Is Surface Roughness in Pure Steam Systems relevant to GMP?

In the pharmaceutical industry, pure steam is a critical medium - especially in sterile and aseptic manufacturing, where it is used for the sterilisation of surfaces, containers, and process lines, among other things.

A frequently discussed question concerns the specification of the internal surface roughness (Ra) of pipes and components: Is a defined roughness - e.g., Ra < 0.8 µm - really a GMP requirement for a pure steam system, or rather an industry-standard design requirement?

Regulatory requirements - a gray area

Specifically, Part 1 of the EU GMP Guidelines stipulates the following for equipment:
3.38 Equipment should be installed in such a way that there is no risk of error or contamination.
3.39 Equipment used for production should not pose a risk to the products. No part of the equipment that comes into contact with the product may interact with it in such a way that the product quality is impaired and a risk arises (whether reactive, additive, or absorptive).

Pure steam systems only meet these requirements to a limited extent: All of them exhibit rouging, and the reddish-black iron oxide corrosion particles represent an unavoidable contamination of the steam. Depending on the process design and/or product, they can pose a risk to the product and patients.

Neither the EU GMP Guidelines nor the US FDA requirements define a specific surface roughness for pure steam systems. The requirements for materials and design of pure steam distribution systems are primarily based on principles such as:

• Suitability for the intended use
• Cleanability
• Avoidance of contamination risks

The requirement for a specific roughness (<0.8 µm) is therefore not a direct GMP requirement, but has rather been transferred from the standards for WFI and PW systems - where microbial contamination can play a role. This does not apply to the same extent to pure steam, which is an inherently sterile medium.

What are the arguments in favour of a roughness specification?

• Design control: A defined Ra specification prevents different qualities from being mixed and ensures uniform processing.
• Cost-benefit analysis: Ra < 0.8 µm corresponds to the standard "2B" finish in accordance with EN 10088 - economically sensible and widely used.

How useful is electropolishing for pure steam?

Electropolishing can delay the formation of rouge somewhat and generally reduce surface defects in stainless steel.

It is possible, and has already been observed, that loose rouge layers (flakes) form more easily on electropolished surfaces, which are then torn off by the steam as large particles. In contrast, "only" mechanically polished surfaces would not show this 'flaking' effect, or would show it to a lesser extent, because "micro-roughness" ensures better adhesion.

Against this background, the evaluation of electropolishing for components in pure steam systems is controversial among experts.

Filters as an integral part of the system

Since rouge particles cannot be completely avoided in the pure steam system, steam filters at the point of use (PoU) can provide a remedy, e.g., 0.1 µm or 10 µm stainless steel sintered cartridges (depending on the process design). These:

• capture detached particles
• protect product and process systems
• can help minimize pressure fluctuations in the network, as they also indirectly act as pressure reducers, thus helping to prevent pressure surges

Conclusion: Technical standard with pragmatic application

A specified surface roughness in the pure steam system is not a GMP requirement, but has established itself as best practice.

Recommendation:

• Use a controlled specification (e.g., Ra 0.8 µm) to demonstrably meet the state of the art in this sensitive area.
• Use PoU filters as a preventive measure to minimize particle load at critical connection points, regardless of the surface quality of the system.