Lubricants used in the steel industry must rise to meet a wide range of challenges. An integrated steel plants functions span operations from iron making and casting to rolling mills and finishing lines. High speed machinery operations, extreme shock loads, wide temperature ranges, dirt and dust, water ingress and vibrations must all be managed to maintain efficient operation.
Proper selection of lubricating greases, therefore, is crucial for equipment reliability and smooth operation. Timely lubrication maintenance is also important to prevent equipment damage and downtime; however, greasing machinery operating at extremely high temperatures presents grave safety hazards.
As a result, theres growing demand in the steel industry for heavy-duty industrial lubricants that can increase machine efficiency, said Manjunath Somshekar, senior manager – lubrication, at global producer Tata Steel Ltd.
Due to extremely hazardous conditions in the steel industry, there is an increased demand to reduce man-machine interface, he told attendees at the annual meeting of the National Lubricating Grease Institutes India chapter in Amritsar in February.
Tata Steel, one of the leading steel manufacturers in India, came across such a challenge with the down coiler in its hot strip mill in Jamshedpur-a vital plant for the companys flat product segment.
A down coiler wraps the finished steel strip onto a high-temperature mandrel while eliminating telescoping coil sides and irregular wraps. The strip enters the down coiler from a pinch roll and is immediately formed around the mandrel using wrapper rolls. The lubrication system in the down coiler is divided into two parts: an outboard greasing system and a centralized system for the mandrel sliding track.
During its 2016-2017 fiscal year, Tatas Jamshedpur plant lost five hours of production time to roll change delays. Two of those hours were pinpointed to mechanical problems stemming from greasing of the outboard bearing, Somshekar reported.
The outboard bearing supports the mandrel with the help of a support arm. Its location is hazardous for manual maintenance because of operating temperatures over 130 degrees Celsius, as well as steam and hot water that accompany the coil as it is rolled.
The operators had been reapplying a lithium grease every three days to keep the equipment running smoothly. Outboard bearing greasing is done manually during a roll change or any downtime, resulting in unsafe activity and man-machine interface, Somshekar stated. However, a centralized greasing system is not feasible in the existing mechanism.
Drawing Out Maintenance Intervals
The Mumbai-based firm conducted a study with the goal of increasing lubrication efficiency and extending the greasing interval. The study analyzed performance properties of the incumbent lithium grease and compared them to the tribological requirements of the outboard bearing. Results showed that the system requires grease with very high oxidation stability and a very high dropping point.
Based on these requirements, the steelmaker created a grease made from a polyurea thickener to perform better at high temperatures, and a synthetic base oil for better oxidation stability. The new grease also had a high dropping point, which is necessary for high-temperature conditions. The company tested the properties of the new grease using ASTM International and Institute of Petroleum bench test methods, then compared the results with the existing lithium grease. Both greases were NLGI Grade 2 consistency. Somshekar noted that the new grease demonstrated superior performance.
The IP 239 test method for load carrying ability revealed that the weld load capacity of the polyurea-synthetic grease was 400 kilograms, but only 285 kg for the lithium grease. ASTM D566 (Standard Test Method for Dropping Point of Lubricating Grease) showed the new greases dropping point was above 240 C, as compared with 180 C for the lithium grease.
Results also demonstrated that water washout and work penetration properties were better in the polyurea-synthetic grease, Somshekar said.
Tata Steel prepared 1 kg of polyurea-synthetic grease, which filled 70 percent of the total free volume in the outboard bearing, for a field trial that began in July. The company initially ran the trial for seven days, then conducted a Fourier-transform infrared spectroscopy analysis to evaluate deterioration of the grease. The results were satisfactory, and no abnormality was found in the bearing temperature or operating sound, Somshekar said.
The maintenance cycle was increased to 15 days, and a used grease sample was again collected for FTIR analysis. Still, no depletion of performance properties was found. The re-lubrication cycle was then stretched to 21 days to match the monthly maintenance shutdown schedule of the plant, and the results were positive.
No abnormal rise in temperature was noticed during the entire duration of the trial, Somshekar said. He added that the new grease not only offered better oxidation stability and load bearing capacity, it improved water washout and flow properties.
Somshekar concluded that the new grease helped the company save 67 minutes of downtime and about 1.34 million rupees ($19,600) in lost production. Further, unsafe working conditions have been reduced without compromising equipment life or performance, providing a boost to employee morale.
Quenching Fire Risk
At the same conference, Eltepu Sayanna, chief technology officer at Haryana, India-based Siddharth Grease & Lubes Pvt. Ltd., highlighted the need for high-performance lubricating greases in steel mills to combat stringent operating conditions and fire risk.
Grease accounts for only 3 percent of global lubricant demand, estimated to be about 42 million metric tons in 2017, but around 70 to 80 percent of bearings in industry are lubricated with greases, Sayanna said. In the industrial grease market, the largest chunk-about 40 percent-goes to primary metals manufacturing, including steelmaking. General manufacturing uses about 15 percent of industrial greases, followed by mining (12 percent), off-highway transportation (8 percent), power generation (4 percent) and papermaking (2 percent). The final quarter goes to other applications, he added.
Sayanna noted that fire is another lubrication-related risk in steel plants due to grease leakage from the equipment, accidents or negligence. While grease itself doesnt burn, flammable gas from grease can ignite when mixed with air in the high temperatures of a steel mill. Some of the areas susceptible to these conditions are blast furnaces, stamp charging machines in coke oven plants, and ladle turret bearings in continuous casting machines, he noted.
In addition to the obvious risk to human life, fires can damage equipment and cause production losses for steel companies. These are very dangerous for the industry, and thats why the steel industry wants fire resistant greases, Sayanna reiterated.
Fire resistant greases, also known as self-extinguishing greases, have been developed for mining, die-casting, pressure casting, steam turbine control systems, aviation and steel mill applications.
The use of fire resistant greases can improve workplace safety as well as enhance productivity, Sayanna continued. Certain segments in mills require these specialty greases, as equipment replacement and frequent, unscheduled repair can become costly. For example, bearings in conveyor units in the steel mill are expensive, and replacement is difficult as temperatures in those areas hover around 900 C.
As explained in a paper by Joel Garrett, Brian Kusak and Matt Bailey of Quaker Chemicals Summit Lubricants, most greases made with mineral base oil provide a fuel source for fire. By removing the hydrocarbons from a grease formulation, the product is less volatile and generates less combustible vapors that can ignite and keep a fire burning.
And while fire resistant greases may catch fire, it doesnt spread and burns out within a few seconds. With self-extinguishing greases, gas generation after burning is much lower compared to conventional greases, Sayanna observed. Hence, fire easily extinguishes even if grease ignites.
Phosphate esters are used in most fire resistant greases on the market, Sayanna said. According to the Summit Lubricants paper, these base oils require far more energy than mineral base oils to ignite and sustain a fire.
Grease consistency also has bearing on fire resistance-stiffer greases are less apt to burn-but a compromise must be reached between this property and the pumpability that is required for a particular application, noted Sayanna.
The final product also has to perform in demanding environments: It must deliver reduced wear and friction; prevent rust, even in the presence of water; be non-corrosive to bearing material; resist pressurized water spray; and maintain performance throughout its service life.
To develop its own fire resistant grease, the private label grease and lubricant manufacturer added varying percentages of phosphate esters to greases made with different thickeners. All of these greases were evaluated for fire resistance. Results showed that greases with lithium complex, aluminum complex and polyurea soaps had better fire resistance than calcium sulfonate or clay-thickened greases.
Next, the company added a polyhydroxy compound, which Sayanna noted has been found to enhance fire resistance in greases, to a proprietary complex soap with a high dropping point to allow for operation in high-temperature environments. Further tests revealed the greases made with lithium complex and polyurea had better fire resistance properties than aluminum complex, calcium sulfonate or clay-thickened greases.
The product was evaluated for various properties that are common in all types of greases, and the fire-resistant grease showed good performance for parameters such as heat stability, oil separation, protection against copper corrosion, extreme pressure and antiwear properties.
Fire resistance properties were measured with a wire gauze test, a steel ball test and a direct flame test, Sayanna explained. The tests are used to determine how quickly a grease ignites and how long it takes to extinguish the fire. Results were compared with products used in steel plants, and the new grease was found to be similar to or better than the greases that were available in the market, he noted.
When we compared the grease that is there in the industry with the one that we developed, we found that this grease has lower self-extinguishing time compared to the commercially available products, basically because of the polyhydroxy compound that we have added in this particular composition, Sayanna said.
The grease has been commercialized for use in steel plants and other heavy industries encountering grease fires, he added.