Is machinery safety for the materials handling industry covered by equipment manufacturers?

Background to the article

Solids handling processes involve machines integrated into assemblies or installations known as production lines. The integration of machines into assemblies increases the complexity over individual machines which in isolation would be classified as dangerous machines. The complexity of different products needing to be in different locations within the facility adds to the operational challenges.

The challenges of maintaining the product flow to hit production targets and accessibility to the machine components introduce many hazards to personnel and the facility.

Each year there are incidents which support the basis of this article, with the UK Health and Safety Executive either prosecuting or issuing enforcement notices against operating companies where employees are harmed by solids handling machinery.

A common finding of the prosecutions is that the machines have insufficient protection to restrict personnel from accessing moving parts which can (and do) cause harm to personnel. In late 2020 / early 2021 there were two prosecutions concluded, within the UK courts, which totalled almost £900k in fines and costs.

This article will explore the considerations which should be taken by designers and operators of solids handling equipment to ensure the safety of the production lines to prevent hazards to people from machinery.

What are the legal requirements?

The legal requirements vary by country but within the European Union and United Kingdom there is the requirement to ensure that the machines are safe to operate and maintain. The Machinery Directive (2006/42/EC) is the main EU directive that must be complied with to introduce a machine into the market. This requires that a risk assessment is conducted on the machine and that any risks which could cause harm are controlled or mitigated to ensure that the risk level is tolerable.

The requirement for assessment covers both the individual machines and production lines of integrated machines (or partially built machines).

The UK regulations which apply include:

• Provision of Work Equipment Regulations (PUWER) 1989
• Management of Health and Safety at Work Regulations 1999
• Health and Safety at Work etc. Act 1974

Where the equipment handles combustible dusts, there is also the requirement for the manufacturer to comply with The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 2016 and for operators to comply with the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR).

But only machine manufacturers need to think about this, don’t they?

This is partially correct.

The consideration of the Machinery Directive is the responsibility of the machine manufacturer (or importer into the EU / UK). The manufacturer needs to consider the risks of the machine and should work with the intended customer / application to assist in the identification of the hazards to be protected against.

So can an operator miss the manufacturer out and just build something themself? This removes the directives issue, doesn’t it?

INCORRECT – the company now becomes the manufacturer and has all the responsibilities to ensure that the directives are met. The only exception to this is if the person building the machine is the only person who is going to use the machine – almost impossible!

The use of the machine is the responsibility of the operating company, and this is where PUWER (and equivalent regulations in other countries / territories) applies.

The operating company needs to ensure that each machine and production line has been assessed against the regulations for safe operation. This includes Regulation 7, the Identification of Specific Risks and Regulation 12, Protection Against Specified Hazards.        

Specifically in Regulation 12 (3):-

‘(d) the unintended or premature discharge of any article or any gas, dust, liquid, vapour or other substance which, in each case, is produced, used or stored in the work equipment;’

Cited within the considerations to be reviewed is the discharge of materials being handled alongside parts of the machine potentially being ejected.  

• The following sections will review the considerations for two types of recognised risks. Hazardous Area considerations (especially dust generation);
• Safety Interlocks protecting against access to machinery moving parts.

However, these are not the only risks that people should consider with their machinery system.

Hazardous Area Considerations

The nature of solids material handling means that there is a significant risk from dust generation within the machinery and production lines. The machinery considerations are also included with the ATEX directives.

The majority of dusts can form explosive atmospheres in the right conditions (i.e. correct particle size, moisture content, degree of dispersion, etc.). As a rule of thumb, any dusts with an organic basis (food, plastics, wood etc.) should be considered flammable until you can prove they are not. Some finely divided metals are also very explosive, aluminium particularly.

The formation of dust clouds both internal and external to any bulk handling equipment should be considered and risk assessed.

Inside equipment, the lofting of dust clouds may be an inevitable part of the process, especially

• Filling silos
• Discharging of bucket elevators
• Filter regeneration.

Externally, a significant dust cloud is harder to form as the concentration of dust needed to exceed the minimum explosive concentration (MEC) is typically above 30 g/m³. In practice, this is an optically dense cloud that you would struggle to see through (for comparison, occupational exposure limits are generally 10 mg/m³). Such a cloud is easily formed inside equipment, but the review should ask the question:

How credible is such a cloud external to the equipment casing?

However, the greatest danger in many bulk materials handling plants is the hazard of secondary explosion, as occurred in the Bosley Mill (Cheshire, UK) explosion in 2015. The cause is not fully concluded but believed to be wood dust combustion and explosion within the mill building. This primary event escalated due to the lofting of accumulated dust which ignited, resulting in a catastrophic explosion that killed 4 people.

These control considerations to prevent secondary explosions include good housekeeping standards Requiring operators to:-

• proactively preventing leaks,
  • clean up spilled dust quickly and
  • carry out regular scheduled deep cleans of plant to remove dust accumulations.

ATEX / Hazardous Area certified equipment is unlikely to help mitigate the likelihood of a secondary explosion. This is due to the primary event likely to be the ignition source, managing the risk of a potential secondary explosion must be addressed adequately.

A further consideration that should be assessed is the breakdown of large particles to smaller, combustible dusts. Typically, dust is only an explosion risk if the particle size is less than 0.5 mm.

However, if you are handling granules or larger particles, this material can break down and result in fines which could be highly combustible. If this is the case, your main product stream may not require certified equipment, but any dust extract systems, bucket elevators or other processes where the material is broken down or dust can accumulate must be addressed.

A good example of this is in the waste wood industry. The main product flows are large pieces of wood which are fed to furnaces to raise steam for power generation. These large pieces do not present an explosion risk however, the milling and transport of these pieces of wood, especially when dry, results in large amounts of dust which must be managed and mitigated effectively.

If an assessment shows that there is the potential for the facility to have a combustible or flammable dust, it is vital that sufficient safeguards to either prevent or mitigate the effects of an explosion are incorporated into the design of any bulk handling system.

This should be done as soon as possible in the design process as the retrospective fitment of either certified equipment (ATEX or UKCA) or protective devices can prove to be very expensive. The identification of the areas in which safeguards are required starts with the completion of a hazardous area classification study, once the results are known then appropriate safeguards can be implemented within the design.

Safety Interlock Considerations

The purpose of safety interlocks on machinery are to prevent harm to personnel. The process should be reviewed within the Machinery Risk Assessment (MRA). The international standard, ISO 12100 in conjunction with the technical report ISO/TR 14121-2) provides a recognised methodologies for assessing the risk levels.

The process to be followed is shown in the flow chart below :

The safeguards, to prevent access to moving parts, that could be considered include:

• Enclosed machine casing.
• Fixed guarding.
• Moveable guarding (interlock gates etc).
• Area monitoring (light curtains, area scanners etc).

Any safeguards which include the use of safety functions to maintain the integrity of the machine, should be reviewed to establish their required integrity. There are two standards which can be used to determine this integrity, either in protection level requirement (PLr) or Safety Integrity Level (SIL). These systems can be either:

• Electrical Systems.
• Hydraulic Systems.
• Pneumatic Systems.

The standards (and others related to specific machine types) provide guidance for the implementation of the safety interlocks.

The two standards present different methods, but the aim for the different methods is to implement high integrity safety interlocks. The choice of which standard to follow is at the discretion of the company assessing the machine. Generally more complex electrical functions have followed IEC 62061 rather than ISO 13849-1, however both are accepted standards by the regulatory authorities.

ISO 13849-1:2015 (figure A.1) uses a graphical method for the determination of the PL requirement, shown in Figure 1.

Where :-

• S1/ S2 = Severity of harm factors
• F1 / F2 = Frequency of exposure to the harm
• P1 / P2 = Possibility of avoiding the harm
• PLr = Performance Level Requirement (levels a to e)

IEC 62061:2021 (Table A.6) utilises a Risk Matrix methodology to determine the SIL, shown in Figure 2.:

Figure 2 – Risk Matrix methodology from IEC 62061:2021

Where :-

• Fr = Frequency of exposure to harm
• Pr = Probability of harm occurring
• Av = Likelihood of avoidance of harm

Both methods will provide the required levels that ought to be achieved. PLr from c to e and any SIL rating conclude that the safety system should be hardwired and use trusted or quantified components for use in the implementation of the safety interlocks.

Once identified the safety interlocks should be:

• Designed
• Installed
• Commissioned and Validated
• Maintained and tested

These considerations ensure that the safety interlocks provide the required protection levels throughout the life cycle of the equipment.

Where to start?

The two considerations presented (hazardous areas and safety interlocks) within this article are two examples of hazards that the regulations ask machine operators to consider. The assessments should be living documents and altered when the machine is operated, maintained and modified to ensure that learning is reflected in the assessments as experience grows.

The assessments shouldn’t be locked in a draw (or in an archived folder in the system) and forgotten about. The following questions can assist in reviewing the health of your systems.

• Do you know whether the systems you have in place are correct for the hazards that you have now?
• Have you changed the materials being processed within the handling system that may give different hazards?
• Do you have a Machinery Risk Assessment , PUWER and if applicable, a DSEAR Assessment in place?
• If you do are they valid for the current processing conditions? If not, why not?
• Do your staff know what the assessments say and are they trained to follow any safety procedures identified?
• Are you maintaining any equipment installed within your hazardous areas to ensure any ignition source protections are effective and the certification maintained?
• Are the components of your safety interlocks in good condition?
• When did you last test that the interlocks functions performed correctly? Are the tests being conducted at the company determined intervals?

Conclusions

The aim of this article is to identify some areas of potential harm that should be considered by operators of material bulk handling systems and to suggest methods to identify and manage these risks. These requirements apply to any machines, however in some instances the risks of bulk solids handling equipment are not always recognised. Other sectors / machines are seen as much more hazardous (robots, presses, cutting machines etc).

The importance of such requirements has led to the development of standards to assist in the assessment of risk, identification of safeguards, design considerations and methodologies to support on-going maintenance of the protections.

The operating company should ensure that they can demonstrate (and have the evidence and documentation in place) to prove that the risks have been considered and prevention measures implemented successfully.

The aim of these requirements is to protect employees from if something goes wrong. These protections include (not exhaustive):

– Prevention of explosions
– Equipment damage and downtime
– Harm to personnel from interaction, impact or entanglement with moving machinery

Regulators will look at the cited directives and laws when conducting an intervention or prosecuting machine owners, sometimes when an incident has occurred. Employers must ensure that these reviews are in place to prevent any incident occurring. Make sure that your company has adequately addressed the requirements.