Few decisions shape an MEP material submittal like the choice between seamless and welded steel pipe. Get it wrong and you face rejected submittals, cost overruns, or — worse — a pipe that cannot safely carry its service. Get it right and the rest of the procurement process becomes straightforward.
This guide explains how each pipe type is made, what the key specifications (ASTM A106, API 5L, pipe schedules) actually mean, and when to specify seamless over welded — with practical guidance for projects in Saudi Arabia.
How Seamless Pipes Are Made
Seamless pipe starts life as a solid round steel billet. The billet is heated to rolling temperature, pierced through its center (the classic Mannesmann rotary-piercing process), and then elongated over a mandrel or plug mill before being sized to final dimensions. The result is a pipe with no longitudinal weld seam at all.
Because there is no weld, the wall is metallurgically uniform around the full circumference. That uniformity is the whole point of seamless pipe: under high pressure, high temperature, or cyclic loading, there is no seam that could become the weak link.
The dominant specification for seamless carbon steel pipe is ASTM A106, which covers seamless pipe for high-temperature service — think refinery process lines, power plants, and steam systems. Grade B is the workhorse grade you will see on most Middle East project specs. ASTM A53 Type S is the general-purpose seamless alternative, while API 5L also covers seamless line pipe for oil and gas transmission.
How Welded Pipes Are Made: ERW, LSAW and SSAW
Welded pipe starts as flat product — coil or plate — that is formed into a cylinder and joined along a seam. The welding method defines the category, and the three you will encounter on Saudi project documents are:
ERW — Electric Resistance Welded
Steel strip is cold-formed into a tube and its edges are joined by high-frequency electric current combined with forge pressure. No filler metal is added. ERW is the most common welded pipe in MEP work: economical, widely stocked, and typically available from 1/2″ up to 24″. Common specs are ASTM A53 Type E and API 5L (ERW grades). Black steel ERW Schedule 40 is the standard pipe for fire sprinkler systems across the Gulf.
LSAW — Longitudinal Submerged Arc Welded
Heavy plate is formed into a cylinder (U-O or JCOE forming) and the straight longitudinal seam is welded from both inside and outside using the submerged arc process. LSAW serves large diameters — roughly 16″ to 60″ and beyond — with heavy walls for high-pressure transmission pipelines. Almost always supplied to API 5L.
SSAW — Spiral (Helical) Submerged Arc Welded
Steel strip is wound helically and the spiral seam is submerged-arc welded. SSAW is a cost-effective route to large-diameter pipe for water transmission, piling, and structural service, where the spiral seam is perfectly acceptable.
ASTM A106 vs API 5L: What the Specs Actually Mean
This is one of the most common sources of confusion in pipe procurement, so it deserves a clear answer:
- ASTM A106 is a product specification: it defines seamless carbon steel pipe for high-temperature service. If a spec calls for A106 Grade B, the pipe must be seamless — full stop.
- API 5L is a service specification: it defines line pipe for oil and gas transmission, and it permits both seamless and welded (ERW, LSAW, SSAW) manufacture. Grades run from B up through X42, X52, X65 and beyond, with PSL1 and PSL2 defining increasing levels of testing and traceability.
They are not interchangeable. A106 answers “how is this pipe made and what is it for” (seamless, high temperature). API 5L answers “what service does this pipe qualify for” (pipeline transmission, either process). Always specify per the service conditions and the project code — not per habit.
Seamless vs Welded: Head-to-Head Comparison
| Factor | Seamless Pipe | Welded Pipe (ERW / LSAW / SSAW) |
|---|---|---|
| Pressure integrity | Uniform wall, no seam — best for high pressure/temperature | Seam is the critical zone; modern ERW with full NDE is excellent for most services |
| Cost | Higher — more energy and processing per ton | Lower — faster, higher-yield production |
| Availability | Longer lead times, especially in large sizes | Widely stocked in standard sizes and schedules |
| Size range | Typically up to 24″–26″ economically | ERW to ~24″; LSAW/SSAW to 60″+ |
| Wall thickness | Heavy walls readily available (Sch 80, 160, XXS) | Full schedule range, but very heavy wall large-OD is a seamless domain |
| Testing | Hydrotest standard; NDE per spec | Weld seam NDE (ultrasonic/eddy current) standard on quality pipe |
| Typical specs | ASTM A106, ASTM A53 Type S, API 5L seamless | ASTM A53 Type E, API 5L welded |
Pipe Schedules Explained: Schedule 40 vs Schedule 80
Pipe schedule is a wall-thickness designation, not a pressure rating on its own. A higher schedule number means a thicker wall for the same nominal size — Schedule 80 has a thicker wall (and smaller inside diameter) than Schedule 40.
Two things specifiers sometimes miss: first, schedule numbers are nominal-size dependent — Sch 40 in 2″ is a different actual thickness than Sch 40 in 12″. Second, the real pressure rating comes from combining wall thickness with material grade, temperature, and diameter per the applicable piping code (ASME B31.1, B31.3, or B31.9 for building services). Never select schedule by rule of thumb on critical services.
When to Specify Seamless Pipe
- High-pressure or high-temperature services — steam, condensate return at elevated pressure, refinery and petrochemical process lines.
- Critical services where the project specification or the engineer of record simply will not accept a weld seam.
- Small diameters with heavy walls (e.g., Sch 160 / XXS instrumentation and high-pressure lines), where seamless is the practical manufacturing route.
- Sour or corrosive services where material and manufacturing route are dictated by standards such as NACE MR0175 — always follow the project metallurgy spec.
When Welded Pipe Is the Right Call
- Fire fighting sprinkler systems — NFPA 13 permits listed steel pipe, and black steel ERW Schedule 40 is the industry-standard choice across Saudi Arabia. (For fire service, always confirm the pipe is listed for the application.)
- HVAC and plumbing — chilled water, condenser water, and domestic water lines are overwhelmingly welded pipe territory.
- Structural, fencing, and general fabrication — where pressure integrity is not the governing factor.
- Large-diameter water and utility transmission — LSAW/SSAW territory.
- Budget-driven packages where the governing code permits welded pipe — the cost and availability advantage is substantial.
Specifier’s Checklist for Saudi Projects
- Define the service first — fluid, design pressure, design temperature. Everything flows from this.
- Check the governing code — ASME B31 series for process and power piping, NFPA 13 for fire protection.
- Nominate the correct spec — A106 for high-temp seamless, A53 for general purpose, API 5L for line pipe.
- Set schedule and NDE — wall thickness per code calculation; seam NDE (UT/RT) for welded pipe on critical services.
- Demand documentation — most Saudi project submittals require mill test certificates to EN 10204 3.1. Confirm this with your supplier before ordering, not after delivery.
- Confirm availability in-Kingdom — seamless pipe in odd sizes or heavy walls can carry long lead times; verify stock and mill rolling schedules early in procurement.
Frequently Asked Questions
Is seamless pipe stronger than welded pipe?
In a strict metallurgical sense, a seamless wall has no seam that could be weaker than the parent metal. In practice, modern ERW pipe manufactured to ASTM A53 or API 5L with full weld-seam NDE performs reliably in the vast majority of MEP services. Specify seamless where pressure, temperature, or the project specification demands it — not as a blanket rule.
Can ERW pipe be used for fire sprinkler systems?
Yes. NFPA 13 permits steel pipe — welded or seamless — and black steel ERW Schedule 40 is the standard choice for wet sprinkler systems in Saudi Arabia. Ensure the pipe is listed for fire protection service where the specification requires it.
What is the difference between ASTM A53 and ASTM A106?
ASTM A53 is a general-purpose specification covering both welded (Type E/F) and seamless (Type S) pipe for mechanical and pressure applications at ordinary temperatures. ASTM A106 covers seamless pipe only, specifically for high-temperature service. A106 Grade B is the default seamless grade on most industrial specs.
What documents should I request when buying steel pipe?
At minimum: mill test certificates to EN 10204 3.1 (chemical composition and mechanical properties), heat-number traceability, and hydrotest records. For fire protection or Aramco-adjacent work, confirm any additional listing certificates or approval documentation your submittal requires.
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Specifying pipe for a project in Saudi Arabia? Ajal Arabia stocks seamless and welded steel pipes in the grades, schedules, and sizes Saudi projects demand — with mill test certificates to support your submittals. Call us at +966 555 993140 or send your BOQ for a prompt quotation with delivery across the Kingdom.