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topic of discussions on cylinders dieseling.

topic of discussions on cylinders dieseling.

topic of discussions on cylinders dieseling.

(OP)
I saw this topic somewhere else and found it very interesting.
Perhaps it makes sense to hear more from experts' voice in this forum.

"Let's open a thread of discussions on cylinders dieseling. A dieseled cylinder will not operate properly due to burnt holes through the piston seals, requiring to be replaced and repaired.
Dieseling in cylinders is a thermodynamics event referencing the explosion of air pockets and oil vapors that mix together, get compressed to bubble size that are carried on the piston surface- most likely the wear band, and through a low number of cycles accumulate such an internal energy / temperature that the mixture auto ignites, burning holes through seals, or even blowing out metal chunks from the piston, in the vicinity of the O-ring, or seal grove where the explosion happened.
As the title suggests, the phenomenon is similar to the combustion process in a diesel engine, and it needs a couple of elements to happen: Air bubbles, or aeration in the hydraulic system, oil fumes or oil vapors that become a combustible mixture, rapid compression and high velocity reciprocating motion that raises the temperature of the mixture past the oil auto ignition temperature.
The method of troubleshooting and implementing changes, so that such events are avoided or eliminated is as complex as one can imagine.
First, it is worth saying that most likely the big, expensive cylinders from mobile systems are exposed to dieseling, as it is almost impossible to eliminate aeration completely in the hydraulic system. Before getting mad at me for this statement, please consider that a substantial part of the hydraulic circuitry is placed above the free surface of the hydraulic tank, therefore bleeding off air is difficult, and incomplete. Hoses supplying fluid to the head end of the cylinders will be arching close to the top of the machine/cabin level, thus having a high potential of entrapping air. Even if this is not the culprit, or the only one, air can be sucked in the installation through the rod seals, and the discussion can continue.
When the cylinder is cycled at high pressure, and the velocity is high, the thermodynamic cycles seen by the bubble will raise the internal temperature of the mixture very rapidly, reaching within 5 or 6 cycles 500 + degrees F, which is higher than most mineral oils auto-ignition temperature. If you don't agree with this statement, please run an example using the law of perfect gases.
There are always triggering elements that will make some cylinders diesel, and the ones that I noticed were aeration + high velocity at the end of the stroke.
Floor for discussion: there are many possibilities for air to be entrapped in the system, and responding to this question first, leads to how to troubleshoot dieseling cylinders. It is complex, interesting, frustrating, costly, but in the end very rewarding when the solution finally is found and proven."

The original post I saw is much longer and can also be found in:
http://www.linkedin.com/groups/Lets-open-thread-discussions-on-1407697.S.45089082?qid=b14ce70b-a73f-42a0-a68d-97dcee44bd98&goback=%2Egmp_1407697

Regards
Max
 

RE: topic of discussions on cylinders dieseling.

Max, and what do you think?

I've seen dieseling occur in pneumatic hammers when enough lube oil and excessive hammer/piston stroke happened.  What is called dry firing can result in dieseling.

I've not seen it in hydraulic cylinders.

If it takes a few cycles to get the 'bubble temperature' high enough it would seem that not enough exchange of oil in and out of the cylinder is happening to take away the compression heat.

I would think if there is enough air to support combustion, the system response had previuosly exhibited spongy chacteristics and should have been looked at for air ingestion.

Ted

RE: topic of discussions on cylinders dieseling.

Usually the entrained air is as a result of cylinder replacement, not just the machine running under normal operating conditions.  Abnormal conditions such as low reservoir level and pump cavitation will also lead to air entrainment.

A simple solution to the problem is to start the system slowly (under low pressure if possible) during commissioning, to help prevent buildup of the pressures required to cause dieseling.  Additionally, pre-fill of the cylinder will aid in prevention.

Air being drawn in past seals is as a result of design or system failure.  A cylinder should never be allowed to move faster than the oil is provided to it; a system drawing in air will also draw in other contaminants (not to mention possible cavitation of the pump to boot).
 

RE: topic of discussions on cylinders dieseling.

I'm not aware of hydraulic cylinders suffering Diesel events.

I am aware that hydraulic accumulators didn't always have bladders, or diaphragms, or vented seals, or even pistons.
... all of which are present because of Diesel events and collateral damage.

I am seeking citeable reports about the Diesel events so as to prevent more of them.


 

Mike Halloran
Pembroke Pines, FL, USA

RE: topic of discussions on cylinders dieseling.

The term "dieseling" in fluid power is used to describe explosive decompression, the results of pressure trapping and the results of partial dry operation.

Explosive decompression happens primarily to elastomeric materials with too much groove clearance in high (greater than the yield strength of the material) pressure conditions. If you use industry standard o-ring grooves and subject the o-ring to pressures spikes greater than 10000 psi and drop to a negative pressure the o-ring will move in the groove with such force that when it hits the other side of the groove, material will break off.

Pressure trapping frequently happens in hydraulic cylinders with buffer seals in front of the rod seal. The pressure intensifies when trapped between the buffer and rod seal promoting burning at the edge of the rod seal. This can happen in cylinders with two opposing u-seals in high speed, high pressure spike applications.

Partial dry running can be caused by too tight of tolerances on moving parts when moving at higher speeds and when cylinders operate at high temperatures with close tolerances. All polymeric seal have a much greater coefficient of expansion than metals so this needs to be addressed in the design. I have over stuffed seals in higher temperature (170°F) high pressure (5000 psi+) cylinders. The seals look burnt and can have material pulled from the contact surfaces.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: topic of discussions on cylinders dieseling.

Ed;
Thanks for your addition to the topic, very insightful!
Chris

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