Machine Sumps and Coolant Maintenance
Your coolant sumps are a prime cause of coolant failure
By Howard Urdanoff
and L.J. McKinley
The sump is the logical place to treat coolants. Unfortunately, most individual machine sumps damage coolants and shorten coolant life.
A lesson can be learned from the design of dedicated circulating systems that serve a single large machine or a central system serving many individual machines. These systems usually employ a reservoir that encourages gravity separation of light and heavy-phase contamination by providing ample residence time. These reservoirs may be equipped for continuous or intermittent skimming of light-phase contaminants, and conveyors for removal of heavy contaminants.
The alternative is to provide huge filter capacity, in one or more stages, to trap and hold all contaminants. The economics of this type of system are prohibitive due to the high cost of the initial system, filter media consumption, and labor for disposal and servicing the filters. Using a large sump or reservoir for primary gravity separation is a one-time cost that eliminates the need for elaborate filtering.
An emulsion (oil/water) is a mixture of otherwise unmixable fluids, dispersed by the emulsifier and held in suspension by electrostatic forces. The emulsifier breaks down the oil into micron-sized droplets. Droplets remain suspended in the water phase, and the resulting mixture's specific gravity is lighter than water, but greater than that of oil. This combination provides excellent lubrication and cooling properties. With the advent of emulsions, the coolant mixture included four distinct components: emulsion, metallics, tramp oil with additives and oxidation byproducts, and bacteria.
In the process of providing lubrication and heat extraction from the work, coolants are pumped from and returned to the sump. The sump collects the metal removed during machining; large chips, a wide range of fines from coarse to submicron in size, and foreign fluids (split emulsion oil, lube, hydraulic and way oils identified as tramp oils). The sump becomes a catch-all with centrifugal pumps shearing these contaminants into the coolant. Hydraulic mixing in the sump and shearing by the pump result in stratified contaminants.
This intimate mixing of tramp oils and fine particulates creates a mixture that is not economically cleansed by existing processes. Large chips and larger fines readily drop to the bottom of the sump; some small chips may be held at or near the surface, by surface tension. Over time, chips held at the surface become wetted and drop to the bottom of the sump. The smaller fines are coated by and taken into both emulsion droplets and tramp-oil droplets, and may cause tooling wear and rejected parts. These metal fines also weaken the negative electrical charge in emulsion droplets, eventually causing them to split/separate and join the tramp oil.
Bacteria are universally present. The fats and fatty acid additives in tramp oil are food for these bacteria and cause their proliferation. This is the source of problems ranging from foul odors to rashes and infections. Without tramp oil, little food is available for bacteria and they do not multiply excessively. Removing all bacteria encourages the growth of fungi, which are more harmful than bacteria.
A further complication of an increase in tramp oil is the loss of the coolant's heat-extraction ability. A rise in tool/part temperature may affect the life of the tooling and part tolerance/finish, while the higher machining temperature creates oil misting. The results often are tool wear, part rejects, and health considerations (inhalation of oil mists, rashes, and infections).
The configuration of many sumps tends to promote hydraulic mixing. The random falling of coolant into the sump, and the positions of support components within the machine, create an environment detrimental to coolant longevity. Static coolant pockets foster bacteria growth, and high-velocity currents mix the particulates and un-emulsified free oil with the base emulsion. These contaminants are then delivered to the tool/workpiece with negative results.
Currently, coolant is usually maintained by one of the following methods:
- no cleaning/dump when dirty;
- installation of an oleophilic skimmer (belt, disc, rope or tube) in each sump;
- periodically servicing sumps with a conventional floating skimmer and portable treating equipment;
- servicing sumps with an Oscillating Deep Skimmer to remove both surface oil/contamination and "cream" (stratified phases) and treating with a portable coalescer; or
- bringing coolant from each sump to a remote cleaning station where it is cleaned, then returned to the sump.
Each method has drawbacks: dumping is obviously wasteful and costly, while oleophilic skimmers work well only when skimming concentrated contamination--a thick-top, free-oil layer. They may also remove up to 95% of good coolant (which is expensive to replace and haul away), leaving a high percentage of contamination in the coolant. Oleophilic skimmers cannot remove (stratified) cream phases. Conventional weir skimmers remove mostly surfaced material, not stratified cream phases, and a deep skimmer requires a more expensive and sophisticated skimmer than the conventional floating skimmer, plus a larger access area to the sump and an efficient oil separator (usually a coalescer). Taking coolant from each sump to a remote cleaning station requires labor for pumping and transporting of coolants to and from the sumps and treating unit. All sumps, except those treated by deep skimmers, require manual cleaning of oily coatings before recharging with clean coolant.
Sump designs that preclude sufficient primary separation prevent the practical cleaning of coolants. To prolong coolant life and quality, thus reducing cost, both surface and stratified contaminants must be removed.
The least expensive, most effective cleaning method involves three stages: first, encourage gravity separation in the sump of oil and cream phases (prior to homogenizing in the pump); second, remove this light contamination by deep skimming; and third, separation of tramp oil. Physical filtering of the metallics, if required, may follow. Efficient oil separation normally captures and removes most harmful fines and bacteria.
Failing to minimize turbulence and to remove light-phase contaminants will doom the operator to either short coolant batch life or elaborate, expensive, and possibly ineffective treating equipment. No single, economical treatment currently exists for handling a mixture of tramp oil, chips, and particulates in an agitated sump. Unless oil is removed before filtering, filters are extremely high in maintenance cost, and are inefficient for removing oil. Surface skimmers are handicapped by the presence of stratified waste.
Existing sumps are rarely modified. Many operators are reluctant to alter the sump configuration, believing that any modification would void their warranty. This very well could be a misunderstanding, and contacting the machine builder might correct this impression.
Each individual sump should be modified to provide quiescence (a minimum of five minutes residence in a controlled, equalized, and uninterrupted flow pattern is good; longer is ideal). You should operate at the highest possible sump level for the best separation; low liquid level reduces volume (i.e. residence time). Capturing the flow from the work with large-diameter lines that drain to a common area of the sump could help. Splashing and turbulence should be prevented/minimized. Enlarging the sump may be an option, and the whole sump area should be accessible for cleaning.
Coolant maintenance must be scheduled regularly, and all baffles should be removed except for one down baffle and one (short) up baffle immediately before the circulating pump suction. Doing so prevents waste from entering the pump. Unless cleaned frequently, baffles and screens trap the tramp oil, which eventually will be carried into the pump.
In this sump, a coolant deflector delivers dirty coolant to a strainer where solids are collected for manual removal. The separation section provides at least five minutes residence time. A deep skimmer removes surface and cream-phase contamination, which is then sent to a coalescing system. The clean reservoir contains treated coolant, and is the proper point for monitoring concentration and coolant level.
Where possible, an expanded-metal or coarse-screen collector, easily serviced, should be employed to catch and hold coarse chips in the work area. The most effective treatment of these sumps is by deep skimming followed by either a coalescing filter or gravity separation. Contaminants must be captured before they can be removed.
If they employ coolants, the next generation of machines should be equipped with or provide:
- Sump flow patterns encouraging gravity separation prior to coalescing,
- Deep skimmers to capture both surface and stratified contamination,
- Non-shear circulating pump(s), (perhaps diaphragm pumps),
- Built-in coalescers to deal with rare oil leaks and normal "splitting" of emulsion oil,
- Standard filters using selective porosity elements and condition indicators,
- Large chip removal facilities, easily removable containers or automatic conveyors,
- Built-in monitoring equipment for coolant concentration and liquid level, and
- Accessible sumps for ease of servicing.
New machines should be configured to capture coolant from the work areas and route it, via gravity, to a suitably sized sump for a minimum of five minutes controlled residence time. Coarse chips should be collected and removed, and turbulence should be eliminated. Minimum volume in gpm required is 5 X flow (see recommendations from oil suppliers), and should include allowances for varying levels that result from filling machine lines plus evaporation of approximately 10%. When split sumps are used, all sections must be accessible for treatment.
Benefits from the foregoing would include extremely long coolant life (elimination of approximately 90% of waste disposal, sump cleaning labor, and machine outage for cleaning), far fewer rejects/rework, longer tool life, eliminating 50 - 80% of make-up emulsion oil and handling costs, and elimination of "misting" and bacterial problems.
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SME publishes books and videos on metalcutting fluids. Cutting and Grinding Fluids: Selection and Application presents the basics including how fluids function and fluid types. The book Filtration Technology: Second Edition is a fundamental introduction to industrial filtration. It covers fluids, contaminants, media selection, and more. Available on video and DVD, Water Extendable Lubricants Control is intended for training new and existing personnel in the use and control of water-extendable lubricants used in the manufacturing process. For more information or to place an order, contact SME Customer Service at 800.733.4763, or click on "online store" in the box at the left of the screen, to order books and videos.
This article was first published in the May 2004 edition of Manufacturing Engineering magazine.