Fired heaters operate in a number of industries including refining, oil sands
and petrochemical industries. ASME and API standards are used for overall construction
of the pressure side components; API 530 is used for the pressure design thickness design
of fired heater tubes. This standard provides for a range of tube materials but at the
uppermost temperatures, it limits itself to ASTM A608 Grade HK-40 (25Cr – 21Ni) and is
listed as 1,850 °F [1,010 °C]; however, the allowable rupture stress is severely limited
and may lead to impractical tube thicknesses.
Foundries have developed higher alloyed materials, such as A608 HP (25Cr – 35Ni) grades,
often referred to by an industry recognized convention as HP and other variations such
HP50, HPNb modified and HPNb + MA (micro-alloyed); these extend the service temperature
range to 2,000 °F [1,093 °C] with much improved creep rupture strength capability and,
thus, providing more reasonable tube pressure design thicknesses.
Some Disadvantages When Using Higher Alloys
However, there are disadvantages in using higher alloys;
• they are proprietary
• their creep properties are not available in API 530 or API 579
• the creep properties are closely held for competitive advantage
• the foundries are reticent in providing mechanical property information, or
• they require extensive search of the technical literature to retrieve
properties, if available
HK-40 (25Cr - 20Ni) alloys were developed in the 1950’s and their creep rupture properties are
widely presented in the literature, such as API 530. However, the seeming lack of published data
for the HP materials is unusual given that these materials have been in use since the 1970’s and 1980’s.
The absence of essential creep strength and rupture data for these specific materials in WRC 541, API 530
and in the open literature raises concerns for affected facilities with regards to standard
engineering practice and compliance with operations integrity management systems.
Industry trade (e.g., API) and professional associations (e.g., ASME) may wish to address the implications.
API 579 is Overly Conservative
A further drawback is that API 579 provides overly conservative criteria when
attempting to use some aspects of Part 10 Assessment of Components Operating
in the Creep Range for these specific, higher alloyed materials.
As a result, inspectors and owners may severely discount the useful life of
their equipment and may be incurring premature replacement and expense of these higher alloyed tubes.
Effective Creep & Remaining Life Assessment
There are reasonably effective approaches to make creep remaining life determinations.
These make use of well-established, readily available NDE methods combined with the,
sometimes, difficult-to-find, but available material properties. These techniques
will establish an appropriate safe operating envelope to define rational, in-service
monitoring and replacement schedules.
Contact the writer for additional insights and consider our class covering Part 10
John Aumuller, P. Eng., Ph. D.