The purpose of this article is to draw attention to a potentially significant design matter, relating to station pipe designed to AS2885.1 using one of the nominated standards (AS4041 or ASME B31.3). It relates to design using A106 Grade B materials and associated A105N flanges and A234 WPB fittings at design temperatures below ambient.
Clause 2.11 of AS4041 clearly shows that it is not. Design for low temperature is required if the design minimum temperature is less than 0°C. Below this temperature, pipe (and fittings) toughness is required to be demonstrated, with the requirements varying with the design temperature, wall or reference thickness and the pipe grade. ASME B31.3 generally has similar requirements.
As a consequence of somewhat arbitrary nomination of station piping design temperatures, and an assumption that A106 materials are suitable for -29°C, it is likely that station pipe often does not comply with the requirements of the design standard.
Background
Until the publication of the 1997 revision of AS2885.1, station pipe design was covered by the standard. Like its predecessors (tracing back to ASME B31.8/B31.4), the revision permitted (and encouraged) the use of line pipe and high test fittings for design and construction of station pipe.
The 1997 revision of AS2885.1 deleted the previous rules for station piping and instead nominated that it be designed in accordance with an appropriate pressure piping design standard. AS4041 and ASME B31.3 were nominated as appropriate standards.
Unfortunately, the piping standards are intended for general plant use and unlike the pipeline standards, did not provided any incentive for designers to use API 5L grades, because the design stress for grades higher than X52 were pegged at X52.
About the same time, construction contracting strategies started separating the construction of stations from the construction of the pipeline – and the station designer was no longer part of the overall project strategy, which in the past included procurement of line pipe and high test fittings for stations. Consequently, A106B (and associated fittings) became the default material for station pipe.
This “˜default’ became embedded as consultancies moved into more advanced piping design methods, but chose only to populate the CAD design database with pipe and fittings complying with A106B (and associated fitting) materials.
The material difference
The significant difference between A106 and API 5L materials is that A106 pipe is intended for general use at temperatures where the risk of brittle fracture is negligible, and a performance requirement may be resistance to creep.
Pipe manufactured to API Specification 5L (PSL2) is alloyed and processed to produce fine grained microstructure that provides significantly improved mechanical and toughness properties, and is required to have a charpy impact test toughness of at least 27 J at 0°C. (Pipe manufactured to PSL1 may also have fine grained microstructure, but it is not a requirement). Pipe manufactured to project specifications often has higher toughness, and is subjected to increased toughness testing, including drop weight tear tests, to demonstrate its ductility at the design minimum test temperature.
Materials intended for low temperature service (ASTM A333 materials) are also manufactured to deliver controlled microstructure, and are required to comply with minimum charpy impact toughness (for example ASTM A333 Grades 1 and 6 are required to exhibit minimum toughness of 18 J (full size) at a test temperature of -45°C).
AS 4041 2006
AS 4041 – 2006 removed the impediment to using line pipe in station pipe, by recognising the increasing strength of pipe complying with API Specification 5L in grades between X42 and X80. This revision provides significant incentive to designers by allowing station pipe thickness that may be similar to the thicknesses of heavy wall pipe (the design stress permitted varies from 58 per cent of specified minimum yield strength for Grade X46 to 50 per cent of SMYS for Grade X70).
High test fittings
High test fittings are manufactured from pipe or forging material whose properties are consistent with the properties of API 5L materials. While they are not usually stocked in Australia, with a little thought and planning by designers, fittings can usually be sourced from stocks held in the USA, or manufactured to order well within the time frame of practically all pipeline construction projects.
These materials offer significant benefits through the use of greatly improved material properties, but also through improved weldability, provided by the lower carbon equivalent of the high test fittings.
Low temperature fittings
If required, fittings from low temperature materials (ASTM 420 Gr WPL6) and flanges (A350 LF2), together with low temperature pipe (A333 of the appropriate grade) may be available from Australian stockists or may be sourced from overseas suppliers – there is usually a cost penalty compared with the higher temperature grades.
Designers beware
Designers (and licensees) must take care to understand the obligations imposed by nomination of the minimum design temperature, not only for line pipe design, but also for station pipe design. The research and nomination of the appropriate minimum design temperature must be undertaken carefully. (For brittle fracture to initiate, the stress must exceed a threshold value. This, together with the thermal mass of the pipe, is usually sufficient to accommodate transient temperatures associated with depressurising and repressurising station pipe).
A design temperature lower than ambient (20°C) may require special testing of A106 grade pipe and fittings, and these tests may show that the material does not comply with the design requirements of the piping Standard (AS4041/ASME B31.3).
Where the minimum design temperature requires special testing, designers are encouraged to consider using high quality material complying with a nominated line pipe specification, together with associated high test fittings, because these fine grained materials have significantly improved toughness. With some planning, these can be purchased as part of a project order. Alternatively, pipe complying with a low temperature specification (like ASTM A333) may be appropriate.
Obligation to appreciate the requirements of nominated standards
AS2885.1 nominates a number of Standards as being suitable for design and manufacture of materials, components and fittings.
In specifying materials, components, fittings (and design) to a nominated standard, each designer has an obligation to have read and understood the scope and limitations of the nominated standard as it is applied to his design. Simply requiring compliance with a nominated Standard may result in failure.