Machinery designed for demanding applications, such as the horizontal and vertical grinding mills used in mining, mineral processing and cement production, cannot normally be lubricated with a simple oil bath or grease. Heat generated within the machine by moving gears and bearings, or absorbed from the process itself, increases operating temperatures above the point where it can safely dissipate by convection or conduction.
As a result, this type of machinery requires the use of an oil circulation lubrication system. This provides lubricant at a prescribed temperature, viscosity and flow rate, with the lubricant removing heat as it passes through the machine.
Oil circulation lubrication systems are typically designed to match the needs of each application, depending for example on the size of the machine, the type and number of gears and bearings, and the process and environmental conditions.
Many of these systems are centralised, enabling them to provide lubrication to multiple machines at various flow rates and pressures. The system stores, filters and cools the lubricant before delivering it again to the machine. This type of lubrication system works well in high temperature environments and protects machinery from contaminants such as abrasive particles, water and entrained air.
Oil circulation lubrication systems are also used in the mining, mineral processing and cement sector for lubricating and cooling the bearings of fans and large gearboxes in conveyor drives and crushers. These systems can have reservoirs ranging from 30L in capacity, with flow rates of 0.1L/min, to 40,000L with flow rates of 3,000L/min.
The control of oil temperature and removal of contamination ensures that the operating life of the lubricant is optimised, minimising the need for oil replacement and reducing operating and maintenance costs. Additionally, providing a regulated flow of contamination-free oil to each machine reduces wear and ensures that gearing, sliding components, seals and bearings lasts as long as possible. The continuous flow of oil through the machine also removes wear debris generated by moving parts, helping to extend service life.
Some machines, like horizontal grinding mills, require specialised dual pressure/dual flow lubrication systems. These provide a high-pressure/low flow rate oil supply at start up, to lift the machine off its journal trunnion bearings to start its rotation. Once rotation has begun, a low pressure/high flow rate oil supply ensures that the trunnion bearings are efficiently lubricated and cooled. The same system can also be used to lubricate the mill pinion bearings.
How it works
The main sections of an oil circulating lubrication system are:
- Oil pumping station and oil reservoir;
- Filtration system;
- Heat exchanger;
- Oil flow control and monitoring; and
- Supply and return piping.
Correct specification for each application is critical for system reliability and effectiveness. This includes the configuration and sizing of the pump station and reservoir and the routing of the supply and return piping. Incorrectly designed or installed systems can lead to insufficient or excessive lubricant being supplied to each machine, resulting in excessive wear and damage, or oil leaks.
The required oil flow for each lubrication point is determined by the type, size and heat load of gears and bearings, rotational speeds, the required oil viscosity, maximum allowed oil temperature and the sizes of oil inlets and outlets. Once the flow requirements have been established, the total capacity of the pumping station can be determined, the appropriate flowmeters can be selected and system piping can be sized to suit the oil flow rates to each of the lubrication points. The design and construction of the pumping station will define the cleanliness, cooling and quality of the lubrication oil.
Oil pumping station and oil reservoir
The capacity of the pumping station oil reservoir is based on the total system oil demand. This figure needs to include an allowance for extra volume, to ensure that the reservoir will contain all the oil in the system when the machine is shut down for maintenance.
In traditional circulating oil systems, the circulated oil is retained in the reservoir for between 20 and 30 minutes before it is pumped out again. During this time, water and heavier contamination particles will settle to the bottom of the reservoir and any entrained air bubbles will separate to the top of the tank, reducing the amount of debris that needs to be removed by the filtration system.
The flow inside traditional oil reservoirs is usually controlled by intermediate baffle plates, which are intended to keep the oil flowing equally from the return end of the reservoir to the pump suction side. This does, however, dramatically reduce oil flow efficiency. According to a study by Tampere Technical University in Finland, the flow efficiency in a traditional tank is only 30-50%. In large lubrication systems, this can be a significant economic factor, requiring bigger reservoirs and longer oil retention times.
More efficient tank construction has made it possible to reduce reservoir sizes by 50-70%, without affecting the quality or the life of the oil. For example, the reservoir of the SKF CircOil Flowline system has a circulate construction, which functions differently to traditional oil reservoirs. The returning oil is collected in the top centre of the reservoir and then flows with equal speed radially to the outside of the reservoir. The unit is equipped with a set of separation plates that collect and naturally direct water droplets to the bottom of the tank and air bubbles to the top as the oil moves between them. This system of internal plates allows the circulation of 95% of the oil inside the reservoir, allowing retention time to be reduced to around 10 minutes, and producing a significant reduction in the required volume of oil.
Oil reservoir accessories
Oil normally returns to the reservoir through a return screen, which prevents large particles, impurities and air from entering the system. To minimise the amount of air entering the oil, it is important that the return flow enters the reservoir smoothly, without splashing or foaming. The SKF CircOil Flowline system has a special arrangement for the connection of the oil return line to the reservoir that limits foaming.
Depending on the ambient conditions and the viscosity of the oil, the oil reservoir may require heaters. While traditional steam heaters are still sometimes used, oil is usually heated by a thermostatically controlled electric heating element mounted in a thermowell submerged in the bottom of the reservoir. The thermowell keeps the oil from overheating.
As the oil level inside the reservoir will vary between machine start-up and normal operation, a desiccant air breather filter is fitted to the reservoir. This allows free-flow of clean dry air into and out of the reservoir. The oil level is monitored by a level transmitter and level switch, which will stop the system if the level is too low.
Various types and sizes of filter element can be used, depending on the degree of cleanliness required for the application. Lubrication systems are usually designed with double filter assemblies, each incorporating one or more filter elements. One of the filter assemblies is for primary operation with the other for standby for use when the primary filter needs replacement. Filter condition should be monitored continuously by pressure switches or transmitters.
Another technique to maintain high oil cleanliness is to have a ‘kidney loop’ pump, to circulate a low flow of oil from the reservoir through a fine filter cartridge back into the reservoir. The kidney pump can be fixed to the system or can be a portable unit for periodic use.
In oil circulation lubrication systems, most of the oil is used for cooling. Typically, the oil temperature will increase during circulation by 10-20°C. This heat must be dissipated by the system reservoir itself or with the aid of a heat exchanger before the oil returns to the lubrication points. The heat exchanger can be either a water-oil air-oil heat design.
Oil pressure control
The traditional way to control output pressure in the pumping station is to use a relief valve. In smaller systems this can be a spring-loaded valve; for bigger capacities a pneumatic control valve is used. The pump operates at a constant speed, while flow and pressure is controlled by the relief valve. Because these valves require a continuous flow, the total pumping and cooling capacity must be 10-25% greater than required by the lubrication points themselves, increasing the size, and cost of the system. Moreover, this continuous flow generates heat and energy losses.
A more efficient pressure control solution, adopted in the SKF CircOil Flowline system, uses motors equipped with variable frequency drives (VFD). A pressure transmitter at the pumping station oil outlet is used to determine the rotational speed of the electric motors via the VFD. Such control makes it possible to pump the precise amount of oil required for each lubrication point. The control immediately adjusts to allow for changes in oil flow requirements by the individual lubrication points.
Smaller lubrication units may use gear pumps, while larger units generally use screw pumps. Screw pumps are extremely reliable, generate less noise, have low energy consumption, steady (non-pulsating) oil flow and have a wide rotation speed and capacity range. Pumps will normally be fitted with an internal overflow valve and an external safety relief valve to prevent damage if oil flow is interrupted. Systems generally use two parallel pumps, one in operation and one on standby.
Each oil circulation lubrication system requires precise control. The lubrication system controls can be integrated into an existing supervisory control and data acquisition (SCADA) or automation process control (PLC) system, or the control can be a standalone system. The control system controls the pressure and operation of the pumps, heating and cooling. It also monitors oil pressure, oil level inside the reservoir, pressure drop over the filters, etc. Specific alarms will be activated if abnormal or out-of-tolerance conditions occur.
Modern control systems, combined with VFD controlled motors, simplify critical functions such as automatic cold start-up, making them particularly beneficial for the start-up of large systems. These systems often incorporate digital oil flow monitors, which feed information about the lubrication system back to the operator and to overall plant control systems.
Oil flow control and monitoring
Flowmeters are a critical component in oil circulation lubrication systems. They provide oil flow to suit the individual lubrication point, but also provide key information about flow control. There are several methods for monitoring oil flow. It can be monitored visually, and such flowmeters can be equipped with electric alarms, if required. Usually these meters have a float indicator, which moves according to the oil flow. The float position shows the oil flow through the meter. It is also possible to monitor the oil flow digitally, where all flow information can be displayed directly and also monitored remotely.
For lubrication reliability, the construction of the control valve is critical. In particular, when low volumes oil flows are required, even the smallest of particles can plug traditional types of needle valve. With specially shaped valve spindles it is possible to have a wider flow-through area, and these small particles will not stick inside the valve. Also there are special pressure compensated flow control valves, which could compensate system changing pressure conditions. These are also designed for high operation pressure up to 200 bar.
In oil monitoring and control, it has to be considered that all lubrication systems are sensitive to viscosity changes. Therefore, it is important that the lubrication system oil temperature remains stable during the operation.
Supply and return piping
Oil circulation lubrication systems are often located in corrosive environments; even if the immediate surroundings of the machinery are relatively dry, the return lines will have high humidity levels inside the pipes, which can cause corrosion. Stainless steel piping is normally recommended for these systems. The piping should be seamless and joined using welded rather than threaded fittings to minimise any restriction in oil flow.
The supply lines are routed from the pumping station to the flowmeter panels and then onto the lubrication points. The sizing of the piping will be based on distance and any differences in elevation between the location of the oil system and the machines, the viscosity of the oil and the oil flow rate. Sizing of the return lines is critical because return flow relies on gravity to drain the oil back to the reservoir. Return lines carry hot and humid air along with the oil, increasing the risk of corrosion in the piping. They must be therefore be equipped with ventilation connections, with desiccant air breather filters to reduce water ingress into the system.
The reservoir must be located at a lower elevation than the machine and return piping should pass without rise in elevation between the machine and the top of the reservoir. Where the position of lubrication points means they cannot be connected via the main gravity return line, it is necessary to use separate return pumping stations, or sump units, to collect the oil and pump it to the main return header or directly to the oil reservoir.
Keith Meyers is global industry manager mining, mineral processing and cement at SKF. See: www.skf.com