Cleaning for Oxygen Service: Do it Before or After Assembly?

Oxygen service systems employed in oxygen-enriched environments have no room for error when it comes to being thoroughly cleaned. Even the smallest of contaminants can cause a fire or an explosion. Secondarily, particles can become entrapped in the system leading to equipment malfunction. Because of these risks, Cleaning for Oxygen Service (CFOS) is given extra attention and is considered a specialized subgroup of high-purity chemical processing. CFOS requires specific processes for cleaning, drying and inspection to ensure safety.

To this end, both ASTM International and the Compressed Gas Association (CGA) have published guidelines with best practices that should be followed to ensure that components of an oxygen service system have been cleaned to a state that minimizes the risk. Of the two, ASTM’s “Standard Guide for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments” (ASTM G93/G93M-19) covers applications across a broad range of industries. On the other hand, CGA’s “CGA G-4.1, Cleaning of Equipment for Oxygen Service” is more narrowly focused, describing the “cleaning methods and requirements for equipment used in the production, storage, distribution, and use of liquid and gaseous oxygen to reduce the risk of fire, explosion, or promotion of combustion.” It details the standards for cleaning the oxygen service’s components, but also covers how any equipment involved in providing or transporting liquid or gaseous oxygen should be cleaned as well, up to and including storage vessels and rail cars.

Before or After

Though each has a different scope, both documents are recognized as international standards for CFOS. Additionally, ASTM G93/G93M-19 references CGA G-4.1 as part of its own guidance. The two documents also share another common factor in that they both advocate for oxygen cleaning the individual components prior to assembly. G93 is more explicit than G-4.1, with section 6.1.1 stating, “Assembled systems must be disassembled for cleaning if construction permits.” As an additional consideration, G93 and G-4.1 have different inspection and cleanliness requirements. Those differences are addressed in our article linked above. It is therefore important that the standards appropriate to the system’s application are the ones that are complied with.

While “unassembled” is straightforward enough, in terms of CFOS, “assembled” can mean both putting subassemblies together before delivery as well as installing on-site. There are valid reasons on both sides of the argument about whether to conduct oxygen cleaning before or after assembling components. This debate, coupled with questions about how to meet the requirements of G93 and G-4.1 have resulted in a number of Astro Pak’s clients and others asking our Technical Services Group (TSG) for advice on best practices for CFOS. There is no “right answer” that works for every situation as the trade-offs need to be carefully weighed beforehand. The chart below offers a quick summary of the comparative advantages of performing CFOS before or after assembly:

Comparative Advantages of Performing CFOS on Components Before or After Assembly/Installation Pre-Assembly Post-Assembly
Reduces risk of contaminants becoming caught in entrapment areas.
Maximizes effectiveness of analytical techniques used to determine sufficient cleanliness.
Allows the system owner to apply the lubricants which meet their specification.
Follows the strong recommendations of industry-standard documentation ASTM G93/G93M-19 and CGA G-4.1.
Typically faster, less labor intensive and lower in cost.
Nullify the risk associated with contamination during the assembly or installation process.
Does not require additional soft goods for re-assembly of components.

In general, CFOS before assembling or installing the component maximizes the confidence that the components have been successfully cleaned while adhering to the best practice. By contrast, performing CFOS afterwards maximizes savings in time while also nullifying the risk associated with contamination during the assembly or installation process.

There are additional factors involved which must be considered before making a choice:

Risk of Contaminant Entrapment

At every component connection point is a potential entrapment area which can be difficult to properly clean. The largest example of such are fitting connection points, flanged, sanitary, or other connection types can become entrapment points. Other areas of a system can also present these risks, such as dead legs and multiple piping branches. Fabrication debris, residual cleaning products or other foreign material can remain on the surface and be undetectable and unreachable.

Pre-Assembly Cleaning – Reduces this risk significantly

Post-Assembly Cleaning – Elevates this risk

Cleanliness Verification Testing

As part of the CFOS process, a component should be visually inspected under a white light and blacklight, evaluated using wipe tests, and/or have solvent analysis conducted to ensure that the cleaning process was successful. The above-mentioned entrapment areas can block visual inspection, prevent a proper wipe of the cleaned surface and limit the ability of solvent to wet all surfaces.

Pre-Assembly Cleaning – Greatly eases this process and provides a higher level of assurance that all areas have been inspected.

Post-Assembly Cleaning – The presence of entrapment areas requires a more robust sampling plan to ensure that the CFOS was successful by ensuring a sufficiently representative cleanliness sample has been collected.

Greater Control for End User

Given the level of risk involved, many end users require assurance that oxygen-compatible lubricants have been used in the assembly of components.  Hydrocarbon-based lubricants, as commonly used in valves or other moving parts, will serve as a fuel for an oxygen fire. Fluorinated lubricants, which are designed for oxygen systems, are safer to use in oxygen system components.

Pre-Assembly Cleaning – The end user can be assured, through review of the cleaning contractor’s procedure and records, that hydrocarbon-based lubricants have been completely removed and replaced with suitable, oxygen-compatible lubricants.

Post-Assembly Cleaning – The end user is reliant on the original manufacturer of the components to use the proper lubricant without any additional means of verification.


Pre-Assembly Cleaning – Because the parts are cleaned individually, time must be allotted to assemble them into subassemblies before installation.

Post-Assembly Cleaning – No extra time is needed for assembly and installation.

Adequate Contamination Control Practices

A component is only as clean as its environment. If the components are not assembled or installed in an environment that meets the required cleanliness level (ISO rating), then the CFOS process is essentially nullified.

Pre-Assembly Cleaning – The system owner must have adequate protocols in place to ensure that the cleanliness level achieved through CFOS is not compromised. Re-validation may be necessary if cleanliness practices during installation were questionable.

Post-Assembly Cleaning – Since components are already in place there is no risk of re-contamination.

Replacement of Soft Goods

In situations where the components to be cleaned are disassembled to allow for the process, it is best practice to replace any gaskets or seals before reassembly as they may have been compromised or damaged during the disassembly process.

Pre-Assembly Cleaning – Soft goods should be replaced to ensure proper function of the component.

Post-Assembly Cleaning – Since components were not disassembled for cleaning, no replacement of soft goods is needed.

Additional Considerations

While cleaning components after assembly or installation nominally saves time, the time saved can be quickly offset by the extra time and steps needed to test and validate that the assembled unit has been sufficiently cleaned to the point that the risk of an oxygen fire has been reduced. Similarly, the risk of mechanical failure – a secondary, but still important concern – is similarly reduced as contaminants can also cause valves or other moving parts to malfunction. For those reasons, many consider post-assembly cleaning to be preferred.

Some CFOS standards require that the cleaning solution within the system achieves a certain velocity or turbulent flow to ensure that any contaminants are removed from the surface. Thus, the pumps moving the solution through must be sized accordingly.

To avoid a lot of the pitfalls involved in post-assembly CFOS, the system can be designed in advance to support this. Should the CFOS protocols require solvent extraction, the system should be designed to allow for a comprehensive sample to be collected which covers enough of the internal surface to verify the cleanliness level has been achieved. Where solvent, oil-free dry air, or inert gas is used to achieve the necessary drying of the interior surfaces, locations allowing the easy removal of this chemistry should be included at the design phase.

Because of the difficulties of performing CFOS on a completed system, a hybrid approach is often utilized. For example, a system with welded straight piping with no entrapment areas can be installed and flush cleaned.  Meanwhile, in line components that present an entrapment risk are cleaned separately and then installed after the piping is cleaned.

It’s important to note that variations within systems and components must be evaluated individually to develop a proper cleaning strategy. For example, a compressed inert gas line for industrial use would likely not require the high level of cleaning and cleanliness validation that a high-pressure pure oxygen line used in an aerospace application would. In some cases, it may be beneficial to clean the system both before and after installation to minimize risk to the greatest extent possible.

Additionally, if a system has a pressure regulator or other intricate components that require cleaning, a flow-through approach will most often not be advisable, and the regulator or component should be completely disassembled for cleaning and validation.  Whenever a component is disassembled, it is also important to perform a full functional test to verify proper operation.  Functional components (i.e. valves, regulators, etc.) are reassembled and functionally tested in a clean environment with procedures that will preserve the cleanliness achieved (i.e. clean test gasses, tools, work practices, etc.).

Whatever Your Plan, Astro Pak Can Help

The best time to contact Astro Pak is during the design phase. Our experts can review your plans and suggest the best course of action – including advice on how to design the system for successful initial CFOS, and for regular maintenance cleaning that will ensure a long service life.

Pre-assembly cleaning can be done at any of Astro Pak’s coast to coast cleanrooms. Once cleaned, subassemblies can be put together as needed and the cleaned components are carefully packaged and sealed to maintain the cleanliness during the transport process. Another alternative is for the components to be cleaned on-site.  In some instances, the use of our mobile labs can verify in the field that the system has reached the required ISO level. Removing the transportation factor not only further cuts the risk of a compromised CFOS, but it allows for more flexibility in case an issue during construction requires a change in design.

In cases where CFOS of fully assembled systems cannot be avoided, Astro Pak has the capability to ensure the required cleanliness level is achieved, depending on system design, and to perform CFOS in a manner that complies with the appropriate standards.

Regardless of whether the cleaning is conducted pre-assembly, post-assembly or in a hybrid combination, it will be thoroughly documented to establish compliance with customer specifications.

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Chad Clements

Chad Clements works as the Chemical Engineer for Astro Pak’s Technical Services Group, providing technical guidance to field, shop, and cleanroom locations across the United States. He has a background in precision cleaning, oxygen cleaning, process engineering, chemistry, and bacterial remediation. Chad holds a bachelor’s degree in Chemical Engineering from the University of Florida. Prior to working under the Technical Services Group, he worked as a Process Engineer for Astro Pak’s Florida Cleanroom facility.

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