Saturday, 12 October 2013

How non condensable gases (NCGs) impact on steam sterilization process?

Effect of non condensable gases (NCGs) on steam sterilization process

 Steam Sterilization process is defined as the action of exposing dry, saturated steam on the product to be sterilized for a specified time period under high pressure. This kills off all living microorganisms. In medical and pharmaceutical fields, the steam is produced from purified water which has to comply with the limit values of the EP/USP. 

Non condensable gases (NCGs), have a low boiling point and as the name implies, are not able to condense out of a process at non-cryogenic temperatures (examples of non condensable gases are nitrogen, carbon dioxide, etc.).NCGs can have a serious impact on sterilization process, energy efficiency and lifetime of an autoclave.

In steam sterilization process steam is considered as a condensable gas (water vapor) and anything else that cannot be turned into a liquid using cooling water is considered non-condensable. In steam sterilization process non condensable gas can blanket (insulate) the items to be sterilized, which could inhibit heat penetration to the surface of the object.This may leads to improper sterilization.

Sources of non condensable gases in an autoclave
1. Inadequate air removal from the sterilisation chamber before steam entry. 

2. Leaks: Leaks in door seals, valves or screw fittings, allow air to enter. 

3. The main source of NCGs is the feed water which used to generate steam. These NCGs are formed during   steam generation because of: 

–Dissolved air in the water that is expelled when the water is heated.  

– Hydrogen carbonate salts dissolved in the feed water which when heated disintegrate in to carbonate salts (lime scale) and give off carbon dioxide (CO2) as a NCG. 

Therefore feed water processing is of paramount importance in steam generation. The water should be demineralised and degassed before being fed into the steam generator.

Non condensable gases impact sterilization process in two ways

  • Insufficient energy delivered to the load to sterilize. Gases do not deliver the same latent heat energy as steam.
  • Pockets of gas can form that provide “islands” of un sterility. Unless the indicator is in such an island, their presence will go undetected.

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