The Physical Characteristics of the Steam Supply to Porous Load Sterilizers


Steam sterilization entails the destruction of all categories of microbial organisms contained in medical devices and products using steam at a specific temperature and pressure (AESIC, 2010, p. 1). Therefore, steam is the main sterilant in most clinical sterilizers with high-temperature steam requirements such as the porous-load sterilizers. On the other hand, the porous-load sterilizers are used to disinfect porous materials such as dressings and towels. In addition, the porous-load sterilizer is also used to disinfect medical utensils and instruments, which are wrapped in paper or fabrics (Health Technical Memorandum 2010, 1994a, p. 8).

Therefore, steam that is used in different types of clinical sterilizers must meet the required quality to achieve effective sterilization (Shuttleworth, n.d.). As a result, there is the need to conduct steam quality tests to ascertain the effectiveness of steam in sterilizing different materials and equipment. Thus, the steam quality tests are carried out by the manufacturing, pharmaceutical, and processing companies concerned with medical devices and products (Shuttleworth, n.d.).

Originally, the steam quality tests were devised for the British National Health Service (NHS) but they are also found in the ISO 11134 and EN 285 standards (AESIC, 2010, p. 7). However, before the publication of the Health Technical Memorandum 2010 (HTM 2010), there was no requirement for the concerned parties to conduct regular physical tests on the steam quality such as the dryness value test, the non-condensable gas test, and the superheat test (Health Technical Memorandum 2010, 1994a). Therefore, the purpose of this paper is to present a critique of the afore-mentioned tests as provided in the HTM 2010 (part 1-5) relative to their responsiveness to improving quality assurance and cost reduction during sterilization.

The test methods for determining the physical properties of steam

The Dryness Value Test

During sterilization, wet steam is ineffective in achieving the required sterility when compared to dry steam because the presence of moisture in sterile materials can cause bacterial retention and re-infection (Shuttleworth, 2009). As a result, the dryness value test was devised to measure the fraction of moisture in steam. Here, a value of 0.99 shows that there is 99% of dry steam and 1% moisture. In addition, the quantity of latent heat in steam is equal to the calculated dryness value. Therefore, the dryness value test depends on the quantity of latent heat and the temperature of the steam (Health Technical Memorandum 2010, 1994b).

According to the method provided by HTM 2010, the dryness value is obtained by determining the quantity of steam used to heat a certain volume of water at a specific starting temperature. Subsequently, the quantity of latent heat in the steam is obtained by determining the temperature of steam using steam tables. Finally, the two values are compared to obtain an approximate value of the quantity of moisture in the steam.

Therefore, the HTM 2010 test method is effective in approximating the quantity of moisture in the steam and the latent heat. However, several pitfalls are notable in the HTM 2010 calculation in that the steam sample used to heat water does not account for the quantity of moisture on the sides of the steam supply pipe and the quantity of moisture condensed on the floor of the pipe. Conversely, the HTM 2010 method compensates for the heat lost through the test equipment by introducing a constant, which relies on the type of test equipment used (Health technical Memorandum 2010, 1995).

Overall, the HTM 2010 test method is more accurate than the one provided by the EN 285 standards especially when using the SQ 1 test equipment because it provides the procedure for choosing and establishing the test equipment. In addition, the method accounts for the heat loss as a result of different materials, which are used to construct the test equipment (Health Technical Memorandum 2010, 1996).

The Non-Condensable Gas Test

Additional gaseous components in dry steam such as air and carbon dioxide limit the effectiveness of steam in achieving maximum sterility because the non-condensable gases reduce the quantity of moisture that condenses on the materials being sterilized (Health Technical Memorandum 2010, 1997). Therefore, the sterilization process would need a lot of steam to achieve maximum sterility. In addition, the gases can insulate heat transfer thereby hindering the heating process. Furthermore, the non-condensable gases prevent the supply of steam to all corners of the load thereby affecting the sterilization process.

As a result, the HTM 2010 provides a relevant method used to determine the quantity of non-condensable gases in dry steam. Here, the dry steam is condensed in water and the residual non-condensable gases are collected in a burette and measured (Health Technical Memorandum 2010, 1994b). In this way, the method enables one to determine the levels of non-condensable gases, which can cause failures in the chemical detectors. As a result, additional vents or air removal systems can be introduced in the steam supply system to regulate the level of non-condensable gases in the steam.

However, it is worth noting that the HTM 2010 method is subjective in that its accuracy depends on several factors such as the initial quantity of gases in the cooling water and the speed at which the exercise is conducted. In addition, the initial quantity of gases in the water is not accounted for in the HTM 2010 calculations. Furthermore, reliable data may be lost when replacing the water used to condense the steam. Overall, the test method can be made more accurate by using the SQ 1 test kit, which employs a condenser to cool the steam (Shuttleworth, 2009, p. 8).

The superheat Test

When steam is superheated, its efficiency in achieving maximum sterility is lowered. Therefore, the superheat test was devised to detect steam that is supplied above its boiling point and the maximum temperature allowed (Health Technical Memorandum 2010, 1994b). Here, the test method uses temperature sensors, which determine the temperature of steam before it reaches the loads. The significance of the test method during sterilization is that it enables one to regulate the temperature and pressure drops in order to allow for sterilization to take place. However, the test method depends on assumptions and predictions. Therefore, its accuracy relies on the initial configuration of the sterilizer to regulate the pressure drops and maintain the temperature of the steam within the estimated limits.


The paper presents an in-depth analysis of the methods provided by the HTM 2010 for determining the physical attributes of the steam used in steam load sterilizers. These methods provide guidelines for conducting the sterilization process in the manufacturing, pharmaceutical, and processing companies concerned with medical devices and products. From the discussions above, it is notable that the methods provide the basic foundation and guidance for testing the quality of steam to be used during the sterilization process involving porous loads. Therefore, additional studies and improvements are required to develop accurate methods, which achieves quality assurance and cost reduction.

Reference list

AESIC, 2010. Steam sterilizers with class B cycle are necessary for the sterilization of hollow, porous, and wrapped goods. London: Association for European Safety & Infection Control in Dentistry. Web.

Health Technical Memorandum 2010, 1994a. Sterilization (Part 1): management policyLondon: HMSO Books. Web.

Health Technical Memorandum 2010, 1994b. Sterilization (Part 3): validation and verification. London: HMSO Books. Web.

Health Technical Memorandum 2010, 1995. Sterilization (Part 2): design considerations. London: HMSO Books. Web.

Health Technical Memorandum 2010, 1996. Sterilization (Part 4): operational management. London: HMSO Books. Web.

Health Technical Memorandum 2010, 1997. Sterilization (Part 5): good practice guide. London: HMSO Books. Web.

Shuttleworth, K., 2009. The application of steam quality test limits. Berkshire, UK: Keith Shuttleworth & Associates Ltd.

Shuttleworth, K., n.d. The derivation of United Kingdom physical steam quality test limits. Berkshire, UK: Keith Shuttleworth & Associates Ltd.