks mill production time. For projects with tight schedules, the availability advantage of welded pipe is significant.
LSAW vs SSAW — Choosing Between Welded Types
For large-diameter pipeline projects, the choice between LSAW and SSAW matters:
Factor | LSAW | SSAW |
|---|---|---|
Weld seam orientation | Longitudinal (straight) | Helical (spiral) |
Raw material | Steel plate (individually inspected) | Steel coil (continuous) |
Weld quality | Higher — better control of plate quality and weld parameters | Good — but coil quality can vary |
Dimensional accuracy | Tighter OD and roundness tolerances | Slightly looser tolerances |
Wall thickness range | Wider range — can achieve heavier walls from thick plate | Limited by coil thickness (typically max 25mm) |
Cost | Higher | Lower (20–30% less than LSAW) |
Acceptance by NOCs | Preferred by Saudi Aramco, ADNOC, and most major operators | Accepted for water, structural, and lower-criticality service |
Typical application | High-pressure O&G transmission, offshore | Water mains, piling, moderate-pressure pipelines |
For oil and gas pipeline projects where your end client is a national oil company or major operator, specify LSAW. Many project specifications explicitly prohibit SSAW for hydrocarbon service. SSAW is an excellent and cost-effective choice for water transmission, structural piling, and non-critical service.
Understanding Weld Joint Efficiency
This technical concept directly affects your pipe wall thickness calculation — and therefore your material cost.
ASME pressure piping codes assign a weld joint efficiency factor (E) to different pipe types. This factor reduces the allowable stress used in pressure calculations to account for the potential weakness of a weld seam:
Pipe Type | Weld Joint Efficiency (E) |
|---|---|
Seamless | 1.00 |
ERW (high-frequency) | 0.85 |
LSAW / DSAW | 0.95 |
SSAW | 0.85 |
Furnace-welded (CW) | 0.60 |
What this means in practice: A welded pipe with E = 0.85 can carry only 85% of the pressure that a seamless pipe (E = 1.00) of the same size and schedule can carry. To achieve the same pressure rating, the welded pipe needs a thicker wall — which means more weight and higher material cost per meter.
At moderate pressures, this difference is negligible. At high pressures, the required wall thickness increase for welded pipe can be significant — which is why seamless pipe actually becomes more cost-effective for high-pressure, small-diameter applications despite its higher per-meter cost.
Decision Framework: 5 Questions to Ask
1. What is the design pressure? High-pressure process piping (above ~100 bar for common sizes) → seamless. Moderate-to-low pressure → welded is usually acceptable and more cost-effective.
2. What is the pipe size? NPS 2"–16" → seamless and welded are both readily available; choose based on pressure and code. NPS 16"–26" → seamless is available but LSAW should be evaluated for cost. Above NPS 26" → LSAW or SSAW; seamless is impractical.
3. What does the applicable code require? Check ASME B31.1, B31.3, B31.4, B31.8, or whatever code governs your system. Some services mandate seamless. Others allow welded with the appropriate weld joint efficiency factor.
4. What does the project specification say? EPC contractors and end-users (national oil companies, refineries, power plants) often have their own specifications that mandate seamless for certain services — regardless of what the code allows. Always read the project piping specification before issuing your material requisition.
5. What is the total installed cost? Consider not just the pipe purchase price, but also inspection costs (welded pipe requires weld NDT), delivery timing, and potential for rejection. Sometimes the "cheaper" welded pipe ends up costing more when inspection and schedule risks are factored in.
Common Specifications by Pipe Type
Application | Seamless Specification | Welded Specification |
|---|---|---|
High-temperature process piping | ASTM A106 Grade B | — (seamless required) |
General service piping | ASTM A53 Type S Grade B | ASTM A53 Type E Grade B |
Oil & gas pipeline | API 5L (SMLS) | API 5L (ERW, LSAW, SSAW) |
Low-temperature piping | ASTM A333 Grade 6 | ASTM A333 Grade 6 (welded available) |
Alloy steel high-temperature | ASTM A335 (P11, P22, P91) | — (seamless required for most grades) |
Water transmission (large dia.) | — (impractical above NPS 26") | AWWA C200, API 5L |
Structural / piling | — | ASTM A252, ASTM A500 |
Stainless steel process | ASTM A312 TP304/316 (SMLS) | ASTM A312 TP304/316 (welded) |
Supply from Kasko Makine
Kasko Makine supplies both seamless and welded carbon steel pipe in the full range of specifications, grades, and sizes required for oil and gas, power generation, petrochemical, water infrastructure, and construction projects.
Seamless pipe: ASTM A106 Grade B, ASTM A53 Type S, API 5L (Grade B through X70), ASTM A333 Grade 6, ASTM A335 (P11, P22, P91). Sizes NPS 1/2" through NPS 24".
Welded pipe: ASTM A53 Type E (ERW), API 5L ERW, LSAW, and SSAW (Grade B through X70). Sizes NPS 2" through NPS 60"+. 3LPE, FBE, and bare coating options.
Complete piping packages: Pipe + flanges + fittings + fasteners + gaskets from a single source, shipped together to your project site across Africa, the Middle East, Central Asia, and beyond.
All pipe supplied with EN 10204 Type 3.1 material test reports. Third-party inspection coordinated on request with Bureau Veritas, SGS, TÜV, or Lloyd's Register.
FAQ SCHEMA
Q: What is the main difference between seamless and welded pipe?
A: Seamless pipe is manufactured from a solid steel billet with no weld seam, giving it uniform strength throughout. Welded pipe is formed from flat steel and welded along a longitudinal or spiral seam. Seamless pipe is stronger and required for high-pressure critical service, while welded pipe costs less and is available in larger diameters.
Q: Is seamless pipe stronger than welded pipe?
A: Yes. Seamless pipe has a weld joint efficiency factor of 1.00, meaning the full material strength can be used in pressure calculations. ERW welded pipe has 0.85 and LSAW has 0.95. This means seamless pipe can carry higher pressure than welded pipe of the same size and wall thickness.
Q: What is the difference between ERW, LSAW, and SSAW pipe?
A: ERW (Electric Resistance Welded) uses high-frequency current with no filler metal, available up to NPS 24". LSAW (Longitudinal Submerged Arc Welded) uses submerged arc welding on steel plate, available NPS 16"–60"+, preferred for high-pressure pipelines. SSAW (Spiral Submerged Arc Welded) uses a helical weld on steel coil, available NPS 16"–100"+, cost-effective for water and structural applications.
Q: When is welded pipe acceptable for oil and gas pipelines?
A: API 5L covers both seamless and welded pipe for pipeline service. LSAW pipe is widely used for large-diameter (NPS 16"+) oil and gas transmission pipelines and is accepted by national oil companies worldwide. ERW is common for smaller pipeline sizes. SSAW is generally limited to water and lower-criticality service.
Q: How much cheaper is welded pipe than seamless?
A: Welded pipe typically costs 30–50% less than equivalent seamless pipe. The savings are most significant in larger diameters and standard schedules. However, welded pipe requires weld NDT inspection, and some project specifications mandate seamless regardless of cost — so always check your project requirements before assuming welded is the budget option.
Q: What size is the cutoff between seamless and welded pipe?
A: Seamless pipe is readily available up to approximately NPS 24"–26". Above this size, welded pipe (LSAW or SSAW) is the standard because seamless production becomes impractical and extremely expensive. For sizes NPS 16"–26", both options exist and the choice depends on pressure requirements, code, and budget.
Not sure which type you need? Send us your pipe specification or line list, and our technical team will recommend the most cost-effective option that meets your project requirements. Contact us via WhatsApp at +90 (537) 521 1399 or email info@kaskomakine.com. We respond within 24 hours.