Ketton Cement operates a 2.7MW crusher with speed control provided by a three-section resistor, with each section dissipating 87kW of heat. When this resistor failed one Friday, Cressall was on hand to help get the works moving again.
Ketton Cement in Rutland, near Stamford, is part of the Hanson Group. Hanson is a UK supplier of heavy building materials to the construction industry. It is part of the Heidelberg Cement Group, a global business in aggregates, cement and ready-mixed concrete.
The company operates cement kilns at Ketton in Rutland, Ribblesdale in Lancashire and Padeswood in North Wales, and produces ground granulated blast-furnace slag (GGBS), a cement replacement in ready-mixed and precast concrete at four locations. It also produces a range of bagged cements and aggregates.
Martin Nicholls, sales director of Cressall Resistors says, “Ketton Cement got in touch to say that the three-section speed control resistor on the main 2.7MW crusher motor had failed and asked what we could do. The resistor operates at 2.2kV, conducting a 735A current through a resistance 0.16 ohm to dissipate 87kW of heat energy.”
Ketton Cement has an annual output of more than one million tonnes of cement. Originally established on this site in 1928, it is next to the quarries from which cement's raw ingredient of limestone is still taken. This is a 24 hour a day, seven day a week operation with its huge inclined rotating kiln the major feature.
Ketton Cement supplies cement primarily to locations in central and southern England. The Second Severn Crossing and the Jubilee Line tube extension are among its more high profile successes. It also supplied over 40,000 tonnes of cement for the Thurrock Viaduct section of the Channel Tunnel Rail Link, most of it from the Ketton works, and 250 tonnes of Ketton cement went into the Medway Bridge M2 motorway-widening project in Kent.
Cement is made by crushing and heating limestone or chalk with small amounts of other natural materials, such as clay or shale, in a rotating kiln to a temperature of 1,450 degrees Celsius. This chemically combines the stones into a hard substance called clinker, essentially changing calcium carbonate (CaCO3) to calcium oxide (CaO) which then reacts with silica (SiO2) to form calcium silicates. This is ground to a powder with about five per cent gypsum, added to control the setting time of the end-product.
Before being fed into the kiln the limestone is broken up into small stones by a giant rock crusher with a capacity of 1.6 million tonnes a year. The 258kW three-phase control resistors for the crusher's 2,700kW main motor were supplied in 1985 and finally reached the end of their working life almost thirty years later.
Martin Nicholls continues: “The resistor was originally supplied in 1985 by Cutler Hammer. Luckily we had all the drawings and even more luckily, some of the right resistor material. We ordered the insulating boards from our supplier, which were delivered on Monday along with the broken resistor, which we stripped, rebuilt and returned to Ketton Cement the next day.”
Cressall Resistors owns the intellectual rights for almost all power resistors designed and manufactured in the UK. This enables the company to undertake virtually any maintenance or refurbishment project requiring resistor technology.
New resistors may be required when motors are being repaired or rewound. Cressall has the knowledge, experience and range of resistor technologies to produce cost-effective designs for new or replacement motor control applications, particularly for customers with legacy systems for which the original equipment supplier no longer exists or has ceased manufacture.
Cressall has the details of almost every resistor manufactured by other British suppliers such as GEC, Eaton Cutler-Hammer and TPR since 1950, so it is invariably possible to produce replacements for older resistors when needed.
Danny Osborne, maintenance engineer at Ketton Cement, adds: “We were anxious not to lose production because of the failure. Over the weekend Cressall manufactured 12 replacement Hi-Temp resistor banks to the original drawings and specifications. We stripped out the old resistors and sent them to Cressall's Dereham factory.”
Cressall's supplier Presspahn, located in Bedford, also worked over the weekend, making the insulation boards which fit around the resistor banks. These were delivered to Cressall on Monday and by Tuesday afternoon the brand-new replacements were assembled and had been collected and taken back to Ketton for fitting. The crusher was operational before Hanson's buffer stock of crushed limestone had been used up.
Danny Osborne concludes: “I have worked with Cressall for many years. It has always delivered on time and if we have any issues, they are always resolved very quickly. If there are any design modifications or improvements we want to make, Cressall always acts very quickly in those instances as well. Our works can be harsh environments and the problem of corrosion often arises.”