In retirement, I’ve had time to contemplate the meaning of life and mechanical seals. With mechanical seals, as with life, things have not always worked out the way I thought they would. I’ve come to believe that mechanical seals, like life, are near-chaotic systems.
Chaos is sometimes defined as complete confusion and disorder. If Chaos is complete confusion then Chaos Theory is the mathematics that attempt to explain it – or at least show how Chaos cannot be explained or controlled.
One characteristic, perhaps the easiest to understand, is that chaotic systems have such a strong dependence on initial conditions that the outcome appears to be random. Chaos was summarized by Edward Lorenz as “When the present determines the future, but the approximate present does not approximately determine the future.”
Non-linear systems sometimes respond in an apparent chaotic manner. It is important for engineers to realize that our day-to-day mathematical methods often use extremely linearized versions of non-linear systems. Anyone who applied engineering mathematics using a slide rule can readily appreciate this linear simplification; however, many computer programs still use the same linear simplifications that we fully mature engineers learned to use on our slide rules.
Examples of potentially chaotic simple systems include:
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- Spring-mass system (with non-linear spring) with damping
- Fluid flow, especially for turbulent flow regimes
- Any process relying on friction
- Any process having wear
- A dripping faucet
- Stick-slip sliding
- Thermosiphon systems (flow direction can reverse).
Any of the above examples seem familiar? Let’s see where Chaos Theory might apply to mechanical seals:
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- All seals have some sort of spring; spring force is a function of seal position (setting)
- Metal bellows seals have little damping
- O-rings provide damping but are dependent on lubrication and surface finish
- Fluid flow patterns around seals are dependent on flush rate (and fluid type, rpm, clearances, etc.)
- Seal face friction is dependent on material combinations and lapping (not to mention speed, load, fluid, leakage, etc.)
- Seal face wear rate depends on face load, friction, materials, lapping, fluid, leakage, etc.
- Shrink fits affect seal face flatness and waviness
- Stick-slip sliding between seal faces is a well-known phenomenon, especially with metal bellows seals
- Piping plans for seals sometimes rely on thermosiphon effects.
Not to mention venting!
We certainly want and expect a consistent performance from mechanical seals. Let’s look at how this might be accomplished.
An often overlooked aspect of mechanical seals is the surface finish (roughness) of the faces. Most people know the importance of lapping a seal face to near perfect flatness but the surface finish is also extremely important. Surface finish is measured in millionths of an inch and can vary considerably with the material, manufacturer, even batch. Also, many suppliers do not check surface finish regularly but rely on the manufacturing process for consistency. Some suppliers may not even have the equipment to check surface finish.
The flush rate to a seal is important not only for traditional heat balance considerations but for establishing the flow pattern around the seal. More – or less – flush is often given credit for solving seal reliability problems when the effect may be due to flow pattern and not traditional heat balance.
Other approaches to minimizing chaos in mechanical seal performance include:
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- Designs employing damping (“pusher” seals)
- Monolithic designs instead of shrink fitted designs
- Cartridge seals have a more consistent assembly and therefore consistent spring load
- Consistency in selecting materials for repaired seals
- Don’t rely on thermosiphon effects
- Vent! Vent! Vent! Don’t attempt to startup with air in the system.
A more appropriate title for this post might have been “Mechanical Seals and Chaos Theory”; however, that title appears a bit more academic than this post actually is. Besides, I really don’t know Chaos Theory but surely it applies to mechanical seals! Perhaps someone with more up-to-date skills in mathematics will apply those skills to studies of mechanical seals and provide guidelines to preventing chaos.
Regarding Chaos and Seals, we need some Nomos;
Hello everyone,
I am relatively new to sealing technology, but we do have some very interesting innovations to share with the sealing community.
Our background is from the machine tool and metrology industries. This gives us a very deterministic outlook. That is everything happens for a reason, based on unchanging natural laws and that it is possible to discover the system influences and deterministically achieve a particular outcome.
This is contrary to Chaos theory and so based on the opposite of the Greek idea of Chaos which is Nomos. Unfortunately there have been many books and much commentary on Chaos theory in the last 20 years, yet it’s opposite, the idea of Nomos is almost entirely unknown. You can still find out about it online if you dig past all the watches and Wikipedia’s but the best place to learn about Nomos is old books.
That being said, let’s apply the idea of Nomos to industrial seals. Today depending on the type of seal and piping plan you either have flow from the process across the seal to vent or your buffer/flush flowing across a seal into a process. This flow across the seal face is the proximate cause of contamination getting into the seal face, not to mention issues with process contamination or flaring requirements.
How can we use deterministic natural laws in order to improve the situation? We can use externally pressurized porous gas bearings to create pressures in the seal gap that are higher in the process and the vent. So there is no flow and so no contamination coming across the seal face. Also with two externally pressurized inputs to the seal face, one being process and the other being seal gas like nitrogen. By using a particle counter in the vent the nitrogen pressure can be adjusted to make the high pressure point in the seal face right between the gases. That way all the process gas returns to the process and all the nitrogen flows to the vent, none of the nitrogen goes into the process. And this is done in a single primary seal face. This will dramatically reduce the size and complexity of seals while eliminating sending gases to flare.
Here is what Pumps and Systems has published on the technology;
https://www.pumpsandsystems.com/search/node/drew%20devitt
And Compressor Tech2;
http://digital.ct2.co/html5/reader/production/default.aspx?pubname=&edid=65dec9cd-8fb5-43f6-a3c4-f62c994b15ad&pnum=50
And here is a longer explanation;
Conventional Dry Gas Seals (DGS) have a flow across their face, from the high pressure to the low pressure side. Moisture or oils from the process are naturally carried into the seal gap by the flow from this pressure differential, where they carbonize or boil from the shear and cause reliability issues. Trying to stop this with buffer gas is like trying to stop water from flowing downhill. Although this leakage is small, it is coming under closer scrutiny from the EPA and other regulatory bodies.
Our “New Way Seal” uses Externally Pressurized Gas Bearing Technology (EPGB) through a porous seal face to create a pressure in the seal gap that is higher than the process pressure. The flow to vent is the same, but about the same amount that is vented flows back into the process. The advantage is that moisture will not enter the gap because the gap is at a higher pressure. It would be like water running up hill; it is just not natural for a lower pressure to flow into a higher pressure. So, there is no flow across the seal face from the process, increasing reliability. The process gas required for the externally pressurized bearing is 1/100 of the buffer gas used with Conventional DGS, and the higher pressure differential makes it easier to condition the bearing gas reliably.
Figure 2: New Way Seal
With our “Ventless Seal” we have all the advantages of the New Way Seal, but additionally we can segregate gases in a single seal face. This enables a Zero Emissions Seal (ZES). This is where all the process gas stays in the compressor and all the barrier gas exits the compressor. By using two externally pressurized bearing gases, (process gas is used on the process side of the seal face and a barrier gas on the vent side) their relative pressure may be adjusted to steer the highest pressure point in the gap to be between the gases. At this point, all process gas flows back to process and all the barrier gas exits the compressor, eliminating Fugitive Emissions and flaring. The balance point is determined by a gas detector in the vent that looks for any process gas molecules (say 10 to 100 parts per million as a threshold) with a control to slightly increase the barrier gas pressure and so maintaining the balance at the separation between the gases.