Design and Construction Practices

 

DESIGN GUIDELINES

Recommendations presented in Section 8 apply to produc­ing and gas processing plant operations where the fluids han­dled contain sufficient hydrogen sulfide to be within the scope of NACE Standard MROI75. NACE Standard MROI75 may also be a useful reference when designing and con­structing other facilities handling hydrogen sulfide. Refer to Appendix D for information on defining a sour environment. All pressure vessels should be designed and constructed in accordance with ASME Boiler and Pressure Vessel Code [re­fer to Synopsis of Boiler and Pressure Vessel Laws, Rules, and Regulations by States, Cities, Counties, and Provinces (United States and Canada)]. All piping systems should be designed and constructed in accordance with applicable pro­visions of ANSI B31.3, ANSI B31.4, OR ANSI B31.8.

Manufactured equipment shall be designed, constructed, tested, and approved to meet or exceed system requirements for hydrogen sulfide service and should be installed in com­pliance with applicable codes and industry-accepted stan­dards.

Process and Mechanical Considerations

Factors to be considered in facility design include, but are not limited to, the hydrogen sulfide concentration and impact of atmospheric and operating temperatures, system pres­sures, pH and water content of system fluids, mechanical stresses imposed on system components, operating and physical strength changes of system components resulting from corrosion and scale deposits, and conditions peculiar to individual processes that could be of a detrimental nature to the system.

Design Considerations

To minimize internal corrosion, piping and vessels should be designed and installed in such a way that areas of insuffi­cient flow (including dead-end piping) are eliminated. Where this cannot be accomplished, provisions should be made for draining of accumulated fluids. Drain systems for hydrogen sulfide laden fluids should be designed to prevent migration of hydrogen sulfide from one point in the facility to another.

Materials Considerations

When exposed to an environment containing hydrogen

sulfide, many materials may suddenly fail in a brittle manner. Failure occurs by a form of embrittlement known as sulfide stress cracking (SSC). Susceptibility of a given material to sulfide stress cracking increases as strength and tensile stress (residual or applied) increase. Material hardness frequently is used as an indirect measure of strength and sometimes is referenced as a limiting parameter. The failure of certain pro­ducing and gas processing plant equipment components used in the sulfide stress cracking regime could result in the un­controlled release of hydrogen sulfide to the atmosphere. Such components should be made from sulfide stress crack­ing resistant materials (refer to Par. 8.1.4).

Materials Selection

Metallic materials satisfactory for use in hydrogen sulfide environments and the conditions under which they should be used are described in NACE Standard MRO175. The latest revision of this standard should be consulted when selecting materials for use in hydrogen sulfide environments. The pro­visions of NACE Standard MROI75 should be considered minimum standards, with the equipment user free to apply more stringent specifications. The material requirements of NACE Standard MROI 75 offer resistance to sulfide stress cracking (SSC); however, other forms of corrosion and modes of failure (such as pitting, hydrogen-induced crack­ing, and chloride cracking) should be considered in the de­sign and operation of equipment. Control of failures by mechanisms other than SSC should be mitigated by chemi­cal inhibition, material selection, and environmental controls (refer to NACE Standard MROI 75, Par. 1.1). Appendix D in­cludes excerpts from NACE Standard MROI 75, providing the definition of a sour environment and graphs that may be used to determine when sulfide stress cracking resistant ma­terials should be used. Users of this publication should check the latest edition of NACE Standard MROI 75 for revisions to the defining criteria for a sour environment.

 Manufacturer certification of compliance with
NACE Standard MROI 75 for equipment intended for hydro­
gen sulfide service may be required by the user, depending
on severity of the service. Adequate quality assurance proce­
dures should be followed to verify compliance by the man­
ufacturer for the original equipment and for any subsequent
equipment modifications.

 Materials not covered in NACE Standard MROI 75
that have been qualified for hydrogen sulfide environment
service by the user or manufacturer, using recognized and
acceptable testing procedures, may be used. A written agree­
ment between the manufacturer and user may be appropriate.
Recognized and acceptable testing procedures are those that
demonstrate that the material(s) performs as well as or supe­
rior to similar material(s) set forth in NACE Standard
MROI 75, using laboratory procedures or procedures for
which testing has been completed under actual or simulated

environmental conditions. The suitability of materials must be supported by appropriate documentation that should in­clude a complete description of the materials, processing, and testing procedure. Laboratory, field, or other environ­mental testing results or service performance should be recorded in writing. Full documentation supporting the suit­ability of the material(s) for the selected service should be maintained by the user, manufacturer, or both. Materials use is subject to compliance with applicable regulations.

Site Selection

In selecting a facility site, consideration should be given to taking advantage of the prevailing wind direction, climatic conditions, terrain, transportation routes, and the proximity of populated or public areas. Site selection should consider maintenance of clear entrance and exit routes and should minimize existence of confined spaces. Applicable regula­tory requirements concerning location, spacing, and height of flares or vent stacks shall be considered in site selection.

Warning Method

Facility design should incorporate a means to provide a warning of a hazardous upset or condition. Devices and equipment such as hydrogen sulfide monitoring equipment, hazardous warning equipment (audible or visual), and pro­cess monitoring devices (such as pressure and flow rate sen­sors) should be considered. The operator should specify, and the design should consider, the physical and environmental conditions that will be present at the installation site.

CONSTRUCTION GUIDELINES

Construction of facilities within the scope described in Paragraph 8.1 should be conducted in accordance with the following recommendations, as applicable.

Fabrication and Joining System
Components

Welding rods of proper composition and size, applied at recommended temperature and rate, should be used to join pipe and system components by welding. Preheating, post-heating, stress relieving, and hardness control requirements should be in accordance with a welding procedure specifica­tion (WPS). Proper bolting and gasket materials should be selected in accordance with NACE Standard MR0175. Proper alignment should be maintained for all piping, and all system components should be adequately supported to re­duce stresses.

Qualification of Personnel

Fabrication of system components and joining of pipe should be done by experienced and qualified workers. Piping

system welders should have passed a performance qualifica­tion test in accordance with Section IX, ASME Boiler and Pressure Vessel Code or API Standard 1104. Qualification under API Standard 1104 will satisfy U. S. Department of Transportation (DOT) requirements for pipelines. Welders shall be permitted only to weld on materials, and to use only procedures, for which they are currently qualified.

Handling and Storage of Equipment

Materials and equipment used in facility construction, re­construction, repair, or in routine maintenance should be stored and handled in a manner that will not jeopardize their integrity. Where equipment storage is required or where use after installation is delayed, adequate precautionary mea­sures against corrosion, fouling, deterioration, and other harmful effects should be employed. Reliable warehouse control methods shall be employed to assure that material and equipment unsuitable for hydrogen sulfide service is not inadvertently used in hydrogen sulfide service.

Inspection

Pressure-containing components should be thoroughly cleaned and pressure tested in accordance with applicable construction codes. Welded connections should be tested by non-destructive tests (such as ultrasonic or radiographic methods). Final facility inspection should be conducted by knowledgeable personnel to determine that the completed fa­cility complies with design specifications and material records and that markings indicate materials used are suit­able for the intended service. Refer to API 510 and API Rec­ommended Practice 12R1.

Repairs

Equipment or systems damaged or worn to the extent that their safety and reliability are doubtful shall not be placed or retained in service. Vessel, piping, and equipment repairs shall be made by qualified and, where required, certified per­sonnel in accordance with applicable codes and good work practices. Only compatible materials, suitable for use in the existing hydrogen sulfide environment shall be used to repair or replace equipment for use in this service. Refer to API 510.

ELECTRICAL DESIGN CONSIDERATIONS

In addition to being toxic, hydrogen sulfide is flammable at concentrations of 4.3 to 46 percent (by volume) in air. Electrical equipment installed in areas subject to exposure to a methane-hydrogen sulfide mixture composed of 25% or more (by volume) of hydrogen sulfide should be suitable for Class I, Group С classified areas (refer to Par. 3.5, "National Electric Code Grouping of Atmospheric Mixtures," in API Recommended Practice 500).