Chemical Engineering November 2011 - 50

Feature Report
Engineering Practice
Oil-Mist
Systems
This primer discusses the pros and cons
of open- versus closed-loop designs
for lubricating pump and motor bearings
Heinz P. Bloch
Process Machinery Consultant
or decades, plant-wide oil-mist
systems have been used to lubricate
rolling-element bearings in
high-speed applications throughout
the petrochemicals, textiles,
pharmaceuticals, mining and other
chemical process industries (CPI). Applications
range from small motors to
large rotating cylinders and a variety
of fluid movers. Plant-wide oil-mist
systems provide a centralized way to
supply lubricating oil to all such components,
and often eliminate the need
for frequent maintenance and corrective
action.
Closed-loop oil-mist systems (Figure
1) have been widely used in these
industries for decades. In fact, the CPI
have been leaders in the use of plantwide
oil-mist systems since the 1960s.
Due to their lower initial cost, openloop
oil-mist systems have traditionally
been more popular in the U.S.
While they may be less costly up front,
open-loop systems consume oil in a
once-through fashion (Figure 2) and
they can discharge spent or " stray " oil
mist to the atmosphere, which creates
environmental issues.
By comparison, closed-loop systems
(as shown in Figures 1 and 3) allow
the oil mist that has passed through
the equipment bearings to be collected,
filtered and reused. As a result,
the lifecycle cost of a well-designed
F
closed-loop system is often less than
that of a comparably sized, plant-wide
open-loop system.
As noted above,
one particular
downside of open-loop systems is that
they allow a considerable amount of
stray mist to escape from the pump
bearing housings (Figure 2). In the
face of strict, plant-specific environmental
requirements, many facilities
now prefer to implement closed-loop
technology, whereby the mist is routed
in compliance with API 610.
API 610 is a widely used and
strongly recommended - although
not compulsory - industry specification,
which calls for oil-mist introduction
between a modern face-type
or rotating labyrinth-style bearinghousing
protector seal and the adjacent
rolling-element bearing. This is
shown in Figure 3. The introduction of
an oil mist into this open space prevents
process vapors or contaminated
atmospheric air from accumulating
there. Similarly, the existence of oil
mist in those spaces at slightly over
atmospheric pressures precludes the
ingress of moist or dirt-laden atmospheric
air.
It is worth noting that lip seals -
which are widely used in inexpensive
pumps - are not considered to be acceptable,
per the API standard. It is
generally assumed that elastomeric
50 CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2011
FIGURE 1.
Shown here is a
closed-loop oil-mist system
that provides lubrication for bearings
in process pumps and their drivers. If
desired, the small, blue collection containers
that are shown in this image can
be furnished with automated or manual
means of pumping the coalesced oil into
the spent mist-return header
Colfax
lip seals start leaking after about
2,000 operating hours. In contrast,
some API-recommended face and rotating
labyrinth seals run well in excess
of 50,000 hours. In the modern
face-type bearing-protector seal (as
shown in Figure 3), a closing force is
applied by a series of magnets. These
magnets are equally spaced around
the circumference of the stationary element
of the seal.
In API 610-compliant systems, the
oil mist must flow through the bearing
(or bearings) on its way to the central
outlet port. Central outlet ports
- usually located at the 6 o'clock position
of the bearing housing, as shown
in Figure 3 - can be connected to a
common return header, which operates
at a pressure below that of the
oil-mist supply line. This application
of a slight vacuum will increase the
flowrate through the system.
The spent mist in the drain header
can then be sent through a final coalescer
element, which will capture
residual oil droplets that are still entrained
in the carrier gas. Instrument
air is generally used as the carrier gas.
While other carrier gases are entirely
feasible, instrument air is fully satisfactory
in virtually all plant-wide oilmist
installations.
A closed-loop system that applies oil
mist to the bearing housings creates
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Chemical Engineering November 2011

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