Boss Oring failure

[Piyush582]

We recently conducted a high pressure impulse cycle (50 to 4500 to 50 psig) test on a manifold assembly with boss fittings on it. The port is per AS5202-12 with MS21902J12 fitting and boss O-ring (NAS1612-12) installed in it. After about 9000 cycles at 140F, the O-ring extruded and broke down resulting in leakage through the port. The other ports in the manifolds were checked and even though we did not have leakage through the other ports, we did find O-ring nibbling and O-ring particles in the port implying the O-ring could have failed after a few more cycles. We inspected the port dimensions and the port was iaw AS5202 spec, including all tolerances and roughness. We are in process of inspecting Fitting. We already check the compatibility of the O-ring with respect to Skydrol and found to be good at 140F. What could be the cause for this failure? The unit has to go through 200,000 cycles of impulse pressure cycles with 150,000 cycles at 140F, 20,000 cycles at 212F, 20,000 cycles at 14F and 10,000 cycles at 248F.
Any kind of help, advice is highly appreciated.

 

[Tmoose]

How did the inspection of the fitting go?

"I am more and more inclined towards 304 cres Fitting yielding as the root cause of leakage."

I believe some mechanical testing/checking of several trial assemblies before and after failure by leaking will make it much clearer if yielding or elastic distortion are what is happening. I'd grind a few faces true, and qualify the fitting lengths before assembly, and then after "failure."
I'd rig up A dial indicator grounded close to the fitting, and contacting the upper surface of the fitting. Then watch and record motion / deflection before, during, and after several slowly applied pressure test cycles.


I see the published pressure ratings are basically OK with your 4500 psi test pressure with some O-ring manufacturers, but not everyone.

http://www.applerubber.com/seal-design-guide/sizes-and-how-to-order/tube-fitting-bosses.cfm

Is the o-ring supplier a first tier company?
I'd still show the various failed o-rings to the o-ring supplier for analysis.
And, be sure to provide info about the rate of pressure rise in the test cycle.
I'm really expecting they will know just what is going on.

Page 253 here -

https://www.parker.com/literature/ORD 5700 Parker_O-Ring_Handbook.pdf

"10.1.1.2 Extrusion and Nibbling
Extrusion and nibbling of the O-ring is a primary cause of seal failure in dynamic applications such as hydraulic rod and
piston seals. This form of failure may also be found from time to time in static applications subject to high pressure pulsing
which causes the clearance gap of the mating flanges to open and close, trapping the O-ring between the mating surfaces.
See Table 10-2 for a failure analysis and corrective action discussion. Figure 10-2 shows an example of an extruded
and “nibbled” O-ring."

I find the triangular gland design interesting.
Being stuffed into a 30 degree corner seems a lot more painful than being stuffed into a 90 degree corner to me. And, involve a lot of sliding and extrusion.