Back to main paper OC 690/13
HSE 690/13 (Version 2)
2 This document has been revised and reissued because of recent incidents of over pressurisation resulting in explosions. It takes into account advances in monitoring methods and the findings of research work commissioned by the British Ceramic Confederation and carried out by SIRIUS (Structural Integrity Research Institute University of Sheffield). The Information Document provides guidance:
1) for employers and employees on the safe operation of sealed batch ball mills (BBMs) in the pottery and allied industries;
2) on the steps which may be needed to assess the risks from this equipment;
3) concerning the periodic routine inspection procedures;
4) on methods of monitoring the conditions within a BBM;
5) on modifications which may be necessary; and
6) on matters which should be taken into account when purchasing replacement mills.
4 This guidance is aimed primarily at users of BBMs grinding wet charges in mills with cast iron end plates. Over pressurisation due to the processing of potentially explosive materials or where solvents are used (neither of which takes place in the ceramics and allied industries) are outside the scope of this guidance. Dry grinding which often makes use of steel balls is also outside the scope.5 The flow diagram
6 BBMs are closed and sealed vessels with no permanent outlet to atmosphere during use. Normal grinding processes will usually result in a rise in temperature and hence in pressure. The whole approach to operating most BBMs is to avoid excessive rise in pressures and specifically a rise of 0.5 bar g or more. If pressures exceed this level, the equipment should be considered to be pressure equipment and should comply with the relevant requirements of PER and PSSR. This guidance is intended to help users to conclusively demonstrate that pressures greater than 0.5 bar g are not generated during normal operation.
8 Batch ball mills (BBMs) are widely used in the pottery and allied industries. BBMs range in size from approximately 0.3-3 metres in diameter. Older mills have cast-iron ends and a mild steel casing. They are lined with silex, porcelain or rubber and are used to mill materials such as minerals, frits, glazes and colours. The milling is usually done in an aqueous medium. The grinding medium in these mills may be flint or pebbles. Modern BBMs are usually fabricated entirely of steel.
9 Compressed air may be introduced deliberately into the BBM to discharge the contents or as part of the milling process.
10 Pressure will be produced inside all BBMs due to the rise in temperature which occurs during the processing. It can vary considerably depending on a range of factors including:
1) the type of material being milled;
2) the mill lining material;
3) the grinding media;
4) the running time;
5) the water content;
6) the amount of mill charge.
11 Different combinations of these factors can affect the working temperature and therefore inevitably the pressure in the BBM. Explosions due to over pressurisation of mills from this cause have occurred and can have catastrophic consequences (destruction of the equipment and extensive damage to buildings) which pose significant risks to anyone in the vicinity. The larger the mill, the more serious the likely outcome. The risk of personal injury is self-evident.
12Appendix 4 contains summaries of known recent incidents. The incidents fall into four general categories:
1) failure of mountings;
2) failure of stub-shafts and/or cast iron end casings due to fatigue;
3) explosion due to over pressurisation; and
4) ejection of the contents under pressure as an operator removes the discharge bung or hatch.
Of these, (3) is likely to be the most serious.
13 Factors which can affect the likely risk involved in this process include:
|i.the age of plant||the pedigree of the BBM may be unknown, along with previous service, level of maintenance, possibility of previous over pressurisation, modifications etc.|
|ii.length of grinding period||the longer the grinding period, the more energy involved and the higher the temperature|
|iii.errors in charging||one of the most frequently reported causes of over pressurisation is a failure to charge sufficient water|
|iv.errors in venting||if venting during the working cycle is part of the milling procedure, failure to do so will inevitably result in higher pressures|
|v.production of steam||there is a step change in risk if the temperature rises to such an extent that steam is produced|
|vii.extent of pre-existing cracks||these will inevitably result in weakening of the structure|
14 User should have a clear understanding of the proposed operating conditions and selection of the plant should take into account the safe operating limits for a new installation. For new installations, the best course of action would be to use BBMs constructed from steel which is stronger and less likely to suffer brittle fracture. Where BBMs with cast iron end plates are introduced, this fact must be taken into account when considering the limits to the conditions of operation.
15 All new mills should be fitted with means of monitoring the conditions within the mill and protection systems to ensure that the safe operating limits are not exceeded. These could include:
1) direct pressure measurement;
2) temperature measurement, with calibration to pressure;
3) over temperature and/or overpressure trips;
4) interlocked water metering devices (to guard against insufficient water being charged into the BBM);
16 It is important to note that one system of protection sometimes used - interlocked current overload monitoring - which was thought to detect a condition where insufficient water has been introduced, has been shown to be ineffective for this purpose.
17 New BBMs should be mounted on steel-framed supports, or a reinforced concrete structure. Manufacturers and suppliers should consider the means of support as part of the overall installation and supply civil engineering drawings for the construction of the supporting piers, together with information on dynamic and static loading. Users should seek the advice of a competent architect or civil engineer to decide a suitable form of mounting.
18 In addition a means of safely venting the BBM should be provided.
19 The following measures may help to ensure that unexpected pressure rise does not occur, or if it does, limit the consequences. They may not be relevant to all installations. Users should assess the application of these measures to their own equipment:
1) venting BBMs during the course of a milling cycle will provide an opportunity to:
a) check that the process is progressing satisfactorily;
b) clearly relieve the pressure; and
c) confirm that sufficient water was charged into the BBM at the start of the process.
2) users should ensure that procedures laid down are rigidly followed, paying particular attention to ensuring that venting is not forgotten due to inadequate supervision or training of personnel, problems with shift changeover, staff absences, etc. Where users do not routinely vent at mid cycle, particular attention is required to ensure correct charging takes place;
3) means should be provided for stopping mills after a predetermined time or a set number of revolutions. Such devices are a useful back-up to correct operating procedures and their use is strongly recommended;
4) careful attention should be given to the correct loading of mills, both with large and small grinding media, to ensure that the mill is run under ideal and balanced conditions;
5) users should keep records of desired run-off temperatures and grinding times. Any increase in temperature above the norm for a particular process provides an early warning of potential problems and should be investigated;
6) where processes are to be varied, or new materials milled, the preliminary operating criteria should be determined from experience of similar materials. Close supervision of initial batches should take place in order to establish correct and safe operating procedures;
7) the water level in the mill should be positively monitored either by manual means (ie dipping of the BBM) or automatically (ie via water meters providing a reading of the water actually pumped into the vessel);
8) some companies have fitted current overload detection systems in the belief that a lack of water would normally result in a higher than expected motor current. It has been shown that this method is unreliable;
9) existing mountings of older BBMs should be inspected periodically as it is possible that any cracks or faults occurring in the mountings could result in a mill falling during operation;
10) operators should be properly trained. The training should include the identification of unacceptable excursions from routine operating, safe venting procedures, the importance of over temperature or overpressure trips, etc;
11) BBMs should not be sited near to other potentially hazardous installations, eg storage of liquid oxygen, LPG or highly flammable liquids. So far as is reasonably practicable, such installations should be sited so that in the event of a BBM failure they would not be affected.
12) to conform to normal practice, equipment similar to BBMs should be fitted with two lines of defence - a control mechanism to avoid excessive pressurisation and a safety device which would operate in the event of a failure of the control mechanism. In the past, the operating conditions within the mills, coupled with the absence of suitable technology made this impossible. However, developments have recently taken place which make these objectives attainable. The operating conditions within test ball mills have been monitored on a real time basis, making use of either:
a) direct measurements of pressure; or
b) direct measurements of temperature (which is directly related to the pressure). This technique makes use of a thermocouple probe set in the lining 3 mm short of the internal surface, together with a battery powered transmitter on the outside of the cast iron end plate and receiver unit arranged to give a temperature display.
Other developments have concentrated on the issue of providing a safety device which would operate in the event of the temperature and hence pressure exceeding the desired conditions. A fusible plug, fitted into the charging cover (suitably protected to prevent uncontrolled ejection) is being actively pursued.
20 Following an explosion in a 7ft diameter Boulton type mill with 12 ribs SIRIUS (see Appendix 2) carried out an analysis of the failure and discovered that the materials of construction in the end plates were very low in strength. This means that the conclusions from the analysis can be applied more widely, since it was clear that other mill end plates could not realistically have poorer metallurgical properties. The analysis also indicated that BBMs with readily visible cracks in their end plates may still be safely run provided:
1) routine inspection and measuring of the cracks takes place;
2) the mill is run at pressures below 0.5 bar gauge; and
3) ongoing measurement of internal conditions confirms 2) above.
Before embarking on the inspection regime set out below, a thorough initial inspection is necessary together with repeated inspections every week for the first month to set out a clear baseline. Guidelines on the frequency of inspection after this initial stage are given below along with suggested actions.
|Number of cracked ribs||Frequency of inspection|
|No discernible cracking||Every 6 months||Continue routine inspection if no change detected|
|1||Every 3 months||Reduce frequency after 3 identical measurements|
|2||Every month||Reduce frequency after 3 identical measurements|
|3 and above||N/A||Remove from service and repair or replace|
There is a range of different designs of mills (eg different number of webs, different diameters, etc). Based on the available information, the above regime is felt to provide a realistic basis on which to build an inspection regime, provided competent engineers oversee the arrangements and make adjustments as necessary.
|Number of cracked ribs||Frequency of inspection|
|No discernible cracking||Every 6 months||Continue routine inspection if no change detected|
|1||Every 1 month||Reduce frequency after 3 identical measurements|
|2 and above||N/A||Remove from service and repair or replace|
21 The preliminary inspection techniques do not require special equipment, but employees required to carry out this task must clearly be competent. Guidelines on these matters are included below:
1) the end plates must be prepared for examination (eg wire brush);
2) a good standard of lighting will be required;
3) only persons competent to do so should carry out this work;
4) the results should be logged; and
5) a defect reporting system should be introduced and used. This system should include guidelines on what to look for and what steps to take if cracks are found. Clearly, a crucial part of these guidelines will be the action to take if changes in the crack size are identified (consultation with competent engineer, etc).
Further more detailed guidance can be obtained from professional bodies such as The British Institute of Non-Destructive Testing, is 1 Spencer Parade, Northampton NN1 5AA, e-mail firstname.lastname@example.org. They publish an authoritative guide The NDT Yearbook which is used extensively not only by those working in NDT and Condition Monitoring, but by users and potential users of equipment and services. This regular reference work contains sections on materials testing methods and capabilities.
22 The repair of cracked cast-iron end plates must only be undertaken by a competent engineer with an understanding of the repair of cast iron. If these repairs are to be considered, for each particular case a very careful assessment of the nature and extent of the damage and the suitability of the repair method must be made. In addition, the tests which will be required immediately after repair and any further tests and examinations which are necessary should also be specified.
23 Some of the matters which will need to be taken into account include:
1) costs of repair against the remaining useful life of the BBM;
2) the existence of appropriate repair techniques;
3) whether the examination procedures will need modification after the repair;
4) an estimation of the remaining safe life.
24 Some of the methods used for loosening old linings prior to removal could result in damage to the structure of the BBM. In particular:
1) striking the outside of the BBM on the cast-iron ends is likely to crack them;
2) hitting the lining on the inside of the BBM where it covers the end plate; or
3) using a pneumatic drill in the same circumstances could also result in damage to the end plate castings.
Methods that could result in damage to the end plate castings should not be used.
25 Relining operations could also produce more immediate hazards to the workers involved, in particular from dusty residues of toxic materials in BBMs, portable electrical equipment, etc. Appropriate precautions should be taken. Seek further advice on suitable precautions if necessary. Before relining takes place, a thorough internal inspection of the cast iron end pates should take place.
26 Certain precautions need to be taken where work has to be undertaken in a confined space in which dangerous fumes are liable to be present to such an extent as to involve risk of persons being overcome. In these circumstances the access hole used for entering the confined space should be not less than 460 mm long and 410 mm wide, or if circular, not less than 460 mm in diameter. Where appropriate, employers should ensure that the work is carried out in accordance with the Confined Spaces Regulations 1997. (oc288/7) .
28 In cases where an existing ball mill is sold on and where the operating parameters or fundamental use of the mill is changed then consideration should be given to the application of The Pressure Equipment Regulations 1999 which are concerned with the design, manufacture and supply of new pressure equipment. The PER will only apply to pre-owned equipment where the equipment has undergone extensive modification and/or repair, such that the BBM can now be considered to be new equipment. (HSE guide to the inspection of manufacture and supply - Chapter 4.9). This will need to be determined on a case by case basis.
(See also Commission Working Group 'Pressure' Guideline 1/3 reproduced below).
|Question:||Are replacements, repairs or modifications of pressure equipment in use covered by the directive ?|
|Answer:||1) Entire change: the complete replacement of an item of pressure equipment by a new one is covered by the PED.
2) Repairs are not covered by the PED but are covered by national regulations (if any).
3) Pressure equipment which has been subject to important modifications that change its original characteristics, purpose and/or type after it has been put into service has to be considered as a new product covered by the directive. This has to be assessed on a case by case basis.
|Reason:||The directive applies only to the first placing on the market and putting into service.|
29 This problem is mainly confined to the grinding of bone, when aluminium ear tags have been included in the bone. The small number of users involved in this trade now generally purchase from countries not thought to use ear tags, with the back-up of a visual examination of the material together with a metal detector fitted to the loading belt. Wet grinding of any material with steel balls and lining will result in hydrogen evolvement and must not take place.