Smart Oil Skimming Cuts Costs
Oily contamination of parts washers creates unnecessary costs, poor quality parts, and costly environmental problems. This article presents a simple approach for skimming and capturing quench oil from heat-treating parts washers, allowing operators to take control of this bothersome process step. New skimmer/separator technologies continuously skim and capture floating oil from parts washers (Fig. 1).
Oil contamination is costly
There are many costs associated with oil-contaminated washers including:
- Cost of poor quality parts resulting in scrap, rework and loss of process control
- Cost of treating and disposing contaminated fluids; most washer "dumps," or drainings, create unnecessary treatment and disposal costs
- Downtime for wash-solution replacement (unnecessary downtime increases many production and labor costs)
- Cost of shot blasting and rework due to oil contaminated wash fluids (oil stains and baked on grit arise from poorly cleaned parts)
- Cost of smoke remediation equipment (high capital cost, high operational cost, high labor and maintenance costs). Oily contaminants from the parts washers burn off in the draw furnaces creating smoke and other environmental problems (Fig 2).
- Cost of replacement cleaners (frequent bath dumps create unnecessary costs for replacement cleaners)
- Cost of labor for washer management (maintaining inefficient old skimming systems creates high labor costs)
- Water and sewer costs (poor skimming and frequent dumps create high fluid service costs)
- Health and safety costs (unnecessary fluid handling, smoky environments, and poor housekeeping practices due to poor oil skimming all create health and safety issues)
- Environmental costs (all environmental and quality programs call for continuous improvement; effective oil skimming and capture offers many opportunities for big gains)
Skimming technologies include overflow, single stage, and two stage. Table 1 compares different washer treatment methods and devices.
Overflow weirs: A rapidly declining practice is to simply overflow the top surface of the washer tank for treatment or disposal. Every one gallon/minute (gpm) overflowed from a washer creates more than 500,000 gallons of oily wastewater per year. In one application, a single parts washer was overflowing more than 4,000,000 gallons per year just trying to remove floating oil.
Single-stage devices: Drag-out devices such as belts, disks, drums, ropes and mops rotate in and out of the bath. Any oil that it attracts gets dragged out of the tank. The problem with this technique is that these devices do not remove enough oil, and they drag out valuable source fluids. In heat-treating facilities, this approach does not solve the problem, but instead, it simply creates unnecessary oily wastewater. Says one veteran of the heat-treating industry about drag out devices: "We've been doing the same thing for 30 years, and it's still wrong."
Two-stage designs: A more effective approach uses two stages; the first to skim, and the second to separate. A floating skim head inside the wash tank skims the oily surface. Skimmed fluids are transferred to a separator where oil is captured and clean fluids are returned to the washer.
Gravity separators are open-channel oil separators. When designed right, they will not clog or "blind over" like coalescers that use high-maintenance media to remove oil. There are no media and no consumables used with gravity separators. Skimmed oil is separated from the wash solution by gravity only as it passes through the separator (Fig 3).
Evaluating skimmer designs for two-stage systems
For oil-skimming systems to work properly, they must continuously remove surface oil. Skimmers deployed on hoses or mechanical pivots typically fail due to problems with wash chemistry, heat and debris. A more effective approach is to use a skimmer deployed on a fixture that keeps the skim head operating at the surface. These "over-the-edge" type of skim heads do not require tank draining for installation (Fig 4). A new, patented skimmer design removes oil by suction (Fig 5).
Transfer pump issues for two-stage systems
In most two-stage systems, a transfer pump moves the skimmed fluid from the tank skimmer through the separator. Positive displacement pumps, such as air-operated diaphragm (AOD) pumps are recommended. They have relatively high specifications for pumpable solid size and produce a small amount of oil shear during transfer. Centrifugal pumps shear the oil into smaller droplets, making them harder to remove in the separator.
Captured oil management for two-stage systems
Unlike belts and disks and drums that remove a lot of wash solution with a little oil, two-stage systems should remove a minimum amount of wash solution while producing a dry oil for resale or recycle. Good separator design concentrates the captured oil, allowing wash solution to pass.
Factors for successful skimming
Issues related to successful skimming include smart cleaner choices, resell/recycle captured quench oil, selling or recycling skimmed quench oil requirements and oil recycling limitations and workarounds.
Using a cleaner that causes oil to separate and float so it can be skimmed is smart. Using a cleaner that causes the oil to emulsify or go into solution creates poor cleaning, as well as expensive treatment and disposal options. It simply hides the problem, making it difficult to manage effectively. Every wash in emulsifying cleaners increases the percent of oil in your bath until it must be dumped. Parts are being cleaned with an increasingly oily solution. Fluids dragged out of the washers into the tempering furnaces burn off as smoke leaving contaminants on the parts. Switching to an oil-splitting cleaner, especially those closest to pH neutral, allows the oil to float where it can be managed effectively and much less expensively. This greatly increases cleaning performance. It also extends bath life, cutting all related washer costs. While the costs of oil-splitting cleaners may be higher than inexpensive emulsifiers, their cost per use is significantly lower.
What should be done with captured quench oil? Captured quench oil can be sold or recycled economically if the amount of water in the captured oil is small. Manual decanting requires large storage areas and high labor inputs. Modern skimming and separation systems automatically deliver a captured oil product that is "dry" enough to sell or recycle without surcharges for water removal.
What are the requirements for selling skimmed quench oil? Captured quench oil having less than 10% water content can be sold for secondary uses. The same hauling service that now charges to haul oily wastewater will pay for captured oil with low water content.
What are the requirements for recycling skimmed quench oil? Mobile on-site recycling vendors can visit your plant and reprocess your captured oils to a standard that meets or exceeds new oil specifications. On-site recycling is valuable because you are assured that it is your oil and your additive package that is returned to your quench tanks. On-site recyclers will certify your oil for water content, quality, and additive package for about half the cost of new oil.
What are the oil-recycling limitations and workarounds? If a cold oil and a hot oil are both skimmed from a common wash tank, the skimmed and recycled oil can typically be reused for jobs requiring the cold oil specification. This can lead to significant savings in new quench oil purchases.
The role of human support
Heat-treated parts washers are among the messiest processes on any plant's floor. Nothing this messy can be fully automated. Good skimmers and separators can be deployed, cleaners can be optimized, but nothing can ensure success better than good human support. The time spent per tank will be small, but the value is large. Daily maintenance checks to observe skimmer operation, oil capture and separation, and transfer pump operation are key. Many operators are surprised by the amount of oil captured. Barrels for captured oil can fill quickly. A good plan for moving the captured oil from the skim site to a storage area for resale/recycle is important.
Case Study: Transmission Plant
Daimler Chrysler Kokomo Transmission Plant (KTP) is among largest heat treat facilities in North America. Plant manager Bob McCulley honed his processes over the years, but felt there were big cost savings and big environmental gains to be made through better parts washer management. McCulley's goals were to increase effective bath life of his heat treat washers. The company also set a goal to skim and capture quench oil and have it recycled and certified for reuse. McCulley followed a strict plan to achieve his goals including installation of a prototype skimming system, implementing the process, analyzing results and documenting savings.
A Universal Separators SmartSkim(r) skimming and separation system was installed on one washer initially. This allowed KTP to evaluate skimmer effectiveness and to begin quality tests of recycled oil. What made the project a success was the team approach Bob McCulley built into the implementation process. All heat treat operators and support staff were involved with the testing and implementation stages. This ensured a good understanding of the goals set for the team.
Results have exceeded expectations. Effective oil skimming and capture has resulted in much longer bath life and cleaner parts. There was a significant reduction in smoke due to effective oil removal from the washers. Captured oil contained insignificant amounts of wash solution. An on-site recycler was hired, and tests of recycled oil showed results exceeding new oil specifications at about half the cost of new quench oil.
Nine more SmartSkim systems were added, and documented savings was calculated to be $206,520 annually. Payback for the skimming equipment was about 3 months. Tens of thousands of gallons of quench oil per year, previously sent to central treatment for separation, are now captured and recycled on site at the heat-treating department (Fig. 6).
DaimlerChrysler KTP recently be-stowed an environmental award on this project as "Significant Contributors" to environmental excellence. Signed by DaimlerChrysler chairman Dieter Zetsche, this effort has earned global recognition for its innovative approach to a costly problem. Initially proven as a cost-saving measure, the SmartSkim systems delivered benefits in many other areas. Chrysler Group Executive Vice President-Manufacturing Tom Lasorda acknowledged the unit's impact on ergonomics, environment and safety.