O-RING Groove design for hydrostatic testing 1

[NBR90]

I'm having trouble with coming up with the right groove design. Briefly, I do hydrostatic testing on valves from 2" to 36". For 2" - 12" I need to test them for leakage up to 5000 psi water pressure.

I have a 200 ton hydraulic clamping system with 200 mm cylindrical rod in the middle, sandwiching the valve are two plates (36" wide) top and bottom (see pic attached).

Success so far:

* Last year, I cut groove to 7.0 mm wide, 5.0 mm deep (Using NBR 70D, 7.0 mm wide O Rings). The grooves are cut "round" as opposed to "retangular U shape" , giving it even less volume.

I was able to test to 3000 psi water pressure for 12" valves using about 90-100 ton clamping force.


* This year, I cut another plate with 6.5 mm wide groove and 6.0 mm deep (Using NBR 70D, 7.0 mm wide Orings). The grooves are cut "rectangular" giving it more volume than 'round' cut. I was able to test up to 4000 psi water pressure for 4" valves using about 50 ton clamping force.

But with the 6.5mm, 6.0mm cut, anything above 6" fails miserably at 1800 psi and below. Sometimes it pops out, most of the times it just leaks.


So I need to re-cut my grooves.

Some have suggested that I go with a groove that is 8.0 mm width and 5.0mm deep (7.0 mm Oring) - enough volume in the groove to swallow all of the Oring.

The problem I may encounter is that my clamping force is not uniform & exacting like a bolting series. I'm relying on a 200 mm wide cylindrical rod in the middle to 'clamp' it together. Therefore, any gap inside the groove may cause all sorts of unplanned leakage, especially at higher pressure (2000-4000 psi). I'm sort of relying on the extruding O ring for 'tolerance' adjustment, sort to speak.

On the other hand, I know that elastomer are incompressible. And that I may have different problems such as extrusion, swelling, etc, etc, if I don't give it enough volume for it to do its work (I've read that Oring should only be 80-90% maximum volume of Groove Volume).

That said, I do have empirical data that putting the Oring inside groove it will not fit and under 100 ton of compressive force DOES NOT destroy it. Nor does it make the seal fail. In fact, it was sealing 12" valves pretty nicely up to 3000 psi (I'v never tested beyond0. Again, in that set up, I cut the Oring groove 'round', 7 mm wide, 5 mm deep using 7 mm NBR 70D Orings.

So, I know I need to re-cut my plates. The question is, how?

I'm thinking either:

7.0 mm wide, 5.0 mm deep, rectangular U shaped. Using 7.0 mm NBR 70D Orings.

or

8.4 mm wide, 6.0 mm deep, rectangular U shaped. Using 8.4 mm NBR 70D Orings.


Any help, suggestions is appreciated.


Yes, I should use my old plates, but they are too small, I designed the new bigger plates according to a 'suggestion' for better seal by a specialist... well..

 

[EdDanzer]

Look in the Parker book. Neproene may be a good lower cost choice than Viton.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[byrdj]

while the original question was o-ring groove design, and the replies have been mainly make the groove as per recomendations so it will act as an o-ring and not a gasket, your clamping force my be inadaquate. a correct oring will seal IF the clamping force is greater than the seperating force.

ie 12" diameter at 3000psi gives 170 tons.

 

[btrueblood]

At room temperature, NBR is as good as any other common rubber materials for water sealing, though it may swell a little over long-duration soaks, and that swelling can cause issues for sealing. EPDM is a better choice if that is happening.

But, NBR (or even SBR) have better resilience (won't take a set as easily), and wear resistance, and so may be better choices for tooling applications.

 

[Sweguy]

Hi

I have not read all the converation but I`m quite sure that my tip is not discussed.

I`m working as an engineer at a valve making company, we stays at maximum Ansi Class 300 pressure class, pressure test is then 78bar, 1130 psi.

When I design any special pressure test equipment, I always seal it on the inside diameter of the valves endings.

We have plates like you but they don`t have any o-ring grooves, instead they have four threaded holes, we bolt a thin round plate onto the big plate, the thin round plate has the same outer diameter as the valve size and has an o-ring on the outside. With this design the plates can deflect much without any leakages and don`t demand any pre compression from the clamping equipment.

 

[NBR90]

Sweguy,

Thanks for your input; your ideas sound great. I have a few questions:

How thick are your thin round plates? What is your maximum valve size you can test to?

I suppose you need to drill two holes in each side of the thin plates for air/water passage?

How did you cut your O-Ring groove (width, height and O ring width)?

How would you design it for higher pressures (ANSI Class 600 and Above)?

 

[Sweguy]

Hi NBR90

Actually we us similar method as you for our standard valves but when we are making big valves that are very rare for us we don`t have any pressure machine that can take the force, so then we have blind flanges with the design as I have described. Thickness are as a standard blind flange for the biggest size that the are plate will be used for.

One blind flange can be drilled for a couple of sizes and pressure classes.

The thin round plates that we are using are roughly 20-25 mm thick. and have a groove on the outside for a 6mm o-ring, compression roughly 5-10 % in this design the o-ring will be seal better with high pressure, when the pressure rises the o-ring will be forced against the outer and inner diameter, so the compression rate is not so important.

We only test with water and has a single hole in the middle of the plates, a small distance out from the middle we have four bolts that holding the small plate against the big plate, note that the threaded holes in the big flange shall not be drilled to deep, there is still pressure working in this area. Outside those holes there is a o-ring groove with an o-ring that seals between the big and small plate. Note that the diameter where this groove is must be smaller than the valve size, this lets the pressure compress the o-ring instead of what happens in your design where the pressure is forcing the plate away from the flange.

If I should design it for higher pressure classes, I should make sure that the gap between valve bore and the small plate outer diameter to be at a minimum, I should probably design it with a gap of 0,5mm on the diameter.

If you are worried over the compression rate for the o-ring, I have an example of minimum compression but tight anyway. On a diameter of 800mm I installed a o-ring that was 9mm thick, it was compressed 0,2mm. 2-3 % compression and I tested it up to 28 bar without problem.