lifetime of an o-ring 4

[offonoff]

I thought i posted this already but an hour later it isn't in the forums. Must have pushed the wrong button or something.

We've got a 6" port that needs sealing on a 1500 psi, under 100 C vessel full of super critical carbon dioxide and dissolved oils, terpenes, ethanol and others. A grooved flange and an o-ring is the likely solution.

the port will be opened daily and under these conditions i am curious how long before the o-ring will need replacing. The chemicals involved limit our material choices. Teflon and Viton (a fluoroelastomer) seem to be the options. Any other materials to consider?

both are similarly priced. any ideas about expected life times under daily seal breaking and making? Pressure builds over the course of a minute or so, and discharges more slowly (~10 minutes).

 

[MikeHalloran]

Take a look at capturing the seal with a dovetail groove. I think it's covered in the Parker book. Use the Parker dimensions. Provide brass picks for replacing the seal so your operators don't scratch the groove.

Solid PTFE will work exactly once.

Teflon- covered elastomer seals will give you a few cycles, more if the elastomer itself can deal with the sealed fluid.

Bal-Seals would probably give you quite a few cycles, but may be a special in that size.

I might try teflon string/ tape/ cord 'packing on a roll' with plain (but deeply textured) flanges before going to o-rings. Disassembly destroys it, but the stuff is relatively cheap, and if you set up the stationary flange with studs, you can rapidly wrap the stuff around the studs. You'll still need a brass pick to help get it off.







Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

Make sure you check for CO2 compatibility at those pressures. I've pulled apart some CO2 injection wellheads and the o-rings snapped, crackled and popped like Rice Krispies once the pressure was relieved (it was really cool to see though). CO2 seemed to dissolve into the material at the higher pressure and, thus, may actually pass through it. The o-rings didn't seem to be damaged until the CO2 started expanding out of them and bursting bubbles of rubber. If your ring will see alternating high and low pressures you may experience this.

 

[offonoff]

yes, i've heard of the problem with pressure changes (which is why i mentioned the delta pressure rates). Good to hear your experience with it. I dont know if some materials don't do it, or what depressurization rate is adequately slow enough to avoid the issue.

mike:

good ideas. I've ordered a copy of the parker book and sent an inquiry to the bal-seal company. they look like excellent seals, but i expect them to be expensive. maybe they last long enough to be worth it...

as for the teflon string idea. you mean lay the string down in a tight spiral pattern (with a half dozen or more revolutions) and then bolt the flange down? So like an o-ring but sloppy and redundant (hopefully the two cancel each other out), and without a groove? really? have you ever seen this work?

I've heard of teflon tape over threads, and that works. so maybe... certainly worth a shot. and if not we can machine a groove and use an o-ring.

when i get some prices and life estimates, i'll report back on the economics of that strategy.

 

[MikeHalloran]

No, I mean one turn around a stud, then straight to the next stud, then one turn around that, and repeat, until you've constructed a polygon circumscribed by the studs, decorated with a loop around every stud.

Crane used to sell a white, smooth round filled ptfe cord about 3mm diameter that they suggested as both emergency gasket replacement and valve packing. I've used it as both, and it works, at least at modest pressures. I have some in a toolbox somewhere, but I lost the container, so I don't know what they called it.







Mike Halloran
Pembroke Pines, FL, USA

 

[HamishMcTavish]

Just out of interest why are you opening the port once a day and is there an alternative means of achieving the same end - instrumentation etc?

Cheers, HM

No more things should be presumed to exist than are absolutely necessary - William of Occam

 

[vpl]

offonoff

Your other post is still there -- thread794-235279. There's a "my thread" link on the upper left side that will show you all (or the majority for frequent posters) of the threads you've started.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[unclesyd]

In deference to MikeHalloran's post I don't believe that I would use any Gortex style material where there is daily making and breaking of a heavy flange. Also I don't believe that you can achieve sealing of a Class 900 flange in the manner proposed.
I've had very poor results with these materials in conditions where it requires replacing constantly as in your case, to many chances for a poor make up. Our internal piping standards limits Gortex material to Class 150 flanges to 350F.

Here is the options by Gore.

http://www.gore.com/en_xx/products/sealants/gaskets/gaskets.html

I would stay with a captive gasket if at all possible.


Here is another gasket we have used in some tough installations.

http://www.selcoseal.com/index.htm

Here is source of metal Orings, Crings and self energised seals.

http://www.helicoflex.com/products/orings/

You have a distinct advantage in finding a sealing method as there are numerous SCCo2 systems operating. The only rub comes with the daily making and breaking of the flange.

 

[offonoff]

yes, apologies for the multiple threads. I intended this to be a different topic: the life of an o-ring in this situation, and suitable cheap o-ring rather than the other thread which established the o-ring as a possible solution. But both threads have become the same. sorry.

The seal is broken because its a batch process and solid material needs to be put into and taken out of the chamber. I assume most scCO2 processes also have the making/breaking issue because i see no other way to fill and empty the chamber with plant material or greasy metal parts or what have you.

Although there are many scCO2 operations already, most are proprietary and i assume wouldn't hand out their intellectual property. Putting some letters into research institutions might be a good idea though.

As far as the teflon string idea, I'm not visualizing the stud part. are you talking about welding short little stubs on the flange face? the stubs would need to be shorter than the thickness of the compressed string. wrapping around the studs would cause leaks due to the irregular thickness (from overlap). a little bit of leakage is ok, but i imagine an overused o-ring would seal better than the string. still an interesting idea to keep in mind.

unclesyd: thanks for the links. These gaskets all have good properties, but if they are more expensive than an average o-ring and don't last for more cycles, then I do not benefit from their advantages. I've inquired to gore and selco. did you suggest helicoflex rings because you expect them to last more cycles than others?

thanks

 

[unclesyd]

Check out the line up from Greene Tweed.
They have several elastomers, Fluoraz or Chemraz, that might do the job and be somewhat cheaper.
GW has all type of sealing systems also, though I'm a little worried about you pressure. GW has a special seal for the above mentioned dovetail groove.

http://www.gtweed.com/85256D3C0038D8DA/vwContentByKey/N25PFPN9221ELETEN

Look at the list toward the bottom of the page at the following site you will see several possible approaches to your problem with quick opening closures.

http://rmenergy.com/products.html

No matter which way you go gaskets, Orings, etc. make sure when you get a quantity pricing, ones and two's will get into you pocket book.

Have you look at a valve for this purpose?

 

[HDS]

bribyk

That is called explosive decompression. The same thing as getting the bends scuba diving.

 

[offonoff]

There are obviously many types of seals, with all sorts of internally energizing springs and specialized cross sections. Many of the fancy seals suggested so far are more expensive and the companies that have responded to me say the seals are designed for one time mating, though customers have reported getting a few more out of them.

My question is: in this application, is there any reason to go a route other than a widely available (circular cross section) o-ring?

Unless an alternative seal is good for dozens of cycles, it seems best to me to just take an off the shelf 2 dollar viton o-ring and try to get a few cycles out of it. (i don't mind a little leaking).

unclesyd: a valve for this 6" port, or valves for the process? I dont know what a valve for the port would be. for the rest, seems like regular ball valves and needle valves from mcmaster carr are around $15 and will hold the pressure. we'll see what happens when the solvent has its way with their insides...

 

[Bribyk]

Has an API ring gasket been ruled out as a solution? It sounds like the right pressure range and I've seen them reused multiple times (probably more than the should've been but they held in hydrostatic testing). That's also what they use on wellhead flanges so they should hold up to CO2 and hydrocarbons.

 

[unclesyd]

I personally don't think any flanges should be designed where leaking is tolerated because little leaks turn into big ones in a hurry. One point to make is that at your pressures a very small leak is capable of injecting fluids into one's finger, fluid embolism. Fluid embolism is not a pretty sight and I understand it hurts like hell too.

I grant you that an Oring closure is very simple and inexpensive compared to some of the installed costs of some posted solutions.
I hope by now that we have just been finding the range in the dark as we have very few particulars on your design. We all know money enters into the pictures, but intangibles like how good are you maintenance people and operators, any size constraints and accessibility to name a few. I think all contributors have made an earnest effort to let you converge on a solution that you can live with.

Here is some information if you ever decide to go the valve route.
A lot of companies have very good valves for your service. Of many Velan and Habonim both have several figure vales that will handle your process conditions. You best bet would be to get with a manufacturer and let them with all your process conditions give you some recommendations.
The only thing I can tell you is that these valves will not be at Wal-Mart prices. With a properly selected valve it is one of those things you get what you pay for.

http://www.velan.com/products/pdfs/vel-ms-web.pdf

http://www.habonim.com/3 piece valve/pdfs/CATH27P-106I

[301108].pdf

 

[offonoff]

I hope i didn't sound unappreciative. There is quite a bit of useful information on this page, and i will likely come back to this thread in the future as a reference for other situations. this forum and its participants are excellent. I am very happy to have found this,

I am curious though, if there are so many seals that all seal well once, and are of questionable reuse value, is there any reason to spend 30.00 dollars on a seal over the two dollar one? for the flange solution, it appears to me that a less expensive o-ring is superior to a very expensive one because each will only be used for a few days. there are likely factors i am over looking, which is why i turn to this forum for council.

Outside the flange solution, there are other options which require more expensive gaskets (because they are less common geometry or are proprietary). I don't have any experience with them. are the only concerns ease for the operators? Would a quick release be less likely to squirt dangerous streams of fluid on failure? are they less likely to fail?

API rings have not been excluded. I like the idea of a metal gasket due to the reactivity of the fluid contained. I'll read more, thanks.

 

[unclesyd]

There is absolutely nothing wrong with a well designed Oring seal for your application. One just has to look at all aspects of the design like is the ring compatible with the process fluids and can it be installed by anyone with out damage. A Oring seal isn't a free ride or panacea and the successful use of an Oring seal requires due diligence on everyone's part.

Your mention of a two dollar Oring brings up several points that I like to make in discussions of Orings.
We used a very special EPDM Oring that was called out by a specific part number from only one company, no exception. It cost $4.50 ea. A purchasing agent and vendor were looking at the Oring inventory and noticed the EPDM Oring at $4.50 ea. From the vendor, "I have the exact Oring in stock for $0.22 ea." The end result was $7,000,0000 fire.

At different times I kept track of reported incidents at our site where the misuse of Orings resulted in process excursions. Ignoring the incremental cost the downtime alone was over $500,000 in 6 month period. The Oring problems covered all facets of misuse like reusing a damaged Oring, the wrong ring, improper installation, etc.

We use hundreds of RTJ flanges but we don't reuse the rings. There are some people that do reuse bull rings, like the US Navy. They just take particular care when breaking the flange and use something to store the rings in other than a bucket. RTJ flanges are not normally used for frequent make and break situations. A bull ring if it's of the Armco iron persuasion is easily damaged. The making up of an RTJ flange requires a little finesse on the mechanics part. He has to keep the flanges parallel while making up the joint and not overtightened the joint.

Remember you may or may not have the best possible solution at the start, so keep your mind open to improvements. Make sure you get the people involve that will make and break the joint get on board. And remember you can's make it idiot proof all you can do is minimise the damage.

Comeback with any questions and please let the Forum know the approach you used.

 

[tbuelna]

offonoff,

I'd avoid teflon o-rings for two reasons:

A) teflon o-rings cannot be stretched at installation
B) teflon will cold-flow over time

If you really need teflon for material compatibility, use something like a spring-energized teflon Omniseal. They're designed to fit into standard o-ring glands.

http://www.seals.saint-gobain.com/D..._line_edit.asp?ele_ch_id=L0000000000000001583

Good luck.
Terry

 

[offonoff]

Goretex products are _expensive_ and are one use only.

The bal-seal unclesyd is likely good for dozens and dozens of cycles. It looks like an excellent product.

me: If the port is opened daily, can you estimate a life for a seal?

bal-seal: Just about forever. There’s no wear occurring on the seal.

 

[btrueblood]

If you talk to Parker, or (better) the folks at Green Tweed, about decompression limits on elastomers, they can coach you through some procedures that will limit the degree of decompression damage that can/will occur on your seals. Generally, the slower the dumping of high pressure to low, and dumping pressure at the higher process temperatures, reduces the effects of ED.

 

[dsheets05]

Offonoff,

Carbon dioxide is the most aggressive of the common gases in terms of its ability to
cause explosive decompression damage in elastomeric seals. It has a great affinity
for many elastomeric compounds and is capable of being absorbed in large
quantities. Such absorption can cause considerable swelling of seals as well as
damage as it attempts to diffuse away after a decompression event.

I’m not sure an O-Ring will give you the life you are looking if this system is opening and closing daily. Check out the attached seal design. With an ED resistant O-Ring material and the right lead in chamfers this seal should provide you with a long service lifeh. Please let me know if you have any additional questions…