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Valve stem seals
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Tanks A Lot
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While there are many excellent points in this discussion, I believe some aspects may benefit from clarification as they differ significantly depending on the cylinder valve design.
Left: Plain stem | Right: Split stem
We should distinguish between two primary valve stem designs. Designs with a plain stem, as shown in the original post, (right picture) and designs that use a Split Stem with a machined lip (left picture). The O-ring's fundamental purpose is to seal, but the mechanism and forces involved differ in each design.
If the thrust washer seals the outside diameter, what is the O-ring's purpose? It achieves a seal against the inner diameter of the O-ring and the stem. The thrust washer seals the OD of the packing nut, while the O-ring seals the ID of the stem.
Then why not seal directly seal the ID only against the packing nut and leave the thrust washer? This seemingly roundabout approach is necessary due to the extreme forces involved. A packing nut with an 11.0 mm inner diameter subjected to 300 bar generates approximately 290 kg (That's 640 lbs for the Americans!) of force. An O-ring cannot withstand such a force, especially when it involves twisting motions, without being severely compromised. By relying on the seal between the O-ring ID and the stem (<6.0 mm), the resulting force is reduced considerably to about 90 kg (200 lbs).
The O-ring on the right has long disintegrated and it's black bits can been seen all over.
Even 90 kg is a fair amount of force. Technicians who frequently service these valves know that this O-ring often disintegrates, save for a small portion defined by a raised lip at the end of the stem. This lip is designed to allow just enough of the O-ring to survive the compression.
The thrust washer is crucial: it provides a vital barrier between the stem and the packing nut, which are both made from chrome-plated brass. When pushed together with 90 kg of force and twisted, these two metals will abrade one another, making the twisting motion increasingly difficult and inevitably damaging the stem and/or packing nut. It's one of the things I see consistently, when "field-technicians" service these incorrectly, the orifice on the packing nut is destroyed, necessitating replacement of the entire packing nut. Most often the thrust washer was forgotten or it fell out, after all, we need to seal it and that means an O-ring, right?
From an engineering perspective, the plain stem design is fundamentally flawed. The unprotected O-ring is compressed between two parts with considerable force and subjected to a twisting motion, ensuring its short lifespan. Some manufacturers have implemented workarounds. For instance, Rotarex valves for DiveTeam used a very thin 1.0 mm cross-section O-ring that recessed into a raised lip on a stainless steel stem, combined with a wider packing nut ID. To this day, one of my favorite designs in terms of simplicity.
The O-ring (typically a BS1806-011, 7.65 x 1.78 mm | 70S) is often a softer durometer (70S), making it prone to extrusion. Therefore, it is always accompanied by a back-up ring to prevent this failure. This design does provide a seal against the inner diameter of the packing nut. The packing nut still features an orifice and seals against the thrust washer, primarily to prevent water from entering too deeply into the mechanism and causing corrosion.
This system is vastly superior, as the O-ring is protected from the crushing force between the stem and the packing nut.
Both the plain and split stem systems still leave room for improvement. M.D.E., for example, uses a superior stem setup that replaces the flimsy fin on seats with a solid, square mating surface. Other designs eliminate the O-ring entirely, instead using two raised lips separated by a thrust washer. In this configuration, one lip seals the inner gas path against the stem, and the other seals the outer gas path against the packing nut, with the thrust washer pulling double duty for sealing and friction reduction.
The common advice that you can take everything apart up until the seat (parts 5-9 in the original post) is mostly correct. This is possible because the seat's seal faces the high-pressure side. The pressure itself is crucial, as it imparts a significant force onto the seat, actively helping to keep it secured. This means there is virtually zero risk of accidental unscrewing or loosening due to bumping. Furthermore, should the seat begin to unthread, the worst immediate effect is a sharp hiss of escaping gas as it bypasses the partially opened seat and threads. This noise will immediately become noticeably louder the further the seat is unscrewed, providing clear and ample audible warning before any hazardous ejection could occur. While complete ejection is technically possible by ignoring these warnings and continuing to unthread the seat, it would require deliberate and extreme action (or stupidity...).
However, this is not universally true. With certain Z-valve designs, the sealing surface of the seat does not face the high-pressure side, but rather the ambient part of the system. In these specific designs, the packing nut must not be removed while the cylinder is pressurized, as the entire mechanism (stem, thrust washer, and O-ring) is constantly under pressure.
Despite this, a technician will receive a clear warning if they attempt to remove the packing nut. Upon loosening, the outer O-ring of the packing nut will begin to extrude and release a sharp hiss of gas, providing an immediate and clear audible indication that the high-pressure seal has been compromised. Even if this warning is ignored, fully unscrewing the packing nut typically only results in a high-flow nuisance, not catastrophic failure. The threads of the seat, while not designed for sealing, provide significant restriction to the gas flow, preventing rapid gas release.
O-ring and thrust washer
Left: Plain stem | Right: Split stem
We should distinguish between two primary valve stem designs. Designs with a plain stem, as shown in the original post, (right picture) and designs that use a Split Stem with a machined lip (left picture). The O-ring's fundamental purpose is to seal, but the mechanism and forces involved differ in each design.
Plain Stem Designs
Plain stems generally use a BS1806-010 O-ring (6.07 x 1.78 mm | 90S). As the image shows, this O-ring is typically not wide enough to seal against the inner diameter of the packing nut. That outer seal is achieved by the thrust washer in conjunction with the orifice machined into the packing nut.If the thrust washer seals the outside diameter, what is the O-ring's purpose? It achieves a seal against the inner diameter of the O-ring and the stem. The thrust washer seals the OD of the packing nut, while the O-ring seals the ID of the stem.
Then why not seal directly seal the ID only against the packing nut and leave the thrust washer? This seemingly roundabout approach is necessary due to the extreme forces involved. A packing nut with an 11.0 mm inner diameter subjected to 300 bar generates approximately 290 kg (That's 640 lbs for the Americans!) of force. An O-ring cannot withstand such a force, especially when it involves twisting motions, without being severely compromised. By relying on the seal between the O-ring ID and the stem (<6.0 mm), the resulting force is reduced considerably to about 90 kg (200 lbs).
The O-ring on the right has long disintegrated and it's black bits can been seen all over.
Even 90 kg is a fair amount of force. Technicians who frequently service these valves know that this O-ring often disintegrates, save for a small portion defined by a raised lip at the end of the stem. This lip is designed to allow just enough of the O-ring to survive the compression.
The thrust washer is crucial: it provides a vital barrier between the stem and the packing nut, which are both made from chrome-plated brass. When pushed together with 90 kg of force and twisted, these two metals will abrade one another, making the twisting motion increasingly difficult and inevitably damaging the stem and/or packing nut. It's one of the things I see consistently, when "field-technicians" service these incorrectly, the orifice on the packing nut is destroyed, necessitating replacement of the entire packing nut. Most often the thrust washer was forgotten or it fell out, after all, we need to seal it and that means an O-ring, right?
From an engineering perspective, the plain stem design is fundamentally flawed. The unprotected O-ring is compressed between two parts with considerable force and subjected to a twisting motion, ensuring its short lifespan. Some manufacturers have implemented workarounds. For instance, Rotarex valves for DiveTeam used a very thin 1.0 mm cross-section O-ring that recessed into a raised lip on a stainless steel stem, combined with a wider packing nut ID. To this day, one of my favorite designs in terms of simplicity.
Split stems
Split stems have become more common as a direct solution to prevent the O-ring from being squashed. They utilize a machined step in the stem itself. This step, combined with the thrust washer, takes the high compression forces, leaving the O-ring to perform only the sealing function.The O-ring (typically a BS1806-011, 7.65 x 1.78 mm | 70S) is often a softer durometer (70S), making it prone to extrusion. Therefore, it is always accompanied by a back-up ring to prevent this failure. This design does provide a seal against the inner diameter of the packing nut. The packing nut still features an orifice and seals against the thrust washer, primarily to prevent water from entering too deeply into the mechanism and causing corrosion.
This system is vastly superior, as the O-ring is protected from the crushing force between the stem and the packing nut.
Both the plain and split stem systems still leave room for improvement. M.D.E., for example, uses a superior stem setup that replaces the flimsy fin on seats with a solid, square mating surface. Other designs eliminate the O-ring entirely, instead using two raised lips separated by a thrust washer. In this configuration, one lip seals the inner gas path against the stem, and the other seals the outer gas path against the packing nut, with the thrust washer pulling double duty for sealing and friction reduction.
What Can Be Taken Apart Under Pressure
The common advice that you can take everything apart up until the seat (parts 5-9 in the original post) is mostly correct. This is possible because the seat's seal faces the high-pressure side. The pressure itself is crucial, as it imparts a significant force onto the seat, actively helping to keep it secured. This means there is virtually zero risk of accidental unscrewing or loosening due to bumping. Furthermore, should the seat begin to unthread, the worst immediate effect is a sharp hiss of escaping gas as it bypasses the partially opened seat and threads. This noise will immediately become noticeably louder the further the seat is unscrewed, providing clear and ample audible warning before any hazardous ejection could occur. While complete ejection is technically possible by ignoring these warnings and continuing to unthread the seat, it would require deliberate and extreme action (or stupidity...).
However, this is not universally true. With certain Z-valve designs, the sealing surface of the seat does not face the high-pressure side, but rather the ambient part of the system. In these specific designs, the packing nut must not be removed while the cylinder is pressurized, as the entire mechanism (stem, thrust washer, and O-ring) is constantly under pressure.
Despite this, a technician will receive a clear warning if they attempt to remove the packing nut. Upon loosening, the outer O-ring of the packing nut will begin to extrude and release a sharp hiss of gas, providing an immediate and clear audible indication that the high-pressure seal has been compromised. Even if this warning is ignored, fully unscrewing the packing nut typically only results in a high-flow nuisance, not catastrophic failure. The threads of the seat, while not designed for sealing, provide significant restriction to the gas flow, preventing rapid gas release.
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This is definitely one of those once you fully understand the mechanism it is much less scary.
In order for the seat to become a projectile you either have to strip all of the threads or it has to make about 5 full revolutions to back out. If the threads were stripped then the tank would be empty already anyway because the threads on the seat are the only thing actually holding the gas back when the tank valve is closed.
As far as making 5 full revolutions out, usually you are replacing that part in <1min and you never walk away. You can bump it all you want but it's holding back pressure so the threads aren't exactly willing participants in turning and without the stem itself you're going to have a hard time reaching in there. All that to say that you effectively can't accidentally screw it out.
To each their own as far as what they're personally comfortable doing but I've changed at least a dozen of those while fully kitted up and ready to get in the water over the years and certainly wouldn't hesitate to do it again.
OP
@Tanks A Lot Wow amazing post!
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