O-ring hardness for SAE ports 2

[horstr1]

OK, this isn't a pump question, but there isn't a general hydraulics category, and I see someone else's O-ring question was addressed nicely.

I"m replacing a bunch of #&%! NPT plugs with SAE J1926 ones in a hydraulic system. They're just closing off crossover holes in blocks of steel. I don't want to assume that the O-rings already installed on off-the-shelf plugs will be adequate for my pressure. Does anyone know how I can tell what durometer will hold a given pressure in this configuration? My basic operating pressure is 3000 psi, and sometimes we do things at 5000 or more. We may even upgrade the system later to 5000 psi operating pressure, with proof testing to at least 7500, so I need to be sure one of these O-rings won't embarrass me.

 

[Oldhydroman]

There is a fluid power forum - that covers hydraulics.

You should look for O rings that are 90 Durometer.

Your pressures are not particularly high for a hydraulic system and the standard O rings that come with the plugs should be perfectly adequate. Do check the pressure ratings for the larger sizes - it starts to tail off a bit at the larger end of the scale.

Ideally the plugs should be stored without the O rings on (even though this makes purchasing a bigger pain) because the long term storage requirements for the metal bits are very different from the long term storage requirements for the rubber bits. It really doesn't matter how old the plugs are when your supplier delivers them, but you do want nice fresh O rings on them when you plug your holes.

Here's a page from the Parker catalog giving some more information - you can see the pressure ratings on this chart.
DOL

 

[hydtools]

The o-rings are fully contained when the fitting is assembled, i.e. there is no seal extrusion gap. Preventing extrusion is the usual reason to use higher durometer o-rings. The standard o-rings on the fittings work just fine.

Ted

 

[horstr1]

Thanks for both helpful answers. Since I already decided to buy the O-rings separate and scrap whatever comes on the plugs, I'll specify 90 durometer. I agree with hydtools that extrusion is moot, but it doesn't seem like a harder O-ring would hurt anything. And it might help add some safety factor, especially on the larger sizes.
BTW, I did have a case in the forklift world where some SAE plugs in aluminum valve bodies were chronically leaking. The customer's maintenance guy's answer was to put in oversized O-rings and "torque the (expletive deleted) out of them."

 

[hydtools]

Per this data 90 durometer is standard:

http://www.goodyearrubberproducts.c...0/Parker Tube Catalog 4300examin1.Page477.pdf

Ted

 

[Oldhydroman]

Here's the thing, and I'm not decrying the continued existence of bespoke and innovative engineering effort, but if you take it upon yourself to change the component parts of a well proven and well respected sealing system then you should also take it upon yourself to acquire all the calculations, analysis and test results (including long term fatigue tests) to be able to prove to a bunch of "paid-by-results" [non-engineer] lawyers and/or a court of law that YOUR decision was indisputably NOT the cause of...[think of the worst case scenario and type it here]!

I know we're only talking about an O ring (shuttle SRB?) but is it really worth the effort to depart from the standard product? There is already an adequate factory of safety built into the design of the sealing system and putting in a harder seal won't improve that because you can't quantify any improvement in sealing ability and you have done nothing to improve the mechanical strength of the metal parts. As Ted says, a harder ring improves resistance to extrusion but extrusion isn't a risk here. In any event, the harder ring seals at higher pressures but it is sometimes at the cost of the ability to seal when the pressures are low. Often a PTFE or PEEK back-up ring is used with the "normal" rubber O ring to help guard against extrusion - harder and harder rubber compounds are sometimes counter-productive. Many hydraulic engineers and maintenance technicians will agree that the leaky parts of a hydraulic system are often the drain and return lines because there just isn't enough pressure to force the O rings into the microscopic gaps you want them to seal.

There are many reasons why this SAE/ISO sealing system uses 90 Durometer rings. I don't pretend to know all the details but I do trust my thousands of colleagues in the fluid power industry (present and past) do have done a professional job of the design and I wouldn't presume to change anything on a whim. Incidentally, you only need the "working" pressure rating of your plug to match the working pressure of your system. The plugs have a proof pressure capability of their own (typically 150% of working pressure - but do check the SAE/ISO standard or your supplier's datasheet for confirmation). Also pay particular regard to the specified tightening torque (over tightening can actually REDUCE the pressure capability of a fitting) and ensure that you have the right conditions of the thread for that torque figure to be valid (is the thread lubricated or dry).

Presumably you are having to redesign your block because of the change of thread form - have you considered using a permanent closure technology such as a Lee plug, CV plug or Koenig plug?

Just read that tirade back and it comes over a little strong [sorry]. But I decided not to change it anyway, so please feel free {anyone} to shoot me down in flames ;-)

DOL

 

[hydtools]

In my past life, I found leaky o-ring port fittings to be the result of installing the fitting in NPTF ports. The installer put lots of tape and sealant on the threads, but the d##&! stupid o-ring fitting still leaked! No kidding... still leaked. Stupid fitting! Not the installer's fault, heck no.

Please use fittings 'as designed'.

Ted

 

[horstr1]

Thanks for your candor, Oldhydroman. Please don't water down any 'tirades' in the future. Worst case in my case would be another Macondo-type situation, so there's no room for compromise or a fragile ego.

I'm not trying to reinvent any wheels here, I just want to be sure I have an adequate O-ring. If 90 duro nitrile is standard, that sounds fine to me. But if I get a plug out of a box with an O-ring already on it, I don't know anything for sure about that O-ring. I'd rather buy my own and have traceability, know how fresh it is, etc. that's all.

I hired in here last Fall, an oldish engineer but new to this industry, to pick up this development project someone else left behind. The first prototype was already built, so it's pretty far along, but needs a lot more refinement. And every test I've done to date has been interrupted by an NPT plug springing a leak. They have not yet failed to fail. I think it has to do with a hole in a huge solid block not stretching like a pipe joint does. So along with a lot of other design updates, I'm getting rid of all NPT's.

Whatever I use has to be removable, so a truly permanent seal is regrettably out. And I have to consider field service hands that might not recognize a Lee plug and know how to undo it. So SAE looks like the best answer. I haven't designed with SAE ports before, and I have seen them leak, so I just want to make sure I'm doing it right.

Note to hydtools: the disconnect between Engineering and the shop is alive and well. I just got a call because parts wouldn't fit together right, because they weren't putting them together the way I explicitly specified in the assembly drawing. I'm trying to quit smoking, but when strangling is off the table it's tough.

 

[Compositepro]

There have been several previous threads on leaking NPT fitting. The problem is usually improper sealant or wrong thread geometry. NPT threads are by far the most common thread for sealing pressure and have been in use for over a century. They seal very reliably when put together correctly by knowledgeable people. So I'm very skeptical when I hear someone say that NPT threads are no good. The way they work is more complicated than most realize, and without fully understanding how they work, it is difficult and frustrating to solve leak problems. There is a reason why your threads leak. You just haven't identified it yet.

 

[horstr1]

Compositepro, I'm sure you're absolutely right, and I'm not trying to badmouth NPT connections in general. There must be billions of them performing admirably right now, and lasting for decades. But I can't bet this farm on them being "put together correctly by knowledgeable people." Even if we get it right in our manufacturing shop, someone else of unknown competency will work on it later in its service life. I think my best solution for this application is to go with the SAE scheme, also well-established as a reliable means of sealing. Thanks for your input, though.

 

[Compositepro]

Fair enough. But no design is idiot-proof. O-rings can be damaged in handling and must be scrupulouslly clean when installed. Many rubbers will degrade after a few years.

 

[hydtools]

horstr1, if you do purchase o-rings separately make sure they are for the SAE ports and not a close dash number.
I dislike NPT ports for hydraulic components. Wherever I could, I specified the SAE o-ring port where removal and reinstall was necessary and eliminated problems.

Ted