In his presentation at VMA’s 2017 Technical Seminar, Kurt Larson, a process control engineer for Air Products, spoke about the inherent danger of the oxygen production business and how it is particularly important for end users and valve manufacturers to work closely together. Additionally, it’s necessary to have organizations that work to standardize and ensure the safety of plants and people.
Standards for Safety
A number of industry and company standards committees including EIGA (European Industrial Gasses Association) and CGA (Compressed Gas Association) meet regularly to establish requirements for those working in this space.
Larson noted this industry traditionally did not treat liquid oxygen (LOX) as severely as it did gaseous oxygen (GOX). However, there was a serious incident involving a butterfly valve in service downstream of a pump. It resulted in an explosion that killed several people and was later diagnosed as being caused by a lubricant that was approved for oxygen use, but had been contaminated with some kind of hydrocarbon that got down into the valve.
In another incident, a gate valve on a pumped liquid oxygen service failed. In this case, some bearings appeared to be at fault. They were made of a material that was appropriate, but silicon oil was found in the valve. As a result, the industry instituted G4014 (EIGA 200) to address requirements for handling liquid oxygen. These accidents became an opportunity for everybody in the industry to address many concerns in one document.
How Dangerous is Oxygen
Oxygen is not combustible alone. However, if there is a combustion event, high-oxygen content means that combustible materials do burn much faster. Particle impact, rapid pressurization, or compression of materials can result in heating that could cause combustion. Contamination and mechanical energy such as friction can also cause ignition and result in fast, hot fires when more oxygen is present. The higher the concentration of oxygen, the greater the risk of combustion.
Larson pointed out that, while certain precautions must be taken with liquid oxygen, it is even more important to be vigilant when working with it in the gaseous state.
To minimize the risk of fire, it is important to choose highly compatible materials for valves—both metals and soft goods. It is also important to minimize ignition mechanisms. That can be done by minimizing soft goods and limiting the use of lubricants. It is also essential to utilize best practices—from design to manufacture, to getting the product to the site, to operating it.
Choosing Metallurgy for Valves
Larson itemized several factors to consider when choosing the metals for valves in oxygen service.
- Burn resistance: Choose alloys that, after exposure to an ignition event, either do not burn or tend to quench the burn, resulting in minimal combustion.
- Exemption pressures and minimum thicknesses: Alloys have been classified by their resistance to combustion in relation to material thickness. This is a compilation of industry knowledge, published for use in the industry. According to Larson, this means that the end user must know the geometry of every component of the valve to determine its suitability for oxygen service.
- Pressure/velocity criteria: These are less stringent for non-impingement sites as particle impact is not likely. An example would be a modulating butterfly valve in which the valve body may be considered a “non-impingement site” while the disk, portions of the seat and retainer, stem, etc., would be considered subject to “impingement.” Thus, use of a stainless-steel body could be permissible, but a more burn-resistant material such as Monel would be required for the internals. Tables exist to determine what is permitted.
- The evaluation of a valve for oxygen application requires understanding of valve geometries and the minimum thicknesses of all components.
Larson observed that soft goods are the first thing to burn in a valve. They may be exposed to ignition due to adiabatic compression, flexing due to vibration or friction or mechanical impact. He noted that “the best solution is to minimize the use of non-metals.” However, where the use of non-metals is unavoidable, such as for seals, packing, gaskets, lubricants, etc., the risk can be reduced by taking precautions such as surrounding the non-metal with metals to serve as a heat sink, keeping any soft goods from direct exposure to the flow stream and preventing excessive movement.
Choose materials that are physically and chemically stable at process conditions. The preferred soft good materials include Teflon, PTFE, PCTFE, Kalrez and Viton. “But you should note,” said Larson, “impurities or variations in composition can severely lessen a material’s burn resistance, so be aware.”
“Lubricants are very difficult to incorporate in oxygen service. Most of the EIGA documents say that we don’t want any lubricants in our valves in oxygen service, but we know that’s not easy to do,” said Larson. However, if you do need lubricants somewhere in the assembly, it’s okay as long as any lubricants are cleaned out once the valve is assembled.
Ball and rotating plug valves are inherently quick-opening, leading to concerns with adiabatic compression. They may also may have sharp leading edges. As a result, these are generally not preferred in GOX service. Use is more liberal in LOX.
In the case of butterfly valves, when they are open, the body is considered non-impingement but the disk, seat and stem are all impingement sites. Thus, they can be used in throttling service under specific operating conditions.
Gate valves have similar construction considerations to butterfly valves but are not designed for throttling service.
“Many customers don’t have any oxygen experience—so manufacturers need to know how they are going to use the valves,” warned Larson. And it’s crucial to have “proper cleaning facilities and procedures from component level to final assembly.” Additionally, practice judicious use of approved lubricants and ensure valves for oxygen service always receive proper handling and packaging; in addition, keep tabs on quality control of sub-suppliers, especially those providing soft goods.
End Users’ responsibilities
It’s essential that end users maintain cleanliness through installation and operation and limit operation to defined operating cases. “And it’s essential that end users are diligent maintaining valves in oxygen service!” Larson warned.
Valve manufacturers must understand how end users are going to be using the valves that are supplied. End users must ensure the valve manufacturer from whom you are purchasing has proper cleaning procedures, has reputable, reliable suppliers and can identify all materials in the components. With proper specification, installation and maintenance, valves in oxygen service can supply years of safe operation.