Pharmaceutical Steam Review | Steam Quality Test
"Steam" is a type of utility that is used for much more industrial purposes. For pharmaceutical industries, steam generation systems are a must. Steam is indirectly used to produce heat by heat transfer mechanism and also steam is directly used for many industrial purposes such as steam sterilization. Though it is used directly, steam has its own requirement. That is the raw steam is not used for direct purposes, instead, the steam purification is done. Hence before using, the steam quality test is done to ensure saturated steam dryness, superheat level, and for ensuring non-condensable gaseous. So, this post is on the steam quality test. Before that, we must understand "The steam". Though steam is used in the pharmaceutical company, steam is also used for agriculture purposes, domestic purposes, chemical processing, electricity generation, energy storage, etc.
What is steam?
In short, the steam is water in the gaseous phase. Generally steam is coming under thermodynamics. Steam generates when water is heated above 100°C.
Steam Generation:
When water is heated it evaporates, i.e. it changes to water vapor and expands. At 100℃ it boils, thus it rapidly evaporates and at boiling point, the invisible gas i.e. steam is created. The visible steam is called wet steam.
Saturated steam:
Saturated steam is pure steam in direct contact with the liquid water from which it was generated and at a temperature of water at the existing pressure. For example, saturated steam at 50 PSIG has a temperature of 298°F.
Steam Quality Test:
“A continuous supply of saturated steam is required for steam sterilization. Hence steam quality test is more important. Excess moisture carried in suspension can cause damp loads, while too little cannot prevent the steam from becoming superheated during expansion into the sterilizer chamber. The accurate measurement of the percentage of moisture content in the steam is difficult and the traditional methods where the constant steam flow is required are not suitable for sterilizers. The test method described should be regarded not as measuring the true content of moisture in the steam, but as a method by which the provision of acceptable steam quality can be demonstrated” - EN 285.
Many times I heard the below questions regarding steam.
- Why steam quality is important in sterile pharmaceutical industries?
- What is superheated steam?
- What is the test for steam quality evaluation?
- Basics of steam quality test.
Introduction to steam Quality Test:
The testing involved in pure steam validation requires a continuous supply of saturated steam; preferably sourced from the actual line that supplies your sterilization systems.
A too high level of non-condensable gases will prevent the attainment of sterilization; too little moisture carried in suspension may allow the steam to become super-heated during expansion into the chamber, while excess moisture may cause damp loads.
Where steam systems are either routinely or irregularly shut down, large quantities of air will be present in the distribution system on restarting. Hence, it is recommended that in such circumstances a comprehensive and validated venting procedure should be applied and testing steam quality should be routinely executed.
Why Steam Quality Test is required?
- To Detect Non‐Condensable Gas, Dryness, Superheat level of steam.
- To qualify plant/utility/clean/pure steam generators, steam distribution systems, and steam supplies to autoclaves in accordance with cGMP (Orange Guide), HTM 01, HTM 2010: 1994, HTM 2031: 1997, EN 285: 2006, AAMI ST79, ISO 14937:2000, ISPE Baseline Guide for Steam and Water and PDA Technical Reports No. 1 & 48.
- To test physical steam quality with assured and repeatable results in no time at all.
Being a steam sterilizer, steam is the most important part of the autoclave. Hence steam quality is periodically evaluated with chemical and microbiological aspects. During the steam quality test, below mentioned tests are being performed;
A. non-condensable gases test of Steam:
Method:
- The non-condensable gas test is carried out using simply by condensing steam and collecting any gases that may be present in a burette.
- By comparing the volume of gases collected with the amount of condensate we collect, we can calculate how much gas is present in the steam.
- In steam, CO2 is present as gas.
Testing Location:
- In the steam supply line close the user point
- The testing ports should be installed upstream of any sterilizer control valves in order to assure that the testing is performed at the supply header pressure rather than the pressure present within the sterilizer.
- The header pressure should be approximately 50PSI to 80PSI.
Why non-condensable gas (NCG) measurement is required?
In steam, if non-condensable gas is present more than 3.5 %, the steam penetration will not be achieved, leading to sterilization failure. Instead of penetration, the non-condensable gases have accumulated in the center of the bowie dick test pack.
Even rather small fractions of NCGs (below 1 %) seriously hamper steam penetration in porous materials or devices with hollow channels. Hence, a non-condensable gas measurement is must require.
B. Dryness test of Steam:
How to determine steam dryness?
Steam dryness is calculated by measuring the temperature change in a known amount of water in relation to the mass of steam that is required to cause that temperature change.
Ideally, the temperature rise is exactly proportional to the amount of steam delivered to the water to heat it, resulting in a dryness value of 1.0 (i.e. perfectly dry steam with no liquid water content.)
Steam Dryness Acceptance Limit:
Normally, the dryness value is less than 1.0, as there are thermal losses in any piping system even if it is well insulated. Because the dryness value of the steam at the chamber entry point can be quite a bit lower than the dryness value in the sterilizer. Measurements of steam dryness should be made at both locations.
The acceptance criterion for steam dryness (the fraction of steam relative to water – 1.0 = all steam, no water) is at least 0.95, or 95% by weight. However, steam below this value is considered to be wet steam.
What is the significance of the steam dryness test?
Wet steam does not deliver as much energy to the load as >90% saturated steam and can cause the resulting condition “wet packs or damp load”. If the steam is wet, or if the saturation level has decreased since the last validation, the expected Sterility Assurance Level is probably not being achieved. This is especially important for bio burden-based validations since overkill cycles have more of a safety margin by their very nature.
C. Superheat Test of Steam Quality Test:
What is superheated steam?
Superheated steam is a clear color-less gas that will not condense until its temperature drops to its boiling point. Until this occurs the moisture necessary for sterilization cannot be produced and therefore presents a risk to the process. Superheated steam acts as hot air and requires sustained high temperatures and long hold times before sterilization can occur. It is essential in the facility, pure steam validation to verify that the steam being tested is not superheated.
Superheat testing is performed to determine if the steam is at a temperature above its boiling point for a given pressure. Superheat can be produced as a result of excessive pressure drop. When steam goes from high pressure to low pressure, its energy level will remain the same. The steam is sampled in free expansion into the ambient air. The maximum temperature measured at a precise location in the jet is the temperature upon which the superheat analysis is based.
When the temperature and moisture content does not match up, two things can happen:
- If the moisture content is higher than saturation for the temperature, wet loads occur.
- When the moisture content is lower than saturation for the temperature, the condition is called superheat. In superheat, the steam is too dry and its energy content is too high. When the steam condenses on the load, the energy released is enough to melt plastic packaging and actually char paper packaging. Neither is a good outcome.
Superheat test acceptance limit:
The amount of superheat present in the steam should be no more than 25 degrees Kelvin (~25 degrees Celsius) above the temperature in free expansion into the atmosphere at the current atmospheric pressure. For all intents and purposes, this corresponds to an upper limit of 125°C in the measurement.
Test Frequency:
At the time installation and during periodic performance evaluation (biannual)
Steam Quality Test Acceptance Criteria:
Steam Dryness. : >0.95 w/w
Non-condensable gases : ≤3.5% v/v
Superheat. : ≤25°K
Non-condensable gases : ≤3.5% v/v
Superheat. : ≤25°K
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