The "harsh" realities of pressure on Regs

[jhelmuth]

I'm not well schooled on the actual adverse issues that higher tank pressures bring to bear on regulator 1st stages, so here is my question...

What are the changing affects and issues surrounding the pressure differences from cylinders (LP/HP; HP/HP+) to the regulator's 1st stage at the following pressures increases: 2400 -> 3000; 3000 -> 3500?

Some inforamtion(?) I've read indicates that the higher pressures are "bad" for the regulator 1st stage. But how bad? I know that the higher pressure drop brings with it some additional drop in temperature, and this is probably a bit harder on components (but to what degree?). I've not heard much on this while lurking around, and no one else has ever mentioned it to me as I have discussed the purchase of cylinders. Anybody out there have some specific technical knowledge on this?

 

[FredT]

Seat unit pressure is the mythical issue. Any reg designed for reliable operation 3Ksi wil handle 3500 with no sweat. Parts replaced at repair may have a slightly deepr set, but all seats take a set. Service safety factors in the design process will handle any slight overage. IMNSHO Anything over 1.2x rated pressure is pushing it farther than I'd want to take it.

FT

 

[sndt1319]

Does anyone out there know what the factor of safety for fracture is for a tank, valve, 1st stage or 2nd stage? I know in airplanes it is a little bit above 1.

 

[FredT]

Airplanes are significantly weight limited. Diving gear is no so much so, with the exception of being too light.

Rule of thumb would be 1.5-4 for non-pressure housings. Pressure housings should be about 4 to failure, at least 1.6 to first yield.

FT

 

[DA Aquamaster]

The pressure forcing the hard seat against the soft seat ina first stage is basically constant regardless of pressure, at least in a balanced reg, as it is determined by the balance of the forces in the reg - in particular the IP and ambient pressures balancing the downstream force (from the tank) and mainspring pressure. So essentially, whether the tank pressure is 350 psi or 3500 psi, the force on the seat will be the same.

Higher pressures will increase adibatic cooling loads and seat temps may be far colder which can potentially cause materials problems. However this is not normally a problem and while there have been some cases of poorly chosen seat materials, these have been quickly addressed by the companies involved.

The bigger area of concern in a first stage used at very high pressurs are the HP o-rings. For example in a piston regulator, the o-ring can be subjected to HP o-ring pinch where the high pressure begins to cause the o-ring to extrude through the gap between the piston stem and body of the reg to the low pressure side. This can cause cuts on the o-ring and eventual failure.

So higher pressures require tighter tolerances between parts and in some cases harder o-rings. SP went with a replaceable bushing system to allow tighter tolerances to eliminate HP o-ring pinch and to eliminate wear in the first stage body as issues in high pressure applications.

That said, I would probably not sweat 3300 or even 3500 psi with a reg originally designed for 3000 psi provided it is in good shape, well maintained, and is using either a DIN connection (preferable) or a yoke rated to the service pressure being used. (Some yokes are not rated to 3500 psi.) I would however be very careful not to skip an annual service with a reg used in that applicaton.

Tanks are normally hydrotested to 5/3 rds their service pressure. So a 3000 psi tank is expected to hold at pressures up to at least 5000 psi and a 3500 psi tank would be expected to hold at pressures of at least 5833 psi. A 2400 psi tank will be expected to with stand pressures of at least 4000 psi.

Most first stages are also designed with a fair margin of saftey and many are designed for higher service pressures than they will probably ever see in actual service. For example the SP Mk 15, 20 and 25 were designed for use at service pressures up to 4350 psi so there would be a considerable margin of safety even when used with 3500 psi tanks.

Yokes also have a margin of saftey but you do not want to push it as the consequences can be nasty. I have seen older yokes rated for 2250 psi fail violently at 3000 psi. The margin of saftey is greater on newer yokes, but I would still not push it as the forces involved are quite large and the potential exists to launch the reg through the back of your head in the event of a failure.

 

[jhelmuth]

Airplanes are significantly weight limited. Diving gear is no so much so, with the exception of being too light.

Rule of thumb would be 1.5-4 for non-pressure housings. Pressure housings should be about 4 to failure, at least 1.6 to first yield.

FT

Sorry for being ill-informed - but what does this mean in English?

Also... just wanted to clarify that my focus is on the harsh affect of higher pressures to the 1st stage. Can anyone quantify that? I'm unfamilliar with the engineering, but surmise that the more severe pressure drop is the main variable that has to be delt with. What are the physics involved (other than the temperature - heat loss), and the engineering problems which play a role in the design?

 

[FredT]

The number is theoretical or tested failure point vs the "warranted" failure point.

As an example take a sling used for overhead lifting. To carry a 1000 pound working load, the minimum test load that must be applied to each sling is 4X the working load, or 4000 pounds. This is a minimum safety factor of 4. The "design load" for that sling will be about 6x the working load, being that about half of the slings will not fail at a 6000 pound load.

BTW steel 2250 psi working pressure 72's have been pressurized to above 10,000 psi without bursting. They had yielded significantly and no longer looked like a scuba tank, but still held pressure.

FT

 

[DA Aquamaster]

Hmm...if I put 10,000 psi in a steel 72, that would be 288 cu ft. No need for bigger tanks, I just need a beefier compressor.

 

[AtomicWalrus]

Hmm...if I put 10,000 psi in a steel 72, that would be 288 cu ft. No need for bigger tanks, I just need a beefier compressor.

Better yet, just take liquid O2 & N2, mix them underwater and vaporize them as necessary!

 

[DA Aquamaster]

I remember in the early 70's seeing something in a book regarding developing technologies along those lines with a picture of a nice looking female diver with a LOX type tank and some associated plumbing. It obviously never went to market.