Due to the hazardous nature of sour service applications, which contain hydrogen sulfide (H2S), the National Association of Corrosion Engineers (NACE) has issued a standard recommended practice that includes minimum requirements to resist sulfide stress cracking. While not mandatory, many petroleum and natural gas producers are using the current NACE MRO175 guideline to improve the safety of their facilities and employees.
Petroleum and natural gas facility operators cannot afford to take sour service applications lightly because H2S is extremely hazardous. Any type of failure in the pressure boundary area of a level control – such as a displacer switch – can result in the release of concentrated sour gas vapors, which can be fatal within only five minutes at a concentration of 800 ppm.
Ensuring Compliance with NACE MRO175
When using a displacer switch in a sour environment that contains water with H2S present (or sour gas where H2S is present), the manufacturer must specially treat the materials used to construct the switch to ensure that the material will not fail.
To prevent sulfide stress cracking, the material in a displacer switch must not be “brittle.” The manufacturer must test the brittleness or “hardness” of the material to ensure that it meets the industry guidelines for sour service. Hardness is measured by the Rockwell Hardness Scale (HRC) or, as an alternate, by the Brinell (BHN) Hardness Scale.
In most cases HRC 22 or BHN 237 is the maximum hardness acceptable for carbon and low-alloy steels. One exception is A-105 forged carbon steel. This material must meet the more stringent requirement of hardness at or below 187 HB. Hardness testing consists of pressing a diamond shaped indenter or a steel ball against the metal and measuring the indentation.
To ensure compliance with NACE MRO175, the following requirements must be met:
- Approved mild carbon steel or fully annealed 316 stainless steel materials
- ASME approved welders and weld procedures
- Hardness testing to maximum HRC 22
Installing a flanged-cage float switch that has had the carbon steel chamber thermally stress relieved, such as a Magnetrol® Series 3, is a proven way to comply with these requirements. Although Series 3 displacer switches come standard with approved materials as well as ASME welders and weld procedures, MAGNETROL substitutes a 316 stainless steel “E” tube mounting nut and a 316 stainless steel sheathed attraction sleeve to ensure NACE compliance.
Following the stress relieving process, the manufacturer must test hardness in four locations: E-tube mounting nut, top flange, and the circumferential welds at the weld neck flange and bottom weld cap. The E-tube and flange tests should consist of three impressions. The circumferential weld tests should consist of five-point hardness traverses on three locations around the weld circumference.
Performing hardness testing on the process connections of production chamber assemblies may yield inaccurate results because the shape of the couplings and associated fillet welds limits access by the test equipment. However, heat treating the entire chamber removes all of the detrimental effects of welding in all of the affected areas – and eliminates the uncertainty of hardness readings in these areas – to ensure full compliance with NACE requirements.
For more information about displacer switches, you can also download the Buoyancy Products technology bulletin.