Direct pressure blasting equipment
OC 634/7
This 2-part OC and its attached Information Document (ID) describe the design and operation of mobile abrasive blast cleaning equipment, used in the process known as direct pressure blasting (DPB). The OC discusses DPB pressure safety hazards/risks and the information document advises upon the application of the Pressure Systems and Transportable Gas Containers Regulations 1989 and the Provision and Use of Work Equipment Regulations 1992. The ID may be copied and given to interested persons outside HSE.
Introduction
1 Abrasive blast cleaning, known as direct pressure blasting (DPB) is a process where surfaces are cleaned by the action of particles of grit striking at high velocity. The grit, also referred to as abrasive, is fed into a container known as a blasting kettle which is then pressurised. Kettles may be pressurised automatically or manually, the kettle design is the same, but the pipework and valving arrangements differ (see Figures 1 and 2 of the ID). A flow of abrasive controlled by a metering valve feeds into a moving air stream, this is conveyed through a blast hose to a nozzle. The nozzle is directed toward the dirty surface and cleaning takes place. This OC provides information on the process and complements OC 634/8 which solely addresses health risks associated with this process arising from exposure to noise and dust.
2 The ID addresses:
1) legal requirements;
2) standards of equipment construction; and
3) work methods.
3 The equipment used for this process is described at paras 5-11. It is frequently obtained by contractors from a variety of suppliers. Invariably the compressor, blasting kettle, and hoses are owned by different suppliers, and are normally connected together by the contractor. This document presumes all systems are 'mobile' and advice contained in it should be interpreted accordingly.
4 This diversity of supply frequently leads to confusion amongst contractors as to their legal obligations. The ID explains the responsibilities of owners and contractors operating mobile pressure systems.
Description of equipment
5 Air compressor/receiver : Overpressurisation of the air receiver is prevented by the provision of a pressure relief valve. Compressed air is fed through a pressure regulator into a take-off manifold. At this point the receiver and the manifold are at a maximum pressure set by the regulator. The pressure relief valve should be capable of venting the flow of air in the system to safety.
6 Air delivery hose: The connection between compressor and kettle is via the air delivery hose. The air delivery hose connects to the take-off manifold of the compressor.
Frequently a non-return valve forms part of this attachment, isolating the 2 components from reverse flow.
7 Blasting kettle: The kettle is portable. The main body is fabricated in steel. It has incorporated into its structure a dished roof (semi-elliptical), with a central circular aperture, to allow filling with abrasive, and a conical base which channels abrasive toward the abrasive metering or discharge valve. The maximum safe working pressure of this equipment should be clearly marked on the vessel body.
8 Ancillary valves and pipe work are fitted externally to the kettle. They include (refer to valve numbers on Figures 1 and 2 of the ID):
1) remote valve and silencer; which allows air to enter and pressurise the kettle, and the release of pressurised air from the kettle to permit recharging (Fig 1 valve 3);
2) dump valve and silencer; which releases pressurised air from the kettle to atmosphere, thus permitting recharging (Fig 2 valve 3);
3) abrasive metering valve; through which abrasive passes into the air stream which carries it to the blasting nozzle (Figs 1 and 2 valve 4); and
4) rigid steel pipework which connects the valves and kettle.
9 Blasting nozzle: Nozzles are manufactured from a range of materials and are of several types of construction. Further details can be found in suppliers' literature. The main sub-types are straight bore and venturi. The blasting nozzle may also be fitted with a remote control handle also known as a dead man's handle (see para 11).
10 Blast hose: Also known as a hogger, it connects the kettle and the nozzle. This hose carries abrasive grit in compressed air. Where multiple lengths of blast hose are connected together, suitable blast hose couplings should be used. Where the hose couplings fit around the blast hose, a rubber gasket is fitted to protect the internal shoulder of the clamp. Clamps connect to each other by interlocking locators secured in a twisting action. No metallic internal part of the connectors should be in contact with the abrasive, due to rapid wear of the component when contacted by grit in air.
11 The remote control: (also known as the dead man's handle) comprises:
1) a manually operated remote control handle (see Fig 1) fitted to the blast nozzle;
2) a remote valve fitted to the kettle body (Fig 1 valve 3) which allows or prevents air from passing into the kettle; and
3) 2 lengths of narrow bore hose, one feeding air to the remote handle and the other returning air to operate the remote valve.
Outline of the principles of operation of dpb equipment
12 Figure 1 represents the configuration in which equipment is commonly found. It is likely that more than one kettle and nozzle will be run off a single compressor. Typically, blasting kettle systems operate at 7.5 bar. However, more recent developments have resulted in blasting kettles rated at up to 10 bar being introduced.
13 If the rated air consumption of the system is less than the capacity of the compressor, then the system will pressurise to the safe working pressure of the compressor. If the system demand exceeds the supply from the compressor the pressure will not be maintained (see paras 20-27).
14 A pressure regulator is normally positioned down stream from the air receiver and before the air take-off manifold. This regulator has a range of adjustment, ie for a 11.6 bar compressor the range will be approximately 8.5 bar-11.6 bar. Should further pressure reduction be required then the regulator should be changed. However, a 19.7 m per min compressor, which is a commonly used size, would normally be set at an operating pressure of 11.6 bar; the pressure regulating valve on the compressor would have to be reset to a level appropriate to the operating pressure of the blasting kettle. The readjustment should be specifically requested by the hirer. A pressure-indicating device would confirm the correct regulator setting. An alternative to the above is to include a pressure reducing valve in line between receiver and blasting kettle.
15 Abrasive is fed manually into the top of the blasting kettle which is then pressurised with air from the compressor. The opening through which the abrasive is loaded is closed from inside by means of a rubber cone. This component, known as a popper valve, lifts into position as compressed air is bled into the kettle, closing the filling aperture thus pressurising the kettle (see Fig 1 valve 1).
16 An additional problem exists with certain designs of popper valve. The conical head of the valve is made from a high density rubber which is compressible. The base of the cone is normally attached to a plate of a diameter matching that of the cone base. In operation the cone is lifted by the air pressure exerted to its base. The cone compresses down on to the base plate, as it is forced against the sides of the filling aperture, thus forming a seal.
17 However, there are cones in use which do not incorporate this base in their design. The lifting pressure is concentrated at the centre of the cone. The cone can deform when forced against the filling aperture, and can be ejected by extruding its way out. This should not occur at the recommended operating pressures of blasting kettles.
18 In operation, the pressure in the kettle above and below the abrasive is equal. The abrasive passes under gravity through an abrasive metering valve which is located at the base of the kettle. The metering valve (see Fig 1 valve 4) may be manually adjusted to give the prescribed flow of abrasive. The abrasive drops from the valve into a stream of moving air at working pressure and it is then carried through the blast hose to the nozzle.
19 Abrasive flow is controlled either:
1) automatically, by the remote control valve fitted to the blast nozzle; or
2) manually, by an operator, known as the kettleman, opening and shutting control valves on the kettle.
In the manual technique there is no remote valve or remote control and the kettle valve arrangements are as shown in Figure 2. This technique fails to danger regarding the system of work. (the operational technique is described at paras 32-37.)
Air requirements
20 The following are basic pointers in the calculation of the size of compressor required and compressed air requirements for DPB set up. They are included in this document to give a fuller understanding of air requirements. Systems can be readily and inadvertently over pressurised in normal use, this has led to popper valves being extruded through the filling aperture, and other instances of gross overpressurisation of systems in an attempt to improve cleaning rates.
Compressed air requirements
21 A blasting kettle, typically has a safe working pressure of 7.5 bar. Air flow through an open nozzle is a function of variables such as bore diameter and air pressure. A 6mm nozzle at 7 bar passes air at approximately 2.3m per minute. A 12mm internal diameter nozzle operating at 7 bar passes air at approximately 9.5m3 per minute an increase of over 400%.
22 The process is sensitive to the pressure at the nozzle outlet. Based on an operating pressure of approximately 7 bar measured at the nozzle, it is reported that for every 0.07 bar drop in pressure there is a 1.5% drop in cleaning efficiency. This leads to greater use of abrasive per unit area cleaned. Higher pressures produce a greater efficiency with the reduction in the amount of abrasive used. (Cleaning efficiency is a function of abrasive velocity and is proportional to the pressure and volume of air passed through the nozzle).
System losses (pressure drops) in the compressed air system
23 In a blasting system, 'pressure drop' is the difference between the pressure of air leaving the compressor and that measured at the nozzle. Factors contributing to pressure drop include the length of hosing, air leaks, diameter of hose etc. For example:
A compressor operating at 7.5 bar delivers air to a blasting kettle by a 20m length of hose with a diameter of 38mm. The blasting kettle is in turn connected to a nozzle by a blast hose of the same dimensions. The pressure at the kettle would be approximately 6.8 bar and pressure at the nozzle will be approximately 6.5 bar. The pressure drop between the compressor and the nozzle via the kettle is one bar which is equivalent to a loss of 21% blasting efficiency.
24 It is essential to transfer the high pressure and large volumes of air generated by the compressor to the nozzle with minimal pressure losses, so maintaining cleaning efficiency.
Compressor selection
25 Using the information at paras 12-24 the operator can match the nozzle size to the output of the compressor. For example:
Typically, contractors hire a 19.7m per min (700 cfm) compressor. A 12mm nozzle operating at 7 bar passes 9.5m per min (338 cfm). A set up operating 2 blasting kettles, each feeding a nozzle of 12mm in diameter will consume 19m per min (676 cfm). Additional air consumption will be required to the airline fed blasting helmets, allowing for a blasting helmet air consumption of an estimated 0.3m per min (11 cfm) x 2 helmets. This would give a total compressed air requirement of 19.6 m per min (ie 2 x nozzles @ 9.5 m per min each, plus breathing air).
26 Compressors are fitted with a facility known as range control. This allows the compressor to measure the through-put of air and respond to high or low demand by varying its engine revolutions, ie low output low rpm, high output high rpm.
27 It is the pressure that is relatively constant and the volume which is variable.
Method of operation
28 Blast equipment may be operated automatically or manually. The description which follows refers to the equipment illustrated in Figure 1.
Automatic method
29 The blast nozzle has a manually operated spring loaded remote control handle (dead man's handle) attached to it. When compressed air flows into the remote valve (valve 3) it is released through the dead man's handle to atmosphere. All the air flowing to the remote valve at this stage is released to atmosphere; under such conditions the blasting kettle is not pressurised.
30 When the remote handle is depressed closing the aperture the air is returned to the remote valve. This closes the valve and air is directed into the blasting kettle. The popper valve (valve 1) rises, the kettle pressurises, abrasive passes via gravity to the adjustable abrasive metering valve (valve 4) and enters the main air stream in a venturi effect, from where it is carried along the blast hose to the nozzle.
31 Frequently, in both the automatic or manual systems, abrasive blocks up the metering valve. Such obstructions are relieved by increasing the air pressure upstream of the metering valve and blasting away the blockage, using a choke valve (valve 2). Choke valves are illustrated in Figures 1 and 2. Whilst this valve is closed the total air flow passes through the metering valve.
Manual operation
32 The manual method does not utilise the remote valve referred to at para 29. In its simplest form, the manual method is a 2-person operation; one attending the blasting kettle (the kettleman), and the other directing the air nozzle (the nozzleman) (see Figure 2).
33 Initially, air inlet valve 1 is in a closed position, choke valve 2 is in open position. Blow-off valve 3 is in an open position.
34 Abrasive is charged to the kettle past the popper valve.
35 The nozzleman takes hold of the nozzle and directs it at the surface to be cleaned. The kettleman closes valve 3 and opens valve 1 (valve 2 remains open).
36 The popper valve (valve 5) lifts sealing the abrasive charging port, pressurising the kettle. During the course of the blasting operation, the compressed air, air hose, kettle and blast hose are at or near the output pressure of the compressor.
37 Upon emptying the hopper of grit, valve 1 is closed and the remaining air within the kettle is released via dump valve 3. Dump valve 3 should be fitted with a silencer which reduces the noise caused by the release of the compressed air.
Initial integrity of blasting kettle
38 The primary legislation which concerns the initial integrity of the blasting kettle is the Pressure Systems and Transportable Gas Containers Regulations 1989 reg.4. (Further information on integrity is given at the ID para 8). However, HSW Act S.6 may also be relevant. The Provision and Use of Work Equipment Regulations 1992 reg.5 imposes duties on the user to ensure that the kettle is 'suitable', whilst reg.12 requires precautions to be taken against the rupture or disintegration of the kettle.
39 Some suppliers of blasting kettles have affixed the CE mark to their kettles. As neither the Simple Pressure Vessel (Safety) Regulations 1991 (as amended) nor the Supply of Machinery (Safety) Regulations 1992 as amended by the Supply of Machinery (Safety) (Amendment) Regulations 1994 apply to blasting kettles, then they should not be CE marked. This is because there is currently no Article 100A Directive to which they can 'conform'.
40 Suitable standards for the construction of blasting kettles are given at the ID para 8.
Action by inspectors
41 Particular attention should be given to the arrangements to ensure that over- pressurisation of the system does not occur as part of the operational cycle, and to the following:
1) compressor and blasting kettle are matched to prevent overpressurisation of the kettle - paras 12-24.
2) clear indication of safe working pressure on kettle - para 7 and ID para 12.
3) written scheme of examination for the pressure vessels which form part of DPB set-up - ID para 13.
4) equipment, manufactured to recognised standards and well maintained - ID paras 8 and 27-29.
5) provision of pressure indicating gauge - ID para 12.
6) proprietary hose connectors should be - ID paras 21-22.
7) no risk of entrapment between popper valve and kettle body - ID para 16.
Enforcement recommendations
42 Enforcement action should be in line with the following.
1) A prohibition notice should be considered where equipment is not properly maintained.
2) An improvement notice (IN) should be considered where no written scheme of examination exists.
3) Where no pressure indicating device is fitted an IN may be appropriate (ID para 12).
4) Where new equipment does not have dead man's/remote handle fitted consider IN. A letter strongly advising that such a device is fitted should be sent to supplier as a minimum (for further information see OC para 29 and ID para 24).
5) SAPID forms should be sent to PI supplier and copied to Shipbuilding, Heavy Fabrication and Welding NIG, indicating the names of those suppliers and manufacturers of blasting kettles produced after April 1996 with no pressure relief system or pressure indicating device.
6) If a blasting kettle is CE marked then inspectors should ignore the effect of such marking and should consider compliance with health and safety legislation, working from first principles (see ID paras 8 and 12-22).
27 May 1997
(220/FOD/115/97)
Disc ref: J:/Editors/CA1/ ocfiles/634_7.sam
ASI headings
Abrasives: air receivers: blasting: cleaning: enforcement: kettles: pressure systems: Pressure Systems and Transportable Gas Containers Regulations 1989: pressure vessels: pressurised systems: shipbuilding and ship-repairing.
Health and safety executive
Information document
HSE 634/7
Direct pressure blasting equipment
1 This Information Document (ID), consisting of text plus Figures 1 and 2 for illustration purposes, contains internal guidance which has been made available to the public. This guidance is considered good practice but is not compulsory. You may find it useful in deciding what you need to do to comply with the law. However, the guidance may not be directly applicable in all circumstances and any queries should be directed to the appropriate enforcing authority.
2 This guidance addresses the physical risks and hazards associated with direct pressure blasting (DPB) equipment. Information Document 634/8 Parts 1 and 2 address health issues arising from exposure to noise and dust.
Background
3 The equipment used for DPB is frequently obtained by contractors from a variety of sources. Very often the compressor, kettle, hoses and blaster's helmet are owned by different suppliers. They are normally connected together by the contractor. This frequently leads to confusion amongst contractors as to their legal duties.
4 This ID explains the responsibilities of owners and contractors operating mobile pressure systems.
Summary of contractor responsibilities for a mobile system
5 Legal requirements: Current legislation places increasing emphasis on the assessment of workplace health and safety matters. Relevant health and safety legislation includes:
1) the Health and Safety at Work etc Act 1974 (HSW Act):
2) the Management of Health and Safety at Work Regulations 1992 (Management Regulations);
3) the Pressure Systems and Transportable Gas Containers Regulations 1989 (PS and TGC Regulations); and
4) the Provision and Use of Work Equipment 1992 (PUWER).
(The above list is not exhaustive).
6 The purpose of risk assessment is to determine what measures need to be taken to comply with health and safety law. The actual measures will derive from compliance with the legislation indicated at paragraph 5.
7 The compressor, blasting kettle, air delivery hose, blast hose and other ancillary equipment are connected together and so become part of a system. Where a contractor hires or leases the kettle, air receiver and hoses from different suppliers and assembles them on site, then the contractor creates a 'pressure system' as defined in the PS and TGC Regulations regulation 2. However, the PS and TGC Regulations, regulation 8 will not apply because the contractor is not the owner of this mobile equipment and the system is not 'installed'. The act of assembling the equipment by the contractor to undertake DPB does not create a new owner as defined in the PS and TGC Regulations.
Duties of the owners of air receivers and kettles
8 The air receivers and kettles should be properly designed and properly constructed (PS and TGC Regulations, regulation 4). The following standards are considered to be suitable standards of manufacturing for blasting kettles:
1) BS 5169: 1992 Specification for fusion welded steel air receivers;
2) ASME Viii Boiler and Pressure Vessel Code 1989; and
3) BS 5500: 1997 Specifications for unfired fusion welded pressure vessels.
9 Whilst it is not a legal requirement to use these standards, they do give an indication of good manufacturing practice. Manufacturers using alternative standards should be able to demonstrate how they ensure that the equipment is safe, so far as is reasonably practicable, when properly used at work.
10 Appropriate identification plates, containing the information listed in the PS and TGC Regulations schedule 4, should be fitted to receivers and kettles, unless they are less than 250 bar litres. Safe operating limits (SOLs) should be clearly identified and the SOLs should be either marked on the vessels or the information provided to users.
11 The PS and TGC Regulations, regulation 4 requires that the kettle and receiver be provided with protective devices as may be necessary for preventing danger. Difficulties have been encountered in fitting pressure relief devices to kettles (but not receivers), although suitable overpressure protective devices (eg bursting discs) should be fitted, if it is practicable to do so.
12 A pressure indicating device should be fitted to indicate the air pressure inside the kettle. It may be sited as follows:
1) automatic method - downstream of the remote valve (valve 3) and prior to the choke valve (some suppliers already do this) (as shown in Figure 1); or
2) manual method - downstream of valve 1 and prior to valve 2 (Figure 2).
13 The owner of the compressor which includes an air receiver or a blasting kettle with a capacity in excess of 250 bar litres, should ensure it is subject to a written scheme of examination and that it is examined in accordance with that scheme. The owner of an existing air receiver (>250 bar litres), should not allow the air receiver or blasting kettle to be operated unless the compressor and air receiver system has:
1) a suitable written scheme of examination (WSE); and
2) a current report of examination.
Duties of the users of air receivers and kettles
14 Contractors (users) should ensure that the equipment they use is suitable for the work to be undertaken as required by PUWER. Regulation 5(1) of PUWER, requires that every employer shall ensure that work equipment is so constructed or adapted as to be suitable for the purpose for which is used or provided. Regulation 5 is at the heart of PUWER. It addresses the safety of work equipment from 3 aspects:
1) the initial integrity;
2) the place where it will be used; and
3) the purpose for which it will be used.
15 In seeking to achieve compliance with PUWER regulation 5(1), contractors should establish that equipment owners have complied with the PS and TGC Regulations. In particular, contractors should obtain evidence from the owners that the reports of the WSEs are current before the DPB equipment is assembled and they should check that the hoses are of the correct pressure rating and that they are in good condition. If the suitability of the kettle design is in doubt, then further guidance should be sought from the supplier.
16 Contractors should ensure that operators are protected against specified hazards. Such hazards are specified in regulation 12(3) and include:
1) articles ejected from equipment;
2) rupture of blasting kettle or hose; and
3) unintended discharge of air and blasting grit.
A fixed guard or grid should be fitted over the hemispherical roof of the kettle to prevent access to the trapping point between the popper valve and the kettle body. Further information on the precautions which contractors should adopt is given at paragraphs 17-23.
Standards of equipment construction
17 The following is a summary of the basic requirements of the standards of manufacture essential to ensure the initial integrity of pressure blasting equipment. A fundamental requirement shared by all key assemblies of equipment, ie hoses and kettles is that they should be pressure rated.
Rubber hose for compressed air
18 The hose linking the compressor to the kettle should be manufactured to BS 5118: 1980 Specification for rubber hoses for compressed ai r, or an equivalent standard. In BS 5118, hoses are classed in 3 types which depend upon maximum design working pressure:
- Type 1: Light duty ........ 10 bar
- Type 2: Medium duty ... 16 bar
- Type 3: Heavy duty ...... 25 bar
Blast hose
19 A specification for blast hose is BS 5121: 1975 Specification for rubber sandblast hose. This specifies a hose design working pressure of 6.30 bar (90 psi). The typical modern blasting kettle operating pressure may exceed this level. Therefore, hoses in compliance with BS: 5121 would be of insufficient operating capacity for the rigours of industrial abrasive blasting. Higher rated hoses should be used, eg 10-12 bar. Contractors should ensure that blast hose is of a satisfactory standard.
Blasting kettle
20 The kettle should be properly designed and be properly constructed from suitable material, as required by PS and TGC Regulations, regulation 4and as specified in schedule 4. Suitable standards of manufacture to ensure the basic requirement of the blasting kettle are listed at paragraph 8.
Hose connectors
21 Connections should be of the twin lock type, or similar, with positive locking of the components and secure attachment to the respective hoses. There should be no internal metallic connector surfaces exposed to abrasive in compressed air.
22 Anti-whip devices which prevent the separation of hose couplings whilst the system is pressurised should be fitted. Only anti-whip devices supplied or approved by an equipment manufacturer should be used.
Blasting nozzle
23 There are no particular standards of manufacture, for what is a wide range of product. Nozzle selection is based on blasting pressure required and the volume of compressed air available.
General
24 When working by the manual method, effective means of communication is necessary between the blaster and kettleman to prevent inadvertent operation of the blast resulting in whiplash injuries by impact with the worker, or from the abrasive projected from the nozzle. Systems of work should be employed which prevent inadvertent pressurising of the blast hose, and which ensure pressure is switched off in the event of an emergency.
25 The lack of a 'dead man's valve' at the blasting nozzle results in a fail to danger system of work. The blaster is reliant on the secondary system of the communication arrangement to turn off a potentially lethal hose blasting out high volumes of high pressure air. If the secondary system fails, the operator is stranded. Operators should employ an automatic system incorporating a dead man's handle at the nozzle end of the blast hose, and a remote valve on the kettle body.
Training
26 In order to comply with PUWER regulation 9, all operators of DPB equipment should be trained in its use. The training should include operational methods and associated risks.
Maintenance
27 Both PUWER regulation 6 and the PS and TGC Regulations, regulation 12, require that equipment is maintained.
28 All pressure blast kettles, blast hoses, and associated fittings are subject to abrasive wear. The degree of wear varies with the rate of abrasive use and operator technique. It is essential that a proper maintenance programme is established and followed which includes both inspection and repair of equipment.
29 Equipment suppliers can advise on such programmes and usually suggest that the key stage of maintenance are:
- at setting up;
- after 4 hours of use;
- after 40 hours of use; and
- at monthly intervals.
Suppliers can also provide maintenance lists to assist in the establishment of a proper maintenance programme.
May 1997
