dryness test - keith shuttleworth associates€¦ · specifically the differences for the dryness...

$: Dryness Test - Keith Shuttleworth Associates€¦ · Specifically the differences for the dryness test are: ITEM HTM 2010 KS & A ... These indicated a dryness fraction of between$

Keith Shuttleworth & Associates Limited

Dryness Test

“A continuous supply of saturated steam is required for steam sterilization. Excess moisture carried in suspension can cause damp loads, while too little can not 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 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 285:2006 (E) 22.2.1.

The dryness test is carried out using calorimetry. We measure the amount of energy that is present in the steam supplied to our sterilizer and from this calculate how much moisture is present.

This entire tutorial should be read before commencing testing. A number of tips/discussion items are present and these not need be in the exact order in which testing will be carried out. The manual with the SQ1 provides extracts from EN 285:2006, which is the definitive, but concise approach with no background or guidance. This tutorial is intended to supplement and not replace the manual.

While every effort has been made by Keith Shuttleworth & Associates Ltd to ensure the accuracy of the information contained within this document, the user must verify fitness for use.

Keith Shuttleworth & Associates Ltd grants no warrants, express or implied, by statute or otherwise, regarding this document, related materials and calculations, the fitness for any purpose, the quality, the merchantability, or otherwise. In no event shall Keith Shuttleworth & Associates Ltd be liable for any special, consequential, or other damages for breach of warranty.


Health & Safety

• Read the Installation Guide first

• Steam is hot!

• Steam pipes and fittings are hot!

• Steam leaks may not be visible to the naked eye!

• Isolate steam supply and check that noresidual pressure remains before fitting test points.

• Pipe insulation can be an irritant.

Health and Safety

Care should always be taken when working on or near steam pipes due to

the very high temperatures involved. It is recommended that thermal

gloves, overalls that cover arms and eye protection are used.

Steam contains @ 5 times the energy of boiling water at the same

temperature which makes it a dangerous medium to work with. The steam

issuing from pitot tubes for the dryness and superheat test can be invisible

to the naked eye and poses a particular risk.

Do not dismantle steam pipework with any pressure within it!

Special care must be taken when fitting test points or testing steam.

The insulation surrounding steam pipes may be made from a variety of

materials which can cause irritation to the skin, in some instances. It is

therefore recommended that gloves or a good quality barrier cream be

applied as a precaution before handling the material.


We reference EN 285

• Less rigid on test equipment requirements:

• Impact on Dryness Test

– Allows the use of more robust/safer equipment

– Allows the use of alternative (better) methods

– Requires comparison with standard methods

HTM 2010 is a UK NHS (National Health Service) document to provide guidance to operate sterilizers in a compliant fashion. The underlying standard is EN 285:2006 which defines the European Standard, which therefore takes precedence. To provide uniformity within a largeorganisation, HTM 2010 is more specific in a number of ways than EN 285:2006.

The test equipment described in HTM 2010 is more specifically described which effectively prevents the use of alternative and more robust equipment or the use of better and more accurate methods. By contrast EN 285:2006 does not precisely specify the equipment and allows alternative methods provided that they are shown to be comparable.

The equipment provided by KS & A is made to be robust and to provide repeatable results. Specifically the differences for the dryness test are:

ITEM HTM 2010 KS & A

Vacuum flask Glass Stainless steel

Dip tubes Glass Stainless steel

Tube securing clips None Provided


Comparison Data

• Side by side trials

• HTM 2010 average = 0.993464

• KS & A average = 1.002668

• Adjusted KS & A average = 0.995664

• Difference in methods = + 0.0022

• Difference is not significant

• Must use the adjust heat capacity value of

0.23 and not the standard 0.24.

*Four side by side tests were performed using a glass thermos-flask and glass dip tubes as described in HTM2010. These indicated a dryness fraction of between 0.992874 -0.98858, giving an average dryness fraction of 0.993464

The dryness fraction tests performed using the steam quality equipment supplied with the KS & A steam quality test kit using stainless steel thermos flask, stainless steel dip tubes and pipe securing clips produced results that varied between 0.995848 & 1.005398 with an average of 1.002668.

It can be seen that the test equipment used supplied with the KS & A steam quality test kit produced results that were higher by an average 0.009204.

The heat capacity compensation factor in the calculation was then adjusted from 0.24 to 0.23, which modifies the average to 0.995664, which reduces the difference to 0.0022, which is not deemed significant.

On completion of the tests the stainless steel vacuum flask was completely dismantled and together with the other components, was weighed and the heat capacity calculated from the mass and materials used. The value obtained corresponded with the value of 0.23, which was obtained above by calorimetry.

The results demonstrate that when the heat capacity compensation factor of 0.22 is used with the KS & A test kit, it will provide the same results, within acceptable limits of accuracy.

Failure to use the modified factor of 0.23 will provide results that are 0.1 higher than the standard method.

* The tests were conducted on 30th September 1999 using calibrated temperature and mass measuring equipment.


Frequency/No. of Tests

• The only reference to test frequency is to be found in HTM 2010 and is:– On initial validation/commissioning of sterilizers

– Annually as part of the validation of each sterilizer

• It is generally accepted that annual Point of Use testing is sufficient.

• The number of dryness tests needed is one(HTM 2010/EN 285). However, most companies will carry out a series of 3 tests.

The only documented test frequency is detailed within HTM 2010 and this details the need to test the steam quality as part of the original

validation/commissioning of a sterilizer and annually thereafter, as part of

the re-validation programme.

Generally, annual testing is deemed adequate for sterilizer point of use testing.

When qualifying new steam generators, some regulators may require

more frequent testing, under different flow regimes. Subsequently, annual testing would normally be deemed appropriate.


Test Conditions: HTM 2010/EN 285

• EN 285:2006 requires that the test should be carried out following a warm up run and with a small test pack (linen).

• HTM 2010 requires that the test should be carried out with a chamber that is empty other than the normal chamber furniture – no warm

up run.

• Consistent approach is indicated

Our experience is that the most important factor is consistency of approach.

If carrying out multiple runs, a warm up before each test may not be

appropriate.

Our approach would be to use an empty chamber as linen test packs are

sometimes not available, particularly in the pharma industry.

The worst case condition would be with a load having the greatest heat capacity (mass x specific heat). Given that this is not deemed necessary,

our normal approach would be to use an empty chamber without a warm

up run, followed by two repeat tests, where deemed necessary. Our

experience has been that no discernible difference is evident where the

steam quality is consistent.


Dryness Testing Essentials

• Do not improve on the test method!

• Attention to detail

• Repeatable technique

• Accurate temperature measurement (to 0.1o C)

• Accurate mass measurement (to 0.1g)

Calorimetry is rarely used today and needs a precise and repeatable approach to provide

consistent results. It is not difficult, but small errors can be easily magnified.

Do not attempt to improve on the methods described without validating the impact of any

change. Most of the errors we find in obtaining satisfactory results are due to poor sample

point location, closely followed by “improvements” to the standard method. Small changes

may not be deemed important, but on balance will be seen to impact on results.

We suggest that an identical approach is adopted to every test. It will be seen later that

the test can be influenced by starting/end temperatures and masses and can cause

variances in results. It is therefore essential that you use an identical approach in order

that you may determine if steam quality is improving or deteriorating.

There is a very small band of acceptable results (0.95 – 1.00) and a 1 gram error can

cause a 0.02 error which will result in a marginal pass result become a failure. Similarly, a

1o C error can impact on the results by the same amount. Therefore, it is necessary to

measure accurately temperatures to 0.1o C and masses to 0.1g.

Some measurements are more critical than others and these will be given particular

attention later in the Tutorial.

The use of grams instead of kg in the calculation will usually provide a correct looking

result that will be invalid. The correct units must be used at all times.


Theory - 1

• Saturation Temperature (B)

• Sensible Heat (B – A)

• Latent Heat of Evaporation

(C – B)

• Dry Saturated Steam (C)

The graph shows a temperature/enthalpy (energy) diagram for steam.

When water is heated, the energy that results in a temperature increase is known as

sensible heat (B-A) and this is denoted in steam tables by hf. The temperature increases

until the boiling point is reached (B). This is also known as the “saturation temperature”.

The saturation temperature is dependent upon the pressure of the system. The higher the

pressure, the higher the saturation temperature - boiling point and vice versa.

Once the boiling point is reached, the temperature remains static as the amount of energy

applied is equalled by the amount of energy leaving the water in the form of steam.

Increasing the amount of energy applied results in an increased rate of boiling and

therefore evaporation and not and increased temperature.

The energy utilised in converting the water to steam (C-B) is known as “the latent heat of

evaporation” and is often shortened to “latent heat”. This is denoted in steam tables by

hfg.

At point B (the boiling point, before the addition of any latent heat), 100% of water will be

present. At point C (after the full quotient of latent heat has been applied) all of the water,

all of the water will have been converted to steam. At point C the steam is described as

“dry saturated.” This indicates the absence of any moisture (hence dry) and at the boiling

point (saturated). Between points B and C some moisture will be present and the steam

may be simply described as saturated (at the boiling point with an unquantified amount of

moisture present. Saturated steam will be visible due to the moisture present reflecting

light, and is typically as that seen coming from a kettle. By contrast, dry saturated steam

is a clear colourless gas.


Theory - 2

• Superheat (D)

• Dryness Fraction (X)

If we continue to heat steam once it has reached the point of dry saturation (C), its temperature will increase rapidly with the application of

only small amounts of energy. In this condition (temperature above the

saturation temperature - boiling point), it is said to be “superheated”.

Superheated steam will not condense until its temperature reduces to the

boiling point and will not provide the moisture necessary for sterilization to occur. In the superheated state, steam acts like hot air and requires very

high temperatures and long hold times to sterilize.

The amount of latent heat present in steam also describes the amount of

moisture present. Where zero latent heat has been added the condition will be 100% water and 0% steam. Where the full quotient of latent heat

has been added the condition will be 0% water present and 100% steam.

The term “dryness fraction” describes the ration of steam to water present

where unity (1.0) describes steam with 0% moisture. Steam with adryness fraction of 0.5 (X) will have 50% steam and 50% moisture.

Therefore, steam with a dryness fraction of 0.95 will have 95% steam

present and 5% moisture. The higher the dryness fraction, the less

moisture present. If the dryness fraction is greater than unity (1.0) it

indicates that the steam is superheated.


Theory - 3

• Pressure drop

• Dryness Fraction (X)

If the pressure of steam is reduced from a high pressure/temperature to a low pressure/temperature by, for example, passing the steam through a

pressure reducing valve (or orifice) the amount of energy present will

remain the same, as no work has been done to reduce it.

It will be seen above that if we reduce the pressure of steam having a nominal dryness fraction of 0.5 from a high pressure/temperature to a

lower value, its dryness value will increase from 0.5 to @ 0.65. Because

the energy remains the same, the excess energy will be utilised in

evaporating moisture until equilibrium is restored. Therefore, it will be seen

that the effect of a pressure drop will be to dry the steam (improve its quality).

If we reduce the pressure of steam that is dry saturated, the excess

energy present will result in the steam becoming superheated.

EN 285:2006 recommends that the steam supply pressure is not reduced

by ratio’s greater than 2:1 to avoid the risk of superheating. In practice,

unless the pressure drop is immediately before the sample point,superheat will not be evident as excess energy will be lost to the steam

pipe walls.


Theory - 4

• Dryness Value?

• Calculating

When the dryness test was developed there was concern that the method was not truly accurate. For example, the test takes a sample from the

centre of the steam pipe and does not take account of moisture on the

pipe wall or at the bottom of the pipe. For this reason, the term

“dryness value” was utilised to demonstrate the difference between the

approximation used and the dryness fraction which is an absoluteconcept. For most purposes the terms dryness value/fraction are

interchangeable.

In order to calculate how much moisture is present in the steam we are

testing, we need to know the two variables:

1. The steam temperature. This is obtained from a direct measurement of

the steam temperature using a temperature sensor.

2. The amount of energy (latent heat) present in the steam. We establish

this using calorimetry. When these two conditions are known, it is

possible to calculate the proportion of energy present compared to the

total present in dry saturated steam. From this we are able to establish the dryness value/fraction.

T


Calorimetry/Calorimeter

• Calorimetry is the science of measuring the heat of chemical reactions or physical changes.

• A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity.

Calorimetry is the science of measuring the heat of chemical reactions or physical

changes. Calorimetry involves the use of a calorimeter. The word calorimetry is derived

from the Latin word calor, meaning heat.

A calorimeter is a device used for calorimetry, the science of measuring the heat of

chemical reactions or physical changes as well as heat capacity. The word calorimeter is

derived from the Latin word calor, meaning heat. Differential Scanning Calorimeters,

Isothermal Microcalorimeters, Titration Calorimeters and Accelerated Rate Calorimeters

are among the most common types.

A simple calorimeter just consists of a thermometer attached to an insulated container. To

find the enthalpy change per mole of a substance X in a reaction between two liquids X

and Y, they are added to the calorimeter and the initial and final (after the reaction has

finished) temperatures are noted. Multiplying the temperature change by the mass and

specific heat capacities of the liquids gives a value for the energy given off during the

reaction (assuming the reaction was exothermic.) Dividing the energy change by how

many moles of X were present gives its enthalpy change of reaction.

This method is used primarily in academic teaching as it describes the theory of

calorimetry. *It doesn’t however account for the heat loss through the container or the

heat capacity of the thermometer and container itself. In addition, the object placed inside

the calorimeter show that the objects transferred their heat to the calorimeter and into the

liquid, and the heat absorbed by the calorimeter and the liquid is equal to the heat given

off by the metals. This shows that the matter can be neither created nor destroyed.

(Wikepedia)

* This is accounted for in the calculation used for calculating the dryness value.


Dryness Testing Objectives

• To measure the latent heat in the steam supplied to a sterilizer by:– Measuring how much steam is required (kg) to heat a

known quantity of cold water (kg) through a known amount (o C) i.e. from ambient to say 80o C and taking account of any heat losses from the test equipment.

– This tells us how much energy/kg the steam has and by comparing it to the amount of latent heat in dry saturated steam (dryness value 1.00) at the same temperature, we are able to calculate how much moisture is present.

If we have a known mass of cold water at a known temperature and we condense steam into it, its temperature will increase together with its mass as a result of the steam condensing.

Using simple mathematics we are able to calculate how much energy is in the steam:

Energy = Mass of water x temperature change x specific heat of water

Mass of steam

From measuring the steam temperature present during the test we are able, from steam tables, to determine the amount of energy that is present in dry saturated steam and make a comparison. The amount of latent heat in steam is directly related to the amount of moisture.

Steam with its full latent heat quotient has a dryness value of 1.00 (zero moisture and denoted as “dry saturated”). If only 50% of the full latent heat quotient is present the steam will have a dryness value of 0.50 (50% steam and 50% water).

The calculation is further modified to take account of heat losses from the flask/system i.e. energy that has gone into the system from the steam, but has not resulted in an increased water temperature.


Fit the pitot tube & steam temperature

sensor

• Take all the necessary safety precautions and wear the appropriate PPE

• Shut off the steam supply to the sterilizer and fit the test elbow

• Install the correct pitot tube and the steam temperature sensor

• Turn on the steam supply

• Watch/listen

Select the correct pitot tube from the table below based on the steam supply pressure. The SQ1 pitot tubes have the size mechanically etched on the external part of the tube. NB. It may be necessary to change the pitot tube, depending upon the time the test takes and the results obtained.

Steam Pressure (Bar G) *Pitot Hole Size (mm)

Up to 2 bar 0.80

Up to 3 bar 0.60

Up to 7 bar 0.40

* EN 285: 2006 – “The values given in the table are for guidance only”.

Install the steam temperature sensor in the centre of the steam pipe and ensure it is working correctly (steam issues freely and horizontally). This may be a good time to install the non-condensable gas test point.

It is assumed that the installation of the sample elbow and pitot tube will be the subject of local procedures which will have been assessed for Health & Safety.

When the steam supply is turned on it is possible that a considerable amount of condensate may have accumulated and will issue from the pitot tube. Apart from the temperature/water hazard, this could spray onto local electrical/electronic components with the attendant hazards and risk of damage.

After a period of time any water should cease to issue and steam will exit the pitot tube which need not be visible. Great care should be taken not to get in the way of this jet of steam as a serious burn could result.

A subjective assessment of the steam quality is possible by watching and listening to the jet of steam emitting from the pitot tube. Water droplets will be evidenced by a spitting sound and the steam will be made visible as a result of the water vapour present. With practice, it is possible to assess if the steam is likely to meet the necessary standards, simply by observation. Do not commence testing until all residual condensate has been removed from the steam pipework.


Sensor installation

Pitot tube

Pitot tube

Steam temperature sensor

See also “Installation” guidance for alternative configurations.


Pre test preparations

• Prepare the electronic balance

• Weigh the empty flask

• Pre fill the flask with water at ambient temperature

It is assumed that the electronic balance has been levelled and that it is on a flat, solid surface that is free from draughts or other effects that can impact on the results. The reading should be verified as being zero before commencing.

The test process commences by measuring the mass of the test equipment in an empty condition and recording the value in kg to a discrimination of 0.1 g (0.0001 kg).

The flask and all associated components should be dry and free from any visible droplets of water or moisture before weighing (inside and out) and must be weighed with the bung and dip tubes, the connecting rubber hose and hose clips. If a strap or hanger is used to support the flask during a test do not include this or any temperature sensor/cable.

A known quantity of cold water is then added to the flask, typically 650 ml +/- 50 ml using a measuring flask, else the condense collection cylinder supplied with the SQ1.(< 27o C –see next slide on water temperature). NB no special requirements exist for the water, which is typically mains or tank fed cold water. The partially full flask is then reweighed and the mass recorded. A quick manual calculation should be carried out to confirm that the fill volume is correct and is the fill volume required. Ensure that the stainless steel dip tube that passes through the rubber bung is well below the water level. If not, adjust.

Treat the flask with care and do not allow the water to splash either out of the flask or up the sides/onto the rubber bung etc. The calculations assume that all of the water is subject to the heating action of the steam. Any water that is lost from the flask or is not subject to heating by the steam will result in errors.


Initial fill volume & temperature

• Test method requires 650 ml +/- 50 ml

• If the fill volume is either 600 or 700 ml the amount of time the test takes will differ if the water is to be heated to the same temperature, typically 80o C.

• The water temperature should not exceed 27o

C.

• If the water is 10o C the test will take longer to heat the water to 80o C.

The impact of the fill volume utilised and the temperature of the water used can have a

profound effect on the time that the test takes. The end point of the test is when the water

temperature has reached @ 80o C after being heated by the steam.

The extreme conditions are therefore - a fill volume of 600 ml with water at 26.9o C and a

fill volume of 700 ml at a temperature (say) less than 10o C. This should not matter, but in

practice is does!

The longer the test proceeds the greater the thermal losses are from the flask/ system

which are not fully accounted for by the calculation used. Ideally, the test should be

completed in @ 5 minutes and if this becomes extended to, say, 8 – 10 minutes poorer

results will be obtained (indicating wetter steam).

A further problem in using water at a temperature significantly either greater or less than

the ambient temperature, is that it will either heat or cool the flask/system. We are

seeking to determine accurately the temperature increase in the water as the result of

steam heating it and if energy is either being supplied to the water from the flask or

transferred from the water to the flask an error will occur. The most suitable water

temperature is therefore @ ambient where it will not change significantly without the

addition of energy from the steam. If necessary bulk water for the testing should be

allowed adequate time to equilibrate with the local environment.

Our recommendation is to pre fill the flask with 650 ml of water at ambient temperature as

accurately as possible from the measuring cylinder in order to obtain a fill volume of 650

ml +/- 10 ml (or less). Verify the amount is correct before proceeding!


The Dryness Value Test

• During the test you will:

– Measure the start temperature of the water in the flask

immediately before the test commences

– Connect the flask to the pitot tube using the rubber

tube when the steam to chamber valve first opens

– Observe/log the average steam supply temperature for the duration of the test

– Observe the temperature of the water in the flask during the test to establish when the test must end

The test requires a degree of coordination to ensure that all data is collected at the correct time which may appear disconcerting.

In practice, ample time is available and practice helps.

It is suggested that dry runs are carried out without the autoclave running

to check coordination. Also, it may help, in the first instance to use two

people, one to deal with the flask and water temperature and another to

monitor the steam supply temperature.


Temperature Measurement

• We use a hand held dual input temperature meter with a logging feature

• One sensor to measure the flask water temperature

• One sensor to measure the steam supply temperature

When we carry out steam quality testing we use a hand held dual input temperature meter which will also carry out the averaging calculation needed for the steam temperature. One probe is used to measure the water temperature and the other is used to measure the average steam temperature. Please see our website for further details.

The calculations used for the Dryness Test do not take account of losses as a result of the temperature sensors used. Due to the low mass involved we suggest that you use either a bare thermocouple or stainless steel sheathed thermocouple/RTD with a diameter not exceeding 3 mm (1/8”).

We use hand held meters as they are more portable to carry and easy to use. If necessary the data held can be downloaded to our laptops if required.

Generally, if the steam supply pressure and therefore temperature does not vary significantly, there will be little or no impact on the calculation. As the steam temperature increases, the latent heat reduces and vice versa. This has the effect of cancelling small temperature variations and we suggest that you try inputting different values into the Excel spreadsheet calculation to assess the impact. It will be seen that the steam temperature measurement is of *less importance than the water temperature measurement, which is critical! You may deem that sufficient accuracy is obtained by observing the maximum and minimum temperatures and selecting the “average”temperature as being half way between these extremes, after inputting the values into the calculation to see if they have any impact at all on the outcome. Alternatively, you could consider observing the temperature during the test and make a pragmatic assessment. Data loggers, such as the Kaye for example may also be used or results recorded manually for averaging.

* There is a requirement in the Superheat Test that the steam temperature measured during this test and the Dryness Test are within 3o C of one another. This is to ensure that the steam supply is relatively consistent. EN 285:2006 requires the steam supply not to vary by more than 10% when a cycle is being run.


Dryness Test

• Insert temperature sensor into the flask

• Start the sterilizer

• Do not extend the rubber steam tube!

• Start the test

Fit the rubber tube to the dip tube that goes below the water level and secure with the clip provided (if not already fitted).

The temperature sensor used to measure the start/end temperature of the water should be inserted into the flask. It should be inserted the same distance into the flask for each test, typically 25 mm from the flask bottom. NB consistency of approach is important and it is suggested that the sensor is marked and secured in position (with a “tie wrap”, rubber band or tape) to ensure a consistent insertion depth.

The temperature sensor in the steam supply should be checked as being operational and that if logging the result the logger is set up and ready to record the results. If manually observing and recording the results

The next key item is to measure the water temperature prior to starting the test. If the temperature of the water differs significantly from that of the flask it will either be heating or cooling and thus be a moving target. The flask and water temperature must be similar before starting the test. When satisfied that the water temperature is consistent start the autoclave.

When the steam to chamber valve first opens take the water temperature (and record) and connect the rubber tube to the pitot tube and secure with the clip. Ensure that the flask is lower than the pitot tube so that any moisture will drain into the flask. Steam will enter the flask and will be heard bubbling inside. Agitate the flask gently to minimise the impact of stratification where the heated water will go to the top of the flask and the cold to the bottom. Excessive agitation may allow steam to pass into the flask without being condensed if the dip tube outlet is allowed to become uncovered by water. Observe the water temperature.


When to start

0

1000

2000

3000

0 5 10 15 20 25 30

Steam

valve first opens

The test should commence when the steam to chamber valve first opens. This may be an on/off valve, a control valve or both.

This is often (wrongly) confused with the start of the heat up. The test should start when steam enters the sterilizer chamber for the first time in the process.

This usually reflects the following conditions:

The vacuum is often the deepest

The vessel and pipework are at their coolest

This would normally result in the greatest steam demand. While this aspect need not be critical a consistent approach is highly recommended.

It will be seen that the conditions sampled will vary on machines having different cycles and/or being of different sizes. That is to the sample taken on a small machine with a fast process could be most of the vacuum/pulsing stage, whereas on a large machine the sample could be taken of a single pulse.


When to start the test

0

1000

2000

3000

0 10 20 30 40

When steam to chamber valve

opens for the first time


Dryness Test

• Complete the test

• Allow temperature to equilibrate

• Record temperature result and re-weigh the flask

The test should be ended when the water temperature reaches @ 80o C. As previously

mentioned, the duration of the test can have an impact on the results, with the longer the

test proceeds the worse the results may be, if it is prolonged. We therefore suggest a

consistent approach is adopted. We end the test the instant we see an 80o C reading on

our temperature sensor. The test is ended by removing the clip and tube from the pitot

tube, taking care to avoid any hazards resulting from the hot tube and issuing steam.

Suitable gloves must be worn. At the same time any recording/logging activity associated

with the steam supply temperature measurement is stopped.

We do not record 80o C as our endpoint temperature, but agitate the flask more

vigorously than before to ensure that we have a homogenous temperature throughout the

water. We take care to ensure that no water is spilt or allowed to leave the flask. This step

is critical to obtaining satisfactory results. We continue to agitate while watching the

temperature reading and when consistent results are obtained we record the result.

Remove the temperature sensor taking care not to extract any water from the flask and

remove any fixings used to keep the temperature sensor in place.

Ensure that the balance is reading zero and re-weigh the flask with exactly the same

equipment as the initial measurements and record the results to 0.0001 kg (0.1g). Do not

remove the water or dismantle the equipment, in case you need to re-weigh it.


Dryness fraction calculation

( )L

CTT

LM

ACMTTD

s

c

wo 11 ))(( −−

+−=

D = Dryness Fraction, T1 = End Temperature of Water, T0 = Start Temperature of Water

C = Specific heat of water, Mw = Starting mass of water, A = Heat capacity of the flask

Ts = Average Temperature of Steam, L = Latent Heat Steam and Mc = Mass of Condensate

Carry out the average steam supply temperature (and record) and enter all of the data into the calculation either using the SQ1 Excel spreadsheet

supplied with the test or manually.

If carried out manually due to local validation/procedural concerns, we

suggest that you still use the spreadsheet to confirm the results. Review the manual calculation if any discrepancy exists.


Acceptance Criteria

• Dryness Value must be > 0.95 if processing metal goods

• Dryness Value must be > 0.90 if processing porous loads

• Values > 1.00

Most sterilizers will be used to process metal goods and therefore the higher limit will be most appropriate.

There is no maximum value specified and therefore values in excess of

unity (1.00) could be deemed satisfactory. However, such values are

indicative of superheat and a small value in excess of unity could indicate a high level of superheat.

We accept that the technique used may not be perfect but our experience

is that we can repeat tests with a consistency of @ 0.01 following the methods previously described. If we treat this as the experimental error we

would allow values to exist to 1.01 without comment. We would not allow

the same leeway at the lower limit as we would say that this was indicative

of a problem with the steam supply.

If the value exceeds 1.01 we would look at the pressure drop regime in the

steam supply to see if superheat were a realistic probability. Genuine

superheat will result from a pressure drop close to the point of testing (say

less than 3 m (10 feet), typically greater than a 2:1 ratio. If this is the case, there is a real risk that the result is correct, in which case remedial action

may be needed.


Fail results

• Values < 0.90 or 0.95

• Real or technique?

Test results that show a fail condition can be due to the quality of the steam or a poor technique. Poor quality steam can be seen and heard when the outlet of the pitot tube is observed after it is installed. If the jet of steam is not visible and there is an absence of spitting/spluttering, the probability is that the issue lies with the technique as the steam is likely to have a dryness value of > 0.95. This is particularly so if the results are below 0.90 which indicates the presence of 10% of entrained water – steam of a very low quality.

If the result is a “one off” where a series of tests show satisfactory and repeatable results but one shows a poor result, the indication could be that a slug of water has passed through the distribution system. You can only become confident with your results after performing a number of tests with consistent and repeatable results. In this fashion, based on your own confidence, you are able to establish that a problem is real or technique based. For this reason we recommend that you conduct a number of practice tests before commencing formal testing for the first time.

Test the calculation to ensure that you have correctly entered the right values and have used the correct units (kg and not g for example).

Where a marginal failure occurs, say just below 0.95 this could be the result of a poor technique or the duration of the test. As mentioned previously, the duration of the tests can have an impact on the results obtained and generally result in lower rather than higher values. If the tests have been conducted as previously described with 650 +/- 10% ml of water at @ ambient temperature and the test duration exceeds 5 minutes, try using a larger sized pitot tube. This will cause the tests to be carried out quicker and the impact of heat losses to the equipment reduced. This is not cheating the system or a “fiddle.” You are seeking to measure the energy in the steam and you cannot create additional energy by changing the flow rate. Indeed, if the flow rate is too high it may cause localised boiling and all of the energy will not end up in the water. This will result in worse rather than improved results. This is often required where the steam supply pressure is only marginally greater than the sterilization pressure of say 1 bar (15 psi) as there is an assumption that the steam pressure is > 3 bar – see Installation Guidance.


Failed tests

• Steam traps – operational?

• Steam traps – sufficient and correctly located?

• Pipe falls 1:120 – 1:100

• Insulation

• Sagging pipes

In the event of failed results we would review the steam distribution installation. Consideration needs to be given to if the results are

consistently bad or intermittently? This can provide some indication of the

cause.

Are the steam traps that are fitted working? If not rectify and retest.

Are sufficient steam traps fitted? They should be fitted at every level

change and every 30 – 50 m on straight runs of pipe. They should be

installed in accordance with good steam pipework practice. Often, steam traps are not fitted at the base of vertical sections of pipe and.

Are the pipe falls correct and in the right direction? The steam pipe should

fall @ 1:100/120 in the direction of the steam flow to allow condensate to

run to traps.

Is the steam pipe insulated for all of its length? If not properly insulate.

Is the pipework sagging or has been stood on in one or more locations? Sometimes a sag in the pipework will allow good quality steam and then

occasional slugs of water will be driven through the pipework system.


Multiple Test Issues

• Flask, dip tubes, steam tube, clips and bung will be hot

• Cool by filling flask with cold water and place other components in cold water and allow to cool

• Do not re-test until the temperature of the equipment is @ ambient

If carrying out multiple tests it will be necessary to allow the test equipment to cool to ambient temperature. When we carry out multiple

tests, after confirming the results from the first test we will dismantle the

flask and bungs and fill the flask with cold water and place all the other

components into a bucket of water to cool.

Before re-use we will ensure that the equipment has cooled to ambient

temperature. Our experience is that we are normally usually able to

achieve this while the previous sterilizer cycle finishes and we write up our

results.

No not rush this process as a hot flask will cause problems on the next

test. This will be evident by the failure to quickly obtain a static

temperature before the next test commences.

Before retesting ensure that all the components are dry inside and

externally. Any excess moisture will be evident when the empty equipment

is reweighed at the start of the next test.


Qualifying Steam Generators &

Distribution Systems

• Reference in HTM 2031

• Usual to test at generator at min/max flow rates

• May need to exhaust steam to atmosphere to generate max flow

• Test distribution system under min/max flow rates at extreme points

Ensure that the operating pressure of the generator does not exceed the rating of the SQ1 test kit. If this is the case, please notify us.

Dryness Test – 6 BarG (165o C)

Superheat Test - 6 BarG (165o C)

Non-Condensable Gas Test - 5 BarG (160o C)


Combined Tests

• With practice, it is possible to carry out dryness and non-condensable gas tests at the same time.

When carrying out contract steam quality testing we are usually able to carry out both the dryness test and the non-condensable gas test at the

same time. This significantly increases productivity, but needs some

practice. As previously stated, dry runs, without an autoclave running will

allow the technique to be perfected.


If in doubt – contact us

Web www.ksapharma.comKeith Shuttleworth & Associates Ltd

D5 Basepoint Business & Innovation Centre110 Butterfield

Luton

LU2 9HEUnited Kingdom

Tel + 44 (0)1582 433723/Fax +44 (0)207 1173251E-mail [email protected]

dryness test - keith shuttleworth associates€¦ · specifically the differences for the dryness...

Documents