Succeed at steam sterilization

Follow some do’s and don’ts to keep from making common mistakes.

By William D. Wise, Eli Lilly and Company

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Obviously, any deadleg below horizontal is unacceptable, because condensate would not drain. For vacuum pumped systems I have seen L/d ratios of <greater than sign>>6 used as a suspected deadleg. Twenty years ago, L/d = 6 was the limit of technology; today, L/d  2 often is achievable.

2. Don’t use common drains. There is a temptation to tie drain lines together to save piping costs. Do not do this or at least minimize this prior to a steam trap. In parallel lines, sufficient steam may flow through one line, but not the other. It is acceptable to tie drains together after steam traps, if the line is large enough to carry the condensate and some of the steam that bleeds through without creating measurable pressure in that line.

I’ve seen experienced engineers troubleshoot sterilization problems for days until they eventually found that the root cause of the failure to sterilize was two drains tied together prior to a common steam trap.

3. Don’t create stagnant high points. Such points are just potential deadlegs. Steam usually is introduced into vessels through nozzles that are often distant from other nozzles at the top of the vessel. Air seems to be stubborn at high points and is not always predictable. I’ve seen high points that have a 3 L/d ratio not get up to temperature while ones with a 9 L/d ratio did. Due to this unpredictability, it is best to design for positive sweeping steam flow at all these points.

4. Don’t believe that air will flow down based on gravity. Air’s molecular weight is 29 and steam’s is 18. Hence, some people believe that air will naturally fall from high points and be replaced by steam. Don’t believe it! My experience is that it takes steam turbulence to dislodge air and steam flow to carry air out of the system in the sterilization timeframe.

I once visited a company that was designing a new cell-culture reactor. Examining the piping design, I immediately told the designer that the unit would not sterilize properly due to air pockets. The individual explained that the areas I had pointed out would get up to temperature due to air being heavier than steam. I requested he place thermocouples at those locations when he started up the equipment. A few months later the designer informed me that the equipment did not get up to temperature and he had to redesign the piping.

5. Don’t create superheat conditions. Steam sterilization with superheated steam will require dry-heat temperatures and time conditions (much higher temperatures and longer times) and will not follow moist-heat kill kinetics.

Two ways to create dry-heat conditions are:

• Using an external source that heats the saturated steam to superheat conditions. An example that we constantly guard against is the autoclave with a steam jacket. Controls must be put in place to ensure the steam jacket temperature is always lower than the steam sterilization temperature within the autoclave chamber to avoid superheat conditions inside the chamber.

• Subjecting steam to a quick pressure drop (such as via a pressure reducing valve). Even though the steam is at a lower pressure, it still can maintain its heat, creating a superheat condition. Usually, normal heat loss from the system brings the steam back to saturation prior to reaching the equipment being sterilized. I have never encountered dry-heat conditions in equipment being sterilized, but this is what I advised an associate who had: Remove insulation from piping near the reducing valve (if it not likely that someone can be burned); move the reducing valve further up the line; and reduce the steam pressure in stages or put a cooling heat exchanger in place to knock out the superheat.

Not a sterile experience
Designing steam sterilization systems has been a joy over the years. At times, though, it seemed that the processes were like children, always presenting new challenges and reinforcing that I be well disciplined in the basic principles.

The numerous special situations and unique problems in steam sterilization that I have encountered over the years cannot be shared in this short article. However, I hope you’ll find these do’s and don’ts helpful.

William D. Wise is an engineering consultant for Eli Lilly and Company, Indianapolis, Ind. E-mail him at wise_william_d@lilly.com.

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