NACE MR0175 Sour Service Heat Exchangers: What Hardness Limit Applies and How Is It Proven?

Wet hydrogen sulphide service is where ordinary heat exchangers fail fastest. Yuhong Group has completed qualification of its shell and tube line for NACE MR0175 / ISO 15156 environments and NACE TM0284 hydrogen induced cracking resistance, opening the sour gas, hydrocracker and amine regeneration segments to customers who previously sourced these units from Europe.

The qualification is not a paperwork exercise. Sour service failures are almost always hydrogen related: atomic hydrogen diffuses into the steel at the wet H2S surface and recombines at inclusions, blistering plate or cracking welds and heat affected zones. Preventing it requires control of chemistry, hardness, inclusion shape and welding procedure simultaneously.

• Plate and pipe: HIC tested plate to NACE TM0284 Solution A (pH 2.7 to 3.3; H2S partial pressure at or above 2,300 ppm; 96 hour exposure), with sulphur content and calcium treatment controlled at the mill; through thickness testing available on request.
• Hardness control: maximum 22 HRC, i.e. 248 HV10 (approximately 250 HV), per NACE MR0175 / ISO 15156-2, Table 1, applied to welds and heat affected zones and verified on each production weld.
• Weld procedures: low hydrogen consumables, controlled preheat and post weld heat treatment, with procedure qualification including hardness mapping.
• Material options: carbon steel with stainless cladding, SA213 TP316L, duplex S31803 and super duplex S32750 tubes, Monel 400 and C70600 tube bundles, SA182 F11 and F22 tubesheets.
• Bolting and trim: NACE compliant fasteners and gaskets, with sour service qualified alloy trim on nozzle and flange connections.

For sour service, the documentation package is part of the product. Yuhong Group supplies HIC test reports on the actual heat of plate used, hardness survey records, PWHT charts, NDT reports, hydrotest certificates and full EN 10204 3.1 certificates traceable to heat numbers stamped on the unit. Where the client nominates third party inspection, BV, TUV, SGS or Intertek witness points can be arranged for fit up, welding, testing and final painting.

One process detail worth noting: a sour service exchanger is not simply a standard exchanger in better material. Nozzle reinforcement, baffle clearance and welding sequence all affect residual stress, and residual stress is where cracking starts. Our design team reviews each sour service enquiry against the specific partial pressure of H2S and the pH of the aqueous phase before selecting material, rather than applying a single default specification.

Sour service shell and tube heat exchangers are quoted from 30 to 60 working days depending on cladding scope, with expedited handling for turnaround projects. Buyers are invited to send process conditions, H2S partial pressure, chloride level and design pressure and temperature for a material recommendation and preliminary thermal design.