Mill coolant system design
Lubrication is more than meets the eye
What kind of coolant system you construct for your tube mill or roll forming operation is just as important as what type of coolant you use.
Coolants are used to wash away oils, oxides, slivers, and dirt generated during the forming, welding, and sizing of tubes and roll-formed profiles. These coolants are a mixture of water and refined chemicals that help cleanse the process, protect part finish, extend tool life, and inhibit rust and corrosion on the profile and machine.
Typical applications of these coolants include pre- and postpunch operations, roll forming, weld zone flushing, impeder rod cooling, weld scarf removal, profile cooling, sizing, and saw-type cutoff.
The high-volume, medium-pressure, continuous flow of water and chemicals over tools and workpieces creates large volumes of impurities such as sludge and oil that must be removed to prevent contamination of the coolant and tool/profile interface. These contaminants are removed continuously by batch purification processes.
By knowing a variety of mill specifications and production parameters, you can design a flood coolant purification system to meet or exceed the necessary requirements. Such a system could include a pump; fluid transfer hardware; a fluid containment reservoir; water purification, filtration, and oil skimming equipment; temperature controls; and dilution control (as necessary).
Flushing by Design
Much like a river carries solids and liquids, a coolant system in roll forming and tubemaking moves contaminants from the point of entrance to a point of removal. Hoses spray coolants like rain over tools and profiles to keep them clean and remove heat generated in the process. And just like rain into a river, if coolant is deposited directly, the resulting current then can carry contaminants away.
Coolant systems work in a closed loop, so contaminants must be removed using strainers, filters, skimmers, and other mechanical methods. If they are not removed, contaminants build up quickly and can have deleterious effects on profile production, storage, fabrication, and finishing processes.
Plumbing, Reservoirs, and Pumps
|Tube Mill Flow Calculation|
|Application||Number of Stations||Flow per Station
A simple coolant system needs a total circuit flow of 42 gallons per minute. The system is plumbed to provide an appropriate flow rate to each station.
Pipes and Reservoirs. Simple coolant systems use a pump to generate coolant flow and pressure, plumbing to carry and dispense liquids to a point of application, and troughs to contain and return the coolant to a reservoir. A simple system like this requires specification of coolant flow to provide proper flushing and cooling of each production process, with the total flow requirement being equal to the sum of the individual requirements (see Figure 1). This calculation is important because the components used in coolant purification all are flow rate-specified. This number is used to develop pump specifications, reservoir capacity, plumbing diameter, containment/collection troughs, and any inline filtration.
Adding to the complexity, this flow rate ideally should be 25 to 30 PSI at the point of dispensing because pressure is lost within each plumbing component. The mill reservoir should be placed close to the mill and use larger-diameter pipe over shorter distances to provide the least change in coolant pressure from the pump to the mill.
A rule of thumb with plumbing is to maintain the largest required plumbing diameter as long as possible when carrying coolant to the mill because any reduction in diameter limits flow and pressure at the application point. Include butterfly or ball valves throughout the plumbing to allow for pressure limits at specific zones and the ability to shut off coolant during setup and observation of the mill in operation.
Troughs, splash guards, ID plugs, return flow impeders, foam pads, and air blowoffs all are used to collect coolant and related solids from the forming process and route them back to the reservoir. Systems should be designed with easy access for cleaning and uninhibited flow back to the reservoir. Fluid containment should be designed so that no "dead zones" exist within the return flow system.
Flood-type coolant tanks used in welded tube production can range in capacity from less than 100 gallons up to tens of thousands of gallons. Tank volume is based on flow, cooling requirements, and space constraints, with a starting point minimum of five times the flow capacity the pump.
If one central tank is used to feed multiple mills, then a smaller catch tank with its own pumping mechanism must be maintained at each mill. Tanks should be accessible for maintenance, viewing, and repair. A containment dike no less than two-thirds of the overall system capacity is recommended and may be required by law under many circumstances.
At the point of application, use 1/4-inch NPT connectors; this provides versatility in the nozzle/hose type at an economical price and acceptable pressure with minimal flow requirements. Also, include unions throughout the plumbing for quick disassembly and cleaning.
Pumps. Most often, electric centrifugal pumps are used to generate pressure because they are extremely rugged and require very little maintenance. Air pumps can be used when a variable fluid pressure is required with each different profile that is formed; however, these pumps require significant maintenance.
If you know the flow and pressure requirements, a pump can be specified using manufacturer performance charts and a little intuition. Specification of an electric centrifugal pump requires choosing model type, motor size and speed, and impeller diameter. Plumbing inlets and outlets typically are intrinsic to each model size. Be sure you do not underspecify, because a pump always can be throttled down but never can be forced to supply higher flow.
Removal of Contaminants
More effective coolant systems include components that remove harmful contamination. Specifying these components requires that you first define the specific contaminants that need to be removed; you then must assess their frequency and quantity.
This list likely will include a variety of factors that affect the coolant: diluting water, the metal being formed, other required inline chemicals, machine leakage, and operator error. Typical contaminants include metal oxides, coil coatings, machine grease, hydraulic oils, gear oils, metal slivers, metal fines, bacteria, fungi, salts, profile coatings, cutoff oils, roll cleaning solutions, machine fasteners, repair tools, and other solids and liquids.
Contaminants typically are categorized as soluble and insoluble in coolant solution.
Soluble Contaminants. Soluble contaminants, if problematic to the coolant solution, must be removed before they enter the system, because they will become a part of the circulating coolant solution and may become concentrated in the solution. Primary sources of soluble contaminants include diluting water, coil coatings, rust preventives applied inline, and cutoff oils.
Coil coatings, rust preventives, and cutoff oil can be reduced systematically, chemically treated out of the system, or eliminated with proper application and containment equipment (or by using compatible chemistries).
Diluting water, however, contributes significantly to buildup of soluble contaminants that cannot be removed. Water is the most abundant compound in coolant solutions and therefore is the greatest potential contaminant. It typically is used as-is from the tap and contains a variety of inorganic and organic salts along with biomass such as bacteria and fungi. These contaminants become concentrated with use and quickly deteriorate most coolant solutions.
Purification is completed using carbon filters to remove soluble organics, inline ultraviolet light for bioactivity, and reverse osmosis deionization to remove soluble salts. Water quality requirements for a given coolant are based on coolant formulation, tube quality expectations, and incoming water quality versus purification costs.
Insoluble Contaminants. Insoluble contaminants are dispersed in solution and usually can be removed easily by mechanical means. These contaminants typically are generated as a part of the process. Common examples include metal oxides, machine grease, machine oils, metal fines, and metal slivers. The solids are removed with screens and filters, and the liquids typically float on the coolant and can be skimmed or vacuumed.
Baffles, perforated steel, wire mesh, roll media, and submersed conveyors are used individually or together to remove large impurities from the coolant before it is sent from the mill (preferably by gravity) back to the reservoir. Particle size can be measured using a variety of methods so that removal of the contaminant can be cross-referenced to a specific mesh size.
Keep in mind that this type of straining usually is used to catch solids larger than 10 microns that are easily visible. The ideal strainer first collects larger particles that then build up to a point that they become filters themselves, creating a molecular sieve.
Bag or cartridge-style filters are used to remove the smaller solid particles that are suspended in the coolant and that often are not seen easily. These types of filters can be used to remove particles as small as 1 micron under the pressure of a pump; however, bear in mind that there will be a significant loss of pressure going through an inline filter as the solids build in the filter media. Five to10-micron filtration is recommended, and using two filters in series (a primary and backup) allows you to change filter media without affecting production.
Tramp Oils. Tramp oils collect on the top of water-based coolants and must be removed or they will insulate the coolant and possibly promote bioactivity. Belt- and tube-type skimmers can be used to remove small volumes of oil; floating vacuum skimmers are used to remove larger volumes of oil that enter the system regularly. In addition, coolant flow can be used to direct tramp oils for more efficient isolation and separation.
A variety of implements can be added to enhance coolant performance further and eliminate manual processes involved in the use and maintenance of coolant systems. They are as follows:
1. Open-surface or closed-loop heat exchangers. These are used to lower coolant temperatures to proper levels if the process generates a lot of heat. Heat exchangers are used quite commonly with new mills, because maintaining coolants at proper temperatures improves their consistency and extends tool life.
2. Automatic coolant dilution meters and level controls. These units are available in a variety of shapes and sizes. Typical installations include floats to add a prediluted coolant as needed.
3. Monitors. Coolant pressure, temperature, and dilution all can be monitored automatically to identify problems before system failure.
Reduce System Load Where Possible
Your focus always should be to reduce and eliminate system load and contamination by ensuring that proper equipment is used for applying chemicals to the profile and machine and by providing setup procedures for operators and maintenance personnel.
You can reduce contamination further by providing documented initiatives for cleanliness and scheduled maintenance and repair of equipment. Using compatible chemicals that come in contact with the mill coolant also will make the process more predictable. Thorough system cleaning before charging also will benefit you.
Finally, a comprehensive system maintenance plan not only includes a review of existing situations but also implements necessary changes with follow-up. Flood coolant systems require ongoing maintenance and testing to ensure that proper coolant conditions are maintained at all times.
Written by James Dyla
Hazel Park Michigan 48030
Jim Dyla graduated 1985 with honors from Western Michigan University with a degree in business oriented chemistry. His university research included purification of alternative fuel sources, with additional experience obtained in paints, coatings, and adhesives. Upon graduation, Dyla began laboratory research for AMCOL Corporation (formerly American Charcoal Company), a company specializing in metalworking fluid systems used in roll forming, tube production, nonferrous extrusion, and nonferrous casting. He has continued in a variety of positions that include technical sales, product development, production, and now president. Over the years, Dyla has traveled worldwide in search of methods to improve safety and performance of the metalworking fluid systems used in these specialized industries. He participates as a presenter and attendee at technical and management conferences sponsored by Society of Manufacturing Engineers (SME), Precision Metalforming Association (PMA), International Tube Association (ITA), Steel Tube Institute of North America (STI), Custom Roll Forming Institute (CRFI), Fabricators & Manufactures Association Intl.(FMA), Aluminum Extruders Council (AEC), and others. He presently holds one patent for an inline profile coating device.