Safety in Rolling Mills
Safety in Rolling Mills
Worldwide, as the rolling speeds are increasing, greater emphasis is being placed on the aspects of safety while designing the mill equipment as well as during the finalization of the mill layouts. Providing high importance to safety is in the best interest of the designers, manufacturers and the users of the rolling mills. Safeguarding of the mill equipment is necessary for ensuring the safe working of the rolling mill after its commissioning.
Manufacturers of the rolling mills have the objective to produce a competitive mill, while users desire to have a highly productive mill. However, before any of these objectives can be met, both the manufacturer and the user are to first determine how to engineer the mill using safe design principles to minimize operator’s risks. Investing in a safer workplace also reduces the expenses of treating injured workers, helps preventing workplace accidents besides boosting employees’ morale by conveying the message that the organization cares about its employees and wants to protect their health and safety. A brief overview of safety requirements for the equipment of rolling mill is given below.
Common safety related definitions
Safety is the ability of the equipment to perform its function while being transported, installed, adjusted, operated, maintained, dismantled, and disposed of under conditions of intended use specified in the instruction manual without causing injury or damage to health of the people carrying out these functions.
Risk is a comprehensive estimate of the probability and the degree of the possible injury or damage to the health in a hazardous situation in order to select appropriate safety measures.
Hazard is a condition or set of circumstances which can cause physical harm to the exposed personnel.
Danger zone is any zone within or around the equipment in which an exposed person is subject to risk to his health or safety.
Operator is a person given the task of installing, operating, adjusting, maintaining, cleaning, repairing, or transporting of the equipment.
Pinch point is an area, excluding the point of operation, which poses a hazard by exposure to moving parts of the equipment, its related equipments, or the material being rolled.
Point of operation is the location in the equipment where the material or a work piece is positioned and work is performed.
Device is a piece of equipment or a mechanism designed to serve a special purpose or perform a special function.
Safeguarding is a method of protecting personnel from hazards arising from the use of guards, safety devices, or safe work procedures.
Guard is a barrier which prevents entry into the point of operation or other hazardous area (zone). Adjustable barrier guard is a guard with provisions for adjustments which accommodates various jobs or tooling setups.
Awareness barrier is an attachment in which physical contact warns personnel of an approaching or present hazard. Awareness device is a piece of equipment or signal which, by means of audible sound or visible light, warns of a present or approaching hazard.
Presence sensing device is a device which creates a sensing field, area, or plane to detect the presence of a part of an operator’s body.
Safety design procedure
The fundamental approach to control hazards (Fig 1) in given order is (i) determination of the limits of the equipment or system, (ii) identification of the hazards, (iii) risk estimation, (iv) to reduce the risk by design, (v) to control the hazard by guarding, (vi) to incorporate warnings, (vii) to use personal protective equipment (PPE), and (viii) to make use of safety training.
Fig 1 Fundamental approach to control hazard
Determination of the limits of the equipment or system
For the elimination of the hazards or reducing risks, both the user and manufacturer are to first determine the intended use of the equipment. This includes defining and discussing different operating modes of the equipment, phases of use, and different intervention procedures for the operator. Speed, production, sizes to be handled and produced, level of automation, and auxiliary equipment are also to be considered. In order to understand total space requirements for the safe operation of the rolling mill, one is to include all of the auxiliary supporting equipments. These equipments include pick and place mechanisms or destacker to feed blanks, coil cars, turnstiles, or cranes to support the handling process. Also included are air compressors to supply compressed air to pneumatic mechanisms, hydraulic power units to support hydraulic function, robots, and automatic roll change over equipment etc. Auxiliary equipment is an extension of the main line. Hazards are introduced if working space around such equipment is insufficient. Hence, to ensure safety, it is very important for the manufacturer and user of the equipment to take into account the space requirements for installation and range of motion of the equipments and their auxiliary supporting equipment. The designer is also required to predict the working life of the critical components and to evaluate the hazards of those components under different modes of operation.
Identification of the hazards/risk estimation
The next step in the process of a risk reduction program is the hazard analysis which is a process which identifies tasks associated with each stage of the equipment’s function and the hazards associated with these tasks. The best source for identifying hazards associated with the equipment is to consult with those who are familiar with the operation of the equipment, for example, the operator, and safety and maintenance personnel. Mechanical hazards associated with the rolling mills occur in three basic areas. The first area is the point of operation. This is where work is performed on the material such as uncoiling, slitting, cutting, rolling, bending, cooling, stacking, or straightening etc. The second area is the power transmission system. This includes all components of the mechanical system, which transmit motion, to the part of the equipment which carries out the work. These components can include pulleys, belts, connecting rods, chains, sprockets, or gears etc. The third area is all other moving parts. This includes all other parts of the equipment, which move while it is in operation. These parts include reciprocating, rotating, or traverse moving parts, as well as feed mechanisms, conveyors, scrap choppers, and other auxiliary equipment. Rotary motion is one of the most common motions found in rolling mill equipment. Even a slow rotating roll can grab a glove or grip clothing, forcing a finger or arm into the danger zone. Rotating rollers and rolls, rotating pulleys, sprockets, and shafts are potential hazard points which need attention. Nip points are present at belts and pulleys, chains and sprockets, rack and pinions, and between multiple rotating rolls or gears. Electrical hazards, thermal hazards, and hazards generated by noise and vibration which are non-mechanical are to be included in hazard analysis. Common hazards associated with the rolling operations include handling blanks or coils with sharp edges or burrs, handling heavy rolls for set up, clearing jam ups, usage of the jog mode for threading coils, and testing rolls etc.
Risk estimation is a process whereby one makes a subjective decision (estimate of risk value) regarding the severity, probability, and frequency of an injury occurring in an unsafe condition. This process need to be supported by qualitative and if possible quantitative methods. Qualitative or quantitative judgments are based on the knowledge and experience of the decision maker in designing, operating, and understanding the history (accidents and problems) of the equipment. A quantitative method makes analyzing and selecting different safeguarding methods simpler.
Risk analysis methods
Many methods are available for the systematic analysis of the hazards usually present in the rolling mills. There are two basic types of risk analysis methods. One is called the ‘deductive analysis method’ (top down) and the other is the ‘inductive analysis method’ (bottom up). In the deductive analysis method the final event is assumed and the events, which can cause this final event, are then identified. In the inductive analysis method, the failure of a component is assumed. The subsequent analysis identifies the events, which can cause this failure. Some of these methods are given below.
- Preliminary hazard analysis (PHA) – It is an inductive analysis method whose objective is to identify a hazard that can cause harm which in turn can lead to an accident.
- Hazard and operability study (HAZOP) is a systematic technique to detect probable deviation, from normal operation which can lead to hazardous events. The ‘what-if’ method reviews each process and ‘what-if’ questions are formulated and answered to evaluate failure of the components.
- Failure mode and effect analysis (FMEA) is an inductive analysis method where the main purpose is to evaluate the frequency and consequences of component failure.
- The DEFI method uses a computerized system, where faults are recorded to determine rate of failure. MOSAR method is a method for the systematic analysis of risks. It is a complete approach to evaluate failure of the system.
- Fault tree analysis (FTA) or event tree analysis (ETA) can be either a qualitative or a quantitative model of all the undesirable outcomes, which can result from a specific initiating event. In this analysis, probabilities are assigned to each event and then used to calculate the probability of occurrence of the undesired event.
- DELPHI-technique is a forecasting method. It is also used for generating of ideas.
Risk evaluation is a process in which one asks simple questions for each hazardous condition and then answers those questions which need to be evaluated. Typical asked questions are (i) if there is an accident, how severe can it be (severity of potential injury), (ii) how often and how long the operator can be exposed to the unsafe condition (frequency and the duration of exposure), and (iii) what is the probability of occurrence and possibility of avoiding the accident in the future (probability of injury occurrence). Risk value is a cumulative estimate of individual answers to these questions. Normally these parameters may be combined to give a gradation or risk on a weighted scale, for an example, 1 to 10 (1, 4, 7 or 10) for severity of potential injury, 1 to 8 (1, 5, or 8) for exposure frequency, and a scale of 1 to 12 (1, 4, 8, or 12) for probability of occurrence of injury. The weighted scale and values assigned can vary and are usually based on the decision maker’s judgment. In this example, a cumulative risk value of 30 is assigned for the worst case scenario.
Guarding by means of a fixed guard is considered one of the main ways of protecting personnel from point of operation hazards in rolling mills. These guards need to be designed to protect the operator’s fingers or hands from reaching through, over, under, or around the guard to the point of operation. Any access through the guard is required to conform to the safety standards. Initially, without the guard, if the total risk level is at level 18, (medium risk hazard), then the same hazard, with redesign and a fixed guard added can have a total risk level of 3 (minor risk hazard). It is important to determine the potential hazards and hazardous situations associated with modes of operation or tasks performed by the equipment of the rolling mill and assigned total-risk values. Each hazard or task is then to be evaluated with the objective of either eliminating the hazard or reducing the risk to a tolerable level by first attempting to eliminate it by design and if this is not possible then reduce the risk in a satisfactory way by the use of safeguarding methods.
Risk reduction by design
Risk reduction by design means the use of safe design principles, utilizing automation and devising work procedures for minimizing personnel exposure to the identified hazards. The guidelines indicating the principles for estimating, evaluating and reducing the risks associated with equipment are described in short below.
All the equipments are to be designed considering every aspect of the day-to-day operations. Hazard analysis guides the designer to apply sound design principles during the design cycle to minimize the operator exposure to hazard. Some examples are (i) to remove any sharp edges, sharp angles, or rough surfaces of the parts of the machinery, (ii) to make use of a larger safety factor during design, (iii) to reduce speed, noise, and vibration and to avoid extreme temperatures, (iv) to follow standards and codes of the industry, and (v) to select the right material and the process for the heat-treatment.
It is also necessary to apply the ergonomic principles. The ergonomic guidelines for the design, installation, and use of equipment are (i) to avoid stressful postures and movements during use of the equipment, (ii) to increase safety by reducing stress and physical effort of the operator, (iii) to provide proper lighting, (iii) to design, locate, and identify manual controls so that they are clearly visible, identifiable, and can be safely operated, and (iv) to design and locate indicators, dials, and visual displays for enhancing safety and for the easier operation of the equipment. The manufacturer of the equipment is required to take into account the constraints to which the operator is subjected because of the necessary or foreseeable use of personal protection equipment.
There is necessity to apply sound safety principles when designing control systems. Inadequate attention to the design of equipment control systems can lead to unforeseen and potentially unsafe behaviour of the equipment. It is essential that the control systems are designed and constructed in a way so that they are safe and reliable. Control reliability is a design philosophy to assure that a given circuit is performing as expected despite the failure of any single component in the system. Controls for setup and maintenance modes are to be designed using ergonomic principles and applicable electrical standards to ensure operator safety. The equipment is not to be capable of starting up unexpectedly and is to have one or more emergency stop devices. The failure of the power supply or control circuit is not to lead to a dangerous situation.
Automation when introduced improves the equipment safety. Several devices are usually used to automate the operations. These devices limit the risk by reducing hazards at the operating points. The need for access to danger zones can be minimized by locating maintenance, lubrication, and adjustment points outside of these zones.
In case of processing lines involving coils, as an example, a jog mode is provided to thread the coil through processing equipment to test the equipment. Here, the operator is exposed to point of operation hazards. To reduce risk, the items needed to be implemented are (i) only the operator is to have control over the jog operation, (ii) the processing line is to be jogged at very slow and controlled speeds, (iii) the jog actuator is to be designed in such a way that it cannot be actuated accidentally, and (iv) unplanned maintenance, jam clearing, and minor tool changes can be done safely with proper training. For example, in case of flat mills, large coils usually require special handling while loading on to an uncoiler and while threading through the processing equipment. A coil car, coil car upender, hydraulic expansion, or overarm hold-downs are usually used to make this operation safe. Normally, the guides to feeders, presses, and rolls are set for a particular width of a coil. Camber, oil-canning, and improper slitting produce variable widths on a single coil causing a jam up at the entry guides, creating an unsafe situation. To avoid this, spring-loaded gauge bars, which adjust automatically to width variations, are usually used. Electronic sensors are also used to detect unacceptable width changes and double feed of the part, and to warn the operator of these problems.
Safeguarding devices are to be used to protect persons (production operator, personnel in charge of setting, training, or maintenance personnel) from the hazards, which cannot reasonably be avoided or sufficiently limited by design. The level of remaining risk determines the level of safeguarding. Normally two broad categories of safeguarding methods are available to protect the operator from hazards created at the point of operation of the equipment.
The first method is the use of barrier guards, which prevents access to the point of operation or other hazards but do not allow operators to feed parts into the point of operation manually and the second method is the use of protective devices, which keeps the operator or other personnel from being injured at the point of operation.
A fixed guard is normally kept in place permanently with fasteners, making removal or opening impossible without using tools. Fixed guards are usually (i) to be securely held in place, (ii) is to be opened only with tools, (iii) adjustments, lubrication, and maintenance points are to be located outside of the danger zones, and (iv) allow adjustments, maintenance, repairs, cleaning, and servicing operations to be performed while the rolling mill is not running.
Interlocked guards are to trip a mechanism which shuts off power when they are opened or removed so that the equipment cycle cannot be started until the guard is back in place. Movable guards can be opened without the use of tools. Some of the standards require that they also be interlocked. An interlocking guard with the guard locked is to be closed and locked to allow the equipment to operate.
An adjustable guard is one to which once adjustments are made, remains fixed during a particular operation. It can be modified according to the thickness of the material being rolled. It provides a barrier which can be adjusted to facilitate a variety of production operations. A control guard initiates the operation after it is closed.
Types of protective devices
There are several protective devices. Some are described below.
- An interlocking device, as used with a guard, can be mechanical or electrical. The purpose of this type of device is to prevent the operation of the mill section under certain conditions.
- An enabling device is a manually operated device, which, when continuously activated in one position only, allows unsafe conditions but does not initiate them. In any other position, unsafe functions are stopped safely.
- A hold-to-run control device is a manually actuated start and stop control device. It initiates and maintains operation of equipment elements only as long as the control is actuated in a set position. The control automatically returns to the stop position when released.
- A two-hand control device requires simultaneous actuation by the use of both the hands in order to initiate and maintain the operation of the equipment element in which the unsafe condition exists. With this type of device, the operator’s hands are required to be on the control buttons and at a safe distance from the danger area while the equipment completes its closing cycle.
- A trip device causes the equipment to stop when an operator or part of his body enters an unsafe limit. Safety trip controls provide a quick means for deactivating the equipment in an emergency. The trip device can be mechanically actuated. Examples are trip wires, pressure sensitive devices, or non-mechanically actuated devices such as a photoelectric device.
- A mechanical restraining device uses a mechanical stop to prevent unsafe motion. Pullback devices utilize a series of cables attached to the operator’s hands, wrists, or arms. This type of device is primarily used on equipment with stroking action. The restraint device utilizes cables or straps which are attached to the operator’s hands and a fixed point. Hand-feeding tools are necessary if the operation involves placing material into a danger area.
- A limited movement control device is a control system, the operation of which permits only a limited amount of travel of a part of the equipment on each occasion that the equipment is precluded until there is a subsequent and separate actuation of the control. This serves to minimize risk as much as possible.
- A deterring/impeding device is any physical obstacle which, without totally preventing access to a danger zone, reduces the probability of access to this zone by offering an obstruction to free access.
Pressure sensitive safety floor-coverings guard the floor area around equipment. Typically used with flexible manufacturing cells or robotics cells, it is intended to be used as an auxiliary safeguarding device. Pressure sensitive edges are attached to moving parts. On contact, a flexible sensitive edge is depressed sensing a stop signal to the controller.
Emergency stops are intended to reduce existing hazards to persons, damage to equipment or work in process. This is to be initiated by single human action when the normal stopping action is inadequate for this purpose. Examples include emergency stop buttons and grab-wire switches.
Cable pull switches are cables of braided or plastic-coated wire, horizontally installed across the points of hazards created by rotating machinery, and conveyor motion etc. These switches are located at the point of hazard for use by the operator involved. The cable is made slack by pushing or pulling. A stop signal then stops the guarded machine. The safety contacts are required to be remained open until the cable is returned to a safe state and the switch is manually reset, allowing operation of the equipment.
Sensing of the presence of operator is another method to ensure safety. There are three methods for doing it. In the first method, a photoelectric light curtain is used on equipment, which can be stopped before the operator can reach the danger area. This device uses a system of light sources and controls, which can interrupt the equipment’s operating cycle. In the second method, radio frequency (capacitance) presence-sending devices use a radio beam which is part of the equipment control circuit. When the capacitance field is broken, the equipment either stops or does not activate. The third method uses electromechanical sensing devices. These devices have a probe or contact bar, which descends to a predetermined distance after the operator initiates the equipment cycle. If there is an obstruction, preventing the bar from descending to its full-predetermined distance, the control circuit does not actuate the equipment cycle.
Selection of guards and protective devices
Exact choice of providing the safe guards for equipment is to be made after the assessment of the risk for the equipment. A fixed guard is simple and is to be used where access by an operator to the danger zone is not required during normal operation. As the need for access increases in frequency, the inconvenience resulting from removing and replacing a fixed guard increases and hence another method is required. In such situation, an interlocking guard, self-locking guard, trip device, or combination of these can be selected as a safeguarding method. When access to the danger zone is needed during the normal operation, an interlocking guard, trip device, adjustable guard, self-locking guard, two-hand control device, or control guard or combination of above is usually selected as a method of safeguarding.
Present trend is to make use of auxiliary safeguarding, in conjunction with a primary safeguarding device, when additional hazards are introduced and when multiple equipments are used in a system. For example, fixed guards, pressure sensitive floor-coverings, light curtains, trip switches, cable pull switches, or presence-sensing devices are normally used for this purpose.
Required characteristics of guards and protection devices
In designing safeguards, the type of guard or protective device and their method of construction are to be selected to take into account the equipment characteristics which can cause injuries to the operator. Guards and protective devices are required to be compatible with the working environments of the equipment, and are to be designed so that they cannot be easily defeated. They are to provide the minimum possible interference with the activities during operation and other phases of machine life. General requirements of guards and protection devices include (i) are to be of robust construction, (ii) not to give rise to any additional risk, (iii) to be located at an adequate distance from the danger zone, (iv) to cause minimum obstruction to the view of the production process, (v) to enable essential work to be carried out during installation or replacement of tooling and during maintenance, by restricting access only to the area where the work has to be done, if possible without the guard or protective device having to be dismantled, (vi) not to be easy to by-pass or render inoperable, (vii) to prevent contact and to eliminate the possibility of the operator or another worker placing parts of their bodies near hazardous moving parts, (viii) to create no new hazards or interference, and (ix) to allow safe lubrication.
Signals and warning devices
Wherever necessary (but not as a substitute for elimination by design or the use of safeguards), the designer usually adds warning signs and devices. Visual signals, such as flashing lights and audible signals (e.g., siren, horn, etc.) are to be used to warn of an impending hazardous event such as equipment start-up or over-speed. Markings, signs, and written warnings are to be readily understandable and unambiguous. These signs concerning the specific function(s) of the equipment, as they are related to readily understandable signs (pictograms), are to be used in preference to written warnings. Labels used are to be designed as per standards. Labels are to have (i) proper use of colour such as red to identify danger and stop, orange to identify warning signs, yellow to identify caution signs, green for the identification of safety, emergency outlet, and the location of first aid and safety equipment, and blue for the identification of safety information used on informational signs and bulletin boards, (ii) use of symbols and pictographs such danger, warning or caution, (iii) message on the label to identify the hazard, and (iv) to notify result of ignoring the warning and suggest how to avoid imminent hazards.
Personal protective equipment (PPE)
For the provision of suitable protection, the PPE selected are always to be best for the situation. It is important to note that PPE itself can be hazardous. A glove can be caught between rotating parts, safety goggles may hinder vision, or loose cloth or jewellery can catch on a part of equipment. That is why training on how to use a protective device safely is very important.
Training is important aspect of safety. Supplier as part of his supplies hands over technical manuals and drawings which provide the necessary information to the users regarding proper operation and maintenance of the equipment and its associated equipment. The user is to utilize the information, to ensure that the minimum amount of risk is maintained.
The user is required to conduct his own risk assessment based on the anticipated tasks and together with the recommendation from the supplier, is to develop a safe working procedure, and train the operators on the proper use of the equipment. Standard shut down procedures are to ensure protection of the operators. When equipment is to be shut down for maintenance, the source of energy (electrical, mechanical, pneumatic, and hydraulic) is to be first isolated and disconnected. Operators are to be trained to follow the standard procedures. In spite of incorporating all possible safety features in the equipment design, there can be some critical function when the operator gets exposed to an unsafe condition working next to the equipment. Hence, it is critical for the operator to undertake necessary training to limit the risk. As an example, lubricant spills around the line create a dangerous situation. Proper housekeeping guidelines help to reduce unsafe conditions. Implementation of a safety/training program for all personnel operating and maintaining the system is essential for safe operation.