PURPOSE This practice provides guidelines for the selection of materials of construction and thespecification of preventative measures for new equipment and piping in amine processunits used for acid gas removal. This practice includes a definition of amine service, to beused for materials and fabrication requirements.

GENERAL This practice provides process and materials selection guidelines intended to minimize therisks of alkaline stress corrosion cracking (ASCC) and metal loss due to acid gas and/orhigh temperature amine corrosion. API RP 945 "Avoiding Environmental Cracking inAmine Units", and the paper by R. B. Nielsen et al., listed under References, provideadditional background information on amine processes used for acid gas removal and onthe corrosion problems typically encountered in these process units.

DEFINITIONS Amine service is defined as any process service containing an amine solution having aconcentration greater than 2 wt. %. The guidelines of this practice may also be applicableto lesser concentrations if there is a concentrating mechanism present.

PROCESS CONSIDERATIONS In addition to the conventional design and operating information supplied by the process

engineer, the following information should be provided and considered during thematerials selection process:

The type of amine and its concentration.

Whether an amine reclaimer, using caustic, is to be used.

The acid gas loading, in moles acid gas per mole amine.

If an inhibitor is to be used, specify the type.

MATERIALS SELECTION General Considerations

If caustic is used for reclaiming amine, materials in that part of the unit should beselected to resist caustic stress corrosion cracking.

The use of copper alloys should be avoided in all amine services.

Amine Processes Used for H2S or H

2S/CO

2 Removal

For lean amine service equipment and piping, carbon steel is the primary material ofconstruction for all operating temperatures up to 300oF (150oC). For rich amineservice and equipment, carbon steel is the primary material of construction for alloperating temperatures up to 230F (110C). Above these temperatures, Type 304LSS (clad construction for equipment) is normally specified. Type 304L SS is alsonormally specified for all heat transfer surfaces operating at temperatures exceeding230F (110C).

Rich amine service should be considered a severe wet sour service. Considerationshould be given to specifying HIC (Hydrogen Induced Cracking) resistant plate andplate products for the major process vessels (i.e., the absorber tower, regeneratorand overhead accumulator) and welded pipe.

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Amine Processes Used for CO2 Removal

Austenitic stainless (Type 304L) or corrosion resistant alloy clad material aretypically specified for parts of the major process vessels and heat exchangershandling rich amine or hot lean amine [i.e., the absorber tower (around feed inlet),regenerator tower (top and bottom sections), rich/lean heat exchanger(s) (tubebundle), and reboiler(s) (tube bundle and shell)]. In areas where austenitic stainlesssteel is not used, carbon steel with 0.25 inch (6 mm) corrosion allowance is typicallyspecified. In some units, the use of austenitic stainless steel and corrosion resistantalloy clad materials may be minimized by the use of proprietary inhibitors. In suchcases follow the inhibitor licensor guidelines for materials selection.

Austenitic stainless (Type 304L) is typically specified for some piping handling richamine or hot lean amine (i.e., feed to the regenerator tower (from and including the letdown valve), hot lean amine to the rich/lean heat exchanger(s), and piping to andfrom the reboiler(s)). In areas where austenitic stainless steel is not used, carbonsteel with 0.25 inch (6 mm) corrosion allowance is typically specified.

All materials exposed to wet CO2 gas or aqueous condensate saturated with CO2(e.g., the regenerator tower overhead), should either be resistant to corrosion bycarbonic acid or should have an adequate corrosion allowance.

PREVENTATIVEMEASURES Cold formed (5% or greater strain), stressed or welded ferritic steel (including

carbon steel) equipment (except storage tanks) or piping components in amineservice should be stress relieved. A stress relief/postweld heat treatmenttemperature of 1175 + 25oF (635 + 14oC) for 1 hour minimum should be specified forcarbon steel. Postweld heat treatment at lower temperatures, as permitted by someof the engineering Codes, is regarded as ineffective for stress relief and should notbe allowed.

When postweld heat treatment is specified, it should include all socket and sealwelds and all external attachment welds to equipment and piping with a wallthickness less than 1 inch (25 mm).

Consider specifying a postweld heat treatment for any ferritic steel equipment andpiping in which trace amounts of amine can accumulate or concentrate. Any suchequipment or piping not water washed prior to steamout should be stress relieved.

The velocity in all carbon steel rich amine lines should not exceed 6 ft/sec (2 m/s).

Valve trim for amine services should be 12 Cr for operating temperatures no hotterthan 212F (100C) and Type 316 SS for temperatures exceeding 212F (100C).High pressure amine let down valves should be stellite hard faced. Typically, Type304L SS is used for the first section of the spool (10 pipe diameters, or 10 ft.,whichever is greater) down stream of the let down valve in order to avoiderosion-corrosion due to flashing. Impingement plates may also be required in theinlets areas of rich amine heat exchangers and vessels.

Lean amine pumps are usually supplied with carbon steel casings and either carbonsteel or cast iron internals for temperatures up to about 175F (80C). Hot (T > 175F[80C]) lean amine pumps and rich amine pumps should be specified with a minimum

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of 12 Cr stainless casing and 12 Cr stainless steel internals. Grade CA-6NM is therecommended material for 12 Cr stainless steel castings.

REFERENCES

American Petroleum Institute (API)

API RP 945 "Avoiding Environmental Cracking in Amine Units"

Published Paper

"Corrosion in Refinery Amine Systems," R. B. Nielsen, K. R. Lewis, J. G. McCullough, D.A. Hansen, CORROSION/95, paper no. 571 (Houston, TX: NACE International, 1995).

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AMINE SERVICES

MATERIALS AND WELDING ENGINEERING