- Email: [email protected]
Research into equipment for protection against molten-metal splash
Journal Elsevier
of Occupational Accidents, 5 (1983) 59-74 Scientific Publishing Company, Amsterdam
59 -
Printed
in The Netherlands
RESEARCH INTO EQUIPMENT FOR PROTECTION AGAINST MOLTENMETAL SPLASH*
T.D.
PROCTOR
Health (U.K.)
and Safety
(Received
Executive,
September
2, 1982;
Safety
Engineering
accepted
September
Laboratory,
Red Hill, Sheffield
S3 7HQ
6, 1982)
ABSTRACT Proctor, T.D., 1983. Research into equipment for protection Journal of Occupational Accidents, 5: 59-14.
against
molten-metal
splash.
The Foundries (Protective Footwear and Gaiters) Regulations were introduced in 1971 to reduce the risk of injury to the lower legs and feet of foundrymen. Surveys conducted by Her Majesty’s Factory Inspectorate in 1972 and 1973, after the introduction of the regulations, showed that although there had been a marginal reduction in burns there were serious inadequacies in the design of gaiters. Eighty-eight per cent of the injuries sustained when approved gaiters were being worn were the result of molten metal entering the top of the gaiter and being funnelled down into the footwear. This design shortcoming, together with problems of comfort and fastening efficiency, had resulted in some workers being unwilling, or even refusing, to wear the gaiters. Research was initiated to overcome these difficulties and after extensive field trials a new style of gaiter has been introduced. Other developments have been taking place in the testing of clothing against molten metal and a draft standard test method is described and discussed.
1. INTRODUCTION
In foundries it has been recognised for many years that the parts of the body most at risk from molten-metal burns are the feet, and legs below the knee. Splashes or pours around these limbs may occur in many ways, one of the most severe hazards being the splitting of improperly assembled moulds. Various designs of personal protection had been developed but until the early 1970s their use was not legally enforced and as a result varied from one foundry to another. During the late 1960s a standard for foundry footwear and gaiters was developed (British Standards Institution, 1971). This was given legal backing with the introduction of the Foundries (Protective Footwear and Gaiters) Regulations (Statutory Instruments, 1971). For the first time it * Paper first presented at the Second International Equipment, Torremolinos, May 1982.
Colloquium
on Personal
Protective
60
became possible to ensure on a legal basis a consistent level of protection for all workmen in molten metal areas. Unfortunately the standard has proved unsatisfactory in some respects. The main difficulties were identified in two surveys of accidents undertaken by Her Majesty’s Factory Inspectorate (HMFI) during 1972 and 1973, shortly after the new regulations came into force. An examination of some 943 accidents showed that the introduction of gaiters had led to only a slight decrease in the numbers of injuries and that the injuries that had occurred were on the whole rather more severe than had previously been the case. Splashing of molten metal into the tops of gaiters was identified as the main reason for poor performance. Metal trapped in this way finds its way down into the footwear. often causing serious injury to the feet and ankles. Over 80% of injuries sustained whilst gaiters conforming to the standard were being worn occurred by this means. In comparison, less than 6% were due to molten-metal burning through the gaiter or footwear. Thus it was concluded that the fit of gaiters needed to be improved but the materials used in their construction were just adequate. These findings were put before the Joint Standing Committee on Health, Safety and Welfare in Foundries which is a body set up by the Department of Employment and includes representatives of employers, employees and the Health and Safety Executive (HSE). A Sub-committee on lower leg protection was recalled to reconsider the 1971 standard and to make recommendations for improvements. The period since 1971 has seen two further developments in lower leg protection. The first of these is a boot with the top extended up the leg to give similar protection to that provided by a gaiter worn over a normal boot. These are now in use in hundreds of foundries. The second development has been towards trouser materials designed to shed molten metal. A test method for such materials has been proposed in the United Kingdom and a provisional standard is nearing completion. In France an experimental standard has been in use for some time and comparisons of these two methods are currently being undertaken by the International Standards Organisation (ISO). These developments are discussed in the latter part of the paper. The long term aim of the research is to bring the various standards together under a set of comprehensive regulations that will allow the foundry industry to have a choice of protection to suit all situations. 2. DEVELOPMENT
2.1 The 1971
OF AN IMPROVED
standard
STANDARD
FOR GAITERS
for gaiters
To understand the reasons for the disappointing performance of gaiters it is necessary to look first at the requirements of the 1971 standard. In this, provision was made for two types of gaiter. One type (300 mm in height) was intended for use in areas where molten iron was being handled, the other type
61
(200 mm) was to give protection against hot sand during knocking-out operations and against non-ferrous metals. The discussions in this paper are confined to the taller gaiter since this is used more frequently and in the most hazardous conditions. This section should be read with reference to Fig. 1 which shows typical gaiters for use against molten iron designed to meet this standard. It was laid down that gaiters had to be constructed with a piece (a “flare”) to cover the instep, had to include a leg section which covered the leg to a suitable height and had to have an underfoot strap which held the gaiter down firmly onto the footwear. Minimum dimensions were specified for the overall height, the overall width when laid flat and the length for the flare so as to ensure adequate coverage of the leg and foot. All seams had to be manufactured so that
Fig. 1. Examples
of gaiters manufactured
to BS 4676:
1971.
62
edges of materials on the outside of the assembly faced downwards, thus minimising the chance of molten metal being trapped. There was also a mandatory test which was designed to simulate an accident resulting from a split mould which was thought to be the most severe case likely to occur. The test gaiter was supported on a metal frame and 2.5 kg of molten iron was poured onto a point 100 mm above the seam joining the leg section to the flare. Penetration of the metal was regarded as reason enough to fail a specimen. This method ensured the integrity of the stitching at the front of the gaiter and also that molten metal was efficiently deflected. 2.2 Examination
of gaiters on sale
The Shoe and Allied Trades Research Association (SATRA) coordinates the testing and quality assurance aspects of footwear standards in the United Kingdom. This organisation was therefore in a good position to obtain samples of new gaiters for assessment, and in 1975 was commissioned to examine a cross-section of the gaiters on the market to determine how well these could be expected to fit the foundry population. At that time no measurements of the legs of foundrymen had been taken but it was thought they should be reasonably typical of young white males. This assumption was made with some reservation since it was known that many foundry workers had Asian origins and consequently might have different limb dimensions from the indigenous population. However, measurements TABLE
1
Leg measurements
of British
and Asian males
Number in survey
Population surveyed
Maximum calf girth (mm) ____~__ Mean
Mean Standard deviation ______~
Standard deviation
100 500 500 100 64
371 363 365 332 373
30.2 25.9 25.3 19.0 23.7
13.5 12.8 11.7 10.2
75 79
359 379
26.1 27.3
~~~ 1 2 3 4 5
British guardsmen Royal Armoured Corps British infantrymen Indian soldiers SATRA staff
6 Foundrymen Under trousers Over trousers Notes:
1 2 3 4 5 6
-
~ Minimum ankle girth (mm) ~_ .._.___-
from Gooderson and Beebee (1976b) from Ince et al. (1973) from Gooderson and Beebee (1976a) from Bharadwaj et al. (1977) from Lacey and Hole (197 5) author’s own measurements (1979)
226 220 220 203
~~~~__
~~
63
obtained much later, and summarised in Table 1, confirmed that the original assumption was quite reasonable. Details of the contours of the calf were not available from any of the published surveys, therefore SATRA took measurements on some of their own male staff (Lacey and Hole, 1975). Where comparable dimensions were available the results agreed well with another survey (Ince et al., 1973): see Table 1. It was found that the average girth of the bare leg at a height of 300 mm from the base of the heel was about 4% smaller than the average maximum calf girth. This height was chosen because it corresponds to the minimum requirements of the regulations for protection against molten iron. When this new anthropometric data was compared with the dimensions of the gaiters on sale it was found that the gaiters would only fit approximately two thirds of the population. This occurred because manufacturers tend to design to the minimum, and hence cheapest, standard. The minimum specified for the circumference around the top of a gaiter was 370 mm whereas 420 mm would be required to just cover the largest legs for gaiters worn outside clothing. It also became evident that the range of adjustment provided on the fasteners was insufficient to fit the smallest leg, leaving a gap at the top which would allow metal to enter. Another important factor affecting the protection afforded by gaiters was that some gaiters did not have sufficient stiffness to remain upright on the leg without appreciable sagging. Quite clearly, due to the taper of the leg towards the ankle, any collapse of a gaiter will increase the gap around the top and thus increase the chance of metal entering. 2.3 Recommended
new design
The sub-committee on discussing these findings asked for consideration to be given to the production of a single design to fit all male employees. It was argued that if such a course were feasible it would avoid confusion amongst manufacturers as to the sizes of gaiter to produce, simplify purchase and issue of gaiters in foundries and ensure that every foundryman had suitable protection available. Of course, the new gaiter also needed to overcome the deficiencies found in previous designs. With the help of SATRA and the Safety Engineering Laboratory of the Health and Safety Executive the gaiter shown in Fig. 2 was developed. It has a band of soft material around the top which is tapered upwards to fit closely to the leg. The fasteners are of the “touch and close” type and run the whole length of the gaiter. They give 100 mm of continuous adjustment in the circumference with about 50 mm of overlap on the fastener material. This range of adjustment, along with close specification of the top circumference, gives a good fit on 95% of the population. The 5% at the two extremes of the leg size distribution can obtain a satisfactory fit with smaller contact area of the fastener material. The stability of the gaiter is improved by having fasteners along its whole length but as an extra precaution against sagging the specifica-
Fig. 2. New gaiter used for field trials (right leg shown).
tion also requires the material to have a minimum stiffness. The gaiter can be removed quickly in an emergency. In order to have a single gaiter design it has been essential to specify more measurements than in the 1971 standard. Furthermore, little dimensional tolerance can be allowed in the design. The minimum height was set initially at 300 mm but it was left for field trials to determine whether this height was the optimum one. 2.4 Field trials of new gaiter Before putting the new gaiter into general use, comprehensive field trials were undertaken to investigate the quality of the fit, the durability, the opinions of the users and the ease of manufacture. Lessons learned from the earlier
65
designs had instilled a cautious attitude. The trials covered a 49-week period between May 1977 and May 1978, Gaiters for this phase were obtained from five manufacturers. Each of them was supplied with patterns to work from so that finished products had identical dimensions. The reason for including a number of suppliers rather than just a single one was to increase the likelihood of identifying manufacturing difficulties. In all, 128 pairs of gaiters suitable for use against molten iron were obtained, with approximately 25 pairs coming from each manufacturer. Since it was not known whether 300 mm was the optimum height, gaiters were also requested with heights of 325 mm and 350 mm for comparison. On arrival at the laboratory the new gaiters were measured to check their dimensions against the specification and to ascertain the manufacturing variations. Results from these measurements were used to set the allowable tolerance in the final standard. To ensure a reliable cross-section of views and to avoid stereotyped responses, 20 geographically well-separated foundries were chosen to take part in the trials. Selection of foundries was made carefully to include the major ethnic groups, working practices and conditions. One hundred and five pairs of gaiters were distributed. Not all foundries were asked to take part for the full trial period. This was because the gaiters were distributed cautiously over a number of months in case problems necessitating modifications to the design were identified at an early stage. By issuing gaiters in several batches it was possible to introduce changes before the later gaiters were distributed. In the event this policy proved worthwhile since, as a result of frequent complaints initially, it was decided to dispense with the underfoot strap. Many users maintained that this strap was awkward to fasten whilst others reported that it often came undone, creating a situation where tripping might occur. Gaiters without straps, tested under simulated conditions in the laboratory, kept their position over the instep with little twisting; comparisons made by a number of foundrymen confirmed that the strap made little difference in this respect. The improved stability of the new design over previous styles, even without the underfoot strap, can be put down to the greater rigidity resulting from the larger area of the “touch and close” fasteners and also the elimination of sloppy gaiters due to the new stiffness requirements for the material. Before gaiters were distributed measurements were made of the adhesion of the “touch and close” fasteners. This assessment was done at three points (top, middle and bottom) of each gaiter. On completion of the trial period, these measurements were repeated to ascertain the durability of the fastener material under foundry conditions. Although some disquiet had initially been felt in using this material in such arduous circumstances, in only two cases was there any appreciable deterioration and then only over a part of the gaiters’ length. It was concluded that this type of fastener material is appropriate for this use provided the contact area is sufficiently large. The findings certainly could not be extrapolated to the small contact areas found in previous designs where loss of adhesion could be expected.
66
Each foundry was visited a number of times. Firstly a visit was made to distribute the gaiters, show men how to fit them, assess the quality of fit and to make some anthropometric measurements on the legs of the wearers. This was how the data referred to at 6 in Table 1 was collected. Further visits were made to interview the participants, the first of these being at least a month after gaiters had been issued. The final visit was to withdraw the gaiters at the end of the period. Where possible, each man issued with gaiters was interviewed in conjunction with a specially designed questionnaire. At some foundries members of the safety staff conducttd these sessions whereas at other foundries it was done by research staff. By using different interviewers it was possible during the analysis to check for bias between interviewers and to treat with caution any findings where such bias existed. In fact little variation was found between the results from the various interviewers. The first part of the questionnaire contained general questions to determine what men thought were the good and bad points of the gaiter and whether they had any ideas for improvement. In the second part the questions referred to specific aspects of the design e.g. the “touch and close” fasteners. The questions were arranged in this way so that the true strength of feeling about the good and bad points could be gauged before opinions could be influenced by discussion. The answers given to the general questions are summarised in Table 2. Apart from problems with the underfoot strap which were discussed earlier the reaction was very encouraging: the two innovations, i.e. the top band and the large area “touch and close” fasteners, both received more favourable than unfavourable comments, which is statistically significant at the 0.001% level. Most of the unfavourable comments in the “others” category could have applied to almost any gaiter and were not critical of this particular design. Table 3 shows the more specific information relevant to the assessment of the top band. Men were generally well satisfied with the comfort, but a few TABLE Summary
2 of general
comments
Feature
Comment favourable to new gaiters
Top band “Touch and close” fasteners Underfoot strap Others -
28
4
32
30 7 34
r 2: 41
35 31 75
99
74
173
Total Number
of men interviewed
= 86
Comment unfavourable to new gaiters
Total
-__
61 TABLE
3
Summary
of responses
to questions
related
to the top band
Response favourable to new gaiter (%)
Response unfavourable to new gaiter (%)
Does the top fit comfortably on your leg?
90
10
Does the gaiter fit well enough at the top to prevent metal entering?
76
24
Question
thought some metal could still enter at the top. A third of the unfavourable responses, however, were qualified with phrases such as “a little” or “not much” metal could penetrate. The result shows a tendency towards satisfactory protection significant at the 0.001% level. Details of minor accidents occurring to men wearing the gaiters suggest that metal is more frequently stopped at the top of the gaiter than was the case with previous types. The results in Table 4, which summarises the specific information gathered on the gaiter fasteners, indicate a favourable response to the new system significant at the 0.001% level. Another important conclusion drawn from the interviews was that men showed a preference for the gaiters taller than 300 mm. In fact, of the gaiters used, the tallest (350 mm), were liked best. Finally, men were asked how the gaiters as a whole compared with the ones they had used before. Some 86% thought they were an improvement, whereas 5% preferred their old styles, again a highly significant result. On the whole their response was encouraging and led to the conclusion that this design represented a step forward in user acceptability. The cost of protecting a workman is obviously an important consideration to foundry managers who have to compete in a difficult economic climate. TABLE Summary
4 of responses
related
to the “touch
Question
and close”
fasteners
Response favourable to new gaiter
Neutral (%)
response
Response unfavourable to new gaiter
(%)
(%)
Does the gaiter stay in place on the leg?
90
10
What do you think of the fastener strips down the side compared with other types of fastener you have used?
a4
13
3
68 TABLE 5 Length of service of new gaiters Number of weeks in use
Percentage of gaiters remaining in use
9 14 29 45
92 75 60 50
to to to to
12 23 31 49
Improving the protection usually leads to increased equipment cost per unit and the new gaiters are no exception to this rule. In fact they cost 40--50% more per pair than the existing gaiters. However, cost per unit is not the only parameter in the total cost equation since the lifetime of the equipment is also of importance. From the field trials it was possible to determine the numbers of gaiters still in use after various periods of time. These results are summarised in Table 5. It should be noted that equipment did not necessarily disappear from use because of deterioration and in fact this reason for disuse was very rare. More usual causes were men leaving a foundry, one foundry closing down, and men losing their gaiters. Check visits to the foundries were deliberately avoided except for the initial and final interviews so that a realistic assessment of the numbers of gaiters maintained in service could be obtained. In fact the table suggests that with normal usage the equipment should last at least six months on average, even at a conservative estimate. It has been found from discussions with foundries that existing types last only one to three months. Thus the trials showed that the extra durability of the new gaiter is enough to offset the increased costs per unit. 2.5 Conclusions
and recommendations
The work described above was used to prepare a draft proposal for revision of the gaiter section of British Standard 4676 (British Standards Institution, 1971). A report of the work along with a copy of the revised standard was put to the Health and Safety Commission’s Foundries Industry Advisory Committee who endorsed the views expressed and asked HSE to approach the British Standards Institution with a view to incorporating the revision in the standard. This revision is now in the committee stages involved in its publication and the final version is expected to appear towards the end of 1982. Briefly, its requirements are expected to be as follows: gaiters must comply with the dimensions shown in Table 6 which should be read in conjunction with Fig. 3. This is the main change from the 19’71 version of the standard but additionally a clause relating to the stiffness of the material used in the construction has been inserted together with a number of other detailed changes. The re-
69 TABLE
6
Main dimensions
of new gaiter
Height Width of top band Reduction in circumference between bottom and top of top band Circumference around top at maximum size Circumference around top at minimum size Length of flare Distance from centre of flare to outside edge of gaiter
350 mm 65 mm 30 t 415 k 315 i 160 i 142.5 +
minimum minimum 5 mm 10 mm 10 mm 10 mm 12.5 mm
-
Fig. 3. Dimensions
of new gaiter (right leg shown
from front).
quirement to test the complete assembly against molten metal has been retained but the method of assessing failure on this test has undergone minor amendment. Since completion of the work on gaiters it has become apparent that some types of boot when worn outside the trousers suffer from similar design deficiencies to those which prompted the work on gaiters. For this reason a section has been included in the new standard to cover this type of boot.
70
3 DEVELOPMENT
OF STANDARDS
FOR CLOTHS
3.1 Background Over the years a good deal of development has gone into the provision of clothing for protecting against molten iron and steel. Tests for materials have been developed in the United Kingdom by the British Steel Corporation and also in the United States of America (Mehta and Willerton, 1977; Barnes, 1979; Catton, 1980). According to these tests either woollen or flame-retardant cotton clothing may be used in situations where moderate splashes of molten iron or steel may occur (Peter, 1978; Barnes, 1979; Catton, 1980), which suggests that against these metals both materials give a similar performance. It has been assumed by many people that the use of a flame-retardant material automatically guarantees protection against molten metal Recent work on the passage of heat through such materials when they are splashed by molten metal shows this assumption to be unreliable, particularly for metals such as aluminium where adhesion to the fabric may occur (Mehta and Willerton, 1977). Such sticking causes excessive heat transfer through the material with the subsequent risk of burns to the wearer and in some cases may result in holing followed by penetration of the hot metal. The criteria for protection have therefore been extended to include the possibility of burns as well as the general requirement of flame resistance. For this reason the performance requirements for materials have had to be reassessed and new methods of test specifically against molten metal have had to be considered. 3.2 Requirements
of a test method
The main criterion to be satisfied by a test method is that it should realistically reproduce the hazard to which workmen are exposed. It is relatively easy to provide a high level of protection by increasing material thickness and hence weight but such a course increases both the physiological load on the wearer and the general discomfort. The latter assumes greater importance as the weight increases until a stage is reached where outright rejection of the equipment occurs. Precisely at what level this will take place is not easy to predict since it depends on the subjective judgement of the wearer. The British Steel Corporation have made an extensive study of this question in relation to the provision of woollen clothing for their furnace workers and have suggested an optimum level of protection (Catton, 1980). Intuitively it seems likely that in making a decision to wear protective clothing an individual balances his own experience of risk against his general well being. If this hypothesis is correct, it implies that the acceptable level of protection is related to the risk and will be lowest in low-risk areas. The test procedure must therefore simulate the whole range of hazards in order to have available a range of protection to suit all needs. Thus, the aim has been to select and develop a method
71
which can be used with as many as possible of the common metals, can reliably assess the protection afforded by different material types and weights and needs relatively small quantities of metal for each test. The latter requirement is made because tests are best conducted in controlled conditions whereas large-scale test methods, such as that described by Mehta and Willerton (1977), require access to an industrial furnace. There is no conflict between the severity of the test and this criterion because the total weight of metal used is not the only important parameter to be considered. It is the weight of metal per unit area which determines the severity so that a small weight over a small area can be just as severe as a large weight over a large area. Experience has shown that there is little likelihood of clothing being required to protect against metal in contact with it for more than a second or two, unless metal adheres to the material. The only occasions when longer contact periods might occur are when men are lying on the floor or when metal lodges in the clothing. It would be unrealistic to expect clothing to protect against such severe hazards as men lying in pools of molten metal. Even with very heavy protection only extremely short exposures could be tolerated. In any case experience suggests that this type of situation occurs extremely rarely if at all. The problems caused by metal lodging in the clothing can be prevented by proper design to protect pockets and eliminate folds. Areas such as shoulders where the clothing surface is nearly horizontal can be of increased thickness if necessary. These arguments suggest that a realistic test should be conducted with the material at a steep angle to the horizontal so as to assess the ability to deflect metal and simulate the nearly vertical clothing surface occurring in wear. 3.3 Test methods A number of workers have proposed test methods meeting the above criteria. Broadly these methods all measure the passage of heat through the material immediately after contact with the metal. Differences arise in the means of detecting the heat flow. Most methods have used a traditional heat sensor, usually a thermocouple attached to a suitable calorimeter, placed underneath the material so that when molten metal comes into contact with the material’s upper surface the rise in temperature underneath is measured (Wren, Scott and Bates, 1977; Jaynes, 1980; Cornu et al., 1981). With this method the design of the calorimeter is critical. Calculations show that even with a simple device such as a thermocouple attached to the back of a single copper sheet, the heat flow varies considerably with the thickness of the copper. Thus such calorimeters need to be accurately specified and constructed. If the calorimeter is not quite flush with the surface on which the test material is supported there may be an air gap between the specimen and the calorimeter. Such a gap will cause appreciable resistance to heat flow and hence to the temperatures recorded. The above factors concern the physical design and construction of the equipment but there are other considerations.
For example, over a period a build-up of charred material tends to take place on the measuring device which is difficult to remove effectively and causes a reduction in the temperatures obtained. A further major problem is that the precise point of impact of the molten metal has to be well defined in advance. This is not easy to achieve with some metals due to the difficulties of ensuring a smooth flow over the lip of the pouring crucible. Variations in the direction of the molten metal stream can occur due to distortion of the crucible by heat, contamination from previous tests and to the existence of a skin of semisolidified material on the metal surface. The latter is particularly acute with aluminium. The other approach to the assessment of heat flow is to use a plastic skin underneath the material so that melting or damage to this skin signifies the transfer of a known quantity of heat (Mehta and Willerton, 1977; Benisek et al., 1979; Benisek and Edmondson, 1981). In the United Kingdom this latter technique has been developed into a draft British Standard following trials within four laboratories. A brief history of this method is useful in understanding its development. In 1977, Mehta and Willerton devised a test using 3 kg of molten iron which was incident on an inclined fabric over a plastic skin. This quantity of metal over the area of the test material was regarded by the foundry and steel industries as being a reasonable representation of the hazard to which workmen might occasionally be exposed. Due to the subjective nature of assessing different degrees of damage to the plastic skin and the quantities of metal needed, a laboratory-scale method was developed (Benisek et al., 1979). In this method the same material was exposed to different weights of metal, and therefore different contact times, until a well defined degree of damage to the plastic skin occurred. The result was then recorded as the weight of metal the material would withstand. Because industry had accepted the larger-scale method it was necessary to confirm that materials ranked in a similar manner in both procedures. The proposed British Standard is based on further development of this method. Small quantities of molten metal are poured onto samples of material inclined at an angle to the horizontal. The angle of inclination used depends on the type of metal poured, a smaller angle giving a more severe test. Sixty degrees has been found suitable for iron, steel and copper but forty-five degrees is more appropriate for aluminium. Assessment is made by examining the surface of a PVC skin simulant placed behind and in contact with the material. Any damage to the PVC skin is noted. Successive samples are tested using different weights of molten metal until the quantity is found which only just damages the PVC skin. The results are recorded as the “molten-metal splash index”. The pouring crucible may be rotated by hand or by using an electric motor driven from an electronic circuit. In both cases the aim is to rotate the crucible at a constant rate through an angle of 110” from the vertical in 2.8 to 3.2 seconds.
13
The test is designed to give an end point that is comparable in terms of heat transfer with the quantity of heat required to produce first degree burns (Mehta and Willerton, 1977). The accepted value for this quantity is 9.41 X lo4 J m-* (2.25 cal cm-“). In fact it can be shown that damage to the PVC skin occurs at a heat flow of 2 to 4 X lo4 J m-* (0.5 to 1 cal cm-*) and this provides a margin of safety (Thompson, 1980). The main disadvantage of this method is that several tests have to be conducted on the same material to find the true value of the molten-metal splash index. With a little practice however this is not difficult to achieve and the method offers considerable advantages to offset this inconvenience. It uses simple apparatus which can be constructed and put into use very quickly. It is suitable for laboratory use since only small quantities of molten metal are required. It has been proved to work for a range of metals. The pour does not need to be accurately directed since the heat sensor underlies the whole of the test specimen. This is an important consideration because it has been found by video recording a series of tests that the point of contact of the molten metal stream on the specimen wanders about considerably during a single test. Variations between tests can be reduced by using a new pouring edge of the crucible for each test. However, even after all precautions have been taken it is not possible to guarantee the exact impact point on the specimen. Where traditional heat sensors are used this has been found to lead to large variations in recorded temperature rises. The PVC skin simulant is not so sensitive to such positional changes. Another advantage is that the heat sensor material is changed for each test so that the cleaning of debris from previous tests is unnecessary. Build-up of debris causing a loss of heat transfer to the sensing device has been found to be a problem with the thermocouple devices. It has been concluded that the considerable advantages offered by this method far outweigh its disadvantages, which explains its choice for standards purposes. Comparisons between the method and that proposed by Cornu et al. (1981) are being coordinated through the International Standards Organisation and involve the cooperation of six countries. o Crown copyright
1982
ACKNOWLEDGEMENTS
The author would like to thank all the many people who have contributed to this work within HSE, and the protective clothing and foundry industries. He would especially like to thank his colleague Mr. H. Thompson who has made an invaluable contribution
74 REFERENCES Barnes, T.E., 1979. Heat and flame-resistant clothing. National Safety News, September 1979, pp. 54-55. Benisek, L., Edmondson, G.K. and Phillips, W., 1979. Protective clothing evaluation of zirpro wool and other fabrics. Text. Res. J., 49: 212-221. Benisek, L. and Edmondson, G.K., 1981. Protective clothing fabrics, Part 1: against molten metal hazards. Text. Res. J., 51: 182-190. Bharadwaj, H., Verma, S.S., Zachariah, T., Bhatia, M.R., Kishnani, S. and Malhotra, M.S., 1977. Estimation of body density and lean body weight from body measurements at high altitude. Europ. J. Appl. Physiol., 36: 141-150. British Standards Institution, 1971. Specification for gaiters and footwear for protection against burns and impact risks in foundries. British Standard 4676: 1971, British Standards Institution, London, England. Catton, J.A., 1980. Personal protection in a high hazard activity. Protection, September 1980, pp. 18-21. Cornu, J.-C., Aubertin, G. and Rapp, R., 1981. Vetements de protection contre les projections de metaux en fusion. Cahiers de Notes Documentaires, No. 102, Institut National de Recherche et Securite, Paris, France. Gooderson, C.Y. and Beebee, M., 1976a. Anthropometry of 500 infantrymen 1973-1974. Army Personnel Research Est., Farnborough, England, Report No. 17/76. Gooderson, C.Y. and Beebee, M., 1976b. A comparison of the anthropometry of 100 guardsmen with that of 500 infantrymen, 500 RAC servicemen and 2000 RAF aircrew. Army Personnel Research Est., Farnborough, England, Report No. 37/76. Jaynes, P.S., 1980. Testing fabrics with molten steel. Professional Safety, October 1980, pp. 15-20. Ince, N.E., Redrup, S. and Piper, J., 1973. Anthropometry of 500 Royal Armoured Corps servicemen 197 2. Army Personnel Research Est ., Farnborough, England, Report No. 36173. Lacey, P.D.A. and Hole, L.G., 1975. Protective footwear for the foundry industry. Shoe and Allied Trades Research Ass., Kettering, England, Report No. M81 FI. Mehta, P.N. and Willerton, K., 1977. Evaluation of clothing materials for protection against molten metal. Text. Inst. and Ind., 15: 334-337. Peter, H., 1978. Criteria for assessing heat-resistant protective clothing. Protection, July 1979, pp. 6-9. Statutory Instruments, 1971. Foundries (Protective Footwear and Gaiters) Regulations. Statutory Instruments 476, London, HMSO, 1971. Thompson, H., 1980. Personal communication. Wren, J.E., Scott, W.D. and Bates, C.E., 1977. Thermal and mechanical properties of aluminized fabrics for use in ferrous metal handling operations. Am. Ind. Hyg. - ASSOC. J., 38: 603-612.
of Occupational Accidents, 5 (1983) 59-74 Scientific Publishing Company, Amsterdam
59 -
Printed
in The Netherlands
RESEARCH INTO EQUIPMENT FOR PROTECTION AGAINST MOLTENMETAL SPLASH*
T.D.
PROCTOR
Health (U.K.)
and Safety
(Received
Executive,
September
2, 1982;
Safety
Engineering
accepted
September
Laboratory,
Red Hill, Sheffield
S3 7HQ
6, 1982)
ABSTRACT Proctor, T.D., 1983. Research into equipment for protection Journal of Occupational Accidents, 5: 59-14.
against
molten-metal
splash.
The Foundries (Protective Footwear and Gaiters) Regulations were introduced in 1971 to reduce the risk of injury to the lower legs and feet of foundrymen. Surveys conducted by Her Majesty’s Factory Inspectorate in 1972 and 1973, after the introduction of the regulations, showed that although there had been a marginal reduction in burns there were serious inadequacies in the design of gaiters. Eighty-eight per cent of the injuries sustained when approved gaiters were being worn were the result of molten metal entering the top of the gaiter and being funnelled down into the footwear. This design shortcoming, together with problems of comfort and fastening efficiency, had resulted in some workers being unwilling, or even refusing, to wear the gaiters. Research was initiated to overcome these difficulties and after extensive field trials a new style of gaiter has been introduced. Other developments have been taking place in the testing of clothing against molten metal and a draft standard test method is described and discussed.
1. INTRODUCTION
In foundries it has been recognised for many years that the parts of the body most at risk from molten-metal burns are the feet, and legs below the knee. Splashes or pours around these limbs may occur in many ways, one of the most severe hazards being the splitting of improperly assembled moulds. Various designs of personal protection had been developed but until the early 1970s their use was not legally enforced and as a result varied from one foundry to another. During the late 1960s a standard for foundry footwear and gaiters was developed (British Standards Institution, 1971). This was given legal backing with the introduction of the Foundries (Protective Footwear and Gaiters) Regulations (Statutory Instruments, 1971). For the first time it * Paper first presented at the Second International Equipment, Torremolinos, May 1982.
Colloquium
on Personal
Protective
60
became possible to ensure on a legal basis a consistent level of protection for all workmen in molten metal areas. Unfortunately the standard has proved unsatisfactory in some respects. The main difficulties were identified in two surveys of accidents undertaken by Her Majesty’s Factory Inspectorate (HMFI) during 1972 and 1973, shortly after the new regulations came into force. An examination of some 943 accidents showed that the introduction of gaiters had led to only a slight decrease in the numbers of injuries and that the injuries that had occurred were on the whole rather more severe than had previously been the case. Splashing of molten metal into the tops of gaiters was identified as the main reason for poor performance. Metal trapped in this way finds its way down into the footwear. often causing serious injury to the feet and ankles. Over 80% of injuries sustained whilst gaiters conforming to the standard were being worn occurred by this means. In comparison, less than 6% were due to molten-metal burning through the gaiter or footwear. Thus it was concluded that the fit of gaiters needed to be improved but the materials used in their construction were just adequate. These findings were put before the Joint Standing Committee on Health, Safety and Welfare in Foundries which is a body set up by the Department of Employment and includes representatives of employers, employees and the Health and Safety Executive (HSE). A Sub-committee on lower leg protection was recalled to reconsider the 1971 standard and to make recommendations for improvements. The period since 1971 has seen two further developments in lower leg protection. The first of these is a boot with the top extended up the leg to give similar protection to that provided by a gaiter worn over a normal boot. These are now in use in hundreds of foundries. The second development has been towards trouser materials designed to shed molten metal. A test method for such materials has been proposed in the United Kingdom and a provisional standard is nearing completion. In France an experimental standard has been in use for some time and comparisons of these two methods are currently being undertaken by the International Standards Organisation (ISO). These developments are discussed in the latter part of the paper. The long term aim of the research is to bring the various standards together under a set of comprehensive regulations that will allow the foundry industry to have a choice of protection to suit all situations. 2. DEVELOPMENT
2.1 The 1971
OF AN IMPROVED
standard
STANDARD
FOR GAITERS
for gaiters
To understand the reasons for the disappointing performance of gaiters it is necessary to look first at the requirements of the 1971 standard. In this, provision was made for two types of gaiter. One type (300 mm in height) was intended for use in areas where molten iron was being handled, the other type
61
(200 mm) was to give protection against hot sand during knocking-out operations and against non-ferrous metals. The discussions in this paper are confined to the taller gaiter since this is used more frequently and in the most hazardous conditions. This section should be read with reference to Fig. 1 which shows typical gaiters for use against molten iron designed to meet this standard. It was laid down that gaiters had to be constructed with a piece (a “flare”) to cover the instep, had to include a leg section which covered the leg to a suitable height and had to have an underfoot strap which held the gaiter down firmly onto the footwear. Minimum dimensions were specified for the overall height, the overall width when laid flat and the length for the flare so as to ensure adequate coverage of the leg and foot. All seams had to be manufactured so that
Fig. 1. Examples
of gaiters manufactured
to BS 4676:
1971.
62
edges of materials on the outside of the assembly faced downwards, thus minimising the chance of molten metal being trapped. There was also a mandatory test which was designed to simulate an accident resulting from a split mould which was thought to be the most severe case likely to occur. The test gaiter was supported on a metal frame and 2.5 kg of molten iron was poured onto a point 100 mm above the seam joining the leg section to the flare. Penetration of the metal was regarded as reason enough to fail a specimen. This method ensured the integrity of the stitching at the front of the gaiter and also that molten metal was efficiently deflected. 2.2 Examination
of gaiters on sale
The Shoe and Allied Trades Research Association (SATRA) coordinates the testing and quality assurance aspects of footwear standards in the United Kingdom. This organisation was therefore in a good position to obtain samples of new gaiters for assessment, and in 1975 was commissioned to examine a cross-section of the gaiters on the market to determine how well these could be expected to fit the foundry population. At that time no measurements of the legs of foundrymen had been taken but it was thought they should be reasonably typical of young white males. This assumption was made with some reservation since it was known that many foundry workers had Asian origins and consequently might have different limb dimensions from the indigenous population. However, measurements TABLE
1
Leg measurements
of British
and Asian males
Number in survey
Population surveyed
Maximum calf girth (mm) ____~__ Mean
Mean Standard deviation ______~
Standard deviation
100 500 500 100 64
371 363 365 332 373
30.2 25.9 25.3 19.0 23.7
13.5 12.8 11.7 10.2
75 79
359 379
26.1 27.3
~~~ 1 2 3 4 5
British guardsmen Royal Armoured Corps British infantrymen Indian soldiers SATRA staff
6 Foundrymen Under trousers Over trousers Notes:
1 2 3 4 5 6
-
~ Minimum ankle girth (mm) ~_ .._.___-
from Gooderson and Beebee (1976b) from Ince et al. (1973) from Gooderson and Beebee (1976a) from Bharadwaj et al. (1977) from Lacey and Hole (197 5) author’s own measurements (1979)
226 220 220 203
~~~~__
~~
63
obtained much later, and summarised in Table 1, confirmed that the original assumption was quite reasonable. Details of the contours of the calf were not available from any of the published surveys, therefore SATRA took measurements on some of their own male staff (Lacey and Hole, 1975). Where comparable dimensions were available the results agreed well with another survey (Ince et al., 1973): see Table 1. It was found that the average girth of the bare leg at a height of 300 mm from the base of the heel was about 4% smaller than the average maximum calf girth. This height was chosen because it corresponds to the minimum requirements of the regulations for protection against molten iron. When this new anthropometric data was compared with the dimensions of the gaiters on sale it was found that the gaiters would only fit approximately two thirds of the population. This occurred because manufacturers tend to design to the minimum, and hence cheapest, standard. The minimum specified for the circumference around the top of a gaiter was 370 mm whereas 420 mm would be required to just cover the largest legs for gaiters worn outside clothing. It also became evident that the range of adjustment provided on the fasteners was insufficient to fit the smallest leg, leaving a gap at the top which would allow metal to enter. Another important factor affecting the protection afforded by gaiters was that some gaiters did not have sufficient stiffness to remain upright on the leg without appreciable sagging. Quite clearly, due to the taper of the leg towards the ankle, any collapse of a gaiter will increase the gap around the top and thus increase the chance of metal entering. 2.3 Recommended
new design
The sub-committee on discussing these findings asked for consideration to be given to the production of a single design to fit all male employees. It was argued that if such a course were feasible it would avoid confusion amongst manufacturers as to the sizes of gaiter to produce, simplify purchase and issue of gaiters in foundries and ensure that every foundryman had suitable protection available. Of course, the new gaiter also needed to overcome the deficiencies found in previous designs. With the help of SATRA and the Safety Engineering Laboratory of the Health and Safety Executive the gaiter shown in Fig. 2 was developed. It has a band of soft material around the top which is tapered upwards to fit closely to the leg. The fasteners are of the “touch and close” type and run the whole length of the gaiter. They give 100 mm of continuous adjustment in the circumference with about 50 mm of overlap on the fastener material. This range of adjustment, along with close specification of the top circumference, gives a good fit on 95% of the population. The 5% at the two extremes of the leg size distribution can obtain a satisfactory fit with smaller contact area of the fastener material. The stability of the gaiter is improved by having fasteners along its whole length but as an extra precaution against sagging the specifica-
Fig. 2. New gaiter used for field trials (right leg shown).
tion also requires the material to have a minimum stiffness. The gaiter can be removed quickly in an emergency. In order to have a single gaiter design it has been essential to specify more measurements than in the 1971 standard. Furthermore, little dimensional tolerance can be allowed in the design. The minimum height was set initially at 300 mm but it was left for field trials to determine whether this height was the optimum one. 2.4 Field trials of new gaiter Before putting the new gaiter into general use, comprehensive field trials were undertaken to investigate the quality of the fit, the durability, the opinions of the users and the ease of manufacture. Lessons learned from the earlier
65
designs had instilled a cautious attitude. The trials covered a 49-week period between May 1977 and May 1978, Gaiters for this phase were obtained from five manufacturers. Each of them was supplied with patterns to work from so that finished products had identical dimensions. The reason for including a number of suppliers rather than just a single one was to increase the likelihood of identifying manufacturing difficulties. In all, 128 pairs of gaiters suitable for use against molten iron were obtained, with approximately 25 pairs coming from each manufacturer. Since it was not known whether 300 mm was the optimum height, gaiters were also requested with heights of 325 mm and 350 mm for comparison. On arrival at the laboratory the new gaiters were measured to check their dimensions against the specification and to ascertain the manufacturing variations. Results from these measurements were used to set the allowable tolerance in the final standard. To ensure a reliable cross-section of views and to avoid stereotyped responses, 20 geographically well-separated foundries were chosen to take part in the trials. Selection of foundries was made carefully to include the major ethnic groups, working practices and conditions. One hundred and five pairs of gaiters were distributed. Not all foundries were asked to take part for the full trial period. This was because the gaiters were distributed cautiously over a number of months in case problems necessitating modifications to the design were identified at an early stage. By issuing gaiters in several batches it was possible to introduce changes before the later gaiters were distributed. In the event this policy proved worthwhile since, as a result of frequent complaints initially, it was decided to dispense with the underfoot strap. Many users maintained that this strap was awkward to fasten whilst others reported that it often came undone, creating a situation where tripping might occur. Gaiters without straps, tested under simulated conditions in the laboratory, kept their position over the instep with little twisting; comparisons made by a number of foundrymen confirmed that the strap made little difference in this respect. The improved stability of the new design over previous styles, even without the underfoot strap, can be put down to the greater rigidity resulting from the larger area of the “touch and close” fasteners and also the elimination of sloppy gaiters due to the new stiffness requirements for the material. Before gaiters were distributed measurements were made of the adhesion of the “touch and close” fasteners. This assessment was done at three points (top, middle and bottom) of each gaiter. On completion of the trial period, these measurements were repeated to ascertain the durability of the fastener material under foundry conditions. Although some disquiet had initially been felt in using this material in such arduous circumstances, in only two cases was there any appreciable deterioration and then only over a part of the gaiters’ length. It was concluded that this type of fastener material is appropriate for this use provided the contact area is sufficiently large. The findings certainly could not be extrapolated to the small contact areas found in previous designs where loss of adhesion could be expected.
66
Each foundry was visited a number of times. Firstly a visit was made to distribute the gaiters, show men how to fit them, assess the quality of fit and to make some anthropometric measurements on the legs of the wearers. This was how the data referred to at 6 in Table 1 was collected. Further visits were made to interview the participants, the first of these being at least a month after gaiters had been issued. The final visit was to withdraw the gaiters at the end of the period. Where possible, each man issued with gaiters was interviewed in conjunction with a specially designed questionnaire. At some foundries members of the safety staff conducttd these sessions whereas at other foundries it was done by research staff. By using different interviewers it was possible during the analysis to check for bias between interviewers and to treat with caution any findings where such bias existed. In fact little variation was found between the results from the various interviewers. The first part of the questionnaire contained general questions to determine what men thought were the good and bad points of the gaiter and whether they had any ideas for improvement. In the second part the questions referred to specific aspects of the design e.g. the “touch and close” fasteners. The questions were arranged in this way so that the true strength of feeling about the good and bad points could be gauged before opinions could be influenced by discussion. The answers given to the general questions are summarised in Table 2. Apart from problems with the underfoot strap which were discussed earlier the reaction was very encouraging: the two innovations, i.e. the top band and the large area “touch and close” fasteners, both received more favourable than unfavourable comments, which is statistically significant at the 0.001% level. Most of the unfavourable comments in the “others” category could have applied to almost any gaiter and were not critical of this particular design. Table 3 shows the more specific information relevant to the assessment of the top band. Men were generally well satisfied with the comfort, but a few TABLE Summary
2 of general
comments
Feature
Comment favourable to new gaiters
Top band “Touch and close” fasteners Underfoot strap Others -
28
4
32
30 7 34
r 2: 41
35 31 75
99
74
173
Total Number
of men interviewed
= 86
Comment unfavourable to new gaiters
Total
-__
61 TABLE
3
Summary
of responses
to questions
related
to the top band
Response favourable to new gaiter (%)
Response unfavourable to new gaiter (%)
Does the top fit comfortably on your leg?
90
10
Does the gaiter fit well enough at the top to prevent metal entering?
76
24
Question
thought some metal could still enter at the top. A third of the unfavourable responses, however, were qualified with phrases such as “a little” or “not much” metal could penetrate. The result shows a tendency towards satisfactory protection significant at the 0.001% level. Details of minor accidents occurring to men wearing the gaiters suggest that metal is more frequently stopped at the top of the gaiter than was the case with previous types. The results in Table 4, which summarises the specific information gathered on the gaiter fasteners, indicate a favourable response to the new system significant at the 0.001% level. Another important conclusion drawn from the interviews was that men showed a preference for the gaiters taller than 300 mm. In fact, of the gaiters used, the tallest (350 mm), were liked best. Finally, men were asked how the gaiters as a whole compared with the ones they had used before. Some 86% thought they were an improvement, whereas 5% preferred their old styles, again a highly significant result. On the whole their response was encouraging and led to the conclusion that this design represented a step forward in user acceptability. The cost of protecting a workman is obviously an important consideration to foundry managers who have to compete in a difficult economic climate. TABLE Summary
4 of responses
related
to the “touch
Question
and close”
fasteners
Response favourable to new gaiter
Neutral (%)
response
Response unfavourable to new gaiter
(%)
(%)
Does the gaiter stay in place on the leg?
90
10
What do you think of the fastener strips down the side compared with other types of fastener you have used?
a4
13
3
68 TABLE 5 Length of service of new gaiters Number of weeks in use
Percentage of gaiters remaining in use
9 14 29 45
92 75 60 50
to to to to
12 23 31 49
Improving the protection usually leads to increased equipment cost per unit and the new gaiters are no exception to this rule. In fact they cost 40--50% more per pair than the existing gaiters. However, cost per unit is not the only parameter in the total cost equation since the lifetime of the equipment is also of importance. From the field trials it was possible to determine the numbers of gaiters still in use after various periods of time. These results are summarised in Table 5. It should be noted that equipment did not necessarily disappear from use because of deterioration and in fact this reason for disuse was very rare. More usual causes were men leaving a foundry, one foundry closing down, and men losing their gaiters. Check visits to the foundries were deliberately avoided except for the initial and final interviews so that a realistic assessment of the numbers of gaiters maintained in service could be obtained. In fact the table suggests that with normal usage the equipment should last at least six months on average, even at a conservative estimate. It has been found from discussions with foundries that existing types last only one to three months. Thus the trials showed that the extra durability of the new gaiter is enough to offset the increased costs per unit. 2.5 Conclusions
and recommendations
The work described above was used to prepare a draft proposal for revision of the gaiter section of British Standard 4676 (British Standards Institution, 1971). A report of the work along with a copy of the revised standard was put to the Health and Safety Commission’s Foundries Industry Advisory Committee who endorsed the views expressed and asked HSE to approach the British Standards Institution with a view to incorporating the revision in the standard. This revision is now in the committee stages involved in its publication and the final version is expected to appear towards the end of 1982. Briefly, its requirements are expected to be as follows: gaiters must comply with the dimensions shown in Table 6 which should be read in conjunction with Fig. 3. This is the main change from the 19’71 version of the standard but additionally a clause relating to the stiffness of the material used in the construction has been inserted together with a number of other detailed changes. The re-
69 TABLE
6
Main dimensions
of new gaiter
Height Width of top band Reduction in circumference between bottom and top of top band Circumference around top at maximum size Circumference around top at minimum size Length of flare Distance from centre of flare to outside edge of gaiter
350 mm 65 mm 30 t 415 k 315 i 160 i 142.5 +
minimum minimum 5 mm 10 mm 10 mm 10 mm 12.5 mm
-
Fig. 3. Dimensions
of new gaiter (right leg shown
from front).
quirement to test the complete assembly against molten metal has been retained but the method of assessing failure on this test has undergone minor amendment. Since completion of the work on gaiters it has become apparent that some types of boot when worn outside the trousers suffer from similar design deficiencies to those which prompted the work on gaiters. For this reason a section has been included in the new standard to cover this type of boot.
70
3 DEVELOPMENT
OF STANDARDS
FOR CLOTHS
3.1 Background Over the years a good deal of development has gone into the provision of clothing for protecting against molten iron and steel. Tests for materials have been developed in the United Kingdom by the British Steel Corporation and also in the United States of America (Mehta and Willerton, 1977; Barnes, 1979; Catton, 1980). According to these tests either woollen or flame-retardant cotton clothing may be used in situations where moderate splashes of molten iron or steel may occur (Peter, 1978; Barnes, 1979; Catton, 1980), which suggests that against these metals both materials give a similar performance. It has been assumed by many people that the use of a flame-retardant material automatically guarantees protection against molten metal Recent work on the passage of heat through such materials when they are splashed by molten metal shows this assumption to be unreliable, particularly for metals such as aluminium where adhesion to the fabric may occur (Mehta and Willerton, 1977). Such sticking causes excessive heat transfer through the material with the subsequent risk of burns to the wearer and in some cases may result in holing followed by penetration of the hot metal. The criteria for protection have therefore been extended to include the possibility of burns as well as the general requirement of flame resistance. For this reason the performance requirements for materials have had to be reassessed and new methods of test specifically against molten metal have had to be considered. 3.2 Requirements
of a test method
The main criterion to be satisfied by a test method is that it should realistically reproduce the hazard to which workmen are exposed. It is relatively easy to provide a high level of protection by increasing material thickness and hence weight but such a course increases both the physiological load on the wearer and the general discomfort. The latter assumes greater importance as the weight increases until a stage is reached where outright rejection of the equipment occurs. Precisely at what level this will take place is not easy to predict since it depends on the subjective judgement of the wearer. The British Steel Corporation have made an extensive study of this question in relation to the provision of woollen clothing for their furnace workers and have suggested an optimum level of protection (Catton, 1980). Intuitively it seems likely that in making a decision to wear protective clothing an individual balances his own experience of risk against his general well being. If this hypothesis is correct, it implies that the acceptable level of protection is related to the risk and will be lowest in low-risk areas. The test procedure must therefore simulate the whole range of hazards in order to have available a range of protection to suit all needs. Thus, the aim has been to select and develop a method
71
which can be used with as many as possible of the common metals, can reliably assess the protection afforded by different material types and weights and needs relatively small quantities of metal for each test. The latter requirement is made because tests are best conducted in controlled conditions whereas large-scale test methods, such as that described by Mehta and Willerton (1977), require access to an industrial furnace. There is no conflict between the severity of the test and this criterion because the total weight of metal used is not the only important parameter to be considered. It is the weight of metal per unit area which determines the severity so that a small weight over a small area can be just as severe as a large weight over a large area. Experience has shown that there is little likelihood of clothing being required to protect against metal in contact with it for more than a second or two, unless metal adheres to the material. The only occasions when longer contact periods might occur are when men are lying on the floor or when metal lodges in the clothing. It would be unrealistic to expect clothing to protect against such severe hazards as men lying in pools of molten metal. Even with very heavy protection only extremely short exposures could be tolerated. In any case experience suggests that this type of situation occurs extremely rarely if at all. The problems caused by metal lodging in the clothing can be prevented by proper design to protect pockets and eliminate folds. Areas such as shoulders where the clothing surface is nearly horizontal can be of increased thickness if necessary. These arguments suggest that a realistic test should be conducted with the material at a steep angle to the horizontal so as to assess the ability to deflect metal and simulate the nearly vertical clothing surface occurring in wear. 3.3 Test methods A number of workers have proposed test methods meeting the above criteria. Broadly these methods all measure the passage of heat through the material immediately after contact with the metal. Differences arise in the means of detecting the heat flow. Most methods have used a traditional heat sensor, usually a thermocouple attached to a suitable calorimeter, placed underneath the material so that when molten metal comes into contact with the material’s upper surface the rise in temperature underneath is measured (Wren, Scott and Bates, 1977; Jaynes, 1980; Cornu et al., 1981). With this method the design of the calorimeter is critical. Calculations show that even with a simple device such as a thermocouple attached to the back of a single copper sheet, the heat flow varies considerably with the thickness of the copper. Thus such calorimeters need to be accurately specified and constructed. If the calorimeter is not quite flush with the surface on which the test material is supported there may be an air gap between the specimen and the calorimeter. Such a gap will cause appreciable resistance to heat flow and hence to the temperatures recorded. The above factors concern the physical design and construction of the equipment but there are other considerations.
For example, over a period a build-up of charred material tends to take place on the measuring device which is difficult to remove effectively and causes a reduction in the temperatures obtained. A further major problem is that the precise point of impact of the molten metal has to be well defined in advance. This is not easy to achieve with some metals due to the difficulties of ensuring a smooth flow over the lip of the pouring crucible. Variations in the direction of the molten metal stream can occur due to distortion of the crucible by heat, contamination from previous tests and to the existence of a skin of semisolidified material on the metal surface. The latter is particularly acute with aluminium. The other approach to the assessment of heat flow is to use a plastic skin underneath the material so that melting or damage to this skin signifies the transfer of a known quantity of heat (Mehta and Willerton, 1977; Benisek et al., 1979; Benisek and Edmondson, 1981). In the United Kingdom this latter technique has been developed into a draft British Standard following trials within four laboratories. A brief history of this method is useful in understanding its development. In 1977, Mehta and Willerton devised a test using 3 kg of molten iron which was incident on an inclined fabric over a plastic skin. This quantity of metal over the area of the test material was regarded by the foundry and steel industries as being a reasonable representation of the hazard to which workmen might occasionally be exposed. Due to the subjective nature of assessing different degrees of damage to the plastic skin and the quantities of metal needed, a laboratory-scale method was developed (Benisek et al., 1979). In this method the same material was exposed to different weights of metal, and therefore different contact times, until a well defined degree of damage to the plastic skin occurred. The result was then recorded as the weight of metal the material would withstand. Because industry had accepted the larger-scale method it was necessary to confirm that materials ranked in a similar manner in both procedures. The proposed British Standard is based on further development of this method. Small quantities of molten metal are poured onto samples of material inclined at an angle to the horizontal. The angle of inclination used depends on the type of metal poured, a smaller angle giving a more severe test. Sixty degrees has been found suitable for iron, steel and copper but forty-five degrees is more appropriate for aluminium. Assessment is made by examining the surface of a PVC skin simulant placed behind and in contact with the material. Any damage to the PVC skin is noted. Successive samples are tested using different weights of molten metal until the quantity is found which only just damages the PVC skin. The results are recorded as the “molten-metal splash index”. The pouring crucible may be rotated by hand or by using an electric motor driven from an electronic circuit. In both cases the aim is to rotate the crucible at a constant rate through an angle of 110” from the vertical in 2.8 to 3.2 seconds.
13
The test is designed to give an end point that is comparable in terms of heat transfer with the quantity of heat required to produce first degree burns (Mehta and Willerton, 1977). The accepted value for this quantity is 9.41 X lo4 J m-* (2.25 cal cm-“). In fact it can be shown that damage to the PVC skin occurs at a heat flow of 2 to 4 X lo4 J m-* (0.5 to 1 cal cm-*) and this provides a margin of safety (Thompson, 1980). The main disadvantage of this method is that several tests have to be conducted on the same material to find the true value of the molten-metal splash index. With a little practice however this is not difficult to achieve and the method offers considerable advantages to offset this inconvenience. It uses simple apparatus which can be constructed and put into use very quickly. It is suitable for laboratory use since only small quantities of molten metal are required. It has been proved to work for a range of metals. The pour does not need to be accurately directed since the heat sensor underlies the whole of the test specimen. This is an important consideration because it has been found by video recording a series of tests that the point of contact of the molten metal stream on the specimen wanders about considerably during a single test. Variations between tests can be reduced by using a new pouring edge of the crucible for each test. However, even after all precautions have been taken it is not possible to guarantee the exact impact point on the specimen. Where traditional heat sensors are used this has been found to lead to large variations in recorded temperature rises. The PVC skin simulant is not so sensitive to such positional changes. Another advantage is that the heat sensor material is changed for each test so that the cleaning of debris from previous tests is unnecessary. Build-up of debris causing a loss of heat transfer to the sensing device has been found to be a problem with the thermocouple devices. It has been concluded that the considerable advantages offered by this method far outweigh its disadvantages, which explains its choice for standards purposes. Comparisons between the method and that proposed by Cornu et al. (1981) are being coordinated through the International Standards Organisation and involve the cooperation of six countries. o Crown copyright
1982
ACKNOWLEDGEMENTS
The author would like to thank all the many people who have contributed to this work within HSE, and the protective clothing and foundry industries. He would especially like to thank his colleague Mr. H. Thompson who has made an invaluable contribution
74 REFERENCES Barnes, T.E., 1979. Heat and flame-resistant clothing. National Safety News, September 1979, pp. 54-55. Benisek, L., Edmondson, G.K. and Phillips, W., 1979. Protective clothing evaluation of zirpro wool and other fabrics. Text. Res. J., 49: 212-221. Benisek, L. and Edmondson, G.K., 1981. Protective clothing fabrics, Part 1: against molten metal hazards. Text. Res. J., 51: 182-190. Bharadwaj, H., Verma, S.S., Zachariah, T., Bhatia, M.R., Kishnani, S. and Malhotra, M.S., 1977. Estimation of body density and lean body weight from body measurements at high altitude. Europ. J. Appl. Physiol., 36: 141-150. British Standards Institution, 1971. Specification for gaiters and footwear for protection against burns and impact risks in foundries. British Standard 4676: 1971, British Standards Institution, London, England. Catton, J.A., 1980. Personal protection in a high hazard activity. Protection, September 1980, pp. 18-21. Cornu, J.-C., Aubertin, G. and Rapp, R., 1981. Vetements de protection contre les projections de metaux en fusion. Cahiers de Notes Documentaires, No. 102, Institut National de Recherche et Securite, Paris, France. Gooderson, C.Y. and Beebee, M., 1976a. Anthropometry of 500 infantrymen 1973-1974. Army Personnel Research Est., Farnborough, England, Report No. 17/76. Gooderson, C.Y. and Beebee, M., 1976b. A comparison of the anthropometry of 100 guardsmen with that of 500 infantrymen, 500 RAC servicemen and 2000 RAF aircrew. Army Personnel Research Est., Farnborough, England, Report No. 37/76. Jaynes, P.S., 1980. Testing fabrics with molten steel. Professional Safety, October 1980, pp. 15-20. Ince, N.E., Redrup, S. and Piper, J., 1973. Anthropometry of 500 Royal Armoured Corps servicemen 197 2. Army Personnel Research Est ., Farnborough, England, Report No. 36173. Lacey, P.D.A. and Hole, L.G., 1975. Protective footwear for the foundry industry. Shoe and Allied Trades Research Ass., Kettering, England, Report No. M81 FI. Mehta, P.N. and Willerton, K., 1977. Evaluation of clothing materials for protection against molten metal. Text. Inst. and Ind., 15: 334-337. Peter, H., 1978. Criteria for assessing heat-resistant protective clothing. Protection, July 1979, pp. 6-9. Statutory Instruments, 1971. Foundries (Protective Footwear and Gaiters) Regulations. Statutory Instruments 476, London, HMSO, 1971. Thompson, H., 1980. Personal communication. Wren, J.E., Scott, W.D. and Bates, C.E., 1977. Thermal and mechanical properties of aluminized fabrics for use in ferrous metal handling operations. Am. Ind. Hyg. - ASSOC. J., 38: 603-612.