Steam Quality Testing
Steam Quality Testing Overview
Steam quality testing is critical when performing any steam sterilization activity with an air removal step, such as autoclaving equipment. Sterilization is driven by the condensation of steam on potentially infected surfaces. As the steam condenses, the latent heat of evaporation is released and the moisture causes the coagulation of protein cell walls. For this process to succeed, several properties of the steam feed must meet specific requirements. The properties are steam dryness, superheat, and purity (or lack of non-condensable gases in the feed stream).
The steam dryness value is a fraction representing the moisture content in the steam. A value of 0 signifies 100% moisture, and a value of 1 signifies saturated steam with no entrained water. There is a direct relationship between the dryness value of steam and its latent heat, meaning that steam with a dryness value of 0.5 will contain only 50% of its potential latent heat at a dryness value of 1. EN 285 defines acceptable values of 0.9 for porous loads and 0.95 for metal loads. However, measuring the dryness fraction through the methods described in EN 285 will not produce accurate values as condensed water or wet films within the pipe are not accounted for, so it is noted that the value derived is a ‘dryness value’ as opposed to a ‘dryness fraction’. It is suitable for the determination of adequate dryness for the process, however.
Superheated steam is steam that is at a temperature above its saturation pressure. This is caused by reductions in pressure over pipe lengths or through orifices, meaning that point-of-use testing should be performed rather than point-of-origin. As the pressure drops, excess energy will cause the evaporation of any moisture entrained in the steam. If the steam is completely dry, the temperature will increase. The increased temperature can not only potentially damage the items to be sterilized, but until the temperature is reduced, the steam will not condense and will act as hot air. Processes with a longer exposure time may allow for the cooling of steam to its condensation point, and it is possible to have superheated steam contain moisture as the heat transfer and phase change processes are not instantaneous, but superheated steam poses a threat to the sterilization process.
The final consideration is the presence of non-condensable gases. Non-condensable gases are contaminants typically introduced through the water fed to the steam generator. As steam condenses on a surface, its volume is reduced, creating an area of low pressure and causing the flow of more steam to the surface. The presence of non-condensable gases prevents the flow of additional steam, effectively insulating the product to be sterilized. In porous loads, the gases can be forced into the product itself.
Steam quality testing will include the verifications of all three parameters listed above. EN 285 provides guidance on the execution of this testing and includes acceptable limits for each test. Our validation staff are well versed in sterilization processes and the execution of steam quality testing when required. We will provide a report indicating the acceptable limits for each test based off of your requirements or EN 285 guidelines. It is also worth noting that steam used for applications with no air removal step, such as Steam-in-Place systems may not require the same limits as autoclaves. This is because there will be residual air in the piping and both the dryness and purity of the steam will not remain constant throughout the system. This is not to say that a well-engineered steam system is not required for these applications, however.