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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Since 1978, I’ve been designing processes, when I helped develop a successful new pharmaceutical route. Later, I spent five years in a pilot plant which ran more than 100 fermenters. Throughout this time, I have had to grapple with steam sterilization issues and have found that while the principles of steam sterilization are well known, designing actual systems can pose big challenges. Following are some do’s and don’ts gleaned from these experiences, as well as the challenges I still encounter in designing steam-sterilizing systems. 

Steam sterilization should ensure that organisms do not contaminate plant equipment and products. Unfortunately, design miscues can compromise performance.

The effectiveness of sterilization is measured in Fo. [Each Fo represents conditions providing a 90% kill or a one log reduction of normal (D = 1.0) Geobacillus stearothermophilus (also known as Bacillus stearothermophilus) spores. Geobacillus stearothermophilus forms heat resistant spores that are used as a standard for moist-heat sterilization; very heat resistant strains with D = 2 are now available.] For a background bioburden of 1 million Geobacillus stearothermophilus spores, 15 Fo’s would ensure that there was only one-in-a-billion chance of a viable spore surviving.

The do’s
The following seven factors are crucial for successful sterilization.
1. Eliminate air from the system. Effective moist-heat sterilization depends on having saturated steam conditions. When air is present in the system to be sterilized, saturated steam temperature and pressure conditions no longer apply.

The air dilutes the steam and a lower temperature than expected is imposed at a given pressure. Trapped pockets of pure air completely prevent steam contact and provide dry-heat sterilization conditions. With saturated steam, one log of kill occurs at 121.1°C for one minute. Lower temperature necessitates much longer times to achieve each Fo. Dry-heat sterilization conditions require much higher temperatures and greater time to create one log of kill.

Two methods often are used for eliminating air:
Pre-vacuum cycles. Autoclaves commonly rely on a vacuum pump to eliminate air from the piping, tubing, equipment, clothing, gauze packing and chamber prior to introducing steam. A cycle of vacuum pull followed by a steam charge is often called a “prevac.” A series of three prevacs usually suffices to remove air, as long as there are no major leaks. The advantage of prevac cycles is that they can remove air from more-porous materials and potentially deeper “deadlegs.” The disadvantage is that a vacuum pump has to be purchased and maintained.

Steam turbulence and flow. When nothing is present that will trap air, vacuum cycles are unnecessary. Many autoclave loads and most fermenters and piping systems can be sterilized using a “gravity cycle.” This method depends upon steam flowing throughout the system in a directional way to ensure air is swept out of the system. Steam flows from points of introduction to steam traps, orifices or through valves that allow the air, condensate and steam to leave the system. The piping is designed with minimal pockets or deadlegs to trap air.

One technique to eliminate air from a stagnant area is to introduce steam in a way that disrupts the air and then sweeps it out of the system. The most common place to trap air is at a high point that is not vented through a trap or orifice. In fermentation, for instance, it is important to ensure the nozzles at the top of the tank, where the agitator seal and other ports exist, do not trap air. Sending steam in through the nozzle is one method to eliminate air. Designing so that steam flows out through a nozzle to a trap is another. A third method is to introduce steam from another port and point it toward the nozzle suspected of trapping air. The steam will create turbulence and disperse the air. Steam flow through the rest of the system (if designed correctly) will carry the air out.

Systems that require sterilization can become pretty complex, especially when piping systems must be included. So, during design, visualizing steam flow and sequencing automatic valves to direct steam flow in defined flow patterns is an important check. Also, choose valves, such as diaphragm ones, that do not have pockets that trap air.

The advantage of using steam turbulence and flow is that no vacuum pump is needed. The disadvantage is that more design skill is required to ensure proper flow of steam.

2. Place traps or orifices at low points to drain condensate and slope lines to them. All true low points in a system to be sterilized either should have a steam trap or an orifice to bleed condensate. If not, steam will cool, become condensate that will fall below the required sterilization temperature and thus fail to sterilize the system. The key is to recognize all low points and design in traps or orifices. Also, sloping lines toward the low points allows minimum buildup of condensate and allows it to drain.

When using saturated steam, condensate is unavoidable, because it is formed whenever steam condenses to heat an object or surface. The condensate, if it remains at the desired temperature for the desired time, will be sterile. When maintaining the temperature of condensate in a low line poses difficulties, a solution may be to raise the incoming steam pressure to the system and hence the temperature of the low line. The pressure of the steam, the rate at which steam and condensate flow through the orifice or  trap, the size and slope of the low line, the amount of insulation and the volume of condensate to be eliminated all play a role in whether that low spot can maintain a given sterilization temperature.

3. Place temperature indicators at low spots and key points. Temperature indicators, usually permanently placed RTDs, provide the process engineer with a way of determining, both in real-time and for after-the-fact troubleshooting, if the system is properly sterilizing during routine production. Traps can fail, valves may remain closed and orifices can plug. Hence, a system that sterilized yesterday could fail today. Placing temperature indicators at each of the low spots where an orifice or trap exists enables detection and automatic alarming when temperature conditions aren’t met. Adding more instruments boosts costs, but also increases sterility assurance. Temperature elements permanently placed in other key places — at the steam source, in the vessel being sterilized or in a location that is a concern — also assist in monitoring the sterilization process.

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