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Best coolant practices make perfect

Don’t forget to consider the coolant system when designing a tube or pipe mill

When shopping for a new piece of equipment or designing a manufacturing system, it’s common practice to make the biggest-ticket item a priority and to work on the auxiliary items, support systems, and consumables later. When it comes to designing a tube or pipe mill, too often the cooling system—that is, the sump, pump, reservoir, filtration system, skimmers, the coolant itself, and the quality of the local water—is regarded as an add-on system that can be dealt with later.

The problem with this approach is that it often leads to an undersized system in an inconvenient location. Considering the cooling system as an integral part of the mill leads to a well-designed cooling system that meets the mill’s needs and makes the best use of the available floor space.

Sump Size and Location

When designing a tube mill coolant sump, keep in mind that the overall size or holding capacity has to allow for a 10- to 20-minute coolant retention time. You can determine the sump’s suggested gallon capacity by calculating the overall flow or pump capacity and multiplying that number by the number of minutes of retention time

For example, if you have a 20-gallon-per-minute (GPM) pump, the smallest reservoir you should specify is 200 gal. (20 GPM x 10). A better option is 400 gal. (20 GPM x 20).

The volume of the sump is an important factor to consider, and larger is better for several reasons. The larger capacity allows metal fines, swarf, and particulates to settle and allows tramp oils to separate out of the coolant, enabling the skimming of these contaminants (see Figure 1).

The larger the sump, the more time the coolant has to dissipate heat and maintain a lower operating temperature. The longer retention time allows more entrapped air in the coolant to dissolve, allowing for greater cooling properties from the coolant and enabling the tubing to be quenched more rapidly after welding. The larger surface area also improves conditions for skimming off tramp oils.

Too often, once the mill is up and operational, the sump is found to be too small. Make sure to size your sump correctly from the start.

Locate the tank away from the mill if possible; this provides for easier maintenance and keeps the critical components of the mill operation within the working vicinity of the mill. Quality check stations, computers, and changeover tables all will have more space if you move the sump behind and away from the mill.

If the sump is an above-ground tank, extend the hoses by several feet to achieve the required spacing. For in-ground tanks, the vital floor space is open, so access to drive components and tooling changes are within the working area.

Clarification

Clarification of the mill coolant is another important factor to consider in the design phase, but it also can be added at a later date.

Figure 1
Skimming tramp oils from the surface requires an undisturbed section of the tank. This “quiet” area can be formed using baffles in the tank.

The simplest way to clarify tube mill coolant is with a baffled tank. These baffles allow for the swarf, particles, and other foreign matter to collect, intact, in areas away from the pump, keeping the impurities in the tank and off of the tubing and mill surfaces.

If the volume of your tank is a few hundred gallons, you can acquire prefabricated sumps from a number of manufacturers, ready to use with minimum setup. If your tank size is several hundred gallons up to more than 1,000 gallons, a custom-built sump may be the best option.

The next step to take in clarifying tube mill coolant is to add a filter media. Paper bed filters or bag filters are low-cost methods to remove contaminants and foreign materials from the mill. When you make your selection, consider the chemistry of the coolant used in the mill; ultrafine filtration may strip the coolant of important additives, so consult your lubricant provider for selecting the proper media.

Figure 2 shows a series of filters and the progression of a coolant from a sample taken from the return side of the sump.

The second stage of coolant clarification is to remove the tramp oil from the system. Tramp oil can come from mill oil and rust preventives on the incoming steel, as well as from the hydraulic systems and drivetrain components of the mill itself. You can remove the floating oils using either a belt or rope skimmer, as long as this is done in an undisturbed area of the mill; if the coolant surface is agitated during skimming, the tramp oil will be difficult to separate from the mill coolant.

A number of belt skimmer manufacturers offer a variety of belt materials, so you might need to try a few to determine which belt type pulls out only the tramp oil and minimizes coolant drag-off. A rope-style skimmer works well for systems with a large surface area. An off-line separator or coalescer can be used as well; an auxiliary pump draws coolant from the main tank to the coalescing unit, then returns it to the main tank. A number of inexpensive separators are available for smaller systems of less than 500 gal.

Chemistry

Water quality plays an important part in the overall health of the tube mill coolant. Many tube mill coolants are just 5 percent to 7 percent metalworking coolant and 93 percent to 97 percent water. That means that a 1,000-gal. system contains 940 gal. of water. Therefore, if your water is bad, your tube mill coolant will suffer.

All manufacturers should know their water source characteristics:

  • What is the pH of the water?
  • What is the water hardness level, and is it from magnesium or calcium? Magnesium hardness is three times tougher on coolant stability than calcium.
  • What are the chloride and sulfide levels? High levels can lead to poor corrosion control, coolant imbalance, short coolant life, and rancidity.
  • What is the conductivity? Higher conductivity can destabilize the coolant and promote corrosion.

There are three coolant chemistries to consider for tube mills: soluble oil, semisynthetics, and synthetics. Soluble oils offer good cooling, excellent lubricity, and high levels of extreme-pressure agents. Waste treating is economical, and they leave a light oily film on the finished product. Semisynthetics provide better cooling than soluble oils because of the reduced oil content, they wash well in aqueous cleaning systems, provide for greater sump life than soluble oils, have better weld-through characteristics, and can be recycled more readily. Synthetics offer superior cooling, clean parts, weld-through without smoke because of lack of oil, longer sump life, and minimal residues. These are summarized in Figure 3.

Which is the best choice for an application? If you need a heavy-duty coolant, soluble oils are the most appropriate choice, followed by semisynthetics and synthetics. However, if process-friendliness is your top priority, then choose synthetics first, followed by semisynthetics and soluble oils. For any particular chemistry, it is recommended that you conduct a value analysis for performance, disposal costs, consumption, environmental impact, health, and safety.

Figure 2
A series of filters capture fines for removal and disposal (left) and return the coolant to like-new condition as shown in three samples (right). An unclarified coolant sample is full of impurities and has a layer of tramp oil at the surface (leftmost jar). The middle jar shows the advantage of allowing the coolant to sit undisturbed, which gives the impurities time to separate. The rightmost jar shows coolant in nearly new condition after it had passed through a series of bag filters.

Conservation

The tube mill coolant is a consumable product in tube producing. By employing a few tricks of the trade, you can reduce the amount of coolant waste.

An air knife or air curtain can help strip the exterior of the tubing of coolant, also making it a cleaner surface for the application of rust preventive if that is part of the process. To collect coolant trapped inside the tube, you can modify the runout table and surrounding areas: A slight elevation of the tube rack back toward the mill will send the coolant into a trough or capture pan, which then can direct it back to the sump.

The chemistry of the coolant can help reduce drag-off as well. In general, use of a semisynthetic reduces drag-off by about 30 percent compared to a soluble oil, because the oil will cling to the tubing. A full synthetic product drags off about 50 percent less than a soluble oil. The chemical makeup of these products can affect drag-off amount positively or negatively, so consult with your coolant supplier to optimize product selection.

Mixing and Control Parameters

For many tube producers, the coolant- mixing procedure is to fill the sump with water and dump in the coolant concentrate. While this might be the quickest method, it is not the most accurate or effective. Heavier, more viscous products will sink directly to the bottom of the tank, and if the sump has a slanted bottom or baffles, limited agitation will occur, possibly leading to overusage and an inconsistent mixture throughout the tank. It is similar to a child pouring chocolate syrup into a glass, adding milk, and beginning to stir. It takes a fair bit of effort to get all of the syrup mixed and leave none stuck to the bottom of the glass. Filling the glass with milk and then adding the syrup while stirring takes less time and effort to produce a chocolatey drink with all the syrup mixing into the milk. The same advice goes for mixing tube mill coolant.

For a 1,000-gal. tank, you will need a proportioner or mixing system. A venturi-type proportioner (see Figure 4) uses water pressure to pull the coolant concentrate and mix it with the water at a set ratio. This type of mixer works well with synthetics and some semisynthetics.

This system is inexpensive but requires some setup and adjustment. You can choose to use two mixers: one to supply a more concentrated ratio for initial filling, and the other at a reduced concentration for daily makeup. If you are using a soluble oil, you might need a volumetric pump.

For coolant makeup purposes, do not add water to the sump. Even if the make-up ratio is 2 or 3 percent, you are putting back in the critical chemical elements that are consumed or depleted during the tube mill operations. Even this low concentration for makeup replenishes needed rust inhibitors, sump conditioners, and lubricity additives to keep the mill at top operational performance.

With the mill filled and ready for production, it is important to keep the concentration within the target range. As the additives in the coolant are consumed, evaporated, and carried off, the concentration will vary. Check the concentration of coolant to water daily to ensure optimal coolant performance.

Also be sure to check the system’s pH weekly and adjust it if needed to prevent biological attack and keep the system healthy. You might choose to use a coolant monitor sheet (see Figure 5) to track the coolant’s concentration and pH. The sheet shows upper and lower limits for the coolant concentration and pH values; action points are defined and recommendations made based on the daily values.

Getting It Right the First Time

Taking all of the necessary parameters into consideration can be a big task, but it doesn’t have to be overwhelming. It’s a matter of understanding all of the parameters, doing the required research, and using that information to specify a cooling system that has enough capacity to suit the mill.

After the mill is up and running, it’s important to develop and adhere to two maintenance schedules, one for the mill and one for the coolant. Daily concentration and weekly pH checks extend the life of the coolant, the coolant system, and even the mill itself. It takes time and money to do the daily maintenance checks, but a regular maintenance routine can pay for itself many times over.

About the Author

Steve Lowery

Vice President and Director of Sales

4300 South Tripp Ave.

Chicago, IL 60606

773-927-6161