Machinery Lubrication July August 2008

CONTAMINATION CONTROL
Managing Water
Contamination to Maintain
Effective Steel Mill Lubrication
BY GREGORY J. SEDELMEIER, SHELL GLOBAL SOLUTIONS (U.S.) INC.
Moisture is omnipresent in steel mills. It comes primarily
from mill cooling water entering the circulating oil
system and from simple condensation during mill downtime.
Although necessary to reduce the extreme heat generated in
the processing of steel, water is among the most destructive
contaminants to the circulating oil system. Water can
degrade a lubricant in a variety of ways, including dissolution
of the water into the oil, the formation of emulsions or simply
the presence of free water.
First and foremost, rust occurs when water attacks steel or
metal surfaces, and this leads to the formation of iron oxide.
Corrosion differs from rust because it occurs when metal
surfaces are attacked by acids. Regardless, whether by rust or
corrosion or both, the end result is the same: damage to the
metal surface. In addition, rust and corrosion prematurely
degrade the lubricant performance unless steps are taken to
minimize the conditions that promote them and a quality
lubricant is selected to ensure long life.
All kinds of critical mechanical parts in the steel mill are
susceptible to water contamination. These parts include roll
necks, journals and roller bearings. Therefore, it is important to
use lubricants that can effectively handle wet environments and
resist quick degradation of quality due to the formation of emulsions,
accelerated oxidation and other factors.
Emulsions
The rolls in the hot-strip mill may be subjected to thousands of
gallons of water every minute, so the bearings can take in significant
amounts of water constantly. The churning of the oil and
water through the bearings can create a milky-looking substance
called an emulsion. An emulsion is a mixture of insoluble liquids
in which one is dispersed in droplets throughout the other.
In mill operations, once an emulsion forms, it is often
difficult to break. Emulsion forming in the lubricating system
will cause the circulating oil to lose its ability to lubricate
effectively, which can result in rusting, oxidation, corrosion
and a general degradation of the bearing life.
Four key elements are necessary to form an emulsion: oil,
water, agitation and an emulsifying agent. Agitation is necessary
to shear the water into small enough droplets that could
get dispersed into the oil phase. The emulsifying agent is a
component that serves to stabilize the emulsion, and is
usually present in small concentrations. It is soluble in the oil
phase and it concentrates itself at the oil-water interface,
26 July - August 2008 machinerylubrication.com Machinery Lubrication