I would like to start this post with a real life story I witnessed a couple of years ago.
A client of ours - a dredging company - was claiming
that the gear pumps we were supplying to them were of a bad quality,
resulting in a very low durability - a couple of hundred hours at most,
which, of course, was unacceptable. When confronted with the standard
oil condition question, the owner of the business assured that the oil
was "used but operational". It was obvious that the problem required
investigation.
The gear pump in question was used to power
hydraulic jacks of a large split barge. A split barge is basically a
barge that's "cut in the middle". The two halves are hinged at the deck
and operated by two enormous hydraulic cylinders, the cargo (rocks,
sand, soil) is carried in between the halves and is dumped by splitting
the whole vessel longitudinally.
Under normal working conditions the pump, run by a
small three cylinder Deutz, worked for about fifteen minutes at most
every couple of hours - the time to split and close the hull to dump
the load. Operating pressure was relatively low, around 150 bar if I
recall correctly, so the pump was working way below its rated
capability.
The inspection of the oil tank revealed milky smelly
liquid, which the crew was persistently calling a hydraulic oil. A
short conversation with the crew members revealed an astonishing and
breathtaking tale about the unhappy day when a steal pipe that ran
along the deck burst, spilling most of the oil over it. The main reason
for the oil spill being so big was the fact that to stop the engine a manual
decompressor had to be pulled, and the lever was mounted directly on
the engine, situated in a small compartment under the deck - quite a
long way from the cockpit. People, who never saw it, can't take their
eyes from the dumping process, however those, who are already thick of
seeing it on a daily basis, look everywhere BUT at the damping, so when
the corroded pipe ruptured, nobody was looking at the deck, and when
they finally saw the oil pool, the engineer had to sprint down several
narrow stairs all the way to the motor compartment to stop it!
The ruptured section was replaced, but the oil level in the tank was dangerously low...
A flashback now, if I may - in the beginning
of my hydraulics career I was employed at a company that manufactured
hydraulic equipment for industrial fishing, and so I spent
many days onboard all sorts of vessels all over Portugal, which gave me
the perfect opportunity to get acquainted with seamen first hand. One
thing I must tell you - if a sailor (or a fisherman) can't find a way out
of a pickle - the world has come to an end!
That time was no different, and the crew quickly came out
with an effective and elegant solution of gathering the spilled oil
with brooms and shovels
from the deck into buckets, and then transfer it from buckets back to
the oil tank, filtering it through a piece of cloth... Of course, when
you are facing a "no other exit" situation and you need the barge
closed to navigate, I guess you might say "to hell with the pump, I
want the damned barge closed", do the broom magic, come back to harbor,
and deal with the situation there... But the thing is... the
predicament had happened more than a year before!!! Ever since
the incident the hydraulics has worked with the same deck-tainted oil!
No wonder the poor pumps wouldn't last. I would guess half of the
hydraulic oil was sea water, the other - sea-water-in-oil emulsion,
with common deck-type anti-anti-wear abrasion additives...
The most astonishing thing was the fact that neither
the crew members, nor the owner of the business, were anxious about the
oil. Their logic was bullet-proof - the water will eventually
evaporate, the particles will eventually filter out, so no big fuss
here... The sudden increase of pump failure rate was explained by low
quality of the supplied pumps, and I'm still not one hundred percent sure
that I managed to convince the owner that the hydraulic oil was no
longer in an acceptable condition.
This story is anecdotic, but I must tell you that I
have seen more than once similar attitudes among hydraulics related
businesses. I am referring here, of course, to the matter of reusing spilled hydraulic oils.
Oil hydraulics is all about oil in huge quantities,
and it is normal that it gets spilled all the time - when you
disassemble a component - the oil is spilled over the bench, when
you connect and disconnect a component to a test bench - the oil gets
spilled, in fact, whenever you do anything around hydraulics -
there's always an oil spill. Of course there are
benches that are prepared to gather the oil spilled over them, and most
of the times it is possible to catch the "runaway" oil from
mounting/dismounting procedures into a tray, but there's still the
question of what to do with it next.
Simply discarding the gathered oil is only viable in case
of very low volumes and occasional spills, but most hydraulic shops
deal with oil spills on a daily basis, so throwing the oil away
can become an expensive practice. Reusing the caught oil, by putting it directly back
into a test rig or a machine is also a very bad idea, and,
unfortunately, is something I see a lot in many shops out there. The
main reason for it, is the fact that gathered spilled oil brings along
huge amounts of particles washed from anything it was spilled over.
With the oil prices constantly growing it is important and
economically justified to reuse "caught" spills, but it is also very
important to filter the gathered oil prior to reintroducing it to a
hydraulic system. From my experience I can tell you that simple single
passing through a filter will not do the trick - the particle
contamination level of spilled/gathered oil is enormous (it is
still better than simply pouring the oil directly to a machine,
though), so it might not be an exaggeration to build a small tank with
a filtering system for exhaustive filtering - which is especially
important for pump test benches, where oil contamination levels should
be as low as possible.
As an alternative economic solution, a devoted spilled oil
tank can be made, which, if designed properly, can use gravity to do
all the necessary filtering. In that case I would suggest a high narrow
tank (like a drum), with the suction line somewhere in the middle and
turned downwards (probably with a screen), a ball valve and a bank of
filters. When the drum is full, the ball valve is opened and the
oil is slowly drained from the middle, leaving all the heavy particles
at the bottom, and most of the fine particles in the filters. The
resulting oil can then be transferred to the respective machines/ test
benches though their return filters using previously mounted fast
couplings.
It is also very important to make sure that there are
no mixtures of incompatible oils. As a rule, you can mix different
viscosity grade hydraulic oils with the same additive package without
consequences, but when not certain, your best bet is still
to discard the unknown oil.
My point is very simple - it is not wise to simply
throw away spilled oils, but it is also unwise to re-use them without prior
filtering. Lots and lots of technical solutions are possible for that
purpose, but it should be noted that simple one filter solution, though
still better than none, will not give satisfactory oil cleanliness
levels, therefore more elaborate filtering solutions should be applied,
without leaving aside one of the best (and free) particle separating
agents - the gravity. In any case the priority should be the
cleanliness level of the outcome oil, and not the filtering speed.
