Idler roller nip hazards on heavy duty belt conveyors
This circular draws attention to the potentially high risks posed by accessible nip points at conventional conveyor idlers on heavy-duty belt conveyors.
Background information and current industry standards
1. The mining industry is already aware of the potential dangers arising from 'nip hazards' on belt conveyor systems and over the last twenty years a policy has evolved of nip point guarding the main system rollers on belt conveyors. These include the drive rollers, delivery rollers, return rollers, loop rollers, and deflection rollers. The process has been a progressive one, usually in response to an accident resulting from ineffective peripheral guarding. The success of adopting nip point guarding at these vulnerable points on the system is demonstrated by the fact that the numbers of nip point accidents have reduced where they have been provided.
2. In addition to the above mentioned main system rollers, nip points also exist at every support idler roller. The risk of serious injury posed by these has however been regarded as quite low and accident experience has supported this view. This perceived low risk and the practical difficulty of effectively guarding every idler roller nip point on a conveyor system, has led to an industry wide toleration of unguarded idler roller nip points, except for certain places where idler roller guarding has traditionally been provided.
3. These exceptions are usually found where the nip force and risk of serious injury is higher than that normally encountered eg where there is increased pressure between idler rollers and the belt, or where there is an increase likelihood of persons making inadvertent contact with the nip point. Hence it has become common practice to guard idler rollers at:
- anti-clines and changes of gradient, where the idlers are more heavily loaded than normal.
- anti-spillage skirts at loading points, where the conveyor belt may be held down against the idlers by the skirt plates.
- under-passes, materials storage areas and maintenance points under conveyors, where the probability of persons making contact with a rotating bottom idler maybe high.
4. At such locations, peripheral guards, or nip point guards, have normally been provided to address the increased risk. The decision as to which type to use has depended on site-specific practicalities and overall effectiveness.
What has changed?
5. Over the past five to ten years, the size of conveyor systems used in the deep coal mining industry has increased significantly. Whereas previously, the average drive power of a conveyor ranged from 60kW (80hp) to 180kW (240hp), it is now commonly 450kW (600hp) to 750kW (1000hp), Other changes include:
- The conveyor belt has increased in size and weight, typically from 0.9m wide type 6 belt, to 1.2m wide type 8 belt and larger.
- The belt weight and mineral load per unit length has increased, while idler roller spacing has remained the same.
- Idler rollers have increased in width, diameter and weight.
- Belt pretensions and the gravitational effects on inclines have increased.
6. These changes have led to an increase in the nip point force on heavy-duty conveyor top ad bottom idler rollers and the greater width of idler rollers, particularly bottom idler rollers, has increased the general accessibility of nip points.
7. The net result is that there is now a significant increase in the risk of injury posed by heavy-duty conveyor idler rollers. This coupled with the industry's reliance on out-dated prescriptive requirements for guarding, is leading to situations where high risk nip points are not being appreciated, identified and adequately addressed.
8. Several accidents have occurred on heavy duty conveyor belt idler rollers, indicating the need to examine:
Under-passes forming part of travelling routes for persons;
- Places where access is needed for maintenance purposes, particularly in front of drives, or behind loops in the conveyor belt;
- Underneath raised conveyors, where materials and tools, or other equipment are often stored;
- Underneath raised conveyors where dinting, or cleaning operations need to be performed;
- In-line bottom belt main-riding boarding platforms, where persons using the platform may make inadvertent contact with a top belt idler roller above them;
- Conveyor bridges between the bottom and top strands of a conveyor, where persons may make contact with at top belt idler roller;
- Fines conveyors, under the bottom strand of a heavy-duty conveyor, where persons carrying out maintenance may make contact a bottom belt idler roller.
The need for a risk based approach
9. A number of factors need to be considered when performing a hazard assessment and determining the control measures needed to address the increased risks on heavy-duty conveyors. As it is not possible to prescribe the exact control measure for every situation, a risk-based approach should be adopted. The two main factors to consider are:
Degree of hazard (likely severity of injury)
This will be determined largely by the pressure between the belt and the idler roller. For example, if the stationary conveyor belt cannot be lifted off the idler by a person using one hand, it is likely that the control measures described in Paragraphs 10 – 12 below will need to be applied.
Likelihood of access to the NIP point
This will be determined by the height of the nip in relation to the activities that could be performed at that location, also, the separating distance between the nip point and the likely position of employees that might make contact with it.
10. The control measures applied to prevent injury to persons will need to be appropriate for the risk and follow the hierarchical approach described in Regulation 4 of The Management of Health and Safety at Work Regulations 1999 and Regulation 11 of The Provision and Use of Work Equipment Regulations 1998. These can be summarized as:
- Where possible, the nip hazard should be eliminated at source.
- Where elimination is not possible, employees should be prevented from gaining access, by effective guarding, (which might be a temporary guard fitted only for the duration of the task in hand), or where this is not practicable, by clear instruction, training, supervision and education to enhance hazard awareness and knowledge of safe working procedures.
11. Where it is necessary for employees to work close to unguarded nips, the work should be controlled by a permit to work system in compliance with the requirements of Regulation 6 and Schedule 1 Part 1 of The Mines Miscellaneous Health and Safety Provisions Regulations 1995. This should ensure that the control measures necessary for a safe system of work are in place.
12. The permit to work system should require appropriate measures to be taken to ensure that the conveyor is stationary and cannot restart or move, by any means, while employees are at risk. This will usually require the conveyor's driving power to be isolated and securely locked off.
For further information, please contact HID SI1 (Mines Inspectorate), Edgar Allen House, 241 Glossop Road, Sheffield, S10 2GW. Telephone 0114 291 2390.
A simple approach to risk assessment of NIP points:
Note: where there is a likelihood of a peripheral guard being removed for cleaning or maintenance purposes, a nip guard should be provided, where a rotational hazard exists in addition to the nip hazard, a nip guard and peripheral guard should be provided.