Updated: Apr 21, 2021
Opinion Article | Open Access | Published 9th January 2021
Wet loads - a phenomenon or by design?
Having been involved in moist heat sterilization and autoclaves for over 40 years it fascinates me
that there still exists in our industry confusion and misunderstanding in relation to wet loads.
I have therefore decided to write this short story to highlight the fact that, despite the occasional (or
should we say frequent) denial within our industry of the existence of wet loads, they do actually
occur in relation to porous/hardware loads autoclaved within the pharmaceutical and biotech
industry. In fact, I would like to receive a pound for every time I have been asked the question,
“Alan, when is wet, wet?”
This story is not to discuss whether loads should be wet or dry, but rather to discuss how to avoid
the manifestation of a wet load in the first place by effective system design and good practice. And
to save production time. Isn’t that worth reading about?
So, where to start? I would suggest that like all good stories, we should start at the beginning…….
Let’s start with the Pure Steam Generator:
It’s a good idea to look at the ‘whole system’ in relation to wet loads, and by that, I mean starting
with the source of the steam, i.e. the Pure Steam Generator (PSG), moving through to the autoclave
and finally the load items.
I think it is reasonable to suggest that the whole steam supply system should be designed to deliver
steam to the autoclave in a reproducible manner and that the steam should be of a known and
suitable quality to ensure the efficacy of sterilization, whilst allowing for the delivery of a dry load on
completion of an autoclave cycle.
For as long as I have been involved in moist heat sterilization, it has been recognized that the steam
produced by the PSG should have known steam quality attributes, specifically in relation to its
dryness, level of non-condensable gases and superheat. In relation to dryness, the current version
of EN 285 suggests a dryness value not less than 0.95; however, from my personal experience I
would suggest that the optimum dryness value should be 0.97 to 0.98 which allows for sufficient
moisture to effect sterilization, without being causal to wet loads.
I would recommend testing the steam quality, including the dryness value, at the source of generation,
i.e. the PSG. However, during my global travels, I have found that this is not the norm, rather it is
only tested at ‘point of use’, e.g. at the autoclave. Whilst the point of use is the most obvious position
for testing, this is not the full picture. If you test at the PSG and point of use, achieving the same
value of, for example 0.97 at each point, then you know the system is ‘ideal’. If the values are
different at both points, then you already know that something has changed and needs to be
investigated. Isn’t that much easier than restricting yourself to point of use testing only, and not
knowing when something is changing in the system or load until you have to stop manufacturing to
investigate the root cause of a fault?
It is therefore recommended to test for steam quality at the PSG, in-line with the flow of steam. It is
also recommended to test under static and dynamic conditions to evaluate how reproducible the
steam quality test results are under varying conditions, including under ‘min and max’ demand. If
you have never undertaken this test regime before, you may be surprised by the results when you try
this procedure. It could be a ‘light bulb’ moment – ah, that’s why I have intermittent ‘wet’ loads! If
all is well and you have the desired dryness value you are looking for, you can move on; if not, best
to stop at this stage and find the cause of the wet steam. It is also recommended to check for
adequate steam trapping at the source of generation, to ensure that condensate carryover into the
steam service supply line cannot take place which could be causal to wet loads.
The Steam Service:
From the PSG, the steam supply is delivered to the point of use, i.e. the autoclave. However, there
may be several points of use, e.g. more autoclaves, freeze driers, SIP stations. The steam system
should have been designed to provide steam to all points of use under maximum demand
conditions, in a reproducible manner, and the steam should be of a known and suitable quality
throughout. It is important that the system is designed appropriately to ensure that this happens.
Basically, it is important to design quality in.
Let us consider a situation where two autoclaves are being supplied by a Pure Steam (PS) system. It
is best to install a ‘Steam Header’ on the mezzanine floor above the autoclaves, so that there is a
reservoir of steam available to deliver the necessary steam supply of the required quality. If the
Steam Header is omitted from the design, this can be one of the prime reasons for an inadequate
It is good practice to install steam quality test points on the Steam Header, which allows the steam
to be tested in a safe and efficient manner. An example of a Steam Header can be seen in, ‘Health
Technical Memorandum 01-01 Part C: Steam sterilization’.
The Steam Header should be fitted with appropriate steam traps, e.g. ‘Balanced Pressure,
Thermostatic (BPT) steam traps fitted with Near to Steam (NTS) elements’. This type of steam trap,
with the NTS element, provides a way of removing unnecessary condensate from the steam supply
which otherwise could be causal to wet loads.
So, having determined the importance of the quality of the steam, let’s continue our story with the
autoclave. I hope your still sitting comfortably.
Steam supply into the Autoclave:
Let’s start with just a single autoclave – although it may be worth remembering that the same
principles apply for more than one autoclave.
As can be seen from our story to date, we have now supplied steam of a known and suitable quality
to the service area of the autoclave. However, we also need to consider the design of the autoclave
and associated pipework, as well as how we get the steam into the chamber, since these will all have
an impact on whether we achieve a wet or a dry load.
When the steam comes from the Steam Header on the mezzanine floor (in our earlier scenario) it
will be supplied at a known pressure, suitable for its intended purpose. It is important to ensure that
the steam supply is, ‘a constant’ and does not fluctuate. If it fluctuates, pressure drops can occur
within the chamber, being causal to excess condensation of the steam which can result in wet loads.
The steam should once again be trapped in the service area, with a BPT trap fitted with a NTS
If all of the above has been well designed and is controlled appropriately, the PS supply will continue
to have a dryness value of 0.97 and that is a very ‘nice’ position to be in.
Now we need to understand the challenge of the autoclave itself……
Let’s consider an autoclave used for component sterilization, utilizing a Porous/Hardware Load cycle.
It has a multi-pulsing air removal stage, heating stage, sterilization stage, vacuum drying stage and
an air vent to cycle complete stage.
The autoclave chamber is surrounded by a ‘jacket’ which is fed with steam to provide uniformity of
heating. It is important that the design and operation of the jacket is fit for purpose. By that, what I
mean is, if the sterilization stage of the cycle is designed to operate at c. 122°C within the chamber,
then the jacket temperature should also be at 122°C. Why? Well, if the temperature of the jacket is
set to a lower temperature, excess condensate will be created as the chamber wall will be cooler
than the chamber environment. This excess condensate will potentially ‘wet out’ the load. Not only
could this impact sterilization efficacy, but the resultant will characteristically be a wet load. Then
typically, to fix the wet load, we employ ‘airwaves’ (air pulsing) during the vacuum drying stage of
the cycle. Whilst this may be an effective solution to the wet load, this has two key disadvantages;
firstly, there is a risk to sterility from wet steam and secondly, it increases the length of the cycle,
therefore using additional energy and reducing production time. All this to fix a problem that was
potentially designed in.
I remember some years ago explaining this to a company who had invited me in to assist them with
their sterile operations. After watching a presentation of their current (at that point in time)
autoclave component cycle, I asked if they would like to understand how their cycle could be
reduced from 4.5 to 1.5/2 hours… The Production Director was first to his feet, “Alan, how can we
do this, please….?”
Whilst we are still thinking about the autoclave jacket, let’s consider another potential design
problem. Many years ago, dedicated Porous Load autoclaves in the UK were designed with
autoclave jackets which had their temperature set the same as the sterilization stage temperature of
the cycle. The ‘steam to jacket’ was always on, which meant that the autoclave chamber was always
hot and at a constant temperature. The doors were also hot when in the closed position.
When a new load was placed within the chamber, the only cold items were the load itself and the
autoclave rack, therefore the condensate that was created from the steam entering the chamber
was created by its contact with the load and rack, not by the inside chamber wall and cold doors.
However, today, it is more typical in the industry to find the opposite as the norm, whereby steam is
turned off to the jacket, either during the drying vacuum stage or at the end of the cycle;
consequently, the autoclave becomes ‘cold’ at rest. Then, at some point, when a new load is placed
into the autoclave chamber, if the jacket and doors are cooler than the steam entering the chamber,
then of course condensate will be created. This is not what we want, since this condensate will lead
to the ‘wetting out’ of the load. Therefore too much condensate will be created at the beginning of
the cycle and the drying vacuum stage of the cycle will be ineffective; the result being a wet load.
So, in effect, the wet load is once again ‘designed in’. This is a major challenge in our industry and
should be a cause for concern.
In our example autoclave, PS is supplied to both the chamber and the jacket; however, not all
autoclaves operate in this manner. Sometimes, ‘Plant Steam’ is supplied to the jacket and PS only to
the chamber; typically due to a supposed commercial rationale to save money – something which in
my experience can lead to more production/product problems, and consequently the financial
savings may only have been short term, at best. But I digress. No soapboxes in this story… keep to
the facts and data Alan. Factually, the use of plant steam can have a negative impact on the
uniformity of temperature within the jacket if ‘scaling up’ occurs within the jacket (think of scale
build up in your kitchen kettle). One way to determine if this is happening is to perform a ‘Jacket
Mapping’ study, whereby thermocouples (TC’s) are attached in varying locations, in intimate contact
to the chamber wall, and a cycle is performed. If lower temperatures than expected are measured,
this indicates that non-uniformity of temperature exists which can/will result in a wet load.
You may now be thinking, “Alan, this is more complex than I thought…!” Well, not really, hang-in
there; this will ultimately make life much easier and more enjoyable if you can deal with those
intermittent (or more than intermittent!) wet loads. So next, let’s go on to the autoclave and cycle
design, as well as the actual operation of the autoclave.
The Autoclave and Cycle Design:
I’m sure you can see from the story so far that the way the autoclave is designed can have a major
impact on whether loads are wet or dry on completion of a cycle. However, the design of the cycle
itself and the operation of the autoclave can also impact on the load wet/dry condition. Let’s
consider a couple of scenarios in a bit more detail – nothing like, ‘real situations’ to understand
Firstly, let’s assume that the steam being supplied to the jacket is not on at all times and in fact,
during the final stages of the cycle, the steam is removed from the jacket and a cooling stage is
employed in the chamber. On completion of the cycle the load is removed from the chamber and a
new load is subsequently placed inside the chamber. We now have a situation where the chamber,
jacket, doors and load are ‘cold’. Then, when the doors are closed and locked, the next cycle is
initiated. At this point, dependent on the type of autoclave and its design, typically steam is
admitted to the jacket. Remember that earlier I suggested that the jacket temperature should be
the same as the sterilizing stage temperature; let’s say in this scenario at c. 122.5°C. However, in
this example, when the temperature in the jacket reaches its ‘designed in’ set point, e.g. 117°C, the
cycle starts. Now clearly 117°C is much lower than 122.5°C; however, if the sensor detecting the set
point of 117°C is positioned at a high level point within the jacket, it would be fair to suggest that in
the lower sections of the jacket the temperatures will be lower; so not even reaching 117°C.
Therefore, when steam enters the chamber during its first pulse, it condenses out on the relatively
cold chamber wall, doors, autoclave rack and load. If too much condensate is created during the
initial stage of the cycle it will not be removed during the drying vacuum stage of the cycle.
I often use the following example when trying to explain this point: “Imagine that I am going to take
my jacket off and soak it with water. I then put my wet jacket (coat for the American reader 😃) in
the autoclave chamber and run a cycle. On completion of the cycle will the jacket be wet or dry?”
In case you are wondering; the jacket will be wet. Why? Because there was too much water
entrained in the material for the vacuum drying stage of the cycle to remove it. So, you clearly point
out to me, “Alan, we are not going to put wet clothing into the autoclave”; however, whatever the
load is, if it’s wetted out during the initial stages of the cycle, the effect will be the same – a wet load
Secondly, let’s assume that we are not going to put a wet jacket into the autoclave, but rather a
wrapped mixed component load. The autoclave and cycle design is exactly the same as for the
previous scenario; however this time the load comprises of tubing, filters and stainless steel items
(one of those complex, tricky loads – but Alan, we have to do this load – and so, all hands on deck to
qualify the load and get into production!).
Following the sterilization stage of the cycle, a vacuum is pulled in the chamber to ‘dry off’ the load.
The principle involved here is that the heat energy transferred to the load during the cycle is now
utilized to ‘flash off’ the condensate during vacuum conditions being applied in the chamber.
However, for this to happen it is wholly dependent on the load item itself, the r