Pressure vessels are the primary component used in a variety of oil and gas systems. They are designed and constructed as per the standards of ABSA and ASME. Carbon steel is a common material used for pressure vessel fabrication. However, sometimes, they are manufactured from stainless steel depending on the process conditions. Most fabricators construct carbon steel and stainless-steel pressure vessels in the same facility. Due to the possible risk of iron contamination of the stainless steel, it is imperative that the fabricators use proven passivation procedures. Read on to learn more about this process.
What is Passivation?
Although stainless steel is naturally corrosion-resistant, it is not entirely impervious to rust. Under the usual conditions, many things may cause it to corrode, including the following:
- Presence of foreign materials during the manufacturing process
- Transfer of iron particles on to its surface from cutting tools
- Sulphides added to the stainless-steel
This is why it is essential to ensure that the stainless steel used for pressure vessel engineering and fabrication is corrosion resistant.
Passivation is a post-fabrication process. It is a finishing method used to make stainless steel more resistant to corrosion and rust. The process typically requires the usage of citric or nitric acid to remove iron particles from the surface of the vessel. It helps to create a protective layer of oxide that is less likely to react when exposed to air.
Passivation Process Steps
Passivation of stainless-steel vessels requires you to adhere to the following:
- Clean the Surface of the Pressure Vessel
The first step should include washing the part or container using an alkaline solution or detergent. It helps to remove grease, oil, and other contaminants from its surface. Also, make sure to wipe off shop dirt and other foreign substances. You could use cleanser or degreaser to clean the parts thoroughly.
- Using Passivating Solutions
After cleaning the vessel carefully, the stainless steel is ready for passivation. Depending on the grade of the material, use citric acid or nitric acid as passivation solutions.
- Test the Passivated Vessels
The last step is to examine and evaluate the result of the passivation process. Make sure that the surface of the parts does not contain any free iron particles.
Things to Consider During the Passivation of Pressure Vessels
Listed below are a few things to consider during passivation:
- Clean and remove all foreign particles from the surface of the vessel before passivating.
- Use clean tap water to rinse and wash the parts.
- Avoid using chlorides in excess quantities.
- Use proper equipment to apply the passivation solution.
- Maintain specific schedules to minimize the chances of contamination.
- Use the right medium to passivate based on the grade of stainless steel.
Preventing Stainless Steel Contamination
Stainless steel can get contaminated when the film or coating on its surface is damaged, leading to corrosion. This can often adversely affect the performance of the equipment, which is why it helps to take measures to prevent such situations during pressure vessel fabrication. Here are some methods that fabricators can use to avoid the contamination of stainless steel:
- Keep A Separate Storage Area
Fabrication shops that have a variety of materials or stock carry a high risk of contamination. This is why it helps to have separate areas for stainless steel, carbon steel, and other types of inventory, as well as their fabrication processes.
- Protect the Material
In addition to maintaining proper storage conditions, it is also essential to handle all the componets used in the pressure vessel adequately. It helps to use the correct form of protection such as plastic wraps for stainless steel material to prevent damage and contamination from iron filings. It also helps to prevent other debris from settling on the surface.
- Eliminate the Risk of Steel or Iron Particles
Free or loose iron particles are one of the most common causes of stainless-steel contamination. Many workshop activities such as welding, grinding, and blasting produce grime, dust, or filings. This debris can land on the surface of the pressure vessel. Thus, it helps to ensure that carbon steel is handled well away from stainless steel materials.
- Use Separate Tools
Using shared devices to handle the pressure vessel components may increase the chances of contamination. This is why fabricators should ensure that separate tools, work benches, equipment and storage fixtures are maintained. It helps to use designated tools for handling and processing stainless steel. It also helps to make sure that they are fit for the purpose.
- Follow Safe Material Handling Practices
As far as safe material handling practices are concerned, it helps to keep the following in mind:
- When handling stainless steel, use plastic or cloth covered ropes or wraps.
- If you are using carbon steel bands for stainless steel vessels, be sure to avoid making any contact with the carbon steel. It helps to place rubber or carpet padding or other similar packaging material on top of the container. Avoid the use of cardboard if the material is stored outside where rain can deteriorate the cardboard.
- Store stainless steel material in a wooden crate or on a wooden pallet. Avoid storing directly on carbon steel pallet rack shelving.
- Cover the inventory to avoid contamination from corrosive particles and airborne debris.
- Use chloride free markers when writing fabrication markings on the pressure vessel components.
- Maintain a Clean Environment
It helps to maintain a high standard of cleanliness in the workshop. This is because quality control markings, fingerprints, paint, grease, and oil may cause damage to stainless steel. Also, it is recommended to use a cleaning agent or acid with due caution and care to avoid any damage or mishaps.
If you are looking for a pressure vessel fabricator who knows how crucial it is to avoid contamination of stainless steel during the fabrication process, we can help you. In addition to complying with the required ABSA and ASME standards for pressure vessel engineering and fabrication, we are experienced with stainless-steel pressure vessels. For more information about our services, feel free to get in touch with us by calling 403-314-5422 or 1-800-993-9958. You could also fill out our online contact form to request a quote.