Oil, grease and other lubricants are used in all types of industries. From lubricating bearings in steel mills, lubricating dies in aluminum wire drawing, lubricating dies in forging, industrial hydraulics and several similar applications.
There are two types of lubricant contaminants – particle contamination and water contamination. Small particles (< 5µ) in lubricating oil result from metal wear, hard hydrocarbons, combustion residue and airborne dust.
Seepage and condensation cause water contamination. These contaminants affect lubricant properties, its life and can result in high preventive and corrective maintenance costs.
Broadly, lubricant contamination leads to 3 types of costs.
1. Product quality issues and premature equipment failure.
As an example, the following chart shows bearing life as a function of lubricant contamination.
2. High lubricant and disposal costs.
3. Environmental cost of lubricant disposal.
Therefore the reuse of these lubricants is desirable, possible and can prove to be very cost effective. Disc-stack centrifuges are widely used and the machine of choice in industrial lubricant recovery.
Accelerated sedimentation through the use of centrifugal force is Centrifugal Separation. A centrifugal separator multiples the natural force of gravity several thousand times. It uses centrifugal force which is generated by rotation of the fluid.
A disc stack centrifuge is a specific kind of centrifugal separator. A set of conical plates (disc stack) is added to the bowl of a centrifugal separator. Addition of the 'disc stack' introduces more settling area. This added surface area speeds up the separation process exponentially.
A disc stack centrifuge separates solids and one or two liquids simultaneously. This is a single continuous process. The centrifuge uses extremely high centrifugal forces to separate the phases.
The denser solids subjected to this force move outwards towards the rotating bowl wall. The less dense liquid phases form concentric inner layers.
The separated solids accumulate around the bowl periphery and are self ejected or manually removed from the bowl.
The separated liquid phase(s) move radially inward towards the bowl center. The liquid(s) overflow through outlet openings in the bowl top.
The clarified liquid phase (or phases) overflow close to the rotating axis, in the outlet area on top of the bowl. In case of 2 liquids, separate outlet chambers let the 2 liquids exit the bowl through independent passages.
In summary, a disc stack centrifuge removes water and solids continuously from the lubricating oil. This means continued production even when large water leaks into the lubricating system. The centrifuge will keep the water content within limits to restore oil properties. Also, it only removes the water without removing the additives.
The following is an example of centrifuge performance on hydraulic oil. This particular oil was in an aluminum extrusion hydraulic press.
Type/Make of Centrifuge: Alfa Laval MAB 104 Disc Stack Centrifuge
Oil Specification: Petro Canada AW 46 Hydraulic Oil
Lubricant centrifuge systems are normally set up in a ‘kidney loop’ configuration. A typical setup diagram is shown below.
A steel rolling mill in the midwest converts steel billets into steel wire. The mill converts 2 ton steel billets into wire by wire drawing. This is a 24/7 operation wherein least amount of downtime is paramount.
The gearboxes and rollers use lubricating oil. Cooling water drips into the oil when splashed on gearboxes. Water contamination of lubricating oil had become a production bottleneck. Contaminated oil results in premature failure of bearings and gearboxes. Production losses related to equipment failure can add up quickly.
Lubricating oil designed to last 3-5 years was being replaced every year! With over 5000 gallons of oil, the oil replacement cost was an extra $100,000 added to their annual operating budget.
Water in lubricants does not evaporate due to the low operating temperature. It is important to remove the water by other methods such as a disc stack centrifuge or a coalescer.
A high-speed, disc-stack centrifuge proved to be the answer. The centrifuge separates all free water and particle contaminants. It can also separate large quantities (> 50%) of water from oils.The centrifuge operates 24/7 to keep the oil contamination at an acceptable level.
The 'self-cleaning' design of the centrifuge allows for fully automatic operation. This minimizes the need for a full-time operator for the centrifuge.
The addition of the centrifuge led to multiple benefits. The lubricant oil replacement period is back to over 3 years now. That translates to over $100,000 annual savings in replacement oil costs. These savings alone paid for the centrifuge within the first year.
Also, the customer realized benefits from elimination of unplanned downtime. This led to higher production efficiencies across the plant.
Sanjay Prabhu MSME is Engineering Manager at Dolphin Centrifuge. The company manufactures centrifuge systems for a variety of industrial applications.