The last unknown in steam sterilization
by Jonathan A. Wilder, Ph.D & Charles O. Hancock, RAC, Stericert Co., division of H & W
team quality may be the last uncontrolled variable in
hospital steam sterilization. Steam sterilizers generally produce sterile
product reliably, but there are times when things go awry for no obvious
The definition of steam quality is
the measureable aspects of steam used for sterilization. These include the
usual measures such as temperature and pressure, and the relationship
between the two. Steam quality also includes other aspects of steam that are
almost never measured in North America. Deviations from established ranges
of these aspects of the steam can result in wet, damaged, or unsterile
When good steam goes bad…
Some of the effects of poor steam
loads and instruments
Sterilization indicator failures and sterility failures
corrosion of instruments and containers
Each of these has a specific cause.
The degree of the problem can be measured, and the situation can be
remedied. The good news is that all sterilizers cleared by the FDA for use
in a healthcare facility can deliver good quality steam to the load and
provide sterile, dry, and intact sterilization loads. The bad news is that
any of them can experience any of these problems, and the cause is not
always something that the end user can predict or determine.
It goes without saying that a
sterilizer must be maintained properly to ensure proper, reliable operation.
This includes preventative maintenance, calibration, and performance
verification as described in AAMI ST79, “Comprehensive guide to steam
sterilization and sterility assurance in health care facilities.” If a
sterilizer is out of calibration, has leaks, or is otherwise not working in
its normal manner, poor steam quality may be a result of those problems
rather than the cause of difficulties. For this discussion, we assume that
the sterilizer is in good repair. If it is not, needed maintenance should be
done before steam quality is tested or blamed for difficulties.
steam quality problems
The four primary failures of steam
quality are listed above. All of them can cause unsterile loads and/or
damage instruments. The cost of repair of a laparoscope or similar device is
upwards of $1,000 per incident. The financial and potential human cost of a
recall of an unsterile load is greater.
The causes of each of these failures
are discussed in detail below.
A wet load can be caused by a number
of things, one of which is wet steam. Steam is composed of vaporized water,
and steam delivered to a sterilizer should have essentially no liquid water
in it. Sterilizers are designed for use with saturated steam and this is
typically specified on the sterilizer manufacturer’s installation drawings.
Steam suitable for sterilization is defined in the European standards (EN
285, HTM2010) as having a dryness value of greater than 0.9 for non-metallic
loads, and greater than 0.95 for metallic loads as delivered to the
sterilizer chamber. The steam dryness value is simply the fraction of dry
steam in the sample measured, with 0.9 dryness corresponding to 10% liquid
water and 0.95 dryness corresponding to 5% liquid water. If the steam
dryness value is too low, wet loads can occur.
Steam dryness is calculated by
measuring the temperature change in a known amount of water and the mass of
steam that was required to cause that temperature change. Ideally, the
temperature rise would be exactly what would result if the energy in
perfectly saturated steam was delivered to the water to heat it. This would
result in a dryness value of 1.0. Normally, the dryness value is less than
1.0, as there are thermal losses in any piping system, and a sterilizer is
no exception. Because the dryness value at the entry point to the sterilizer
chamber can be quite a bit lower than the dryness value of the steam
delivered to the sterilizer, measurements of steam dryness should be made at
the entry point or by sampling the steam in the chamber.
Wet steam can be the result of
engineering issues. These can be:
insulation in the sterilizer, allowing energy loss and condensation,
Low sections of piping between
the boiler and the sterilizer, allowing condensate to pool and be picked
up by steam flowing across the condensate
Too great a pressure drop
across a regulator or between the jacket and chamber, which causes the
“extra” water in the steam at the higher pressure to fall out as
No/clogged steam filters,
either letting condensate pass if no filter, or causing a pressure drop
that causes condensate to fall out,
traps/separators, in either case, condensate in the steam line is not
Steam trap/filter too far from
the sterilizer, allowing condensate to be generated between the trap or
filter and the sterilizer,
Constriction in the flow path
from the boiler to the sterilizer, which can also cause a pressure drop
Other causes of
Other causes of wet loads can be
that they are too dense; that is, too much weight in too small a volume. The
AAMI standard for sterilization containers has a limit of 25 pounds in a
container, with no specification of density. The European standard for
containers, EN 868-8, has a limit of 10 kg (22.4 lbs.) in a “standard
of 30 cm x 30 cm x 60 cm, about 1.9 cu. ft. This is a density of 11.75
lb./cu. ft. Our experience is that if a container’s density is less than
this, there should be no problem with load wetness, assuming that the steam
is suitably dry to begin with.
Damage to loads can take place in
two ways. There can be thermal damage, and there can be staining and/or
corrosion of the instruments and packaging materials.
You might ask how thermal damage can
occur if the instruments are sterilized at temperatures prescribed in the
item’s DFU. The answer is that the steam could be superheated.
Superheat is the situation of having
more energy in the steam than the steam temperature would lead you to
expect. Conversely, superheat is also a situation in which the
temperature of the steam is higher than the saturation temperature for its
actual water content. It can result from the following
Jacket temperature/pressure too
Steam pressure/temperature too
high entering the sterilizer
Steam flowing through a small
orifice between its source and the chamber causing a large pressure
Any of these can cause there to be too much energy in
the steam for its pressure, temperature and water content. If this energy is
released in the load, damage can occur to instruments as their temperature
rises above the recommended processing temperature. The temperature shown on
the sterilizer controls is generally not sensitive to superheat, as it is
measured in the drain of the sterilizer chamber, and superheat will have
been dissipated into the load, chamber wall, or door and backhead before it
reaches the drain.
Sterilization indicators and sterility
A load run with any of the three steam quality problem
listed above may have failed sterilization indicators and also may be
unsterile. For superheat, non-condensable gases, and wet steam, too little
energy is delivered to the load, since steam that is too dry (superheat),
too wet (wet steam), or contains non-condensable gases, has less energy
available than saturated steam to inactivate microorganisms. Non-condensable
gases can also cause air pockets in the load where steam does not penetrate,
meaning that local islands of unsterility may exist in the load. Because
these are localized, if an indicator is not in the “island”, you could never
know that an area is unsterile.
Both of these have similar causes;
something other than water in the steam. These may be impurities like steam
piping treatments, rust in the facility steam pipes or in the sterilizer
jacket or plumbing. If the problem is caused by piping anticorrosion
treatments, the solution is to cut the treatments back or eliminate them.
Anticorrosion treatments are especially problematic with stainless steel
sterilizer jackets and chamber, which tend to pass the treatments on to the
load. Older, tool steel sterilizers are more likely to chemically bind the
treatments before they reach the chamber, since these sterilizers, like
steel piping, have corrosion in the jacket that eats up the treatments
before they can get to the load.
Chemical analysis of condensed steam
can tell you what is doing the staining, and analysis of supplied steam and
steam collected from the sterilizer chamber can tell you if it is problem
with the source or with the sterilizer plumbing or jacket.
Each of these quantities can be
measured and solutions found. The first step is to measure, even if there
are no problems. This should be done at initial installation, or at or
around preventative maintenance to establish a baseline for the system.
Measurements made when there are no problems can also tell you if your
sterilizer is close to having a problem.
If there is a problem, all relevant quantities should
be measured. Persons experienced in steam quality analysis can usually make
cost-effective suggestions to fix the problems, and of course measure to see
if the problem is, in fact, fixed.
A hospital had no problems for a number of years. The
facilities boiler was replaced and wet packs began to occur irregularly. Wet
packs became a regular occurrence as time went by. The problem was observed
in each of three steam sterilizers in SPD, all of which were from the same
manufacturer but were of different models and of ages ranging from 6 to 14
years. The problem was also seen in four additional sterilizers in the OR
suite. The boiler pressure was held at 125 psi, and reduced to 65 psi prior
to being delivered to the sterilizers. Each sterilizer had a steam filter on
its steam feed and the delivery plumbing was of proper design, with adequate
steam traps. In other words, the hospital had an optimal, well-designed
steam system and its delivery to the sterilizers was done “by the book”.
Furthermore, the manufacturer’s service technician carried out a complete
preventative maintenance and evaluation and found the machines to be in good
Using steam quality measurements of steam dryness at
the delivery point of steam to the sterilizer and at the entry point of
steam into the sterilizer chamber, it was found that “perfect” steam, with a
dryness value of at least 0.97 was being delivered to the sterilizers. At
the same time, the steam entering the chamber had a dryness value of 0.84,
that is, was very wet, with 16% liquid water content.
This problem was solved by adding additional insulation
to the sterilizer plumbing and decreasing the steam pressure being delivered
to the sterilizer. The insulation ensured that steam would not condense
before delivery to the chamber and the pressure reduction decreased the
amount of pressure reduction that was required of the sterilizer was less
than two to one for a 270ºF cycle. This is a “magic number” in ensuring good
It was also concluded that the absence of the problem
for all the years before the new boiler was installed and its appearance
after the installation was due to an IMPROVEMENT in delivered steam
quality, i.e., that the old boiler was delivering superheated steam with
lower moisture content, so wet packs did not occur. The new boiler, on the
other hand, was doing a very good job. So good, it seems, that it created
the problem of wet packs.
A hospital had a total of seven steam sterilizers that
were well maintained and fed from house steam. The steam system design was
well done, with proper steam trapping and filtering on the steam lines. No
visible leaks were found in the piping. Yet two of the sterilizers were
found to have high non-condensable gas levels; one, a 30 cu. ft. unit, with
>7% and one, a 3.9 cu ft. unit, with >13% non-condensable gas content. What
was more mystifying was that the unit with >7% non-condensable gas content
was directly adjacent to an identical sterilizer fed from the same steam
line with 0.09% non-condensable gases. What was most distressing for the 3.9
cu. ft. unit was that it was used for flash cycles in the OR. This level of
NCG’s could preclude sterilization, although there had been no difficulty
with the indicators.
The 30 cu. ft. unit suffered from two problems. One was
that its steam filter housing’s gasket was leaking. This was repaired, but
with no effect on the non-condensable gas measurement. The steam trap on the
steam filter was found to be in good working order. Further examination
showed that the jacket steam trap of the unit had missed preventative
maintenance and that there were cracks in some fittings in the steam
plumbing. These can aspirate air into the steam. Once the trap was rebuilt
and the fittings replaced, the unit joined its neighboring unit in the sub
1% range of non-condensable gases.
The 3.9 cu. ft. unit was found to have a different
defect in the steam trap for incoming steam from the building steam lines.
The drain valve from the trap was turned off, making it an expensive
addition to the plumbing that had no effect on the steam quality. This valve
was opened. However, there were still inconsistent readings. Prevacuum
cycles were well within specification. Gravity cycles were not. It was
determined that the port for reading the steam quality was the problem. This
port was about 1” above the feed to the sterilizer chamber, on the far side
of the chamber port from the steam supply line, which was a convenient
location for installing it, but not useful for reading the actual steam
quality. We all have heard that steam and air don’t mix. This particular
measurement proves it. With the trap turned on and the port issue dealt
with, the average non-condensable gas level dropped to 0.13%, an exemplary
This last situation begs the question as to whether
pure flash cycles, with gravity displacement should be used, or whether
“express” cycles with one or two prevacuum pulses are preferred. Since the
amount of time needed for the additional evacuation and pulse or pulses of
an express cycle is not long, if you must flash, and you really shouldn’t,
flashing with an express cycle is by far the better choice, as it will help
the steam penetrate the item, which is what should be happening.
Almost all steam sterilization failures may be
attributed to poor steam quality, as long as packaging and loading are
carried out properly and the equipment is well maintained. Analysis of these
failures is not straightforward for the hospital, and can only be done using
specialized equipment. The practices presented in this article will help
avoid steam-quality related problems, but do not substitute for actual
analysis of the steam quality parameters. Although the US does not have any
requirements for steam quality analysis, if you don’t know why it isn’t
working, and steam quality was never checked, now may be the time.
Definitions of steam quality
parameters and effects of their deviations from accepted values
The measure of the water
content of steam. Acceptable values are ³0.9 (<10% water) for non-metal
loads and >0.95 (<5% water) for metal loads.
Wet steam can cause an
unsterile load in two ways:
delivered to the load to sterilize.
“wet packs”, making the
sterile barrier material surrounding the load less of a barrier and
compromising sterility assurance.
A situation in which the
temperature of the steam is higher than the saturation temperature for
its actual water content. (This is the opposite of wet steam)
Superheat has two potential
Unsterile loads due to
insufficient energy being delivered to the load, since the steam is
Damage to the load if the
superheat is generated where the temperature reached by the load is
higher than its materials can withstand.
A measure of air or other
gases entrained in the steam. Expressed as a percentage by volume of gas
in the steam.
High non-condensable gas
content can cause an unsterile load in two ways
delivered to the load to sterilize.
Gases do not deliver the same latent heat energy as steam.
Pockets of gas can form
that provide “islands” of unsterility.
Unless the indicator is in such an island, their presence will go
 The standard
sterilization volume is the basic building block of European sterilizer