Is Your Grease an Explosive Risk?
By Lisa Tocci
Lubricating
grease isn't often the stuff of drama or danger. But under rare conditions, it
appears that certain PTFE thickened greases being applied with high-pressure
pumps can actually explode, warns the research head of one of Europe's largest
specialty lubricant manufacturers.
Speaking last
week to the National Lubricating Grease Institute's annual meeting in
Scottsdale, Ariz., Martin Schmidt-Amelunxen said that on at least three
occasions, silicone-based lubricating grease containing PTFE has exploded
during seemingly normal operations at end-user facilities. As head of research
and product development at Kluber Lubrication in Munich, Germany, he
investigated some common links in the explosions -- the type of grease, the PTFE
additive, and the use of high-pressure pumps. Now he wants end-users and grease
suppliers to be alert for the conditions that may lead to similar unexpected
blow-outs in their equipment, and to take steps to avoid them.
"Several
years ago, an explosion took place in a customer facility during a
grease-filling procedure," Schmidt-Amelunxen told the conference. This
highly viscous silicone oil based grease was applied with a standard
high-pressure grease pump. No high temperatures or other unusual operating conditions
were present as the grease was being pumped, yet a sudden explosion rocked the
room. One worker was injured, and others described an extremely loud bang,
followed by a black cloud of smoke.
"Due to
the high pressure of the explosion, objects -- even quite heavy ones -- were
blown through the room," reported Schmidt-Amelunxen. "A piece of hose
immediately after the outlet of a high pressure grease dispenser was determined
to be the source of the explosion. Surprisingly, the hose was not torn off completely,
and only a relatively small hole in the hose was found." The white,
food-grade grease inside the hose was clean, without black particles, but the
wall nearby was blackened by carbon residue.
Silicone
greases like the one involved in the explosion are niche products, used in
specialty applications such as sanitary, food and beverage filling
applications, or on beer keg valves, for example. Such greases often are
thickened with PTFE, to add extreme pressure properties. However, both silicone
oil and PTFE are inert raw materials, unlikely to explode spontaneously.
Unable to
pinpoint a cause, the accident investigators finally assumed an old or wrong
type of hose had been used, and closed the case. Best they could guess, a
"diesel" effect may have been caused by small air bubbles in the
grease, or perhaps improper cleaning of the equipment had led to the blast.
A couple years
later though, two similar accidents happened at other production sites. Again,
the explosions were located directly after the grease pump outlet and happened
during a high-pressure grease filling process. "The power of one of the
explosions was so heavy that a metal fitting of the grease pump was torn off,
and another piece of metal was stuck in the concrete wall several meters
away," said Schmidt-Amelunxen. "Luckily, no one was hurt by these
accidents."
Kluber began a
deeper investigation, with the aim of averting such problems in the future. It
wanted to know the source of the black residue from the explosion, and why the
explosions were so very focused.
Digging
deeper, Schmidt-Amelunxen and his Kluber colleague J. Muhlemeier saw some
links: The site of the blasts were all just behind the grease pump. Only highly
viscous PTFE thickened greases were affected. No flames were observed, but
there was strong pressure and a heat wave, combined with a loud
"bang." The flung-out grease was charred like carbon black, but the
grease inside the equipment was still white and unchanged. Pressures in the
pumps shot up by 400 to 500 bar on start-up. And in most cases, the explosions
occurred after a new grease container was hooked up, or after the weekend.
A close look
at the grease itself was puzzling. "No exothermic chemical reaction is
known between dimethyl polysiloxanes [silicone] and PTFE,"
Schmidt-Amelunxen stressed. No evidence of impurities or irregularities in the
grease was found. The black residue was identified as carbon black -- yet the
grease was all white.
The company
questioned if some kind of chemical decomposition reaction were taking place,
involving the depolymerization of the PTFE into tetrafluorethylene (TFE), which
in turn breaks down into tetrafluoromethane (CF4) and carbon black (C). TFE is
very difficult to handle and its decomposition can lead to violent explosions,
but normally such decomposition requires several hundred degrees Celsius to
take place, Schmidt-Amelunxen explained.
In fact, that
decomposition may be occurring, Kluber's subsequent research suggests.
"The presence of air bubbles is a key factor in the decomposition of the
PTFE, and there have been cases where adiabatic compression of air bubbles can
lead to very high temperatures of several hundred degrees C,"
Schmidt-Amelunxen said. (A simpler example can be seen in hydraulic and
compressor applications, where the air and pressure can mean higher oxidative
degradation of lubricants.)
To test this
theory, Kluber sponsored "falling hammer tests" on grease samples,
conducted under the eye of Martin Gosewinkel at the Wilhelm-Jost-Institute in
Hamm, Germany, a safety testing and explosion prevention center. Here,
silicone/PTFE greases containing varying amounts of air bubbles were subjected
to heavy blows with weights; after each blow of the hammer, the grease was
inspected and the presence of black spots was interpreted as a reaction caused
by the energy impact. Sure enough, greases with air bubbles exploded and threw
off carbon black, and did so at much lower-pressure blows from the hammer than
greases which were free of air.
"It was
clearly seen that the presence of air bubbles caused the safe energy level to
drop dramatically," the testers concluded. The grease was degrading due to
air -- but whether that came from oxidation or thermal decomposition is still
unknown.
In the end,
the falling hammer tests may explain why some PTFE greases explode, while
others seem to stay intact. Silicone greases without PTFE don't appear to be a
danger, nor are silicone greases with only small amounts of PTFE. The
triggering factor is the presence of air bubbles in the grease, which may be
introduced when the drums are filled -- or especially by lifting and lowering
of follower plates that are used with high-pressure grease pumps when the
grease is dispensed. Another source of air bubbles might be certain homogenizer
equipment used in grease manufacturing.
In any case,
Schmidt-Amelunxen advised, the source of the adiabatic compression is probably
a high pressure pump, where shock-like pressure peaks can trigger the oxidation
reaction and lead to the bubble implosions.
To combat this
risk, he advised pumping equipment be thoroughly degassed before each use, and
pump pressures and speeds be reduced as well. Anywhere grease is handled, there
should be protection against drawing in of air, and avoidance of shock-like
pressures. Kluber has already added these and other safety recommendations on
its MSDS, product information leaflets and labels.
"This is
not a quality problem or a problem of a specific grease supplier,"
Schmidt-Amelunxen emphasized, "but concerns all PTFE thickened silicone
greases of all manufacturers." One silicone grease supplier in the NLGI
audience was quick to demur, pointing out that the key factor seemed to be
PTFE, not silicone. Maybe so, said Schmidt-Amelunxen, but that was the only
grease type in each of the cases Kluber investigated.
To a follow-up
question, Schmidt-Amelunxen replied that he believes other grease types with
high PTFE levels could see the same violent reactions, if subjected to the same
air bubbles and high-pressure shocks. In fact, he suggested such explosions may
have already occurred, but the link to PTFE decomposition has been overlooked
by investigators. More research and openness may bring other cases to light, he
hopes, so the risk can be understood and avoided in the future.
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