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Steam Quality Testing - Hints and Tips
Who
needs to test steam quality?
What
is the difference between EN 285 and HTM 2010?
Testing frequency
Test Procedures
Regulatory
Approach
Validation of the SQ1 Portable Steam Quality Test
Kit
Accuracy
- Dryness Value & Superheat
Accuracy
- Non-condensable gas test
Background
De-aerators
The Non-Condensable
Gas Test
Pressure/Temperature Comparison
Background
Calorimetry
Acceptance Criteria
Methodology Problems
Causes of Wet Steam -
Plant Steam
Causes of Wet Steam -
Pure/Clean Steam
Background
Acceptance Criteria
Methodology Problems
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Who needs to test steam quality? Manufacturers and processors of sterile products and medical devices within Europe and those who supply Europe. The requirement is restricted to the porous loads/dry goods/equipment processes, which impact on the sterility of finished products. What is the difference between EN 285 and HTM 2010? HTM 2010 is a UK National Health Service guidance document. It was produced in anticipation of EN 285 and seeks to provide guidance to hospitals to allow their compliance with EN 285. EN 285 is a European standard for Sterilization – Steam Sterilizers – Large Sterilizers, which describes the steam quality tests and is the definitive reference. Testing frequency
The only references to
the frequency of steam, quality testing are to be
found in HTM 2010, where it is indicated that steam
quality should be tested as part of the annual
revalidation exercise for each sterilizer. Where
steam systems are either routinely or irregularly
shut down, large quantities of air will be present
in the distribution system on restarting. It is
recommended that in such circumstances a
comprehensive and validated venting procedure should
be applied and testing for non-condensable gases may
be appropriate.
Test Procedures
The standard test
procedures require the steam quality to be sampled
when steam is first admitted to the sterilizer
chamber after a cycle is started. While this
provides a reference condition, it may be inadequate
to fully characterise the steam system which may
perform differently under different flow conditions.
It is suggested that the steam supplies should be
tested under both low and full flow conditions and
particularly for the non-condensable gas test,
include conditions where the feed water pump
switches on and off (where applicable). Where
aerated water is present, the worst case condition
is invariably when water enters the steam generator.
Regulatory Approach As far as we understand, our equipment has not been the subject of specific European regulatory scrutiny, though the results generated by it has, without comment on the equipment or indeed methodology. The approach of the regulators would seem to be: -
Validation of the SQ1 Portable Steam Quality Test Kit The SQ1 Portable Steam Quality Test Kit is supplied with a Certificate of Conformity that details that the equipment supplied dimensionally complies with the requirements of EN 285. It will be seen that the test methods are relatively simple and cannot be referenced to standards. The equipment may be used as supplied with confidence and no validation of the SQ1 Portable Steam Quality Test Kit is required. While the non-condensable gas test equipment is fitted with a contact thermometer, this is for indication only and does not require calibration. Associated data logging and weighing equipment must be be calibrated. See statements on accuracy, below. Accuracy - Dryness Value & Superheat The equipment supplied complies exactly with EN 285. These tests are approximations and inferential respectively. The tests are conducted in accordance with EN 285 or HTM 2010 and the results used accordingly. It is therefore not possible to benchmark these tests with any standard. Accuracy - Non-condensable gas test For non-condensable gas testing we state that our equipment complies with EN 285 as this standard specifically allows the use of alternative methods provided they have been shown to give comparable results. This comparison work has been undertaken and the results showed that the difference between the EN 285 method and our system for non-condensable gas tests was 0.03% over five tests with values between 0.4 and 1.6%. While the results provide some value, it should be remembered in the case of the non-condensable gas test that we have a system capable of producing consistent results which we have to compare with a subjective test method ! The test objective is very simple, steam must be condensed and any gases resulting collected. Our system performs this function consistently and repeatably from test to test, irrespective of the skills of the person carrying out the test.
Background
Non-condensable gases
result from the
water from
which the steam is generated.
The solubility of gases in water reduces
with increased
temperature. It will be seen if water is heated
in a saucepan, gas bubbles are generated
which adhere to the bottom and wall of the
saucepan. If the same
water is heated inside a boiler or steam generator
these bubbles of gas are entrained in the
steam.
These gases will usually
be air, though carbon dioxide may be present in
relatively large quantities as the consequence of
certain water treatment processes, typically water
softeners. This is exacerbated by excessive aeration
that can exist in many pharmaceutical water
treatment plants where water is constantly
recirculated and sprayed into the top of storage
vessels. The effect of such gases being present in
the steam supply to a sterilizer can be the same as
inadequate air removal, that is to say instead of
components being heated by steam condensing on them,
they are heated by a mixture of steam and other
gases. When steam condenses its volume reduces
dramatically causing more steam to flow into the
void left. If gases are present in the steam, it
will be seen that the flow of steam forces the gases
towards load components where they can accumulate.
The presence of these gases causes two problems:
The Bowie Dick Test
result shown below is a result of a sterilizer where
some 6.5% of non-condensable gases were present in
the steam supply. In order for a uniform colour
change to occur, the correct temperature must be
present for the correct time and moisture must be
present. The failure of the centre portion to change
colour clearly shows the presence of air or other
gases. The clear circle in the centre graphically
illustrates how residual air or non-condensable
gases are forced to the centre of load components by
steam flowing towards the load.
While non-condensable
gases are almost always the result of supplying
steam generators with cold feed water, poorly
designed steam distribution systems may allow the
relatively small volumes of gases that are
inevitably produced with the steam to accumulate in
the distribution system which may be displaced in
high concentration packets. This may be resolved by
the installation of well placed steam traps and air
vents.
Generally, if poor
quality steam is generated, it will not be improved
by the distribution system. While air vents and
steam traps may eliminate non-condensable gases
under low or no flow conditions, under high flow
conditions such gases will invariably be entrained
within the steam and will be carried with it to the
point of use. Typically, steam flows at 25 - 35 m/s
(56 - 78 mph) and it is hard to conceive it
separating under such conditions. It is for this
reason that testing at the point of use is deemed
necessary.
Most steam generators
have integral feed water heaters as part of their
design. In most cases, these are present to recover
heat from plant steam condensate and blow down heat
exchanges. In these cases, unless any gases are
allowed to leave the feed water system, they will be
carried by the flow of water into the steam
generator. Simple heating processes such as these
will have no impact on the level of non-condensable
gases in the steam. The water must be heated
externally to the steam generator and gases allowed
to leave. This indicates an external tank.
De-aerators
Commercial de-aerators
atomise water to present a large surface area and
the resulting aerosol is heated with a contra flow
of low pressure steam. The steam heats the water
close to the saturation temperature (boiling point
for the pressure present) and physically strips away
the resulting air bubbles on the water surface.
Often a vacuum pump will be fitted to draw off the
resulting steam/gas mixture.
Other causes of
non-condensable gases are:
The Non-Condensable
Gas Test
The method of conducting the non-condensable gas test described in HTM 2010/EN 285 requires considerable skill to obtain repeatable test results. The water used for the tests should be degassed by boiling and allowing to cool in a closed container. If this is not done, gases will be released from the water in addition to that coming from the steam supply. This may result in high values being encountered. This is exacerbated by the speed at which the test is carried out, the rate determining whether high or low results are obtained. Generally, the faster the test is conducted, the higher the test result. In many respects the test method is subjective. The test has to be completed when the cooling water temperature reaches 70o C. It will be seen that to properly test the steam, it may be necessary to test under different flow conditions. The time involved in draining and replacing the cooling water may cause vital information to be lost and an incomplete picture formed. These problems are avoided when using the SQ1 Portable Steam Quality Test Kit, which utilises a condenser. This prevents the steam coming into contact with the cooling water and avoids problems due to its aeration. The only source of gases can be from the steam. Also, the tests can be carried out for an indefinite time, allowing the steam supply to be tested under a range of flow conditions, in addition to the reference conditions described in the test methodology.
Using the SQ1 Portable Steam Quality Test Kit, the test simply condenses a sample of steam taken from the top of the steam supply pipework and collects any resulting gases in a burette. The test result is expressed as ml of gas/ml of condensate which is described as a percentage. The amount of gas collected per 100 ml of condensate should not exceed 3.5 ml, described as > 3.5%. The same limit is applied by both HTM 2010 and EN 285. It should be noted that 100 ml of water will generate 169.4 litres of steam at atmospheric pressure and while the result is expressed as a percentage, by volume of gas:steam the actual value is 0.00206%, a very small value. Pressure/Temperature Comparison It is often believed that a simple pressure/temperature comparison using steam tables will allow the presence of non-condensable gases to be discovered. If 1% of air by volume were to be present in the steam supply, a value many times in excess of the 3.5 % limit (0.00206% by volume), using Daltons Law, it will be seen from the table below that the resulting temperature depression will only be 0.33o C. Given the differences in response times and calibration errors between pressure and temperature instruments, it will be seen that such a comparison will only detect very large and wholly unacceptable levels of gases (between 1 and 10%).
The theory of steam
flowing through a pipe indicates that any gases
present will be adjacent to the pipe wall. Within
this will be a layer of condensate and further
condensate will be present on the bottom of the
pipe.
It will be seen that if the sample point is larger than specified a greater proportion of gas will be collected and tend to give a higher result. For this reason the dimensions of the sample point should be as specified. Similarly, if the sample is fitted at the bottom of the pipe the result is likely to be lower because of the larger volume of condensate present.
Background
Wet steam is undesirable
as it has less energy than dry steam and more
importantly can cause wet loads. The packaging used
for sterile products prevents reinfection when dry,
but its bacterial retentive properties will be
adversely affected by the presence of moisture. Wet
loads can be considered to be unsterile.
The amount of moisture
present in steam is measured by the dryness
fraction, which is directly proportional to the
amount of latent heat present. The dryness fraction
describes how dry steam is with a value of 1
representing steam that is 100 dry and therefore
free of entrained moisture. 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.
If we measure the latent
heat present in steam that has a dryness fraction of
0.99 we will find that it possesses 99% of the full
quotient of latent heat. By establishing the amount
of latent heat present in steam we can determine its
dryness fraction.
Calorimetry
We measure the latent
heat in steam by condensing a sample in a known
volume of water having a known starting temperature.
The increase in mass of the water represents the
amount of steam utilised to heat the water to its
new, higher temperature. From this simple exercise
we can calculate the amount of energy in the steam.
If we also measure the temperature of the steam
supply we can determine from steam tables the latent
heat heat that would be present if the steam was
100% dry. By comparing the two values we establish
the dryness fraction of the steam sample. Because
the steam is sampled only from the centre of the
pipe and does not take into account moisture on the
pipe wall or condensate at the bottom of the pipe,
the test is deemed to be an approximation rather
than an absolute value. For this reason, instead of
using the term dryness fraction, the test method
uses the term Dryness Value, and this term is
always used when describing test results for steam
for sterilization.
The calculation provided
by HTM 2010 takes account of the heat loss from the
test kit by the use of a constant that is dependent
on the test equipment used. When using the SQ1
Portable Steam Quality Test Kit this constant has
been modified to take account of the stainless steel
vacuum flask and dip tube construction. This
variation is detailed in the calculation in the
manual and in the Excel calculation provided on
floppy disk. EN 285 does not specify the
construction of the test equipment that should be
used or provide any information on how the constant
is calculated.
Acceptance Criteria
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. Invariably this means the latter limit is
applicable. In any event, in plant steam terms,
steam containing 5% of moisture would be seen to be
of poor quality and a dryness value of 0.99 would be
more commonly seen to be acceptable.
Methodology Problems
Out of specification
results are often caused by the test method not
being strictly followed. Where the test point is not
as indicated problems can easily result. Similarly,
modifications to the test points by the installation
of valves and/or additional pipe fittings etc. can
result in additional heat losses being encountered
which are not taken into account by the calculation.
The start and end
temperatures within the flask should be established
by agitating the flask and water until a constant
value is reached. The test should be completed when
the water temperature reaches 80o C. If
the temperature is hotter or localised boiling
occurs, energy will be lost in the form of steam
venting from the flask and misleading results
obtained. To avoid this the flask should be
constantly, but gently agitated during the course of
the test, and preferably the test completed before
rather than after the 80o C limit is
reached. It will be found that the effects of
agitation following the test will tend to result in
an increased temperature rather than a lowering. The
use of a sheathed temperature sensing probe results
in a relatively slow response time for small changes
in temperature and time must be allowed for the
sensor to stabilise.
If the test is carried
out too slowly, the heat losses tend to increase and
have a greater impact as time progresses. The
purpose of the pitot tube is simply to provide a
controlled flow of steam into the vacuum flak. It is
our experience that the use of a pitot tube one size
larger than that specified by the standard test
method will provide a suitably fast test to avoid
such problems. Provided that the water in the flask
is not allowed to boil and heat to be lost from the
system as steam/vapour, the size of the pitot tube
is immaterial. When we conduct the test, we aim to
complete it within 10 minutes.
Great care should be
taken with mass measurements and weighing equipment
must discriminate to 0.1g. Water droplets on
surfaces of the flask that is not subject to the
heating effects of the steam can affect the results
if present in sufficient quantities. In between
tests it is recommended that the flask is dried
internally and externally and that fresh water is
added in such a fashion that it is not splashed on
the outer surface of the flask. When agitating the
flask, care should be taken to prevent any loss of
water which will affect the outcome of the test.
The temperature of the
steam supply should be logged in order that its
average temperature may be calculated for the
duration of the test. While the pressure of the
steam supply would not be expected to
fluctuate by more than 10% (EN 285) any fluctuations
not recorded will cause misleading results to be
generated.
Causes of Wet Steam -
Plant Steam
Wet steam may be caused
by excessive pressure drops on the boiler due to
high demands. As the pressure drops, the size of
steam bubbles increase in turn increasing the volume
of water in the boiler and causing it to be closer
to the steam outlet. The increased size of the steam
bubbles results in a more aggressive boiling action,
which causes more/larger droplets of water to leave
the water surface and enter the steam space and thus
be carried over into the steam.
Steam at a low pressure
occupies more space than steam at a high pressure
and a further affect of a pressure reduction is to
increase the velocity of the steam leaving the
boiler. This can reach such a velocity that it will
take some of the boiler water with the steam.
Certain contaminants in
the boiler water can cause a froth to form on the
water surface, again allowing moisture to enter the
steam supply.
Once in the distribution
system, the quality of steam is likely to
deteriorate as the result of heat losses causing
further condensation. To minimise such
deterioration, the steam distribution system should
be well insulated and be provided with a well
designed and installed condensate removal system
(steam traps and separators). Pipework should always
have a fall towards steam traps. A common problem
that causes wet steam is where pipework is sagging.
This allows pockets of water to accumulate until
they are sufficiently large to occlude the steam
pipe, causing the increased steam velocity to carry
them to the points of use in discrete slugs.
Causes of Wet Steam -
Pure/Clean Steam
Pyrogen free steam
produced by a clean/pure steam generator should be
dry saturated (dryness value of 1). That is to say
it should be dry and at its saturation temperature
(boiling point for its given pressure). Pure steam
generators are normally fitted with a pressure
sustain valve which prevents excessive pressure
drops and therefore the potential to carry over
water with the steam. This valve will prevent
pressure drops at the generator by maintaining the
generators internal pressure at the expense of the
distribution system. As with plant steam its quality
can only deteriorate within the steam distribution
system as described above, where the same design
requirements apply to insulation and condensate
removal.
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Background
Superheated steam is steam at a temperature above its boiling point for its pressure. Superheated steam is a clear colourless 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. While superheated steam is not usually intentionally generated in the healthcare or pharmaceutical industries it can be produced as the result of excessive pressure drops. If we reduce steam from a high to a low pressure its energy level will remain the same. This high energy level will initially result in any moisture present in the steam to be evaporated. Any additional energy will then result in a temperature increase in the steam and the superheat phenomena will become evident. Because the superheat will reduce as heat is transferred to the load, this is generally a temporary phenomena at the start of the sterilizing period. Superheated steam has the greatest adverse impact where high temperature/short time sterilizing cycles are used, typically 134o C for 3 minutes as commonly used in healthcare applications. In this case, should the duration of the superheat last the full 3 minutes, sterilization would not occur. Should the same phenomena be present for 3 minutes of a 30 minute sterilization period, the impact is restricted to the initial 10% of the holding period. Despite, the impact being duration dependent, good practice indicates that superheated steam should not be tolerated. EN 285 indicates that pressure drops should not exceed a ratio of 2:1. If the pressure drops occur sufficiently far away from the sterilizer it will be found that any superheat generated will diminish as it loses energy to the pipe walls and any moisture present. Acceptance Criteria When steam is reduced from line pressure to atmospheric using the pitot and expansion tube shown, the temperature measured should not exceed 25o C above boiling temperature for the atmospheric pressure at the test point (typically the measured value should not exceed 125o C). It is stressed that the limit describes the maximum temperature and that no minimum value applies to this test. The assumption made by the standards, but not specified, is that if this limit is not reached, when the steam expands into the chamber its condition will be satisfactory. In this respect, the test is predictive and its worth is dependent upon the specific configuration of the sterilizer with respect to the pressure drops involved after the test point and any further conditioning that may occur from steam separators etc. Further information Methodology Problems The temperature sensor should be sufficiently small to not represent a large heat sink which will dissipate any superheat. A bare thermocouple is best in this respect. The thermocouple should be moved across the steam jet issuing from the pitot tube until the highest temperature is reached. The value achieved will depend on the dryness of the steam and the size of the pressure drop involved. |
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