Using 300 Series Stainless Steel in Wet H2S Service

1. Purpose

This procedure establishes the policy for using Austinetic (300 series) stainless steels for instrumentation in wet Hydrogen Sulfide (H2S) services, and replaces the long-standing practice of using Monel for instrumentation in these services.

2. Policy

The metallurgy specified for instrumentation in wet or dry Hydrogen Sulfide (H2S) services, THAT ARE CHLORIDE FREE, will be 316 stainless steel.

3. Historical Use of Monel

Monel is indicated on the Inflection Point Engineering P&ID for the internals of piping valves in cold (less than 400°F) services containing H2S and water, because the metallurgy of the piping valves internals (generally 11-13% Cr) is attacked by this combination of materials which eventually will cause the valve internal to fail.

The historical practice of the Instrument Group for determining when to use Monel instead of 300 series stainless steels for thermowells, orifice plates, and control valve internals was based upon a 1957 memo by Dave Boyd, which generally followed the P&ID designation of Monel for piping valve internals. In process units such as Platforming and Hydrotreating, Monel was always specified for instruments in cold flowing streams containing chloride, H2S, and water. Instrumentation in non-flowing services (delta P and pressure instrument diaphragm, pressure gauge bourdon tube, level displacer, etc.) were assumed to be not affected by the chloride, H2S, water combination and continued to be specified as 316 stainless steel.

The detrimental effect of a wet chloride environment on Austinetic (300 series) stainless steels is well documented. What is less clearly defined for Austinetic (300 series) stainless steels, is the effect of a wet H2S environment. The Instrument Group traditionally has not differentiated between wet chloride and wet H2S environments when specifying the metallurgy of an instrument, because chloride was generally present in the makeup gas for those process units producing H2S.

4. Process Design Change

The current designs of Reforming units generally remove the chlorides from the net hydrogen product. Therefore, process units such as Hydrotreating and Hydrocracking units that use chloride free makeup gas should not require Monel in those services that now contain only wet H2S.

5. Reference Data

Although there does not appear to be any specific metallurgical documentation or data that provides the type of information upon which to make an unchallenged decision for choosing 300 series stainless steels instead of Monel in wet H2S service, the following data are available:

• Inflection Point Engineering metallurgical documentation recommends that Monel be used instead of 400 series stainless steels (11 - 13% chromium) in a wet H2S environment because of hydrogen embrittlement causing inter-granular structural damage.

• Inflection Point Engineering metallurgical documentation recommends that Monel be used instead of 300 series stainless steels (18% Cr - 8% Ni) in an aqueous environment containing chlorides because of hydrogen embrittlement causing inter-granular structural damage.

• Inflection Point Engineering metallurgical documentation recommends that 316 stainless steel be used in a wet H2S environment such as the feed lines to a sour water stripping unit.

• Thermowell vendor literature indicates that 316 stainless steel is suitable for service in both wet and dry H2S environments.

• Hydrocracking and Desulfurizing units owned and operated by UNOCAL do not contain special metallurgy related to wet H2S service.

• Gas concentration units associated with Fluid Catalytic Cracking normally contain large quantities of H2S and free water. This process unit has been using stainless steels for both instrumentation (300 series) and for piping valves internals (400 series) for many years without any reported equipment failures caused by wet H2S attack.