Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During a Shutdown of Refinery Equipment
|Publication Date:||10 September 2018|
This standard practice provides mitigation methods to protect austenitic stainless steels and other austenitic alloys from PTA SCC that can occur during a shutdown of refinery equipment.
For the purposes of this standard, a shutdown includes the actual downtime period and the contiguous periods required to shut down and start up the equipment.
For the purposes of this standard, the term austenitic materials includes austenitic stainless steels and other austenitic alloys.
For the purposes of this standard, the term other austenitic alloys refers to those alloys of nickel, iron, and chromium that may be susceptible to PTA SCC.
For the purposes of this standard, the term purging is defined as a flow of inert gas (in this case dry nitrogen) to free the system of impurities (oxygen).
This standard is directed toward preventing SCC caused by PTA that can be formed by reaction of oxygen and water with sulfide corrosion products (i.e., metal sulfides) or with other oxidizable sulfur species (e.g., sulfur, H2S). Appendix A (nonmandatory) provides background information about PTA SCC, including factors that contribute to PTA SCC, and where and under what conditions PTA SCC has been experienced.
The critical levels of sensitization and tensile stress required to initiate PTA SCC are not well understood. Therefore, austenitic stainless steel and other austenitic alloy process equipment that may be exposed to PTA should be protected using one or more of the PTA SCC mitigation methods presented in this standard, except in those cases when the equipment operates below the sensitizing temperature range and the austenitic material has not been sensitized by any prior fabrication practices (e.g., hot forming, welding, heat treatment). PTA SCC mitigation methods are listed in Paragraphs 1.3.1 through 1.3.4, and more details on each mitigation method are provided in later sections of this standard. Users may select one or more of these mitigation methods depending on their needs and assessment of exposure risk, which includes the ability of the selected mitigation method(s) to reduce the PTA SCC risk and the possibility of creating additional exposure risks when implementing the selected mitigation method(s).
Selection of Materials and Fabrication Practices
Selection of materials and fabrication practices are made that result in a fabricated component or process equipment resistant to sensitization, supported by an assessment of the risk of PTA SCC associated with such selections. When the risk associated with potential PTA SCC is judged to be acceptable, the user may not require the application of other mitigation methods.
Nitrogen Purging to Exclude Oxygen
A dry nitrogen purge is used to exclude oxygen from the equipment. Use of a dry nitrogen purge may also exclude water from the equipment.
Alkaline Washing of Equipment Surfaces
Alkaline washing of equipment surfaces is used to neutralize any PTA that may form. Field experience has demonstrated that austenitic stainless steels and other austenitic alloys are effectively protected when alkaline wash solutions are properly applied to all equipment surfaces.
NOTE: The user must consider other factors such as the effect of the alkaline chemicals on catalysts, as well as the appropriate means and protective equipment required for handling these chemicals.
Dry Air to Prevent Liquid Water Formation
The use of dry (dehumidified) air for protection from PTA SCC is acceptable if the dew point temperature of the air entering the equipment is maintained a minimum of 22 °C (40 °F) lower than the internal surface metal temperature.
Regardless of the mitigation method(s) selected, the user must take appropriate confirmation steps to validate compliance with the requirements of this standard to ensure that protection from PTA SCC is provided.
If process equipment remains unopened and "hot" (i.e., above the water dew point temperature of the vapor in the equipment), additional protection from PTA SCC is unnecessary.
The PTA SCC mitigation methods described in this standard are not designed to remove process-related chloride deposits, such as ammonium chloride (NH4Cl) that can cause corrosion or chloride SCC, from the equipment. The alkaline washing method of PTA SCC protection entails some risk of chloride SCC. However, the alkaline washing procedures described in this standard should minimize the likelihood of chloride SCC by the alkaline wash solutions.
Heaters used in process units often have austenitic stainless steel or other austenitic alloy tubes, which are subject to coking, for preheating reactor feed or recycle gas, or both, containing H2S and other sulfur compounds. The austenitic stainless steel tubes in these services can be susceptible to internal PTA SCC. If not decoked, the heater tubes should be kept dry; effectiveness of alkaline wash is low when coke is present. If the heater tubes will be internally exposed to air (for maintenance work) and alkaline washing is the selected PTA SCC mitigation method, decoking of the tubes should be done prior to or concurrent with the alkaline washing to allow the alkaline wash solution to reach the internal tube metal surfaces.
Thermal decoking procedures should ensure that the heater tubes are not subject to condensation of water prior to completion of decoking, and PTA SCC protection should be provided immediately after decoking.
Pig decoking procedures should use alkaline wash solutions during and after decoking.
There have been numerous cases in which alkaline wash solutions have been very difficult to fully drain from vertical heater tubes. Boiling/evaporation of the residual alkaline wash solution during startup has resulted in concentration of carbonate and chloride salts, leading to SCC or solids plugging (tube overheating from loss of flow). Alkaline washing of vertical heater tubes should be done only if effective draining and drying procedures such as nitrogen sponge pigging are developed. Another option is to circulate hydrocarbon (turbulent flow required) to flush out alkaline wash solutions.
PTA SCC protection of the external surfaces of austenitic stainless steel and other austenitic alloy heater tubes should be considered when sulfur-containing fuels have been used for heater firing.
In many applications, combustion conditions in the firebox result in formation of oxide scales on the external surfaces of the heater tubes rather than formation of the sulfide scales that are a precursor to PTA formation. External surfaces of austenitic stainless steel and other austenitic alloy heater tubes with oxide scales do not require PTA SCC protection.
When combustion practices (e.g., fuel-rich combustion) lead to reducing conditions that generate sulfide scales externally on heater tubes, moisture exposure during turnarounds should be minimized. PTA SCC protection during a shutdown should be considered based on a risk analysis. If warranted, one of the following PTA SCC mitigation methods should be used:
Alkaline wash the external surfaces of the heater tubes.Prevent moisture condensation on the external heater tube surfaces by maintaining tube and firebox temperatures above the water dew point. Depending on the dew point temperature, this may be accomplished either by keeping pilots burning or keeping a burner at minimum firing level when personnel entry into the firebox is not needed and safety procedures allow. External or internal electric heaters may also be used to heat the firebox. Heater tube temperatures should be monitored to ensure they remain above the water dew point temperature.2
Use dehumidification equipment to maintain the firebox above the water dew point. Appropriate monitoring should be used to ensure that the atmosphere in the firebox remains above the water dew point. Industry experience with this technique is limited.