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Be careful using PVC with compressed air.[0] It can shatter into hundreds of tiny daggers flying through the air. [0] https://www.engineeringtoolbox.com/pvc-cp

by yardie 3y ago

Be careful using PVC with compressed air.[0] It can shatter into hundreds of tiny daggers flying through the air. [0] https://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-d_796.html https://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-...

amluto 3y ago
This is sort of a general issue. Compressed air is much more dangerous than water under similar pressure, because air is far more compressible. If you pressurize water to, say, 100 psi, you’re compressing the water a tiny bit (water is not very compressible) and stretching the container a bit. The stored energy is roughly 100 psi * (change in volume) / 2, and the change in volume is small. If you hit a water pipe. with a shovel and break it, and your pipe has a rather high 100 psi of water in it, you will regret it: you will get wet, you’ll have a muddy hole, and you will be sad because you have to repair it. If you compress air to 100 psi, you are compressing it to a rather small fraction of its original volume. So the change in volume is greater than the entire volume of compressed air and is almost as large as the uncompressed volume, so 100 psi * (change in volume) / 2. 1 cubic foot of air compressed to 100 psi is something like 10 kJ. Beware! If you winterize your irrigation system with compressed air to 100 psi (terrible idea), and you hit a pipe with a shove while it’s still at 100 psi and break it, you will release a lot of energy.
- Certified 3y ago
  Generally you are not compressing water though if dealing with compressed fluids (unless you are trying to simulate deep ocean depths for pressure testing.) You are compressing Hydraulic Fluid which can cause Hydraulic Injection Injury which can lead to loss of limb [1]. [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532970/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532970/
  - amluto 3y ago
    You’re generally not compressing water or hydraulic fluid because they’re not very compressible. (Although I think many hydraulic fluids are actually more compressible than water, but still barely compressible. They’re used for other reasons. The ideal hydraulic fluid for most applications is entirely incompressible.) But systems using hydraulic fluid to carry power have something that keeps it pressurized as it moves: that is, something that can supply power. Often very large amounts of power. (So do water pipes that have some form of supply, of course, but the pressures are much lower.) So if you pressurize fluid in a pipe, you are storing some energy in the compressed fluid. You may also have a lot of energy stored or power available in some other component of the system. In any event, the type of injury you’re describing seems like more of a velocity issue. When you allow fluid to escape from a high pressure environment to a low pressure environment through an opening that doesn’t have a whole lot of friction, the stored energy converts to kinetic energy. Some basic physics says that a small volume V of fluid has potential energy PV and mass rho * V. Conservation of energy says that, when that potential energy is gone (the fluid escapes), it has the same energy, but now it’s 1/2 m v^2. You can solve for velocity, which scales like sqrt(P). (The real story is more complicated. Pressure in fast-moving things is a bit messy due to Bernoulli’s Law, and different disciplines use annoyingly different terminology. Static pressure, total pressure, stagnation pressure, just plain old pressure, oh my!) Water (or air) jetting out of a hose through a tiny hole at even 100 psi is not likely to inject itself into a person.
darkstar999 3y ago
The article addresses this. These launchers operate at significantly lower pressures than PVC is rated for.
- dwpdwpdwpdwpdwp 3y ago
  One should also be aware that the pressure rating is a function of temperature. Extra caution should be taken when operating in cold environments. Also the cooling effect that occurs when compressed air rapidly decompresses should be considered.
  - jaywalk 3y ago
    The article addresses this, too.
dokem 3y ago
In reality this is not much of a concern at ~100psi. The bigger concern is to make sure that you get a good clean glue seal at all of the joints. That is the most likely error and point of failure. Also, don't drop it.
MilStdJunkie 3y ago
Never use PVC pipe marked "DWV" - that's outflow, and it can break at pressures as small as 7 PSI. Schedule 40 is rated for 400-ish PSI but with epoxy, locktite, and some tape, it can go up to 1200. For repeat use, err on the side of underpressure. There's so many people pushing boundaries in "homegunner" circles, but every time I see someone doing it - with a cheek weld - I get the jibblies. "It's . . your face, dude"
contingencies 3y ago
In China it is really common to run compressed air in PVC pipes at reasonably high pressures for large scale industrial automation. It is considered cheap and sub-ideal, but commonly seen deployed. To be fair, circular extrusions are approximately the strongest structures to apply pressure to.

by yardie 3y ago

amluto 3y ago

This is sort of a general issue. Compressed air is much more dangerous than water under similar pressure, because air is far more compressible. If you pressurize water to, say, 100 psi, you’re compressing the water a tiny bit (water is not very compressible) and stretching the container a bit. The stored energy is roughly 100 psi * (change in volume) / 2, and the change in volume is small. If you hit a water pipe. with a shovel and break it, and your pipe has a rather high 100 psi of water in it, you will regret it: you will get wet, you’ll have a muddy hole, and you will be sad because you have to repair it. If you compress air to 100 psi, you are compressing it to a rather small fraction of its original volume. So the change in volume is greater than the entire volume of compressed air and is almost as large as the uncompressed volume, so 100 psi * (change in volume) / 2. 1 cubic foot of air compressed to 100 psi is something like 10 kJ. Beware! If you winterize your irrigation system with compressed air to 100 psi (terrible idea), and you hit a pipe with a shove while it’s still at 100 psi and break it, you will release a lot of energy.

Certified 3y ago
Generally you are not compressing water though if dealing with compressed fluids (unless you are trying to simulate deep ocean depths for pressure testing.) You are compressing Hydraulic Fluid which can cause Hydraulic Injection Injury which can lead to loss of limb [1]. [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532970/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532970/
- amluto 3y ago
  You’re generally not compressing water or hydraulic fluid because they’re not very compressible. (Although I think many hydraulic fluids are actually more compressible than water, but still barely compressible. They’re used for other reasons. The ideal hydraulic fluid for most applications is entirely incompressible.) But systems using hydraulic fluid to carry power have something that keeps it pressurized as it moves: that is, something that can supply power. Often very large amounts of power. (So do water pipes that have some form of supply, of course, but the pressures are much lower.) So if you pressurize fluid in a pipe, you are storing some energy in the compressed fluid. You may also have a lot of energy stored or power available in some other component of the system. In any event, the type of injury you’re describing seems like more of a velocity issue. When you allow fluid to escape from a high pressure environment to a low pressure environment through an opening that doesn’t have a whole lot of friction, the stored energy converts to kinetic energy. Some basic physics says that a small volume V of fluid has potential energy PV and mass rho * V. Conservation of energy says that, when that potential energy is gone (the fluid escapes), it has the same energy, but now it’s 1/2 m v^2. You can solve for velocity, which scales like sqrt(P). (The real story is more complicated. Pressure in fast-moving things is a bit messy due to Bernoulli’s Law, and different disciplines use annoyingly different terminology. Static pressure, total pressure, stagnation pressure, just plain old pressure, oh my!) Water (or air) jetting out of a hose through a tiny hole at even 100 psi is not likely to inject itself into a person.

darkstar999 3y ago

The article addresses this. These launchers operate at significantly lower pressures than PVC is rated for.

dwpdwpdwpdwpdwp 3y ago
One should also be aware that the pressure rating is a function of temperature. Extra caution should be taken when operating in cold environments. Also the cooling effect that occurs when compressed air rapidly decompresses should be considered.
- jaywalk 3y ago
  The article addresses this, too.

dokem 3y ago

In reality this is not much of a concern at ~100psi. The bigger concern is to make sure that you get a good clean glue seal at all of the joints. That is the most likely error and point of failure. Also, don't drop it.

MilStdJunkie 3y ago

Never use PVC pipe marked "DWV" - that's outflow, and it can break at pressures as small as 7 PSI. Schedule 40 is rated for 400-ish PSI but with epoxy, locktite, and some tape, it can go up to 1200. For repeat use, err on the side of underpressure. There's so many people pushing boundaries in "homegunner" circles, but every time I see someone doing it - with a cheek weld - I get the jibblies. "It's . . your face, dude"

contingencies 3y ago

In China it is really common to run compressed air in PVC pipes at reasonably high pressures for large scale industrial automation. It is considered cheap and sub-ideal, but commonly seen deployed. To be fair, circular extrusions are approximately the strongest structures to apply pressure to.