Boss Oring failure
Boss Oring failure
(OP)
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.
Any kind of help, advice is highly appreciated.





RE: Boss Oring failure
RE: Boss Oring failure
RE: Boss Oring failure
To my calculations the force due to the pressure on the fitting can go up to 5523 lbf based on the maximum sealing diameter "A" in AS5202-12. Therefore, the minimum clamping torque (without any margin of safety) is 87.3 lbf-feet based on the mean diameter between the diameter "A" in AS5202-12 to the diameter "E" in MS21902J12.
RE: Boss Oring failure
RE: Boss Oring failure
RE: Boss Oring failure
RE: Boss Oring failure
I'd confirm the o-ring material is proper for the temperature and pressure.
First I'd determine empirically if the flanges are separating at 4500 psi.
If they are, then the tendency will be greater for the o-ring to extrude into the gap and be damaged.
With ~rectangular groove o-rings, "backer rings" are used to prevent o-ring extrusion and nibbling damage in high pressure service.
Your boss o-ring appears to live in a triangular groove, and would be forced by pressure into a 30 degree corner.
I have no idea if high pressure applications require that geometry be modified to prevent damage or not.
RE: Boss Oring failure
I read the Machine design section again, and I confirm that the portion of the total load which is taken by the manifold should be less than the torque preload to avoid any loose contacts. Its not that the Preload force needs to be more than the total load to avoid loosening of contacts.
RE: Boss Oring failure
Your confusion is that the stiffness ratio is relevant only for the "added" force to the initial constant clamping force. For example: if the manifold stiffness was 10 times the fitting stiffness the application of the added force due to the pressure will add to the fitting only 10% of the "initial clamped" force because the fitting will stretch only 10% (because of the manifold high stiffness) compared to initial stretch during the torque clamping. Thereby, add only 10% to the initial clamping force already in the fitting. This is the reason for initial clamping of bolted joints that makes them safe against fatigue during cyclic loading added to the clamped joint. If the bolts were initially tighten lightly instead of full clamping tight then with every load cycle the bolt/fitting would see the full cyclic load instead of only 10% cyclic addition to the initial clamped force.
RE: Boss Oring failure
RE: Boss Oring failure
The torque of 900 in-lb is applied to the fitting which created a preload force of 4235 lbf (tensile in fitting, compression in manifold). The force here will be the same for both manifold and fitting, but the deflection would be different and will depend upon the stiffness.
Now when a pressure load of 4500 psi is applied, it will generate a force of 5523 lbf based on the fitting cross section area. Since the manifold/fitting was under preload, this additional force will create the same deflection for both manifold and fitting, and the force will get distributed in the ratio of stiffness. According to our calculation, the stiffness % for manifold (aluminum alloy) is coming out to be 46% and fitting (steel) is 54%.So, 46% of 5523 lbf i.e.2540 lbf is added to the fitting tensile preload force and 54% of 5523 lbf, i.e.2982 lbf is reduced from the manifold compression preload force. The total tensile force on fitting under pressure load would be 4235 + 2540 = 6775 lbf and the total compression force in the manifold under pressure load would be 4235 - 2982 = 1253 lbf. As long as this net compression force (1253 lbf) in the manifold does not go to zero, we are good.
Are we talking the same thing or are you trying to convey something different? can you point out where am going wrong?
RE: Boss Oring failure
RE: Boss Oring failure
I agree with most of your theory and am on the same page with you, except for the "you "can not" split the force between the manifold and the fitting. Both feel same load. The stiffness ratio only dictates how much the fitting will be stretched when the pressure is applied as long as the force due to the pressure is "less" than the clamping force". The reason I would like to discuss this more is because if the pressure distribution theory is wrong, then basically I am challenging the methodology being used in our company. Secondly, I will have to apply a torque which would be more than the recommended torque value provided in the AS18280 spec. So, I hope you understand my situation and will not get pissed off by my continuous examples, evidences etc.
I have attached 2 pages of that book where they have shown with figures and also force distribution. Page 918 pretty much explain all the discussion we are having. fig 14-24 (a) shows when only preload torque is applied and the corresponding force Fi and the different deflection in both manifold and fitting, and the figure 14-24 (b) is when external load P is applied. According to this book, P will have two component Pb and Pm, where Pm goes to manifold and Pb goes to bolt. and then the last sentence of page 918 where the book has explained that if the applied force P is large enough to cause the component Pm to exceed the preload force Fi, then the joint will separate and the bolt will feel the full load of the applied load P.
Now if you refer back to my calculations, the P force is the pressure load of 5523 lbf, Pb is 2540 lbf for fitting, Pm is 2982 lbf for manifold and as long as the component Pm which is 2982 lbf less than the preload Fi which is 4235 lbf, there should not be any separation. I humbly request you to please spend some time and page 918 (attached)thoroughly. If after reading also you still think that to avoid separation the total pressure load P should be less then the Torque preload Fi, and am wrong in saying that to avoid separation Pm needs to be less than Fi, then let me know. I will consider this as the potential root cause of leakage and will initiate a serious discussion with our internal team and customers.
RE: Boss Oring failure
http://files.engineering.com/getfile.aspx?folder=6...
RE: Boss Oring failure
I understand your point and where it comes from but, I still have a difficult to point exactly what is bugging me. I will try to go deeper at my own pace but not now. I may find that you are correct as it may seam and it is always good to learn new things. Anyway, if the o-ring extruded that means joint separation or low contact resulting force as a result of the applied pressure which could not resist the extrusion of the seal. This can be a result of a difference between your calculated stiffness of the fitting and the manifold and the actual stiffness of the fitting and the manifold. This is why it is a good practice to have a robust design and to be on the side and clamp the joint to at least 5523 lbf. I understand that this raises a problem that the current fitting which is made of a weak material 304 CRES. You can manufacture a replacement fitting from much stronger material such a 15-5PH-H1025 or in worst case Custom 455 H1000 and even stronger metals. This may raise another problem, is the manifold material 7075-T73 Aluminum alloy can withstand the high clamp, so you may need to use longer thread or replace to stronger material. I recall from my 35 year experience working with R&D of much higher pressure manifolds, fittings, valves, etc., that there where times where we had to make the manifold from high strength stainless steel in the cost of larger weight manifold. One more thought to take into account is that because the stiffness ratio is low, with every cycle the fitting sees large force addition, the statement in page 918 "if the bolt doesn't fail when preloaded it probably won't fail at service" no longer apply because it assumes high joint_to_bolt stiffness ratio (close to 10). Therefore, maybe if you solve the o-ring extrusion problem you may find a fitting failure as a result of fatigue.
RE: Boss Oring failure
In that case, you are right, we might have to go with 15-5 CRES steel if we decide to go with higher torque. The margin on the manifold as a result of higher torque is greater than 5 for both compression and shear, we should not have any problem with manifold.
I will check the version of the book tomorrow, its in my office.
RE: Boss Oring failure
2. You may need to use back-up rings for applications that may extrude o-ring from the gap which normally should not be available. Check handbooks/internet for back-up rings for o-ring applications.
3. Check the o-ring material and hardness suitability for the application and conditions.
4. Sometimes the installation can be problem. You may be damaging/scratching o-ring without even noticing, check your procedure for installation.
RE: Boss Oring failure
The backup rings are not required in this case as it is backed up by metal. The O-ring gland is created by metals (fitting and manifold) on all side. As long as the metals stay in contact, o-ring cannot squeeze out.
Already checked the O-ring material. There is only one hardness for this NAS1612 O-ring which is compatible to skydrol. If we want to go to higher hardness, then we are deviating from the spec. Then entire Aerospace industry use this O-ring for Skydrol high pr application.
Already building tools for installation to avoid O-ring contact with fitting threads.
I am more and more inclined towards 304 cres Fitting yielding as the root cause of leakage. Spending a lot of time in fine tuning calculations for fatigue analysis
RE: Boss Oring failure
https://en.wikipedia.org/wiki/O-ring_boss_seal
RE: Boss Oring failure
"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...
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%205700%20Par...
"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.