Jan-2023

Long-term, safe operation of oxygen service valves

Oxygen is not flammable but supports and accelerates combustion. This means that oxygen service valves need to be designed, manufactured, tested and maintained to prevent this by mitigating the ignition source and contact with fuel.

David Anderson and David Escobar
Valmet

Article Summary

Cleaning, inspection, and testing procedures to meet industry standards and customer requirements. The inherent reactiveness of gases and liquids such as oxygen, ozone or chlorine mean they rank among the most hazardous applications for valves. Each stage of the lifecycle of these valves – design, material selection, manufacturing, cleaning, testing, packaging, handling, installation, operation, maintenance and modification – requires validated processes and competent personnel to enable long-term, safe operation at site.

Most of the existing literature on this topic calls attention to proper valve specification. On that account, this article will focus on considerations of cleanliness including keeping valves oil, grease and contamination-free at factory, at site and after maintenance. This article is not however intended as and shall not be understood as a Valmet oxygen cleaning instruction.

Valmet’s flow control business line follows strict cleaning, inspection and testing procedures to meet industry standards and customer requirements. Valve factories are evaluated and validated against these procedures at least once a year. Valmet also works closely with licensors, skid manufacturers and end users to promote good practices.

Oxygen fire hazards
Commercially produced oxygen can be separated by means of cryogenic distillation, vacuum swing adsorption, pressure swing adsorption (from air) or as a byproduct of water electrolysis. Industrial oxygen is then used for many purposes: in a basic oxygen furnace for making steel, in water pollution countermeasures, as a wood pulp bleaching chemical in the paper making process and in chemical processes such as the production of vinyl chloride, nitric acid, epoxyethane and hydrogen peroxide.

Oxygen is not flammable by itself but supports and accelerates combustion. Oxygen concentrations greater than 23.5% create a special fire hazard. There are three elements necessary for an oxygen valve fire to occur, and they are: oxidiser (oxygen), ignition source, and a flammable material (fuel).

In an oxygen-enriched environment (oxygen concentration in the range of 23.5-100%), the oxidiser is present all the time so is important to understand the potential ignition sources and mechanisms present in the valve. It is also important to know the limits for oxygen compatibility of the materials used for the valve components and lubricants because in the event of ignition, even the metal of the valve can become the fuel.

The following ignition mechanisms must be considered in a gaseous oxygen service when the contributing factors are present:
• Particle impact
• Adiabatic compression heating
• Promoted ignition/kindling chain
• Mechanical friction
• Mechanical impact
• Thermal ignition
• Electrical arcing including static discharge
• Resonance
• Flow friction

Cleaning methods
Importance of cleanliness
According to CGA G-4.1, valves used in oxygen systems (>23.5%) must be cleaned using a verified process and be shown to be free of contaminants. Maintaining the cleanliness of valves in contact with oxygen is essential to avoid ignition. The purpose is to eliminate unnecessary fuel in the valve since contaminants may be more easily ignited than the valve components or support ignition in other ways in the oxygen-enriched environment.

There are two types of contaminants, non-volatile residue (NVR) and particulate. If a valve for oxygen service is not cleaned to an appropriate level, there may be concerns of ignition of NVRs by adiabatic compression or other mechanisms and ignition of metallic particles by particle impact.

Hydrocarbon-based greases and oils, organic compounds, nitrates, phosphates, water-based detergents and cutting oils, some acids and solvents, metallic particles from rust or machining chips, lint, fibers, dust, weld slag, metal grindings and fillings are all common contaminants in piping systems.

ASTM G93, harmonised CGA G-4.1 and EIGA IGC Doc 33-18, and MSS-SP-138 provide excellent recommendations for cleaning processes, inspection methods, and various levels of cleanliness requirements. The following paragraphs include some common cleaning methods, inspection methods, and guidance on packaging, storage, installation, operation, and maintenance.

Mechanical cleaning
Mechanical cleaning is used to remove scale, coatings, paint, weld slag, loose material from the process and other solid contaminants and can include grit or ice blasting, wire brushing and grinding.

Aqueous cleaning can be done with hot water and steam cleaning, alkaline cleaning, acid cleaning, and detergent cleaning.

Hot water and steam cleaning is effective against oil, grease, dirt, and loose scale as well as welding and brasing residues and other contaminants. The addition of detergents can improve the performance of this cleaning method.

Alkaline cleaning uses caustic salt suspended in water to create a highly alkaline solution. It is effective against hydrocarbon oils, grease and waxes, and generally is enhanced by agitation and/or jet spraying. Typically, this is used for industrial parts washers. This process is greatly enhanced by ultrasonic agitation, but the solvent residue must be removed to prevent corrosion on some metallic materials.