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antique diver
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J-Valve Seat Extrusion/Blockage - Photo added

Fri Feb 02, 2018 12:40 pm

The subject of air blockage in valves was briefly discussed on the Compressor forum, and continuing here with photo and more information on tank valves.

Many air valves (not just Scuba) are designed to flow air most efficiently in a specific direction. Flowing "backward" is sometimes necessary, as when filling a Scuba Tank. This is not usually a problem, but under certain circumstances the reverse flow can create a restriction or even complete blockage of air flow through the valve. It is generally caused by air pressure getting behind the soft seat and pushing that seat partially out of the threaded brass carrier. An old or badly worn seat may be more likely to extrude.

Some valve designs are more prone to this issue than others. In fact, almost all versions of the US Divers J-Valve seem to be more susceptible to this problem than most other valves that I have experience with. I think it is probably due to the center hole in the seat allowing air from the fill line to get behind the seat and pushing/extruding it out of the carrier if the fill pressure is applied before opening the tank valve. The age and condition of the seat (such as how tight it is held in place in the carrier) will also have something to contribute to the issue. I see this happen in well worn line valves that do not have a hole in the seat center, such as the Sherwood/Dacor line valve. Those even have an arrow cast into the valve body showing direction of air flow. If one of these is on your fill line in a backward configuration you may eventually see a blockage occur there as the worn seat can extrude from the carrier.


Prevention of this malfunction is easy enough in most cases, if you are filling your own tanks. Just be sure that when beginning a fill that you always open the tank valve before applying pressure from the fill line. (Also don't forget that your reserve lever must be in the down position to pass air into the tank!)

If someone else is filling your tanks, politely advise them to use this procedure. It is likely that that more times the seat is accidently extruded the easier it will extrude again.
IMG_3035.JPG
This shows a normal USD J-Valve seat on the left. The other was intentionally exposed to pressure from the fill hose before opening the tank valve. As the tank valve was opened the seat extruded. I did not turn the tank valve back off, or the seat would have been pushed back into normal position. I just bled the air off and disassembled the tank valve with it still in the open position. It clearly shows the soft seat extruded into a position that will block the incoming air flow.
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SurfLung
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Re: J-Valve Seat Extrusion/Blockage - Photo added

Mon Feb 05, 2018 10:31 am

I still haven't got around to taking that J-valve apart (that we discussed in the compressor forum). So I haven't been able to examine it to see what you are referring to when you talk about the "center hole". Can you show a picture of that? And, is this something that we can simply replace with a part that doesn't have a hole?
SurfLung
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Re: J-Valve Seat Extrusion/Blockage - Photo added

Mon Feb 05, 2018 11:11 am

SurfLung wrote:
Mon Feb 05, 2018 10:31 am
I still haven't got around to taking that J-valve apart (that we discussed in the compressor forum). So I haven't been able to examine it to see what you are referring to when you talk about the "center hole". Can you show a picture of that? And, is this something that we can simply replace with a part that doesn't have a hole?
I can't advise replacing that seat with a K-Valve seat that has no tiny center hole. The hole in the J-Valve seat may be there to prevent extrusion during a dive when the reserve is actuated. That is rumored to be the reason that particular seat design was initiated for USD J's. I haven't quite figured out how that could occur yet, but some day I'll sit still long enough to open one up and think about it.

The important thing is that extrusion during fill is easily avoided by opening the tank valve first before opening the fill line valve. If one extruded, and impeded air flow during a dive because of using the K-Valve seat that may be unhealthy.

BTW: I advise opening the tank valve first with any type of Scuba valve. (or for that matter any type valve that is about to be subjected to "reverse" flow... except maybe a ball-valve)

I'm on the road today, but will try to get a photo later.
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antique diver
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Re: J-Valve Seat Extrusion/Blockage - Photo added

Mon Feb 05, 2018 9:18 pm

SurfLung wrote:
Mon Feb 05, 2018 10:31 am
I still haven't got around to taking that J-valve apart (that we discussed in the compressor forum). So I haven't been able to examine it to see what you are referring to when you talk about the "center hole". Can you show a picture of that? And, is this something that we can simply replace with a part that doesn't have a hole?
Eben, here's a photo to show difference in the seats. New, unused seats are shown with the soft seat up.
Top 3 seats in photo are of J-valve seat (0525-19). Other than the approximately 0.0365" to 0.0370" hole in center of seat, extending to the brass carrier, these are identical to the two K-valve seats (0501-09) below.
IMG_3037 (500x500).jpg
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SurfLung
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Re: J-Valve Seat Extrusion/Blockage - Photo added

Tue Feb 06, 2018 12:53 pm

OFPS (Oh Fer Pete's Sake). Thanks for posting that photo. I've never seen a seat like that before. You say the hole was to solve some problem with the J-Valve? Mine is the 1/2" thread J-Valve but it doesn't have a seat with a hole in it. Here's what I have:
Seat1.jpg
The indentation pressed in from the orifice makes such a nice all around seal... I can see how pre-pressuring it from the fill whip makes it stick to the orifice. I put it all back together after this photo and filled the tank by opening the tank valve first. I probably need to order some new seats so that when I see something like this I can just replace it while I've got everything depressurized and taken apart.

Aside from this issue when filling the tank, the J-valve has been working perfectly... I've breathed it down to zero more than a couple of times and a pull on the lever has released the reserve air.
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