Steam Quality Test

Steam Quality Testing

• Steam Quality, Purity and quantity can be affected by the design , use and Maintenance of overall steam system.
• Steam Systems should be designed to ensure that the steam delivered to the sterilizer is saturated steamhaving a steam quality of 97% to 100 %.
• Steam of poor quality can contribute to wet packs and sub-optimal steam sterilization cycle that might not be identified by bilolgical monitoring.
• Steam Provides moistire that allows the coagulation of cell wall proteins and supplies the energy that heats the components and maintains their temperature, the combinationof moisture and temperature results in sterilization.
• The higher the temperature, the shorter the sterilization time required. For proper sterilization, steam requires a number of attributes. 
• For these reasons, steam quality testing should be applied to steam sterilizers

Steam Quality testing is the phrase sometime associated with a series of three physical tests (non condensable gasses, dryness fraction and superheat)

Super Heated Steam:

• Superheated steam is steam that is at an elevated temperature for it's saturation pressure and is usually generated as the result of pressure drops through either pressure-reducing values or orifices. 
• The impact of the pressure drop is to modify the pressure of the steam while it's energy content remains the same. 
• The excess energy for the pressure present will result in any excess moisture turning to steam. 
• If the steam is already dry saturated, or if excess energy is still present after converting moisture to steam, an increase in temperature will be evident. 
• Once steam is dry saturated only a small amount of energy is needed to create high temperatures. Superheated steam acts as hot air and at the temperatures present will have little or no sterilizing effect.
• The excessive temperatures can also cause damage to items and packaging. 
• To test for superheated steam, a small load test is performed where a thermocouple is placed 2 inches above a standard linen pack. 
• According to European standard EN285, the temperature measured by the thermocouple at the start of the sterilization stage should not exceed the control temperature by more than 5ºC and should reduce to less than 2ºC in one minute or the temperature is considered to be acceptable if it is less than 25 ºC above that of the local temperature of boiling water.

 Non-Condensable Gases (inert gases, Mainly NH3 (Ammonia), CO2 , N2, O2 and halogenated Hydrocarbons):

• Non-condensable gases (NCGs) are gases liberated by steam when it condenses. 
• The source of such gases is usually the steam generator feed water, and the impact of such gases is that they modify the steam from being pure water vapour to a mixture of steam and gas, so that they are an unwanted contaminant.
•  If an item is hollow or porous, these gases will be forced to the center, prevent direct contact or insulate the item. Such conditions are identical to inadequate air removal, where small amounts of air remain.
•  European standard EN285 limits NCGs to 3.5% and is expressed in terms of ml of gas collected per 100 ml of condensate.

Dryness Value test:

• To ensure and to test that an acceptable amount of moisture is present in the steam supply.
• For little amount of moisture there is a chance of superheating may occur. 
• Even too little moisture may prevent sterilizing conditions in the chamber. 
• Steam with a dryness fraction of 0.99 consists of 99% steam and 1% water. 
• Similarly, steam with a dryness fraction of 0.95 consists of 95% steam and 5% water. 
• The dryness value of the steam should be equal to or greater than 0.9 for porous loads or 0.95 where metal loads are processed.

Conclusion:

• As indicated herein, dryness value, steam, superheated steam and non-condensable gases all have the ability to adversely affect the sterilization cycle and therefore the sterility of porous loads. 
• The extent to which the process will be affected will be dependent on the extent of the problem and the type of load. 
• Good practice indicates that we should be aware of the condition of the steam used for sterilization of equipment or porous loads. Periodic testing of steam quality is needed to detect any transient or seasonal problems that may exist. 
• Properly designed, constructed and maintained steam distribution systems are critical to meet the demand for proper sterilization.

Reference:

1.  European standard EN285
2. Health and Technical Memorandum 10 (HTM10).
3. ISO 11134 "Sterilization of Health Care Products-requirements for Validation and Routine Control--Industrial moist heat sterilization".