ASME SA691 EFW Pipe

This ASME SA691 (for ASME boiler and pressure vessel codes, see Volume II of the relevant specification SA-691) covers high temperature and high pressure electrofusion welded carbon steel and alloy steel pipes. Heat treatment may not be required in order to achieve the desired mechanical properties or in accordance with applicable specification requirements. The supplemental requirements provided are used when some additional testing and inspection is required.

This sa691 nominally covers an outer diameter of 16 inches (405 mm) and a maximum wall thickness of 3 inches (75 mm). Pipes of other sizes may also be provided provided that it is consistent with other requirements of this specification.

ASME SA691 is a standard specifically for the manufacture of large, thick-walled, high-temperature, high-pressure pipelines and pressure vessels made of welded steel. It differs significantly from common seamless steel pipe standards (such as ASME SA335) and general standards for seamless/welded pipes (such as SA106 and SA53).

Standard Specifications:
Manufacturing Process: Electrofusion welding. It specifies the production of large-diameter steel pipes from rolled or welded steel plates or strips using arc welding processes (such as submerged arc welding (SAW) and electroslag welding (ESW).
Applications: Specifically designed for high-temperature and high-pressure applications, it is a key material for manufacturing heavy equipment such as large pressure vessels, power plant boilers, and chemical reactors.
Specifications: Primarily covers large-diameter (typically ≥16 inches/406 mm), thick-walled steel pipes that are difficult to produce economically using seamless processes.

Main Material Grades:
The SA691 standard includes various grades of carbon steel and alloy steel to meet different temperature and strength requirements. Common grades include:

Carbon steel: such as SA691 Gr.1-1/4Cr (similar to SA387 Gr.11)
Chromium-molybdenum alloy steel: such as SA691 Gr.2-1/4Cr-1Mo (similar to SA387 Gr.22), SA691 Gr.5Cr-1/2Mo (similar to SA387 Gr.5)
Higher alloy steel: such as SA691 Gr.9Cr-1Mo-V (similar to SA387 Gr.91)

Note: SA387 is a plate standard, while SA691 is the standard for welded pipes made from SA387 plate.

Key manufacturing and inspection requirements:
Due to its use in extreme conditions, SA691 has extremely stringent requirements:

1. Weld requirements:
The weld must be a full penetration weld.
The chemical composition and mechanical properties of the weld metal must match or be superior to those of the base metal.
Post-weld stress-relieving heat treatment is mandatory.

2. Non-destructive Testing: 100% radiographic testing (RT) or ultrasonic testing (UT) of weld seams is mandatory.
Magnetic particle (MT) or penetrant (PT) testing of weld seams is also typically required.

3. Heat Treatment: The entire steel pipe (including weld seams and heat-affected zone) must undergo normalizing and tempering or quenching and tempering to ensure uniform microstructure and stable performance.
4. Hydrostatic Testing: Every steel pipe must pass a hydrostatic test.

Applications:
Coal Power Plants: High-temperature and high-pressure main steam pipelines, reheat steam pipelines, headers, etc., for supercritical and ultra-supercritical units. When the pipe diameter is too large, seamless pipes (SA335 P91/P92) cannot be produced or are too expensive, welded pipes of SA691 Gr.91/92 are used.

Petrochemical Industry: Hydrogenation reactors, Fischer-Tropsch synthesis reactors, shells of large heat exchangers, and large-diameter process pipelines.
Coal Chemical Industry: High-pressure, thick-walled pipelines in coal gasification plants.
Nuclear Power: Large-diameter pipelines in some auxiliary systems.

Conclusion: 

ASME SA691 is a bridging standard between "steel plate" and "heavy-duty piping/vessels." It allows engineers to use high-performance plate (SA387) through controlled welding and heat treatment processes to manufacture large welded pipes capable of withstanding the same harsh conditions (high temperature, high pressure, hydrogen exposure) as seamless pipes.

Choosing SA691 over seamless pipe is typically an engineering decision based on providing a reliable and economical solution when the pipe diameter exceeds the economical manufacturing range of seamless processes.

Read more: ASTM A671 vs. ASTM A672 EFW Pipe