[TheForge] heavy metal math/c frame press

[David E. Smucker]

I try again, the first time I sent this message it failed. (For Andy)

> "David E. Smucker" wrote:
> >
> > Andy,  First there is no such thing as NON-compressible fluid
>
> Yes, there is.  A non compressible fluid is one where the
> fluid density is independent of pressure. Water is non
> compressible.  As far as I know there is no terrestrially
> attainable pressure that will compress a body of pure water.

All physical matter has a bulk modulus -- water included and it is
compressible.  Just not very.

And I quote

"The bulk elastic properties of a material determine how much it will
compress under a given amount of external pressure. The ratio of the change
in pressure to the fractional volume compression is called the bulk modulus
of the material.
      A representative value for the bulk modulus for steel is

B(steel) = 160 x 10(to the 9th) N/m2

     and that for water is

B(water) = 2.2 x 10(to the 9th) N/m2


      The reciprocal of the bulk modulus is called the compressibility of
the substance. The amount of compression of solids and liquids is seen to be
very small.

      A common statement is that water is an incompressible fluid. This is
not strictly true, as indicated by its finite bulk modulus, but the amount
of compression is very small. At the bottom of the Pacific Ocean at a depth
of about 4000 meters, the pressure is about 4 x 107 N/m2. Even under this
enormous pressure, the fractional volume compression is only about 1.8% and
that for steel would be only about 0.025%. So it is fair to say that water
is nearly incompressible. Reference: Halliday, Resnick, Walker, 5th Ed.
Extended."

(Website
http://hyperphysics.phy-astr.gsu.edu/hbase/permot3.html  )

      As Tom, relates in another post, in the real world most fluids have
more compressibility that we think.  When dealing with high performance
hydraulics systems and modeling them we would use values of twice the
handbook value.  Most of this we related to entrained gas -- but it matched
the real world performance of these systems.  Such consideration are very
important when designing high performance servo hydraulic system such as
those used in today's rolling mills to provide gauge control.

      > > Another example of high stored energy is when a load is support by
a blocked
      > > hydraulic cylinder such as in a fork lift or pay loader lift.
Failure to
      > > understand this possible stored energy can result in serious
injury or death
      > > to those working on these type of systems.

      > Could you elaborate?  I'm not sure what you mean here.


      When we have a load -- supported by a hydraulic cylinder and held in
place by a closed center valve there is stored energy in the system between
the cylinder and the valve.  On my tractor for example I can support a 1000
pounds in the bucket of the tractor lift and it is held there with a closed
center valve even with the tractor off and the pump not running.  If I
remove the hose between the cylinder and valve under these conditions two
things will happen -- I will have full oil flow -- and the bucket will
"quickly descend".  In my old day job -- I know of several fatal accidents
where equipment moved and crushed someone working under such a suspended
load.

      Now back to blacksmithing.

      Dave Smucker

----- Original Message ----- 
From: "Andy Vida" <osan@netlabs.net>
To: <theforge@mailman.qth.net>
Sent: Tuesday, November 25, 2003 4:16 PM
Subject: Re: [TheForge] heavy metal math/c frame press


>
>
> "David E. Smucker" wrote:
> >
> > Andy,  First there is no such thing as NON-compressible fluid
>
> Yes, there is.  A non compressible fluid is one where the
> fluid density is independent of pressure. Water is non
> compressible.  As far as I know there is no terrestrially
> attainable pressure that will compress a body of pure water.
> If we speak in terms of fifth-state conditions (what was
> once called "super-dense" matter, AKA black holes) then
> the fundamental structures of mundane matter are being
> defeated and all bets are off.
>
> Hydraulic fluids are either noncompressible or virtually
> so.  The amount of stored energy in such a fluid is, for
> all intents and purposes non-appreciable.  As far as I know
> ALL measurable stored energy in such systems resides
> in the containment structures.  Whether micro-miniscule
> compressibility is an issue in the design of high pressure
> systems as you state, I will defer to your assertions as I
> am by mo means an expert in this field.  But I'm curious
> then, because the stored energy in the containment structures
> of the system is still going to be far and away greater.
> Can you say what the precise issues are?  I find this idea
> fascinating.
>
> > Stored energy is no where near what it is in an
> > air or steam system  at low pressure such as 100 psi.
>
> Precisely, and in order to do mass damage, you need volume.
>
> >
> > The amount of compression and stored energy in very high pressure
hydraulic
> > systems at 5000 to 10,000 psi is another story but for systems that
> > blacksmiths and bladesmiths use this would be unrealistic.
>
> Unless you happen into a good deal on a press that uses an
> 11 KPSI Enerpac setup. :)
>
> > The stored energy in a hydraulic press is another issue all together.
As a
> > press is loaded there is store energy in not only the oil but in the
stretch
> > of the press itself.
>
> Well now this is a very different kettle of fish, and I completely
> agree.  The mass of the press constitutes a huge spring, mainly
> due to the tension (as opposed to compression)
>
> > The real danger is from a smaller leak that
> > causes a misting flow of the hydraulic oil -- which that comes in
contact
> > with the hot metal being worked and causes a fire / explosion of the
mist
> > cloud.  Hydraulic oil in liquid form is quite safe but in mist form it
is
> > much more like a gas.
>
> Oh yes, very definitely so.  This was a major problem with
> commercial aircraft regarding collisions.  An aircraft in
> troulble dumps fuel (ideally) prior to "landing" to prevent
> or at least minimize the fire hazard which as I recall is a
> greater threat to survival than initial impact in most cases.
> The problem lay in the hydraulic systems where breaches of
> containment of the high pressure lines would cause atomization
> of the fluids, which would in turn ignite.  The resulting
> conflagration was effectively as bad for the passengers as if
> the fuel had not been dumped.  There are now flame retardent
> hydraulic fluids specifically designed for aviation applications
> but my recollection is that they are not perfect and can in
> fact ignite under certain conditions.
> >
> > The point of my referencing the pump failure that you noted is that it
is
> > very rare and I have never seen one caused by the hydraulic pressure
itself.
> > None the less in the industrial system that this pump fail there was a
large
> > amount of stored energy because this system -- as do most high
performance
> > industrial systems -- included major gas hydraulic accumulators to
provide
> > high volume flow for short duration as required by the system.
>
> Ah, I think I understand you.  Basically you are referring to
> gas cylinder capacitance for low pressure burst flow, no?  I
> gather that the accumulator is solidly isolated from the rest
> of the system via valving during high pressure operation, yes?
> If so, then we are talking about hybrid systems, which are
> different from the purely hydraulic.
>
> >  None the less I think it is important to not give the
> > impression that hydraulic system don't have cases of high stored energy.
>
> Yes, I completely agree here.  I was unaware that there were
> such mechanisms in place, but it makes perfect sense.
>
> > Another example of high stored energy is when a load is support by a
blocked
> > hydraulic cylinder such as in a fork lift or pay loader lift.  Failure
to
> > understand this possible stored energy can result in serious injury or
death
> > to those working on these type of systems.
>
> Could you elaborate?  I'm not sure what you mean here.
> >
> > I still want to come back and once again make the point that there is
real
> > risk of fire / explosion between a hydraulic system and hot metal were a
> > small leak can create a mist that then can become a major fire ball.
Just
> > make sure your fittings, pipe, and hose are correctly rated for the
> > pressures you are using.  You don't want to spray / mist hydraulic oil
on
> > hot metal !!
>
> One solution is to design a failsafe point into
> the system.  That is to say, you over engineer the entire
> system to some degree and place a "weak" element in a
> location where failure is deemed to pose the lowest threat.
> You might, for example, over-engineer all the conduits and
> place the failsafe point inside the reservoir on the high
> pressure side of the pump so that if there is a failure due
> to over pressurization, it will occur at the weak point and
> be contained by the structure of the reservoir.  No mess,
> and damger is contained.  The only mess will be the hands
> of the mechanic that has to replace the failsafe point. :)
>
> I'm not sure there is a lot to be done to protect against
> failures due to damage to the containment structure (e.g.
> driving your forklift into a concrete abuttment and nicking
> a line that fails later on).  Perhaps digital monitoring
> such that any sudden pressure drop that crosses a predetermined
> threshold triggers a release or deluge valve (deluge valves
> are super high speed opening and closing, microseconds from
> full closed to full open) to depressurize the system immediately.
> But things get so over complicated... :(
>
> -Andy
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