| A New Approach to Heat Treating Parts Washing.
New innovations in the management of heat treating parts washers are
yielding powerful, unexpected benefits. Simple, cost effective shop floor
practices are being combined in new ways to deliver big quality
improvements and significant help to the bottom line. Employing these
proven, reliable steps early in the process can dramatically cut waste
hauling expenses and greatly reduce environmental liabilities while
continuously producing cleaner parts.
Typical heat treatment for the hardening of steel would include
heating to some austenitizing temperature then quenching in an oil bath to
harden the steel.
After quenching, the parts are washed and subsequently tempered to reduce
residual stresses. It is the management of the parts washing step - after
quenching and before tempering - that is the subject of this article.
How Parts Washing Impacts Manufacturing
Poorly managed parts washing operations can affect manufacturing in many ways.
Poorly managed parts washing operations can affect manufacturing in many
ways. Quality is perhaps the single biggest issue facing the
manufacturers using heat treating. Tim Hoefft, Heat Treating Engineer at
Caterpillar describes the problem clearly. "Caterpillar continuously
strives to increase quality throughout its operations. An ongoing
evaluation showed that parts washing fluids used in the heat treating
operations could introduce quality problems when not managed properly.
Parts washers covered with oil can cause poor quality washes and create
other manufacturing problems downstream. Clean parts are particularly
important in our gear manufacturing operations. "
Parts washers contaminated with high oil content contribute to quality
problems. In poorly managed washers clean parts are often sprayed with
oily wash solution or pulled through a layer of surface oil and grit prior
to tempering. Gears and other parts which are improperly cleaned carry
oil out of the parts washer. These oils become a baked on contaminant
during the tempering step. Removing baked on oil residues typically
require shot blasting and other labor intensive reworking steps.
Oil dragged from the parts washers into the draw furnaces also raises
significant environmental issues. Oil heated inside the furnaces creates a
smoky environment for the workers in the plant as well as potentially
prohibited levels of hydrocarbon emissions out the stacks.
Oily parts washers also create significant long term maintenance
problems for furnace operators. Oil burning inside a tempering furnace
degrades the fire brick lining, requiring more frequent re-lining cycles.
This is an expensive step, keeping the furnace off line and unproductive.
In years past, quench oil was removed by solvent degreasing. Environmental
and safety concerns about solvents have led to a change to aqueous cleaners. Early formulations of aqueous cleaners were aggressive, high pH sur
factants designed to emulsify as much oil as possible. Oil which would
normally rise to the surface was changed into an oil/water hybrid that was
held in solution. This slowed the accumulation of oil on the surface of
the washer, but it hid the problem in an ever dirtier bath. When the
capacity of the bath to hold emulsified oils was used up, the bath lost its
effectiveness. Spent baths must be hauled away by licensed special waste
haulers due to their high pH and high FOG count (fats, oils and greases).
Costs for these hauling services vary by region of the country, but all add
significant costs to the manufacturing process. Frequent hauling cycles
also result in the need for expensive replacement cleaners.
Current Treatment Methods
Typical treatment methods for keeping parts washers clean have failed to
take a comprehensive approach to the problem. Most employ tools borrowed
from unrelated areas of fluid treatment. These typical treatments also
rely on old assumptions about cleaners. All current treatments result in
an expensive, oily wastewater stream. Management and disposal of this
wastewater is expensive and labor intensive.
A review of typical methods currently in use:
Drag-Out Systems. These include oil belts, discs, drums and mops
and have been the most common in recent years. These devices
have a component which rotates in and out of the parts
washer. Many use an oil- attracting (oleophilic) material
which is supposed to attract oils from the surface.
As these components rotate out of the washer, materials clinging to
them are scraped off into a waste collection barrel. These
devices typically drag out a
waste stream which is about 50% oil and 50% wash solution. Not
only is this solution expensive to haul, it creates an
unnecessary demand for expensive replacement cleaners.
Coalescers. While these devices come in a wide variety of
configurations, most employ tightly packed beds of
coalescing media. Oily washwater is pumped through the media
causing oil molecules to coalesce and grow larger making them
easier to remove.
Coalescers were designed to work in pure oil - water solutions.
Heat treating parts washers often contain a much wider variety of
contaminants, such as grit, scale and soils. When operators
attempt to pump these solids through the tight passageways of a coalescer,
the coalescing media blinds over and becomes blocked. In heat
treating applications, coalescers must be frequently taken off
line and cleaned. Cleanup is labor intensive,messy and expensive.
Off-line gravity separation. This method requires operators pump
out their washers into holding tanks for further separation over time. The
large holding tanks take up otherwise productive floor space and frequent
labor input is required to manage these fluid transfers.
Emulsified oil is still held in suspension by emulsifying cleaners as there
is no mechanism for readily separating the layers of oil from the aqueous
cleaner. Because of this, an oily waste stream is still produced.
Barrier filtration. Bag and cartridge filters have been tested to
solve the problem of oil and solids in the wash solution. These
typically blind over quickly and the cost of consumables becomes
prohibitive.
Membrane Filters. Membrane filters are devices that filter fluids
down to a very tight micron rating -typically 1 micron and under. In
washwater applications they are designed to remove emulsified oil from the
cleaners. Membrane filters quickly blind over with
contaminants, particularly free oil. On the shop floor,
membrane filters can become expensive maintenance headaches. Membrane
filters also damage most cleaners by filtering out valuable cleaner
components such as rust inhibitors and defoamers. Expensive membrane
filters can be avoided, however, with a simple change from emulsifying
cleaners to oil splitting cleaners.
Emerging Best Practice for Heat Treating Parts
Washers
At Treat-All Metals, a commercial heat treating facility in Milwaukee, WI,
a large batch washer equipped with a drag-out separation system was not
able to keep up with the large volume of oil. It required frequent hauling
of oily wastewater and replacement of the bath, according to Maintenance
Director, Ron Barnhart.
Treat -All tried several new methods before settling on a combination of
oil-splitting cleaners, direct surface skimming of the oily wash solution
from the tank, and separation of the oil from the wash solution using an
in-line, open channel gravity separator. See Fig. 1
Fig. 1
The emergence of oil splitting cleaners has been a great addition to the
tool box of heat treating managers. Oil splitters emulsify very little
oil. Oils in these cleaners rapidly separate and rise to the surface,
where they can be easily removed via direct surface suction . See Fig
2, Fig. 3 Newer versions are moving toward pH neutral formulations
which contribute to improvements in worker safety.
Fig. 2
Fig 3 Oil capture inside SmartSkim™ Separator. Clean fluids gravity drain back to washer on the right.
Gravity separators exploit the difference in specific gravity between the
oil and the wash solution. Oil separates from a fluid at a rate determined
by Stokes Law. This formula predicts how fast an object will rise or fall
through a heavier fluid based on the density and size of object and the
distance the object must travel. Open channel gravity separators exploit
both variables of Stokes Law. Oil must rise only a very short distance
before it is captured on the bottom side of closely spaced inclined plates
inside the separator. See Fig. 4
Fig. 4
Once separated from the flow, captured oil with very low water content can
be removed from the gravity separator . This oil which contains less than
5 per cent water, is referred to as dry oil. While oily wastewater
produced by other arrangements must be hauled at the producer's expense,
dry oil can typically be sold for a profit. Gravity separators discharge
dry oil to a collection barrel without any moving parts. The clean wash
water is returned to the wash tank.
Gravity separators also remove solids from heat treating parts washers.
Difficult to remove, lightweight solids can contaminate the wash bath
causing significant quality problems. Those solids are typically made
up of small, light particles of scale, grit, stop off paint, and a variety
of other contaminants. Solids can be sprayed back on to the part from
dirty wash solutions. Parts can also be recontaminated when they are
dragged through grit held in the oil at the surface of these washers.
Removing these types of solids has been difficult and expensive.
Coalescers have been tried but their media are quickly blinded over by
grit. Filtration devices, such as cartridges or bags are sometimes used,
but these require expensive consumables to stay on line.
Gravity separators do not require any consumables and are not blinded over
by solids. Treat-All Metals found that they do not notice any loss of
separation efficiency until their gravity separators become half filled up
- 50 percent of the separator volume-with solids, which typically takes
months.
Treat-All is not the only company which has tried this combination of
cleaners and equipment. Tim Hoefft of Caterpillar faced problems similar
to Treat-All with his company's old drag-out systems. "A review of
existing oil removal methods showed the need for an upgrade," Hoefft says.
"Belt and drum skimmers were not able to remove thecontinuous input of oil
from the washers."
Caterpillar tested a surface suction system similar to the one in place at
Treat-All Metals. "The test showed that all oil was removed from the
surface of the washer on a continuous basis" Hoefft says. Following
successful testing, Caterpillar deployed several of these systems.
"Due to the system's efficient use of gravity separation, very little
maintenance is required. In fact, beyond initial set up, our rule has been
to 'leave them alone'."
The Bottom Line
The combination in place at Treat-All Metals and Caterpillar offers
significant benefits:
- Cleaner parts
- Extended bath life
- Cuts in waste hauling costs
- Decreased cleaner costs
- Profit from a former waste stream
- Worker safety and environmental improvements
- Decreased maintenance requirements
All of this combines into a rapid payback, as opposed to many other
systems currently in use. Caterpillars review of the payback issues has
indicated that their systems will pay for themselves in "well under one
year", says Hoefft.
However, the switch to oil-splitting cleaners, suction skimming and gravity
separators can require adjustments. For example, most oil-splitting
cleaners are not good at removing already baked-on contaminants. Also,
gravity separators must be sized appropriately for the job to be most
effective. Small units can be overwhelmed by the treatment flows common to
heat treat parts washing. However, given the right configurations, this
new combination can yield powerful, enexpected benefits to heat treat parts
washing operations.
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