Difference between pistons and diaphragms in High pressure tanks

[adm3745]

The extra 40 brings in too much equipment for "rec dives". It's why I overfill the 108. I'll eventually buy another 108 for doubles. The only concern for money that I ever presented was the price hike to HP tanks over LP. I have no problem in the thousands im about to spend, but am going to still try and be efficient in spending it. I'm starting my cave training in the fall, so I think it completely appropriate to start planning now. And who said these schenangins were for Rec dives?

 

[cerich]

Hello,

I don't want to start an argument on overfilling..but sometimes for a long dive I run my LP Steels to 3800-4000. Any cave diver in Florida will understand why. Is it more harmfull to a diaphram regulator at these high pressures than it would be to a Piston regulator or is there no difference. Im currently on a Mk25 and really am unaware to how its hurting the Reg if at all. I justify it by saying its only above 3400psi for 10-20 minutes. Would a Diaphram be more susceptible to failure after repeating exposure to say 3800psi? (For DIN connections).

Thanks,

Adam

generally yes a duiaphram will less enjoy those types of pressures than a piston, that however isn;'t true of all diaphrams, depends on the HP seat material.

 

[halocline]

generally yes a duiaphram will less enjoy those types of pressures than a piston, that however isn;'t true of all diaphrams, depends on the HP seat material.

Can you explain this, please?

 

[cerich]

Can you explain this, please?

sure...all generalizations

In the bad old days (say 10 years ago) the majority of diaphram HP seats were a soft material glued somehow to the poppet, under higher pressures they just gave away, pistons HP seats are however normally just a solid chunk of plastic of some type. If they hold they hold, there are some that have been known to crack under the pressure literally but that is rare.

Recently HP seats on diaphram regs ahv ebeen more of a solid material on the poppet than in the past, generally they have been better at holding up to higher pressures, however there are still some modern well respected diaphram regs that have an older style HP seat (soft material glued to poppet) taht I would be concerned running above 3500 psi.

All that said, my LP steel tanks get to 3400-3500 fairly common, they only get above that by mistake (some dude thinks it is cool to fill them to 4000, it isn't)

 

[halocline]

sure...all generalizations

In the bad old days (say 10 years ago) the majority of diaphram HP seats were a soft material glued somehow to the poppet, under higher pressures they just gave away, pistons HP seats are however normally just a solid chunk of plastic of some type. If they hold they hold, there are some that have been known to crack under the pressure literally but that is rare.

Recently HP seats on diaphram regs ahv ebeen more of a solid material on the poppet than in the past, generally they have been better at holding up to higher pressures, however there are still some modern well respected diaphram regs that have an older style HP seat (soft material glued to poppet) taht I would be concerned running above 3500 psi.

All this reasoning assumes that higher tanks pressure results in the seat being subjected to higher pressure, more or less being forced into the orifice with greater pressure. I don't think that's true. It's intermediate pressure that forces the seat into the orifice and creates the seal. The whole idea of balanced 1st stages is that supply pressure has no (or only slight) effect on IP.

I do understand that with higher supply pressure there is a more severe pressure differential across the seat, and I suppose that there is theoretically some seat material that simply would not be able to form a good seal in the presence of this pressure differential. But I would guess that would be evident as severe IP creep right from the start, rather than accelerated wear in the seat.

The thing that determines how much pressure the seat is under is the seating force, which in a balanced 1st stage is the IP minus the force of the spring. That's a pretty low number.

 

[cerich]

All this reasoning assumes that higher tanks pressure results in the seat being subjected to higher pressure, more or less being forced into the orifice with greater pressure. I don't think that's true. It's intermediate pressure that forces the seat into the orifice and creates the seal. The whole idea of balanced 1st stages is that supply pressure has no (or only slight) effect on IP.

I do understand that with higher supply pressure there is a more severe pressure differential across the seat, and I suppose that there is theoretically some seat material that simply would not be able to form a good seal in the presence of this pressure differential. But I would guess that would be evident as severe IP creep right from the start, rather than accelerated wear in the seat.

The thing that determines how much pressure the seat is under is the seating force, which in a balanced 1st stage is the IP minus the force of the spring. That's a pretty low number.

nope, when the valve is open and the oriface is away from the seat material the whole force of the tank pressure is behind the gas ripping by the HP seat as it makes it way to fill the LP side negative pressure caused by the inhalation cycle. Once the LP side of the reg is equalized with ambient the valve shuts and the seat is no longer under as much pressure....on the inside of the area where the oriface makes a seat, the outside is still subjected to tank pressure.

 

[300bar]

nope, when the valve is open and the oriface is away from the seat material the whole force of the tank pressure is behind the gas ripping by the HP seat as it makes it way to fill the LP side negative pressure caused by the inhalation cycle. Once the LP side of the reg is equalized with ambient the valve shuts and the seat is no longer under as much pressure....on the inside of the area where the oriface makes a seat, the outside is still subjected to tank pressure.

Surething and when you purge while opening the valve,you will accomplish this.

 

[halocline]

nope, when the valve is open and the oriface is away from the seat material the whole force of the tank pressure is behind the gas ripping by the HP seat as it makes it way to fill the LP side negative pressure caused by the inhalation cycle. Once the LP side of the reg is equalized with ambient the valve shuts and the seat is no longer under as much pressure....on the inside of the area where the oriface makes a seat, the outside is still subjected to tank pressure.

Sorry, this does not make any sense. The presence of slightly higher air pressure or increased air velocity (which the presence of itself is a very dubious claim) around the seat will not cause any extra wear on it. That's just silly. It's a solid material, not an o-ring. It has to take repeated contact with a sharp edge thousands of times. How would a little extra air pressure, even if its "ripping by" cause any wear? The thing that could cause wear would be increased pressure causing either the orifice or the piston to contact the seat with greater force or velocity, and that just does not happen in balanced stages as long as IP is stable. I'm not sure why you're having trouble picturing this.

Try an experiment; check IP on any balanced 1st stage at 500 PSI, 3000 PSI, and 3500. (or as high as you want) If the IP is the same, that must mean that the amount of force pushing the seat and orifice/piston together is the same. There's no way around that one that I can think of.

Now, if you found that IP creeped quite a bit at high supply pressures like 3500+, then I would agree that there's more force on the seat.

This is a good one for Luis, maybe he'll weigh in.

BTW, I'm not sure what you're referring to with "when the LP side is equalized with ambient"... The LP side of the 1st stage is IP, not ambient. I'm sure you know that. And flow through the seat/orifice valve and into the 2nd stage is also not determined by supply pressure but by IP and the demand placed by the 2nd stage.

I'm pretty sure that the only parts of a balanced 1st stage that will experience more wear as a result of higher supply pressure are soft seals that directly hold supply pressure from ambient. Those would be the hp piston o-ring and the balance chamber o-ring in a diaphragm reg, as well as any o-rings that seal the HP seat retainer and yoke/DIN connectors.