chrisk Posted March 27, 2015 Report Posted March 27, 2015 I've had two incidents in the last year that could have resulted in an Oxygen Emergency under the right circumstances. I'm wondering if others have had similar incidents. The first was a night flight at 12,000 ft. My wife had her cannula plugged into the plane, sitting in the back, but turned off at the flow meter. I of course was wearing mine. About an hour into a 2 hour flight, we hear a LOUD bang. My heart skips, but the plane keeps flying. We have no idea what happened, but assume a can or bag of something in the luggage popped. At the hotel, we find nothing out of order. The next morning, it's readily apparent that all the O2 has left the plane. The pvc tubing burst at the flow meter. The next time was at 19,000 ft on my cannula. I immediately recognized the problem and had my mask on in well under a minute. What is most interesting is the pvc tubing burst in the identical place: just before the flow meter, where the flow control is located. Last week, I called the manufacturer to upgrade the PVC tubing to polyurethane, which is supposed to have a higher working pressure. Interestingly the manufacturer said they have switched their product line from pvc to polyurethane. With all that said, I think I will adopt a new policy for my plane. The pilot wears the mask or cannula where the flow control is located on the scott connector. The pressure in the tubing at that point is maybe 10 psi. The passengers get the cannulas where the flow control is on the flow meter. (i.e. where the tubing has to support ~100 psi between the scott connector and the flow control) Quote
LANCECASPER Posted March 27, 2015 Report Posted March 27, 2015 Cannulas are only good to FL180. http://www.aopa.org/News-and-Video/All-News/2011/February/1/Safety-Pilot-Landmark-Accidents Quote
chrisk Posted March 27, 2015 Author Report Posted March 27, 2015 Cannulas are only good to FL180. http://www.aopa.org/News-and-Video/All-News/2011/February/1/Safety-Pilot-Landmark-Accidents I believe the regulations say a mask needs to be available above FL180. The topic has been debated here a few times. I think the confusion comes from 23.1447 Also, there is a huge difference in the useful consciousness at FL190 and FL270. It the article, it appears the pilot was flying at FL270 and ran out of O2. Quote
LANCECASPER Posted March 27, 2015 Report Posted March 27, 2015 The only point I was making from the article is this quote "Cannulas are only allowed up to FL180; masks are required above that altitude." Quote
carusoam Posted March 27, 2015 Report Posted March 27, 2015 Something sounds funny. Polymers are not known to hold pressure very well over time. The words 'cold flow' are used to describe what happens... 100psi sounds like a lot pressure for PVC tubing. PVC tubing is a plasticized version of PVC pipe. Expect metal tubing to be used for high pressure lines. For flexible lines, polymer lined stainless braids are common. Similar real life type experience: -PBT was used in house construction and suffered from the same type of failure. -Washing machines commonly use stainless braided hose to keep the cold flow failure from flooding a house. -PVC tubing is typically used for fish tank air pumps. A few psi at best... Things to consider... Check the pressure that is really used in your system. Check the recommended tubing that is being used. Check the procedure for turning off the flow of O2. Using the flow meters to turn of flow instead of using a valve on the high pressure side sound like what may have happened. This method can leave too much pressure in the line...(does this ring a bell? Quote
chrisk Posted March 27, 2015 Author Report Posted March 27, 2015 Most of my knowledge in this area comes from SCUBA diving. For SCUBA they have 2 stage regulators. The first drops the tank pressure to around 100 psi. The second stage is what provides breathing capability. I've been assuming aviation regulators are a lot like the first stage of a scuba regulator. That is, they output around 100 psi. I've never measured the output pressure, mostly because I don't have a good way to do so. Anyway, inside the plane it is very simple. A throw leaver remotely opens a valve on the tank. It's labeled "on" and "off". The 1500 psi O2 goes to the regulator. The pressure drops to around 50 to 100 psi, and the O2 is then distributed to plug in connectors via metal tubing. The oxygen masks plug into the plug-in connectors. Somewhere after the plug in connector is a flow adjustment. It's basically a needle valve that changes the size of a hole, which O2 flows through. The effect is to reduce the PSI going to the mask to just 2 to maybe 10PSI. To my knowledge, there is no special procedure to turn on the 02. --Maybe there is an adjustment in the regulator and the PSI can be reduced? I suspect there is a performance spec. The bottom line is some O2 masks and cannulas are made where there is a stretch of tubing that is supporting 50 to 100 psi. These are nice because passengers can adjust the O2 flow for themselves. Turn the needle valve until the ball floats to your altitude. If the valve is on the connector, it is very hard for the right hand passenger to adjust it. However having the needle valve on the connector eliminates the higher pressure hose. Quote
Piloto Posted March 27, 2015 Report Posted March 27, 2015 There is differences between a SCUBA air delivery and pilots oxygen system. On the SCUBA system the first stage delivers 100 psi above depth pressure. The mouth piece is a demand valve that only delivers air when the diver inhale. On the pilots oxygen system the first stage is a low constant pressure with a flow restrictor. But the oxygen always flow even if the pilot is not wearing the cannula or the mask. However a pilot smaller oxygen tank will out last the SCUBA tank due to the lower pressures involved in flying and the oxygen concentration is five times that of air. José Quote
chrisk Posted March 28, 2015 Author Report Posted March 28, 2015 There is differences between a SCUBA air delivery and pilots oxygen system. On the SCUBA system the first stage delivers 100 psi above depth pressure. The mouth piece is a demand valve that only delivers air when the diver inhale. On the pilots oxygen system the first stage is a low constant pressure with a flow restrictor. But the oxygen always flow even if the pilot is not wearing the cannula or the mask. However a pilot smaller oxygen tank will out last the SCUBA tank due to the lower pressures involved in flying and the oxygen concentration is five times that of air. José Exactly. Do you know what the pressure is supposed to be on the low pressure side of the first stage? I've just assumed it is some where in the 50 to 100 psi range. It seems to be bursting the standard pvc tubing on my oxygen cannulas, just prior to the last flow restrictor. --also my built in O2 tank is a little larger than a standard scuba tank. Quote
Piloto Posted March 28, 2015 Report Posted March 28, 2015 Exactly. Do you know what the pressure is supposed to be on the low pressure side of the first stage? I've just assumed it is some where in the 50 to 100 psi range. It seems to be bursting the standard pvc tubing on my oxygen cannulas, just prior to the last flow restrictor. --also my built in O2 tank is a little larger than a standard scuba tank. 50 psi is too high, it is the same pressure at your water faucet. I would expect no more than 10 psi for oxygen systems. Otherwise the hose would burst if get kinked. José Quote
carl Posted March 28, 2015 Report Posted March 28, 2015 The filling pressure of a oxygen tank is in the range of 2200- 2500 psi, but some tanks do have different filling pressures,depending on their material. The working pressure of an oxygen tank should be 50 psi. This should be the max pressure available to power a devise,therefore the working pressure.It is achived usually with one single stage reducing regulator. Your flow limiting devise is the flowmeter, which is a metal needle valve, with this you can stop the flow and hold the 50psi. Beyond this point begins a plastic cannula, simple mask, or rebreather mask hose. You can adjust the flow at the flowmeter, 0.5, 2 , 6 liters per minutes, but you should not have another flow limiting devise beyond this point, since the plastic tubing is meant to be an open ended circuit and not build up any pressure. Splitters , which divide the flow , should be reinforced high pressure hose, and used before any plastic connection is made. You can turn off the flow on high pressure hose , it is designed to hold the 50psi pressure. Also as you gain altitude the ambient pressure decreases on the outside of the nasal cannula tubing, which decreases the internal pressure needed to rupture it. It get easier to explode the higher you go. carl Quote
chrisk Posted March 29, 2015 Author Report Posted March 29, 2015 I should be more clear. The thread has sort of wandered from my intent, which is to see if other have run into issues with their O2 masks/cannulas? Below is one example of a cannula. You can see the needle valve is right on the connector. This will have low pressure in the plastic hose. I have one of these, it will now be reserved for the pilot. My high altitude masks are this configuration too. The other picture is a flow meeter with the needle valve. Typically the Scott connector with this configuration does not have a needle valve. It is this second type that I have experienced bursting on the tubing, immediately upstream of the flow meter. I've sent my examples of these back to the manufacture to have the tubing upgraded on the high pressure side. I plan to use these only for passengers. Quote
carusoam Posted March 30, 2015 Report Posted March 30, 2015 What keeps the tubing on the barb of the 'high' pressure side of the flow meter? Plasticized PVC tubing has a tendency to stretch. Does it slip off the barb? When you speak with the vender... Ask what pressure is normally encountered at that location? I still think that your system is delivering higher pressure than is expected. Best regards, -a- Quote
chrisk Posted March 30, 2015 Author Report Posted March 30, 2015 What keeps the tubing on the barb of the 'high' pressure side of the flow meter? Plasticized PVC tubing has a tendency to stretch. Does it slip off the barb? When you speak with the vender... Ask what pressure is normally encountered at that location? I still think that your system is delivering higher pressure than is expected. Best regards, -a- The tubing is secured with a metal crimp band. It does not slip off. I'll show pictures when I get my flow meters back. Quote
chrisk Posted March 31, 2015 Author Report Posted March 31, 2015 Ok, here is a picture of the O2 lines that burst. The two bursts were near the metal clamp. Hopefully the polyurethane lines will hold up better than the PVC. As can be seen, the line to the flow meter is under higher pressure than the mask. Quote
carusoam Posted March 31, 2015 Report Posted March 31, 2015 I am still perplexed... PU is not much stronger than PVC, but will inflate like a balloon better... It doesn't take much pressure to make O2 flow through a tube. A few psi will do the job... It is similar to pushing fuel to a carburetor, a few psi is all it takes... Can you set the pressure lower at the regulator? Again, I don't have aviation O2 experience. I have extruded miles and miles of PVC and PU tubes. I made the tube, just didn't use it... The nice stainless retainers give the image of strength. Realistically, they keep the tube from blowing off. I am seriously thinking you may have more pressure than you should in that part of the system, unless somebody else shares that same experience... If you get 90psi delivered to the flow meter, the PU tube will inflate to about 2" in diameter... Real life (Boy Scout) experience... Take a 1/4" PU hose (aka surgical tubing), tie a knot in one end. Put a pen barrel in the other end. Use safety wire to hold the barrel in place. Insert pen into a school drinking fountain. Open water valve, inflate tube with water... This is called a 'water weenie', it is better than the best water pistol! Hope that adds a visual aid to the situation. Quote
N201MKTurbo Posted March 31, 2015 Report Posted March 31, 2015 I think the problem is turning off the flow with the flow meter. The system works by having a pressure regulator at the cylinder with an orifice to regulate the flow. Normally the pressure after the orifice is very low, but if you stop the flow with a needle valve, that is shut off, the pressure in the line before the needle valve will reach the regulated pressure. Another possibility is a problem with the pressure regulator. I've seen pressure regulators that had leaky valves. The valve that the diaphragm or piston actuates in the regulator. This valve is the one that must shut off the 2000 PSI pressure. If it has the slightest leak and there is no flow the regulated pressure will eventually reach tank pressure. This will blow up just about anything. You can measure the regulated pressure by connecting a pressure gauge to one of the outlet fittings. Connect only one outlet so there is no flow in the system. The gauge will read the regulated pressure. If this pressure climbs over time you need to get your regulator repaired. Quote
smccray Posted March 31, 2015 Report Posted March 31, 2015 Most aviation regulators reduce tank pressure to 50 PSI then use a needle valve as above to control the flow. Mountain High uses regulators at 25 PSI, so if you're concerned about the pressure call MH Oxygen. Quote
Yooper Rocketman Posted March 31, 2015 Report Posted March 31, 2015 I think the problem is turning off the flow with the flow meter. The system works by having a pressure regulator at the cylinder with an orifice to regulate the flow. Normally the pressure after the orifice is very low, but if you stop the flow with a needle valve, that is shut off, the pressure in the line before the needle valve will reach the regulated pressure. Another possibility is a problem with the pressure regulator. I've seen pressure regulators that had leaky valves. The valve that the diaphragm or piston actuates in the regulator. This valve is the one that must shut off the 2000 PSI pressure. If it has the slightest leak and there is no flow the regulated pressure will eventually reach tank pressure. This will blow up just about anything. You can measure the regulated pressure by connecting a pressure gauge to one of the outlet fittings. Connect only one outlet so there is no flow in the system. The gauge will read the regulated pressure. If this pressure climbs over time you need to get your regulator repaired. Agree with this. I've used my O2 a lot, and have never had an issue. That said, I've never installed the tubing to the ports without the flow meters dialed for flow. If we are done using them, we pull the tubing out of the ports. When the needle valve on the flow control is closed, the tubing will have to withstand regulated pressure, at best, and possibly much more if the tank regulator is not 100%. Quote
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