Info Scuba Cylinder Valve Installation

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Akimbo

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Scuba Cylinder Valve Installation

Very few subjects are more shrouded in diving mystique and folklore than installing Scuba cylinder valves. Casual recreational divers probably don't need to be concerned with this but advanced divers through professionals in the recreational Scuba and commercial diving industries can benefit from a working knowledge. Scuba industry pros might consider training and certificating from Professional Scuba Inspectors.

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(Image Reprinted by permission from XS Scuba)

I find that the XS Scuba PRO Valve has a number of features that I value including:
  • Wrench flats
  • Easy conversion from Yoke to DIN
  • Minimal rotations between full-open and closed (1.5 rotations)
  • Clear working pressure labeling (technically the blow-out plug's working pressure rating)
  • Hand-friendly over-sized knob with On/Off color coding

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Installed Scuba valve cross section, unpressurized. Cylinder wall not to scale. The O-ring gland profile is typical of a steel cylinder. Aluminum cylinders use a flat bevel/chamfer.

I'll limit this post to installing O-ring sealed cylinder valves currently on the market, but we can investigate all the other varieties including tapered threads worldwide if readers are interested.

History in the US
Large high pressure industrial cylinders in the US use 3/4" NGT (National Gas Taper) threads. The NGT male thread has about two more threads at the large end than NPTs (National Pipe Thread) and can be very difficult to tell apart.

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Typical industrial gas valve with a 3/4" NGT tapered thread

Technical Sidebar: NGT versus NPT
Specifications:
NGT (National Gas Taper), ANSI CGA V-1​
NPT (National Pipe Taper), ANSI B1.20.1​
NGT and NPT are very similar, often confused, and share these dimensions:​
  • All Pitch Diameters, 1/2" pipe is 0.840" OD and 3/4" is 1.050" OD
  • Flank
  • Lead
  • Major Diameter Truncation
  • Minor Diameter Truncation
  • Pitch, 1/2 and 3/4 both have 14 TPI (threads/inch)
  • Taper Angle, 1:16 ratio total which equals 1°47' or 1.783° per side
The primary practical difference is NGT threads have 2-3 more threads at the large-end than NPT. When introduced, NGT was used for dangerous/toxic gasses and the specifications include higher precision than NPT, which was primarily for water. Modern manufacturing tolerances for high pressure fittings easily exceed both.​

René Bussoz* chose to use 1/2" NGT threads used on smaller industrial cylinders when he ordered the first "Standard Tank" that met meet the ICC regulations in the US. Before that he was importing the cylinder sets sold by La Spirotechnique in France.

* René Bussoz imported the Aqua Lung into the US, founded US Divers (which is now the company Aqua Lung), and sold it to L'Air Liquide.​

Unlike industrial gas suppliers, Scuba air filling stations didn't have the ability to dry the gas to the same degree. As a result, internal corrosion became a problem and the small cylinder neck made it difficult to inspect and remove rust. They also used plumber's pipe dope to seal and lubricate the threads. US Divers introduced the current standard in the US in 1960

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US Divers' 1960 Catalog, Page 6:
SPECIFICATIONS
3/4" STRAIGHT PIPE THREAD WITH "0" RING ATTACHMENT (Patent Pending) replaces outdated litharge seal*...​

* litharge is a lead monoxide dope (lubricant and sealant) that was used to seal tapered threads before PTFE tape was developed.

As near as I can determine, this O-ring sealed straight thread connection is unique and is not part of any industrial standard of the time. Only the thread portion meets a published standard but not the O-ring gland:
  • NPSM, National Pipe Straight Mechanical thread, current standard: ANSI B 1.20.1-1983 (R1992)
  • Size 12
  • 3/4-14 for 3/4" pipe size that has an actual Outside Diameter of 1.034" or 26.26mm and 14 threads/inch

The design has some similarity to the SAE (Society of Automotive Engineers) Straight Thread Port J1926 and the Metric Straight Thread O-Ring Port ISO 6149-1, but is not compatible.

Diving Trivia, Where did "J" and "K" valve come from?
I know it is hard to believe but the names that stick with us today came from the item designations on Page 5 of the 1953 US Divers Catalog:

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Yeah, it really is that simple

Sorry for the US-Centric perspective. Here is a more comprehensive discussion of all of the different Scuba cylinders, valves, and "neck threads": Wikipedia: Diving cylinder

Installation

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Many divers see O-Rings like this and believe that this damage is caused by incomplete extrusion

Failure Diagnosis and Misconceptions
There is a persistent and often repeated rumor on the Internet that states that Scuba cylinder O-rings should not be lubricated, probably because of O-rings like the one in the image above. Several things can cause O-ring failures like this.
  • Inadequate lubrication when tightening and removing the valve
  • Incorrect O-ring size, material, and/or durometer
  • Incompatible lubrication that causes swelling
  • Incomplete tightening leaving too large a gap between the valve flange and the cylinder. For all practical purposes the gap should be zero or metal to metal all the way around.
  • The O-ring was twisted/spiraled or scared during installation.
For perspective, consider these factors:
  • The majority of O-rings are used in hydraulic applications, which is a fluid with lubricating properties
  • Hydraulic O-rings are heavily greased on installation
  • Dynamic piston-seal O-rings have a relatively large gap where the O-ring could extrude.
  • Many hydraulic applications operate at higher pressures than Scuba cylinders, constantly cycle, and see higher temperatures.
Another factor that makes Scuba cylinder O-rings relatively unusual is they rarely move in the gland because they tend to stay pressurized during their operating life, typically between 300-500 PSI or 20-35 Bar and the cylinder operating pressure. This effectively eliminates wear from consideration and the zero-gap properly installed valves makes extrusion highly unlikely.



Continued in the next post
 

Akimbo

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Continued from previous post



SAFETY WARNING

Always verify that manufacturers rate their lubricants and products for HP (High Pressure) Oxygen Service before using. Specifications and standards may change. Standards for Oxygen Cleaning also vary wildly so proceed with caution.


Lubrication
O-ring lubricants in diving applications should be used sparingly. Typically just a thin film is fine, just enough to make it shiny. You will see a lot of videos showing grease being heavily smeared on O-rings and glands for hydraulic and pneumatic applications. That is valid because great globs of grease in the system just adds to the lubricated media. This is not the case for breathing gas applications or in instruments like underwater camera housing and computers. It doesn’t matter how high the quality of the lubricant is, you don’t want it in your lungs or on the camera lens.

The first consideration for choosing a lubricant must be compatibility with the medium and the O-ring material. You will see a lot of Silicon based "Food Grade" grease being used and it is fine for most applications but NOT for all. The most frequently recommended lubricants for Scuba cylinder O-rings are:

For Air Service Only:
Dow Corning Molykote 111 (previously Dow Corning Silicone 111)

For Nitrox and Oxygen Service:
Engineered Custom Lubricants: ChristoLube MCG 111 and ChristoLube MCG 129
Aerospace Lubricants: Tribolube 71

Quite a few other suitable brands and formulations are available.

O-Ring Installation Techniques

Protecting O-rings when sliding over threads has been an industry "best practice" for decades, but is rarely discussed in recreational Scuba diving.

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O-rings can be protected from thread damage on Scuba cylinder valves with adhesive-free Flagging Tape

Valve Torque
There is a lot of debate over what "proper valve torque". The practical minimum is tight enough to make metal to metal contact between the valve flange and the cylinder, plus what is required to prevent accidental unscrewing during normal use. The practical maximum is whatever the forged brass 1"-14 threads can take without damage. That leaves a pretty wide range.

Luxfer’s Guide to Scuba Cylinder Visual Inspection (Second Edition), Page 21:

"Without a recommended torque from the valve manufacturer, we would recommend a torque of 50 lbf-ft, plus or minus 10 lbf-ft."

XS Scuba recommends 50 Ft-Lbs to their dealers.

It can be difficult to use a torque wrench on Scuba valves because not all can easily be turned with a standard Crowfoot (open-end to socket) adapter. Most mechanics experienced with torque wrenches can probably get pretty close "by feel".

A quick approximation for anyone that is inclined can be achieved by measuring 12" from the center of the valve and marking the lever you are using (wrench, clamped piece of wood, etc.). Attach a luggage scale (set to pounds) at that point and pull to the reading you want. Given the wide tolerance indicated above, it will be more than close enough.

DIY Tips
Divers that occasionally service cylinder valves may not be able to justify the cost of tools that aren't on hand. It can be especially challenging when the valve doesn't have wrench flats or requires a large and expensive wrench that you don’t own.

It is not uncommon to use a rubber mallet to tap on the valve to tighten or loosen but that can be difficult on some valves without hitting the plastic handle. The handle can be removed but that increase the risk of hitting the valve stem with the hammer and damaging it. There are better methods.

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An improvised wrench can be made with some scrap wood, (ideally) a rubber scrap pad, and a clamp. It is especially useful for valves that don't have wrench flats.

You can purchase a nicely-made DIN Valve Removal Tool. Another option is to make one with a G5/8" BSP (British Standard Pipe) male plug or a stainless steel DIN plug welded to a piece of pipe or tubing.

A DIN Valve Removal Tool can also work on yoke valves by adding a DIN to Yoke adapter.

Holding the cylinder can also be difficult when removing cylinder valves. Here are a few options for an improvised a cylinder vise:
  • A wooden fence post and a ratchet/cargo strap
  • Cylinder bands for doubles, if you have them
  • Use cam bands around the cylinder and a piece of scrap wood. A lever can be attached to the clamped scrap wood
Pro Tips
People/companies that service a lot of cylinders should consider some of these tools:
  • Fast-acting cylinder vise. Larger chain pipe vises can be used by adding some rubber to protect from scarring the cylinders but are not a good production solution for larger dive retailers or hydro shops.
  • Cylinder internal inspection lights. Lights can be made from LED rope lights available in most home centers
  • A compressed air nozzle from a clean dry source (breathing air or better)
  • DIN Valve Removal Tool and assorted large wrenches
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The production cylinder vise on the left is pneumatically controlled and can be used on full size industrial cylinders. The vise on the left uses a large cargo ratchet strap.

Special thanks to the Scubaboard Moderators that helped me with this thread and XS Scuba for their technical assistance.



End of Multipart Post
 
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Sam Miller III

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Good Gosh !
Absolutely Awesome

However, I expected nothing less from you...

This should be converted to a Power point and presented to the diving community beginning with DEMA and all the other national and regional dive events

FYI - XS SCUBA was formed with some of the most experienced knowledgeable SCUBA diving minds from basically Orange County, California diving community. With the passage of time most have retired or migrated else where. But they certainly left a mark on the diving world

I have a few historical comments I will post later when my schedule allows

But I will close with BRAVO ! Well done !

SAM MILLER III
 

BoltSnap

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Akimbo

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Thanks for all for the kind words but this is designed as an introduction/conversation starter; far from the end of the story. Some divers will stop at the first post but I'm sure there are plenty who are hungry for more.
 

tphelps

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Thank you for sharing @Akimbo! Can you tell us more about that last vice with the ratchet strap? At my previous establishment we had the V-shaped vice from Global Scuba Manufacturing. Is this the same one?
 

Vicko

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I'll have to dissasemble 40ish cylinders , that vise idea idea with a rachet strap will be a life saver (or a knee saver considering how accurate I am with a hammer).
 
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sealark

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Very good information I like the wood clamped to the top of valve when no flats are there.
 

Akimbo

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Thank you for sharing @Akimbo! Can you tell us more about that last vice with the ratchet strap? At my previous establishment we had the V-shaped vice from Global Scuba Manufacturing. Is this the same one?

Try this:
  • Google: scuba cylinder vise
  • Click: Images for scuba cylinder vise
  • Click on any interesting image. That gives you the option to visit the page the image came from.
It is also something a welder could make pretty easily from scrap. The first dive shop I went to in 1962 was a part-time affair in a plumber's workshop shed. He made a pneumatic version that looked almost identical to the production version above. It was really important then because nearly all tanks had NGT threads.
 
https://www.shearwater.com/products/peregrine/
https://cylindertrainingservices.com/training-program/

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