Getting my Instructor excited by opening the gas valve all the way, fast

OR

Adiabatic Compression, as it applies to scuba divers.

Why does my instructor get excited? What exactly is adiabatic compression and why do I need to know about it?

Your instructor gets excited because of the possibility of fire or explosion! Adiabatic compression is defined as a process where high-pressure gas flows at or near sonic velocity toward the low-pressure end of a piping system with a major restriction, such as a dead-end, a down sizing, or an abrupt turn. The likelihood of a reaction (ignition) increases with increases in the rate of gas compression and gas temperature.¹

So let us break that down into terms that are understandable and apply to scuba diving and let you the diver understand why it is important. We all use compressed gas, usually air, to scuba dive with. Some divers use enriched mixes called nitrox. This is a compressed gas with a higher concentration of oxygen than air or a PO2 greater than 21%. There are also divers who use 100% oxygen (O2).

As you know, when a gas expands (decompresses) it cools. When a gas is compressed, it heats (also known as heat of compression). In a closed system, scuba unit, filling bank, etc… you are allowing the gas to move from a compressed state to a expanded state then back to a compressed state. Most people have seen and felt this when their scuba cylinder is filled rapidly the sides heat up. This is compression of the gas. If you allow the gas to flow freely from the cylinder valve, it expands rapidly as the pressure reduces and there is a decrease in temperature at the valve. It can be so great as to have frost form on the valve. This is expansion of the gas.²

When compressed gas flows from high to low pressure, it can quickly reach a very high velocity, especially at constrictions, valve seats, regulator poppits and angular fittings. In these instances, flow velocities exceeding 148 feet / sec. (45 m / sec.) are considered to be dangerous. There is even greater danger when the speed of the gas flows at or near sonic velocity (speed of sound is approximately 1,125 feet per second at 68o F). When speeding gas strikes an obstruction, it is instantly recompressed and the ambient temperature instantly skyrockets because no heat is lost to the surrounding materials (instantly in this case is not instantaneous but rather very, very fast). The higher the initial pressure, the greater the temperature generated.³ High temperatues can cause auto-ignition of containment material.

With 2,000 psi oxygen (PO2 100%), temperatures well over I,500°F can be momentarily produced. This is sufficient to ignite even such inert substances as Teflon and stainless steel. To state it simply, an uncontrollable fire can ignite and burn, hoses, metal and YOU.

This phenomenon, similar to diesel ignition, is called ADIABATIC COMPRESSION. Great care must be taken to control adiabatic compression.

Scuba cylinders are frequently filled, pressurized and depressurized by the diver. The potential for danger exists with concentrations of oxygen greater than that found in air (PO2 21%) and incomplete understanding of Adiabatic Compression.

How do you prevent mishaps? With careful design and operation of gas flow and filling operations and specific education about gas pressurization and depressurization. Several O2 fires have occurred during the preparation of diving gas mixtures due to lack of understanding and a inappropriate attitude toward adiabatic compression. Mishaps include oxygen hose fires where Teflon-lined hoses with stainless steel over-braid have detonated and burned , throwing molten particles about. Other accidents have included the neck of a scuba cylinder being blasted open and launching the valve.⁴

To prevent accidental detonations, always open O2 cylinder valves very slowly and pressurize systemic components as “gently” as possible. Design your system using slow-opening globe or needle valves that contain seats made from oxygen-compatible polymers. Transfer oxygen gas slowly to prevent excessive heating. The maximum transfer rate for pure O2 is 200 psi / minute and rates during mixing are usually 50 – 70 psi / minute. And lastly, open your scuba cylinders slowly, gently pressurizing the system.

Charles law states: “For any gas at a constant pressure, the volume of the gas is directly proportional to its absolute temperature”⁵ or more simply put “when a gas is compressed, temperature is raised”.

So remember, when working with compressed gases, always open the valves slowly. Gently pressurize systems. Transfer pure oxygen at 200 psi per minute or slower. And if you are involved with mixing gases, do not exceed rates greater than 50 – 70 psi per minute.

It can save your life!

For more information on Adiabatic compression and gas mixing consider taking a Mixed Gas Blender or Oxygen Service Technician course.

Some additional definitions of Adiabatic compression.

Adiabatic compression is a process where there is not heat transfer to or from the system, and all supplied work is added to the internal energy of the gas, resulting in increases of temperature and pressure. Adiabatic compression or expansion is favored by good insulation, a large gas volume, or a short time scale (high power level). In practice there will always be a certain amount of heat flow, as to make a perfect adiabatic system would require perfect heat insulation of all parts of a machine.⁶

Adiabatic compression is a chemical or mechanical process in which there is no exchange of heat with the surroundings (no heat entering or leaving the system). It is applicable to enclosed systems.⁷ The relationship of pressure and volume when a gas or other fluid is compressed or expanded (with no loss or gain of heat) is that compression causes an increase in temperature and expansion a decrease in temperature.

¹ Neal, Jan and Morrissette, D.. (1998). Mixed Gas Blender And Oxygen Service Technician. p. 11. Daytona Beach, Florida: Underwater Dynamics, Inc.

² Taylor, Larry “Harris”, Ph.D. Adiabatic Expansion. A Gas Laws Primer. 9 Mar 2010. http://www-personal.umich.edu/~lpt/primer.htm .

³ Boyd, Richard, Dr. and Kent, Greg. Inherent Hazards Of Pressurized Oxygen. Engineering Nitrox Partial Pressure Blending Systems © 2002 . GMC Library. Global Mfg. Corporation. 9 Mar 2010. http://www.gmcscuba.com/pdf/engineering%20nitrox%20partial%20pressure%20blending%20systems.pdf .

⁴ Boyd, Richard, Dr. and Kent, Greg. Inherent Hazards Of Pressurized Oxygen. Engineering Nitrox Partial Pressure Blending Systems © 2002 . GMC Library. Global Mfg. Corporation. 9 Mar 2010. http://www.gmcscuba.com/pdf/engineering%20nitrox%20partial%20pressure%20blending%20systems.pdf .

⁵ Heine, John, Bookspan, J. and Oliver, P.. (2000). Diving Physics. NAUI Master Scuba Diver. p.89. United States of America: NAUI.

⁶ Adiabatic. Temperature. Gas Compressor. Wikipedia. 20 May 2007. http://en.wikipedia.org/wiki/Gas_compressor.

⁷ Adiabatic. Chemical Molecule Library- Glossary. 9 Mar 2010. http://www.ch.ic.ac.uk/vchemlib/mol/glossary/ .

Dive Safe!

Dean

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