HP/LP Hose Failure?

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Rae HalfTheHill

MSDT/Captain
Staff member
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Divemaster
Messages
263
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Location
Florida Keys
# of dives
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I was watching a new episode of Dive Talk on YT last night (wonderful channel, highly recommend), and they were reacting to an instance where a DM had what appeared to be an O-ring burst at depth. The entire incident was handled very well.

They then went into a discussion of hose failures. The first question to come up was which failure will drain a tank faster, a HP or LP hose. Opposite to what common sense says, the LP hose will actually drain a tank faster due to it's wider diameter (which makes sense as it is supplying the diver's breathing gas as opposed to a HP hose that is just supplying the SPG). The image that was shown appeared to be the endcap of the HP hose, which did have just a pinhole sized opening. My first question is if the entire hose has this diameter opening?

The next topic they were discussing is if the cylinder would empty faster at depth. From my understanding, the ambient pressure at depth would not have an impact on gas coming a failure at the LP or HP hose, as the system is closed until the gas reaches the second stage. They were considering an experiment at 100' of cutting both a HP and LP hose to test how much time it would take to empty a cylinder. Interesting food for thought, and I was hoping to find some more amplifying info here. Thanks!
 
I was watching a new episode of Dive Talk on YT last night (wonderful channel, highly recommend), and they were reacting to an instance where a DM had what appeared to be an O-ring burst at depth. The entire incident was handled very well.

They then went into a discussion of hose failures. The first question to come up was which failure will drain a tank faster, a HP or LP hose. Opposite to what common sense says, the LP hose will actually drain a tank faster due to it's wider diameter (which makes sense as it is supplying the diver's breathing gas as opposed to a HP hose that is just supplying the SPG). The image that was shown appeared to be the endcap of the HP hose, which did have just a pinhole sized opening. My first question is if the entire hose has this diameter opening?

The next topic they were discussing is if the cylinder would empty faster at depth. From my understanding, the ambient pressure at depth would not have an impact on gas coming a failure at the LP or HP hose, as the system is closed until the gas reaches the second stage. They were considering an experiment at 100' of cutting both a HP and LP hose to test how much time it would take to empty a cylinder. Interesting food for thought, and I was hoping to find some more amplifying info here. Thanks!

The hose has a larger ID, but the orifice is the restriction so it does not matter.

At depth, HP would technically be a bit slower as the delta P is smaller. The difference however is insignificant as you are talking about ~60psi increase in ambient pressure against ~1500psi average supply pressure so it is a few percentage points.
Conversely, LP would be drained at a higher rate. The volumetric flow of the gas should remain close to constant, however the gas is 4x as dense at 100ft as it is at the surface so the mass flow is significantly higher and would drain the tank faster. I do not want to do the math on what the difference would be as there are lots of equations involved, but it's enough that it would be considered significant.
 
The internal diameter of the HP hose is small, but depends on the hose layup and material used. Typically the hole is about 1mm diameter.

The hose diameter however makes no diference because it is the smallest orrifice that sets the mass flow and that orrifice "chokes" at a certain mDot meaning no more air can come out for a given deltaP


For a LP hose full burst, it is unlilkely that the 1st stage can actually reach and hold it's IP setpoint (typically 9 to 10 bar above ambient) as the flow losses in the 1st stage at such a high continuous mass flow will effectively choke that 1st stage (especially parts like the sintered debris filter)

I've never actually put an IP gauge on one LP port, and connected a bare hose to another and then turned on the tank valve and seen what the IP reads however.I'm sure it will be very 1st stage architecture dependant (ie a free flowing, large seat/orrifice SP MK25 is going to be able to hold its IP up to a greater mass flow than a smaller, diaphram type regulator)
 
Conversely, LP would be drained at a higher rate. The volumetric flow of the gas should remain close to constant, however the gas is 4x as dense at 100ft as it is at the surface so the mass flow is significantly higher and would drain the tank faster. I do not want to do the math on what the difference would be as there are lots of equations involved, but it's enough that it would be considered significant.
But since the tank is a rigid (almost) closed system, the density of the gas inside doesn't change with the ambient pressure. It only changes after it leaves the tank, and the linear flow velocity would be very slightly slowed by the increased ambient pressure. Coming from a low pressure port, that 60 psi of ambient pressure is a little more significant up against the 135 psi of IP. In any case, the flow rate would be fast enough that the difference between on the surface and at 100 feet deep would be negligible.

I like to end my Scuba Skills Updates with a free flowing reg exercise. I have the students hold down their purge button and breathe off of it while I time them for 1 minute. On Sunday, I freeflowed my reg from 2000 psi to 300 in that one minute in 1 foot of water. I was using an AL50, so that's about 28 cubic feet of lost air, and would be the equivalent of ~1060 psi from an AL80. A freeflow is probably just a little slower than a burst hose.
 
@kmarks the gas in the tank doesn't, but it does in the rest of the system, though your point is valid since the tank valve is usually the restriction for regulator flow anyway so it is unlikely there would be any real pressure inside of the low pressure hoses. A few seconds over a minute can be considered significant though
 
Conversely, LP would be drained at a higher rate. The volumetric flow of the gas should remain close to constant, however the gas is 4x as dense at 100ft as it is at the surface so the mass flow is significantly higher and would drain the tank faster. I do not want to do the math on what the difference would be as there are lots of equations involved, but it's enough that it would be considered significant.
Not correct. The important factors are:
  • How quickly air is pushed out of the tank by pressure.(500 to 3000 psi)
  • How much the external pressure pushes back. (141.81 psi at 100m deep)
Think about this logically:
  • Would air escape the tank faster in a vacuum?
  • How quickly would air escape a 3000psi tank, in a 3000 psi environment?

What is true, is that the bubble or air volume (not quantity, volume) would be less due to external pressure.

edit: It appears I was slightly wrong on one minor point. I didn't consider was how a balanced first-stage works. A balanced first-stage would allow more air to escape from the low-pressure side.
 
Somewhat related to the original topic, I have a theory that if you experienced a failure, and had no other air supply, you could (1) remove your BC, (2) partially close the tank until you had a slower steady stream of bubbles, and (3) breath the bubbles rising while swimming towards the surface with your BC/tank in front of you.

In my first scuba-class we breathed from the regulator below, but out of our mouth, so it seems logical that would work from a cut hose, blown o-ring, etc. It's obviously far less efficient than breathing from a regulator, but a possible backup plan in case of emergency.
 
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