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Tramp oil is an undesirable phase of hydrocarbon oil contaminant found in machining coolants and washer fluids. This floating oil is a by-product of industrial machining operations contaminating the water-based coolant. Tramp oil removal from machining coolants is critical to the maintenance of coolants.
The various components of machine tools leak operating oil that mixes with the machine coolant. These could be way oil, spindle oil, gear oil, etc. Another source of oil is the anti-rust coating on the raw metal used in the machining process.
This oil gets washed into the coolant during the machining process. Over time, this oil accumulates in the coolant sump forming a floating layer known as tramp oil.
Tramp oil in machine coolants is the cause of a variety of issues. Some of the common issues are listed below.
The primary role of the machine coolant is to keep the cutting tool from overheating. The friction in the machining operation causes heat that needs to be dissipated. The coolant’s tramp oil reduces the coolant’s ability to cool the cutting tool, reducing tool life.
The floating tramp oil layer in the coolant sump prevents the coolant from ‘breathing’. The insulation of the coolant from the air causes the growth of anaerobic bacteria in the coolant.
Bacterial growth can lead to hydrogen sulfide gas which emits an unpleasant odor. The bacteria is also harmful to the operators and can cause skin infections such as dermatitis.
The contaminated coolant can cause the tool to overheat. If the tramp oil contacts an overheated tool, it can cause smoke harmful to the operator. In worse conditions, the oil can catch fire, causing safety issues.
There are several ways to remove tramp oil from the machining coolant. Some of these are straightforward tramp oil removal systems. Others are more effective ways that are discussed in detail below.
Simple skimmers remove the floating oil from coolant sumps by skimming the oil layer into a local floating container. This method is ineffective because it depends on the gravity settling of the coolant.
With the constant churning of the coolant, most of the tramp oil stays suspended in the coolant.
A coalescer uses a set of plates or membranes to ‘coalesce’ (bring together) the small tramp oil particles to form bigger droplets that can then be separated. However, using coalescers has its limitations, with large surface requirements for effective separation.
Oil adsorbers and other surface collection devices are also used to collect the floating tramp oil in sump tanks.
An industrial centrifuge configured to remove tramp oil from machining coolants is known as a tramp oil centrifuge. This centrifugal separator generates a gravitational force that is thousands of times that of gravity to separate the tramp oil from water-based coolant effectively.
A tramp oil centrifuge is a three-phase disc-stack centrifuge specifically designed to separate tramp oil from machine coolants. This centrifuge rotates at 8,500 RPM and exerts centrifugal forces over 7,000 Gs.
The heavier coolant gravitates towards the bowl wall while the lighter tramp oil exits the bowl from the bowl center.
An industrial tramp oil centrifuge offers certain key advantages over other methods of coolant recovery.
The disc stack centrifuge has a much higher separation efficiency than other tramp oil separation methods listed above. The extremely high centrifugal force enables the centrifuge to separate particles as small as 1 micron to separate.
This separation efficiency leads to the total removal of tramp oil from the coolant.
A tramp oil centrifuge is a three-phase centrifuge meaning it not only removes the tramp oil but separates the tiny suspended metal particles from the coolant. This particle removal eliminates the need for coolant filtration as well.
Coolant separation from tramp oil is a continuous process through the tramp oil centrifuge. The centrifuge installation is in a kidney loop configuration that enables the centrifuge to remove the tramp oil and metal particles continuously.
This inline process keeps the machine coolant free of contaminants at all times.
The tramp oil centrifuge has a small footprint for its processing capacity compared to the other separation devices. Therefore, one can install tramp oil centrifuges on mobile carts or trolleys that are moved between coolant sumps as required.
Since the tramp oil centrifuge separates the tramp oil from the coolant in the sealed bowl, the coolant and tramp oil’s environmental exposure is restricted.
This centrifuge feature allows the entire tramp oil removal system to be isolated from the environment leading to an odor-free working environment.
As with most technologies, the tramp oil centrifuge also has some inherent disadvantages that limit its applicability. Some of the disadvantages are listed below.
An industrial centrifuge is a heavy-duty centrifugal separator; therefore, the initial cost is higher than that of other tramp oil removal devices. However, a centrifuge does not need any media or dedicated labor, ensuring a quick payback of the initial investment.
A tramp oil centrifuge has rotating parts that involve bearings and other components associated with internally moving machinery. Therefore a centrifuge is more complex than a simple float skimmer or a stationary coalescer.
Like all industrial equipment with moving parts, an industrial centrifuge requires periodic maintenance. Though the centrifuge has very few consumable parts, timely maintenance is essential for the reliability of the equipment.
There are several Alfa Laval tramp oil centrifuge models. The specifications of one of these centrifuges are listed below.
| Centrifuge Type | Alfa Laval WSPX-204 Tramp Oil Centrifuge |
| Configuration | Three-Phase; Coolant, Tramp Oil, Sediment Removal |
| Capacity | 2 GPM on Coolant Separation |
| Bowl Speed | 8,800 RPM |
| Sludge Capacity | 0.5 Gallons |
| Drive Motor | 3 HP; 1800 RPM |
| Size | 3’ x 4’ x 4’ (H) |
| Weight | 2,000 Lbs |
A tramp oil centrifuge is ideal for the efficient and continuous removal of tramp oil from water-based machine coolant. The comparatively higher initial cost is justified by the improved product quality, reduced part rejections, longer tool life, less machine tool downtime, and safe working conditions.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge