NACE and H2S Wet Sour Service Design for Heat Exchangers
Equipment used in oil and gas production and refining is often exposed to extremely corrosive environments containing wet hydrogen sulfide (H2S) gas. This “sour service” causes damage through sulfide stress corrosion cracking (SSCC), hydrogen-induced cracking, pitting, and more. To address these issues, the National Association of Corrosion Engineers (NACE) has developed standards like NACE MR0175/ISO 15156 for material requirements and testing procedures. Proper understanding and application of NACE guidelines is crucial for shell & tube and air-cooled heat exchangers operating under wet-sour H2S conditions.
What is Wet H2S Sour Service?
Wet H2S sour service refers to the presence of H2S gas dissolved in hydrocarbon liquids like crude oil and condensate. This leads to an acidic environment with a pH range of 3-5. Even small amounts of H2S, down to a partial pressure of just 0.00003 psi, can cause sulfide stress corrosion cracking (SSCC) along with other damage mechanisms.
Sulfide Stress Corrosion Cracking (SSCC) is influenced by stresses, material properties, and the environment. High tensile stresses in the material increase the likelihood of crack initiation and propagation. Materials with higher hardness and strength are more susceptible to SSCC due to greater internal stresses and reduced ductility. The presence of hydrogen sulfide (H2S) in the environment is critical; it leads to hydrogen absorption by the metal, promoting embrittlement and cracking. Effective prevention involves selecting materials with appropriate mechanical properties, managing environmental exposure to H2S, and designing to minimize or redistribute stresses within the material.
Sour service conditions require the use of corrosion-resistant alloys and addressing factors like chemistry, hardness, temperature, flow rates and more in the equipment design.
Metallurgy Considerations for Heat Exchanger Components
Selecting the right materials of construction is one of the most critical aspects of equipment design for wet H2S service.
For heat exchanger components such as tubes, shells, and heads, 304L and 316L stainless steels provide good corrosion resistance. Austenitic stainless steels contain nickel and chromium, giving them enhanced SCC resistance. 904L has even higher nickel content for severe conditions. Matching stainless steel tube sheets are also recommended to avoid dissimilar tubesheet joints and galvanic corrosion. It’s also recommended to match the baffle metallurgy to the tubing to avoid galvanic corrosion if the sour process is on the shell-side.
Duplex (S31803, S32205) and super duplex (S32750, S32760) stainless steels provide better protection against SSCC than 300-series stainless steels due to their higher strength and improved corrosion resistance. Their mixed microstructure of austenite and ferrite phases enhances mechanical properties, offering greater resistance to sulfide stress corrosion cracking in harsh environments, including those with hydrogen sulfide (H2S). Super duplex grades, with higher chromium, molybdenum, and nitrogen contents, offer even greater corrosion resistance and strength. These properties make them ideal for applications in challenging sour service conditions where 300 series steels might fail due to their relatively lower yield strength and susceptibility to corrosion.
Carbon steel generally lacks the needed SCC resistance, but grades like SA-516 70N can potentially be used for shells and heads if the carbon equivalent chemistry composition is tightly controlled. Restricting silicon and phosphorus content, while optimizing chromium, improves resistance. Stress relieving and post-weld heat-treatment (PWHT) is crucial to minimize hardness due to forming and welding activities.
Heat Exchanger Bundle Design Factors
The design of the tube bundle itself can influence corrosion and fouling. Tighter tube pitches increase shell-side velocity and reduce fouling, while larger tube pitches decrease shell-side velocity and chances of erosion-corrosion. Tighter clearances between the tubes and tubesheets help minimize crevice corrosion at tubesheet joints. Careful consideration of baffle designs can improve flow and drainage through the shell side to avoid stagnant zones.
Bolting, Gaskets, and Fasteners
For flanged connections and tubesheets, choosing the right gasket material is crucial. Kammprofile type 304L and 316L gaskets are preferred for custom body-flange gaskets, while spiral-wound gaskets in 304L and 316L grades are used for standard nozzle flanges.
Bolting should be resistant to sulfide stress corrosion cracking too, so grades such as SA-193 B7M studs and SA-194 2HM nuts may be considered. Checking flange and fastener standards like ASME B16.5 and B31.3 ensures compliance for materials, strength, and design. Using heavy hex studs instead of bolts enables inspection of the entire stud for cracks.
Inspection Testing and Maintenance
Hydrogen Induced Cracking (HIC) testing evaluates a material’s susceptibility to cracking caused by hydrogen absorption in corrosive environments. This test involves exposing steel samples to a sour environment, typically saturated with hydrogen sulfide (H2S) under controlled conditions. The samples are then examined for cracks or fissures that result from the diffusion of hydrogen into the steel, leading to internal pressure and crack initiation. The severity and distribution of cracks are measured and rated to determine the material’s resistance to HIC. This testing is critical in industries such as oil and gas, where materials must withstand sour service conditions.
Sulfide Stress Corrosion Cracking (SSCC) testing assesses the resistance of metals to cracking under the combined influence of tensile stress and corrosive hydrogen sulfide (H2S) environments. In this test, pre-stressed metal specimens are exposed to an environment containing H2S at specified conditions (temperature, pressure, and concentration). The test duration varies, but it generally continues until either cracking occurs or the test period concludes without failure. The outcome helps determine the material’s threshold stress level for SSCC and guides the selection of appropriate materials for use in H2S-containing environments, critical for industries such as oil and gas.
Enhanced inspection and testing procedures are mandated under NACE MR0175/ISO 15156. Nondestructive examination (NDE) methods like 100% radiographic for internal flaws in welds, and ultrasonic testing to check for delaminations in plates for shells and heads. Materials must be certified as meeting NACE requirements. In sour service, damage and corrosion occur more quickly so regular preventative maintenance and inspection practices must be adapted.
Contact Altex Industries for Design and Services for Shell & Tube Heat Exchangers
Altex Industries provides engineered shell & tube heat exchangers and air-cooled heat exchangers to the oil and gas, chemical, and other industries. With in-house design, manufacturing, and testing capabilities, Altex offers air coolers and NACE MR0175/ISO 15156 heat exchangers purpose-built for wet H2S service.
Contact our team today to discuss the unique needs of your next heat exchanger project.
