O-ring sealing problem 2

[vahabsolgi]

Hi every body
I want to seal an electrode which is installed into a hole with an viton material O-ring. The o-ring size is 3mm as inner diameter and 1.5mm as thickness. The operation pressure is 30bar and the maximum temperature will be 120 degree centigrade. the o-ring is 0.6mm compressed after installation. Other dimensions have been mentioned in the attached link.
After assembling this setup it is noticed that there is a leakage even in lower pressures (10 bar) in the place of sealing o-ring.
I would like to help me and tell me where is the mistake and how should I change the design to overcome this problem.
Moreover is there a standard maximum pressure range which small o-rings can seal?

 

[hydtools]

The surface finish of the counterbore face may be a spiral. A cirular finish would be better. How is the counterbore created?
That seems to be a high compression, also.

Ted

 

[vahabsolgi]

Hi Ted
The groove is made by lathe and I have not measured its surface roughness but it is very smooth visually and the other contacting surface has been grinded and smoothly polished with the surface roughness Ra=0.5
By the way according to the O-ring makers handbook the compression is a little higher than the standard but does is affect the sealing immediately after assembling? I mean the set up have been tested by the pressurized air right after assembling and there is not any effects of temperature or other circumstances over time.
my other question is that is there any standard which tells us that there is a maximum pressure the o-rings with small cross sections can seal? I want to know if I can achieve better sealing by increasing the cross section of the o-ring?

 

[hydtools]

Which side is pressurized? Does the o-ring fit to the ID or OD of the groove in its free state?
O-rings fail primarily by extrusion though a clearance, not by pressure compression.

Ted

 

[vahabsolgi]

the fluid pressure is from the right side (according to the drawing).
the o-ring fits to the outside diameter of the groove in its free state.

 

[hydtools]

So are you saying the seal leaks after passing the initial test? What processes occur after the initial test?

Ted

 

[vahabsolgi]

not actually, the seal leaks during the initial test and can not pass the test.
after the test there is a liquid with maximum pressure 30 bar and maximum temperature 120 Degree centigrade in the right side of the drawing.

 

[desertfox]

Hi vahabsolgi

How are you applying the sealing force? Also have you done a tolerance check to ensure the ‘o’ring always protrudes by 0.6mm beyond recess it locates in, I have this feeling that tolerance build up between the two mating parts might be preventing the correct ‘o’ ring compression.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[vahabsolgi]

Hi
Yes I have Checked the tolerances and there is nothing wrong there. the sealing force is applying directly to the back of metal part.
I think the cause might be because there is a maximum pressure limit for any o-ring size which in my case the test pressure is higher than that. does every body have any idea about that? I did not find anything about it in the literature.

 

[lfw618]

What are you using to apply the sealing force? And how much force are you applying? What fluid are you sealing? Does the o-ring appear damaged after the test? What are the gland dimensions? I 'd make sure you're not exceeding 90% gland fill. O-ring manufacturers generally do not provide a maximum pressure for a particular o-ring, as that will vary with the application, there's a lot of variables that would go into that. They do have technical support that is pretty good about offering guidance/advice in my experience. That said 10 bar relatively is not much, especially with 40% squeeze.

 

[vahabsolgi]

A plate in contact with the back side of the Electrode (the metal part) is applying the sealing force by tightening 4 bolts.
The o-ring does not damage after the test.
the gland dimensions: Inner Diameter: 2mm, Outer Diameter: 6mm, Depth: 0.9mm
the o-ring dimensions: Inner Diameter: 3mm, Outer Diameter: 6mm, Thickness: 1.5mm
so the gland fill is: %98 which is higher than 90 percent

 

[desertfox]

Hi vahabsolgi

Can you show a sketch of how the four bolts are arranged around the electrode? Also do you have a bolt tightening sequence? What method are you using to ensure consistent bolt tightening?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[lfw618]

Not sure if it's causing your leaking issue but 98% nominal gland fill is an issue. Typically you'd shoot for around 75% gland fill nominally, taking into account manufacturing tolerances so that you do not ever exceed 90%.

 

[vahabsolgi]

Dear Ifw618, the information was helpful.
Another question, does big compression (40% which is more than the standard) affect the sealing?

 

[hydtools]

I still come back to the lay of the sutface finishes creating a path across the oring sealing faces. Any finishing process done by traversing across a face creates a texture that crosses the sealing faces.
What is the durometer hardness of the oring?

Ted

 

[vahabsolgi]

Hi Desertfox
The bolts arrangement has been enclosed. As you see, actually we have two electrodes which are being pushed toward each other by two solid plates.
Meanwhile the bolts are passing trough the holes in one of the solid plates and end of them are been fastened to four nuts on the other plate in addition the bolts are been tightened in a Criss-Cross sequence.

 

[desertfox]

Hi again

Is the leaking occurring at one side of the device for example:- it leaks on the side where the bolt heads are?
Also from the tolerance build up what is the maximum and minimum compression of the ‘O’ring.
Hoe Are you controlling the tension on each bolt during tightening? I can see a possibility of some of the compressive bolt load distorting the plate and not contributing to ‘O’ ring compression particularly as the ‘o’rings are directly opposite each other.
It would be better if the compression of the ‘o’ring was independent of each other on each side.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[vahabsolgi]

Dear hydtools
The O-Ring durometer hardness is ShoreA=70.
what will be the results if I use an o-ring with lower harness? can it help? I mean that is there any general rule about choosing the o-ring hardness?

 

[vahabsolgi]

Dear desertfox
It is not Clear that which one of the o-rings leaks during the test! it can be either of them. There is not a huge tolerance buildup here. It ranges from +0.05mm to -0.05mm in the o-ring compression.
The bolts are being tightened in a Criss-Cross sequence and the plate material is very rigid (CK 75 spring steel) so there is not a noticeable distortion in the plate after assembling.

 

[desertfox]

Hi

How do you know when the bolt is tightened enough, I agree with the criss cross pattern of fastening

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein