Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[KBIO]

Hello!
I'd like to have your opinion concerning the  O'rings that we use in steam.
(I do not know if I have to post this in R&D but it looks like there is more people here! )
Some are for using Nitrile and some Viton!
But, I think that we have two different problems in sealing our assy:
1- the boiler = high temp . Not really; as it goes up to 120°C most of the time. So I think that Nitrile is good.
2- The liquid gas in gas tank has a temperature going down to -20° (??) above all when filing up and I think Viton will be more appropriate.
If I look the caracteristics of both material , as we have no corrosif liquid or solvent or other special effluent (like H2S), I read that common viton O'ring has a temperature working range genrally between = -40°C & +120°C.
So I would say that viton fits all our requirements.
But I am not sure and I'd like some more informations about what is the most suitable for steam.
Thank you for your help!
Cheers!

[derekwarner]

KBIO.........we have discussed this a few times.....in my time on Mayhem .....

1. The common model boiler @ 45 PSI~~~~3 Bar will generate ~~~150 degrees C at the boiler discharge valve
2. The boiler discharge valve will have a spindle sealing o-ring which must be good for this 150 degrees C
3. Nitrile [NBR] elastomer o-ring material is rated to 100 degrees C in a dynamic installation & is therefore unsuitable for this application
4. VITON elastomer o-ring material is rated to 200 degrees C in a dynamic installation

So from this we see Nitrile [NBR] whilst listed as suitable for hot water is not acceptable for steam @ 150 degrees C

I fully understand & accept that the steam by the time it is reduced in temperature within the lubricator....will still border on 100>115 degrees C & therefore Nitrile [NBR]
is still NOT an acceptable seal element for steam @ this temperature

Other members may wish to offer comment re the use of Silicone o-rings.........I have no experience here  ...Derek

[andrewh]

My tuppence worth
Confession - I used to work in rubber

All that derek says is true, and that might incline people to use Viton O rings. 
They have 2 issues that might make them less than ideal:
1)   If overheated  (405 degC) they release Fluorine gas, which immediately dissolves in atmospheric water to form HF hydrofluoric acid - this destroys metals, glasses and people and is hazardous even in o ring quantities
2)  Viton is physically weak by elastomer standards
3) its VERY expensive (but that may not be a problem)

Nitrile will probably be fine - the 100 degrees is the PERMANENT temperature it will stand to work 24/7 with no loss of elasticity or other properties.  We don't mind replacing a o ring each year.  Nitrile is hopeless in air (ozone) and especially ozone but both of these are limited in a steam engine

My strong preference would be silicone, where the temperature is immaterial (I use it in CO2 motors at minus 20 or so).  It ccan be made hard enough to reduce friction (about 75 to 80 shore A if interested) and the oils used will just make it swell temporarily

hope this is helpful,
I have probably given the impression that viton has its problems - I don't use it .  Leave it for aerospace where it is essential, and the maintenance chaps know its habits
andrew

[steamboatmodel]

That's a good point Andrew. I knew there was a problem with Viton at elevated temperatures, but could not find a reference to it on the net. Back when I worked in Hydraulics and Pneumatics there was a fellow who was removing and disassembling equipment at a plant that had been through a fire, he got some of the black goo from overheated O rings on his hand and ended up loosing three fingers. After that everyone read and questioned the material safety reports instead of just tossing them.
I found a reference to Hydrofluoric acid,
http://en.wikipedia.org/wiki/Hydrofluoric_acid
When you look down to Safety and read it it should be obvious that Viton may not be the first choice.
"However, because it is absorbed, medical treatment is necessary;[9] rinsing off is not enough. In some cases, amputation may be required.
Hydrogen fluoride is generated upon combustion of many fluorine-containing compounds such as products containing Viton and polytetrafluoroethylene (Teflon) parts. Hydrogen fluoride converts immediately to hydrofluoric acid upon contact with liquid water."
Regards,
Gerald.

[KBIO]

Good evening!
Thank you gentlemen for your advice.
I was not aware that viton could be so harmful at high temp.
Never the less ,we have to take into acount , that we never reach such high temp in our steam assy (<150°C) and unless it goes on fire, I find the viton the most suirable to do the job.No?
Nitrile can be left aside then. Cost must be considered  but we can afford a few viton O'ring though!
The problem I have w/ silicone is that it is very soft and that it will pop off when tightening the safety valve or fill up plug. If I can find silicone O'ring with same hardness that viton, it would be the ideal then.
Thanks again, this is very interesting.
Regards

[JJS]

Hi,

I agree with Andrew, As i have been working for the petrochemical industry selling filtration isystems, we only advised viton for chemical resistance matters. Viton o-rings are 5-10 times more expensive then nitrile. Viton is more stiff then nitrile and needs to be replaced more often. In the pertochemical industry we advise to use viton only once and replace after one use.  Silicon but even nitrile will do for low pressure steam applications (but of coarse viton can be used if you want to spend more money) . Viton decomposing is only likely if it is in contact with  open fire.

jjs

[gondolier88]

Silicon but even nitrile will do for low pressure steam applications (but of coarse viton can be used if you want to spend more money) . Viton decomposing is only likely if it is in contact with  open fire.

jjs

I was not aware that viton could be so harmful at high temp.
Never the less ,we have to take into acount , that we never reach such high temp in our steam assy (<150°C) and unless it goes on fire, I find the viton the most suirable to do the job.No?

A simple and realistic situation for you;

You have a problem with your steering servo in the middle of the lake- you have a full boiler when the boat sets off, but when the problem strikes it's running low, the pressure in the boiler starts to rise rapidly and the safety valve opens. The water level now drops below the water gauge and the gauge glass is open to temperatures well in excess of Viton's safe working temperature. At this point the rescue boat is pushing your boat back to the shore (while people run in every direction away from you!)- you quickly dive in and turn the gas off.

The next time you fill the boiler you notice that the bottom of the gauge glass is leaking, so you strip it down and take out the knackered O-ring and chuck it in the bin OH DEAR YOU NOW HAVE HYDROFLOURIC ACID ON YOUR FINGERS.

This really isn't ideal (understatement), and while it is above it's temperature in the boat it will be releasing Flourine gas (used by the Nazi's in WW1 to drift with the wind across no-mans land and dissolve peoples lungs and blind them and other horrific things).

My advice- keep tyring to find the right silicon ones, or have spare soft ones to replace broken ones at the lake.

Greg

[andrewh]

Hi, KBIO

Good evening!
.
.
 
The problem I have w/ silicone is that it is very soft and that it will pop off when tightening the safety valve or fill up plug. If I can find silicone O'ring with same hardness that viton, it would be the ideal then.
Thanks again, this is very interesting.
Regards

Er, no.
Silicone available from ShoreA 15  - soft as a human body (and used in it) - to 97 - about as hard as a cricket ball, and I've no doubt we could get some as hard as bakelite (98) if we tried.

If it helps anyone interested in the families of elastomers my former employer Harboro rubber do a publication  called "Engineering in Rubber" which is, I think available for download or reading on their website
http://www.harboro.co.uk/engineering_in_rubber.html
A riveting read
andrew