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JohnsonMadukayil (Mechanical)
3 Jun 05 14:56
Is there a Arrhenius Plot out there for EPDM? We are trying to calculate the qualified life of an EPDM O ring in a valve using the following equation:

ln(life) = (E/K)(1/T) + Constant
where E = activation energy
K = Boltzmann's Constant
T = absolute temperature
Constant = Intercept of the Arrhenius line

I do not know what the Activation Energy value would be for EPDM(Ehtylene Proylene Diene Monomer) nor the value of the Constant. Can someone please throw some light on this?

IRstuff (Aerospace)
3 Jun 05 14:58
This is pretty much dependent on the design and manufacture of the object, so the supplier should be the best authority.  

TTFN

Cockroach (Mechanical)
3 Jun 05 15:20
What are you doing?

Elastomer lifetimes are not simply a function of "activation energy", but rather dependent on environment (i.e. pressure, temperature and chemical composition of the working fluid) and manufacturing processes (base rubber composition, mixture additives which are trade secrets, press pressure/temperature).  And this is only the start of a huge list of unknowns.

Elongation of life is typically a laboratory experiment conducted in autoclave studies by the manufacturer.  On ocassion I have had the need to design and perform studies in explosive decompression and degragation in rubber when an unlisted environment pops up in an application.  You cannot simply sit behind a desk and expect to pull a recipe formula from a chemistry book.

IRStuff has it right.  The manufacturer would have the necessary information assuming you have the right chemistry for the working fluid.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

JohnsonMadukayil (Mechanical)
3 Jun 05 15:29
The working fluid is Air, pressure is 160 psig and the temearuter is 49 Celsius normal and 135 Celsius max. The manufacturer (Parker) has no clue. Called up Dupont Engineering to see if they have some idea, and left them a message. The valve is going into a nuclear utility.
unclesyd (Materials)
3 Jun 05 16:05
Like all the previous replies an O-ring is compounded elastomer and in your case the base is EPDM.  EPDM O-rings have about as many varieties as Neoprene O-rings.    

What does the valve manufacturer have to say about the life in air service?  They should have some idea of the number of cycles if the O-ring is dynamic.

Did you call the Parker Tech Service or did you query a distributer?

Parker at one time was the leader in technical service, but that was several years ago.

Another good source "was";

http://www.gtweed.com/85256D3C0038D8DA/vwContentByKey/N25NLPR4971DMPJEN
Carburize (Materials)
7 Jun 05 10:13
Check with Cameron Elastomer Products they have done work in this area for over 20 years.
JohnsonMadukayil (Mechanical)
7 Jun 05 10:30
I had asked Parker Technical support. They did send documentation saying that the EPDM Oring can withstand 10exp6 rads over a period of 5 years. But this info. may not be enough.
proscow (Materials)
27 Jun 05 10:07
"Qualified Life" is a term used in nuclear power industry for equipment used in Pressurised Water Reactors (mainly).  Equipment, eg a valve actuator will be tested by ageing at some temperature higher than the expected service temp to simulate the required lifetime, it will be irradiated to a dose simulating service plus accident, go through operational cycles, be seismically tested and go through a thermal transient to simulate an accident at end of life.  Polymeric components are often the life limiting bits and are given a qualified life, after which they need to be replaced, based on testing of the whole component.  The testing is expensive so if an actuator is to be used in a slightly different application with say a lower service temperature an estimate of the new qualified life (i.e. service interval) can be estimated using an Arrhenius extrapolation.  Activation energies are held in a nuclear industry database.  BUT the activation energy can change with temperature, will be different for different EPDM compounds, will be different for different properties.  There are activation energies for EPDM varying from <0.8eV to 1.48eV.  You can only use the method once you have some ageing data to start with and I would not recommend it unless you understand the significant errors and limitations inherent in the method.  EPRI have written reams on the subject.
What you actually need to do is specify the operating conditions to the seal manufacturer in detail and ask how often the seal should be replaced.  You need to say if the seal is a face seal/rod seal atc, static/dynamic, process fluids (including any contaminants), temperatures with duration and frequency of any cycles, pressure, radiation dose rate (if appropriate).

For the conditions that you quote and if the time at 135C is very limited (a few hours) and very rare (once or twice) and it is a static face seal, I would say that typically qualified life would be 10 years maximum for an EPDM seal.

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