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Mar 18, 2021

Reline your thinking (1/4): Appropriate and safe mill access

Jared Le Cras
Jared Le Cras
Director, Mill reline equipment, Metso Outotec
Mill reline machines provide an efficient and reliable mill relining operation. But with all the advantages that come with this equipment, there is still one focus area that requires consistent attention: Safety. Two of the main safety concerns with mill reline machines are safe mill access and risk of fall from height. In this blog we will investigate two potential solutions to these concerns: Fall arrest systems and access platforms.

Fall Arrest Systems

There are two distinct approaches to fall arrest systems:

  • Fall restraint – this approach prevents a worker from being exposed to a fall hazard.

  • Fall arrest – This system is used where a worker is exposed to a potential fall. A fall arrest system will prevent the worker from contacting the surface below the work platform/area.

metso-outotec-mill-reline-machines-safe-mill-access
Fall restraint (left, AS 1891.4 Figure 2.1(a)) and fall arrest (right, AS 1891.4 Figure 2.1 (d))

Fall restraint is the preferred option, however the narrow beam used for mill access via the reline machine makes it highly impractical to prevent personnel from reaching the exposed edge.

Alternatively, a fall arrest system can be implemented assuming adequate fall clearance can be achieved. Fall clearance is the clear space beneath a working area required for a fall arrest system to perform correctly. It is calculated by considering factors such as lifeline deflection, lanyard length, energy absorber extension, height of person to attachment and residual clearance. All these are covered in standards such as AS 1891.4 and ISO 16024.

Fall clearance definition – AS 1891.4 Figure 7.1(c)
Fall clearance definition – AS 1891.4 Figure 7.1(c)

When applying a fall arrest approach to safe mill access, the system would consist of a horizontal lifeline, safety harness, lanyard and personal energy (shock) absorber. Attempting to utilize a fall arrest system quickly reveals that such a system is not suitable for safe mill access due to the inadequate clear fall distances involved.

A potential fall arrest system implemented for safe mill access
A potential fall arrest system implemented for safe mill access

Even when using an impractically short lanyard, a best-case fall clearance height of 2.1m (AS 1891.4) or 2.8m (ISO 16024) below the main beam is required. This already eliminates most relining applications and certainly does not offer any protection for falls within the trunnion area. It gets even worse if a more standard 2m lanyard is used – then the best-case fall clearance height becomes more than 4m which makes the system completely inappropriate for all relining applications

Even if a novel system could be designed to circumvent this fundamental safety issue, there are still many other general difficulties in implementing and operating fall arrest systems. These systems require regular and often expensive maintenance, specialised storage, comprehensive training, harness wearing, hand tool tethering and exclusion zones around fall risk areas.

Access Platform Systems

Access platforms avoid the issues associated with fall arrest systems by providing a safe means of access without requiring supplementary equipment. However, there are three main design issues for this application that must be considered:

  • The system must fit through the mill opening

  • The system must comply with platform design standards

  • Erecting the system must not endanger personnel

The first issue forces the platform design to be collapsible or assembled from multiple parts. Fixed access platforms are only suitable for the very largest mill openings which makes them unsuitable for the vast majority of applications.

The second issue is perhaps the most critical – the platform design must meet relevant design standards. An access platform that does not meet design standards is more dangerous than having no platform at all. It provides a false sense of security and exposes personnel to even greater risks. The access platform must perform exactly as someone using the platform would expect. If it looks like a guardrail, it must function as a guardrail as per required specifications. This means no wire or chain barriers (expressly forbidden in ISO 11660-1) and any steelwork must fully comply with the structural, deflection and erection requirements set forth.

An access platform that does not meet design standards is more dangerous than having no platform at all.

Finally, since the access platform will require some form of erection, the third design issue revolves around protecting personnel and minimizing the risks involved during the erection process. These protection measures must take multiple hazards into consideration including fall risks when the platform is not fully erected and injury due to improper manual handling of components.

A recent engineering thesis concluded that an access platform design that completely mitigates fall risks is not possible without detrimentally impacting core MRM functions (Louw, 2020). This means that any access platform solution will be a compromise between improving safety and maintaining MRM functionality – it becomes an optimization problem. However, it is critical that any solution meets the three design requirements mentioned above.

An Appropriate Access Solution

Creating a platform system that improves mill access safety while meeting the three design criteria is very challenging. A collapsible system is essential to clear the small mill opening, but when designed to meet the required standards, the solution becomes heavy and difficult to erect. The space restrictions also impede any kind of powered erection system.

A solution is to split the system into two main parts: A fixed guarding system for the trunnion area and a collapsible charge access platform inside the mill.

The access path through the trunnion of the mill can be kept narrow since liners do not need to rotate in this area. By adding guardrails to the outer beam, the fall hazard inside the trunnion is completely mitigated and these rails simply extend with the beam during the usual reline machine setup procedure. Personnel are therefore not exposed to any hazards during erection. Removable grating sections can be reinstalled once the beam is extended to maintain the large flush working area outside the mill.

Charge access platform in mill entry configuration
Charge access platform in mill entry configuration

To avoid lifting the heavy charge access platform inside the mill, a pivoting design can be employed. The platform initially rests on top of the outer beam, but once the beam is extended, the operator can push the guardrail grab point to slide the platform into position and then lift and pin the lightweight guardrail in place. A ladder can be attached to the dedicated ladder attachment point and the access system is ready to use.

Charge access platform erected
Charge access platform erected

The platform provides ample space for personnel to stand while a large liner piece is rotating on the liner cart. It is therefore an excellent vantage point for the liner cart operator or the reline machine crane operator if using remote control for liner pick up.

The compromise in this system is having to leave the opposite side of the beam unguarded. The reline machine crane requires unrestricted access to one side of the liner cart and mill, so it is not possible to fully enclose the area. However, if personnel stay on the ladder side of the beam, then exposure to the unguarded side of the Mill reline machine is minimized.

References

AS/NZS 1891.4:2009 Industrial fall-arrest systems and devices. Part 4: Selection, use and maintenance.

ISO 16024:2005 Personal protective equipment for protection against falls from a height – Flexible horizontal lifeline systems

ISO 11660-1:2008 Cranes – Access, guards and restraints – Part 1: General

Louw, H. (2020). Mill Reline Machine Safe Access System. Curtin University

This blog is part of a series. Continue on to part 2, where we discuss the approaches to implementing automation into your mill reline.

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