Sep 2020

Rolling Mills

High pressure hydraulic, gear and bearing lubrication and rolling fluids are used to operate rolling mills that process primary metals. Mill rolls are driven by large several thousand horsepower motors. Rolling mills are computer controlled with complex automated drive systems. Overall, rolling mill equipment is expensive and can take a long time to repair or replace, even when slight damage occurs.

Types of mills include blooming, bar, rod, plate and strip mills. The primary focus of this article is strip or sheet mills because they present the highest loss exposure of any of the other rolling methods and require specialized protection. There are two kinds of strip mills, hot and cold. Hot mills are used to reduce the thickness of the slab from the caster. Slabs are reheated in furnaces then pass through a series of roughing stands and finishing stands. Hot mills can be several thousand feet long and are critical as they usually feed 100% of downstream processes. Cold mills take the coil from the hot mill for further reduction in thickness. Cold rolling can be accomplished using single or tandem mill configurations.

Hydraulic fluids are either combustible or an approved less flammable type (even less flammable fluids can pose a hazard under the right condition) and used for automatic gauge control, roller bearing thrust, roll change etc. Gear/roll bearing oil is always petroleum based. The gear/roll bearing oil is recirculated from large central reservoirs. Rolling fluids are applied to the sheet for dissipating frictional heat and lubrication as it passes through the rolls of the stands. Water is almost always used as the rolling fluid with hot mill processes; however, certain alloys may require an emulsified solution. Cold steel/copper rolling uses primarily a water emulsion fluid, although plain oil can also be used. Aluminum rolling uses a kerosene type fluid. Cold aluminum rolling has the highest fire loss frequency of all cold metal rolling due to the use of the kerosene-based fluid and the tendency of the strip to break easily because aluminum is soft. Attention should be paid to the tension applied between rolls on aluminum mills – too much tension will cause strip break, resulting in sparks and subsequent ignition of the combustible rolling fluid.

Mills will usually have cellars beneath them housing the oil systems. Provide liquid tight containment for oil reservoirs. Volumes are thousands of gallons and operate at high pressure. Oil leaks will produce an atomized spray due to the high pressure creating a torch-like fire. This type of fire presents little burning surface for cooling or wetting by automatic sprinkler protection and has a very high rate of heat release. Once ignited, a petroleum-based fluid will continue to burn until the oil discharge is stopped or the oil is consumed. Heat detection actuated automatic interlocks should be provided, these are devices used to shut down the hydraulic, rolling fluid and lubrication oil pumps, and stop the flow of hydraulic oil from the accumulators. Remote manual emergency shut switches should also be provided. The only way to extinguish an oil spray fire is to shut off the supply. Occasionally, there will be situations where shutting down the hydraulic and lube oil systems may cause a significantly larger loss and/or exposure. In these instances, the mill should be programmed for a sequential shut down. Tubing is often preferable to pipe because it can be bent to fit in restricted spaces, with a minimum number of fittings, reducing the number of possible leak points, use steel reinforced hose where flexible connections are required. Welded joints in large piping or in connections to hydraulic cylinders or manifolds should be used in lieu of threaded pipe and flange connections.

Cold rolling mills under normal operations produce copious quantities of combustible oil residues that can lead to fast fire spread. Mills using an emulsion as the rolling fluid (usually 5-10% of oil or animal fat in water) will also leave combustible residue because water evaporates due to the frictional heating during the rolling process. Structural building members are especially vulnerable as these are often difficult clean due to heights and proximity over the rolling mill. Cold mills should be provided with noncombustible hoods and exhaust ventilation to control and collect the oil residue that is generated.

The mill stands, housings, ceiling, ventilation systems and all other areas including gear boxes and drive side of the mill should be protected with automatic sprinklers systems. Interface the fire protection system alarm system with the mill computer controls so the mill cannot start unless the automatic sprinklers and associated heat detection interlocks are in service. Mill drive motors are usually located in a room adjacent to the stands. Drive motors can be either AC or DC. Replacement time can reach more than 24 months for very large motors. Spare motors should be provided and rotated on a regular basis to avoid polarization and mechanical stresses. The spares should be electrically tested to assure their operability and usability as a viable spare unit.

Mill computer rooms and control pulpits housing rectifiers, variable frequency drives, programmable logic controllers and other computer equipment are very critical areas and should be provided with automatic fire protection systems. There is usually no possible way of manually operating rolling mill equipment. It should be noted that depending on business interruption potential, individual computer cabinets be provided with in-cabinet fire suppression in addition to room protection.

Overall superior administration of management programs is critical to prevent large losses. Programs for business continuity, preventive maintenance, housekeeping, hot work,
lockout/tag out and oil leak emergency response plan are particularly important.

Risk Logic can recommend and help develop preventive maintenance and property loss control programs at your facility. Please contact us to schedule a property survey at your facility by one of our primary metal specialists.

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