Optimising mill safety and reliability

The Finnish-headquartered minerals and metals processing solutions company Outotec has combined high-efficiency hydraulic technology with advanced ultrasonic bolt load testing to optimise the speed and safety of bolting operations in the construction of SAG (semi-autogenous grinding) and ball mills.

Locally in Australia, Outotec’s use of Enerpac hydraulic torque wrenches and pumps, in combination with Boltstress ultrasonic technology, means each bolt in a machine can be precisely fastened in just a couple of minutes to the exact required tension, saving time and ensuring safety across the hundreds of bolts required on a construction project.

Peter Nilsson, Global Installation Manager - Grinding at Outotec in Australia, said that the technique was employed when assembling the critical load-bearing frames. The bolts varied in size from 30 to 60 mm diameter, with up to 1000 bolts being tightened to the correct tension in each mill.

Outotec’s SAG and ball mills - grinding mills that tumble large ore rocks, sometimes in association with steel balls - are part of the metal extraction process for minerals that include gold, copper, platinum, lead, silver, zinc, alumina and nickel.

“Installations can be undertaken for a wide variety of safety-conscious clients anywhere in the world,” said Nilsson. “Outotec can either construct and install the mills for clients or, alternatively, supply the technology and training, and work in partnership with clients to complete the installation.”

Outotec had employed Boltstress bolt testing for about two years before being able to further enhance the precision bolting task by teaming it with the Enerpac ZU4208TE-Q pump and W-Series torque wrenches and sockets.

W-Series hexagonal wrenches have precisely repeatable torque from 2712 to 20,337 Nm. Coupled with the high-speed pump, they are used to ensure correct power, precision and repeatability in loosening and tightening nuts. The torque wrench pumps incorporate firmware that cuts out an entire step in the bolting process by enabling users to directly display torque (in either foot pounds or Newton metres) instead of having to convert them from conventional pressure output readings. The pumps supplied to Outotec were engineered for optimum compatibility with Boltstress’s ultrasonic bolt tension measurement for safety and reliability.

Nilsson adds: “The advantage of the technique is that our engineers can preset all the parameters - the information that needs to go to the tools. The actual bolting job can then be done safely by a trained operator who doesn’t have to worry about setting up the correct data properties.”

The bolt-loading tool employs ultrasonic measurement of bolt stretch to ensure the correct pre-load is applied, safeguarding joint integrity and preventing bolted joint failure. It is designed to eliminate potential safety hazards and guesswork. Ultrasonic bolt tension measurement assesses true joint performance by using real (not assumed) data.

The Technical Director of Boltstress Ultrasonics, Trevor Robinson, says measuring bolt tension is preferable to relying on tool torque alone, as joint separation has been known to occur, with dire consequences: “We set the required bolt extension on the Boltstress tool, and then send a signal to the Enerpac torque wrench pump. When the bolt tension reaches the programmed level, the signal stops the pump.

“Although we can preset torque on the torque wrench, we can’t use it to preset the bolt tension. That’s the key - we need to know the tension.

“Typically, a mill under construction might have hundreds of bolts that require tensioning in this way. The tensioning and tightening process using the wrench and the Boltstress tool takes just a couple of minutes for each.

“Boltstress sometimes carries out the bolt securing operations itself and at other times supplies the equipment to contractors, training them in its safe use.

“Our partnership with Enerpac using these tools together is fairly new. It’s recent technology that wasn’t available before then. The combination is working well for Enerpac and our contractor customers.”

The wrenches are supplied with a wide range of accessories for safety in particular applications, including twin 4:1 ratio safety hoses.