INTRODUCTION

This procedure provides some general information about the properties of gases. It is not all-inclusive, and the user of this procedure must use common sense when confronted with a specific gas that may not exactly fit the instructions in this procedure.

SAFE JOB PRACTICES

Always wear the company-mandated personal protective equipment (PPE), such as a hardhat, steel-toe shoes or boots, safety glasses with side shields, and earplugs where required. Additionally, in some areas where dust or chemicals are present, an approved respirator must be worn.

PROCEDURE

Gas does not assume a shape like a solid, or confine itself to a tight volume if it is free to expand. Gas molecules are widely spaced and can act independently and move freely. Gases can also be compressed into any space regardless of its size.

Some examples of gases used in many industries include:

• Acetylene—Used for welding and cutting.
• Oxygen—Used for welding and cutting.
• Carbon dioxide—Used in fire extinguishers.
• Natural gas and propane—Used as fuels.
• Water vapor (steam)—Used on steam turbine engines.

Compressibility and Elasticity

The differences between gases and liquids are the elasticity and compressibility of the matter. Gases are highly elastic and very compressible. They can be forced to occupy small spaces and can expand to fill any volume. The combination of compressibility and elasticity in confined gases is called high resilience. Resilience is normally found in pneumatic tires and other inflated components.

Gas Pressure

When a gas is under pressure, its volume is reduced. This means that the gas molecules become more tightly packed, which increases the number of moving molecules that strike the sides of the confining container. This increase is measurable pressure. Examples of pressurized gases include:

• Fire extinguishers.
• Welding fuels.
• Aerosols.
• Pneumatic equipment.
• Hydraulic equipment.

The units for gas pressure are the pascal (Pa), and pounds-force per square inch (lbf/in², or psi).

Handle all containers that are filled with pressurized gases with care. A ruptured container can blow the container apart.

Measurement of Gas Pressure With Gauges

Gas pressures can be measured with the use of a pressure gauge. Common gauges are marked on the scale with units of pressure such as pounds per square inch (psig) or pounds per square inch absolute (psia).

Atmospheric Pressure

The pressure at the bottom of a container of liquid depends on the depth of the liquid. The pressure at the bottom (at sea level) is greater than the pressure higher up.

Common units of atmosphere pressure are the bar and the atmosphere (ATM). ATM is the average pressure exerted by the earth's atmosphere at sea level.

• 1 ATM    = 101.35 kPa.

• = Approximately 1 bar.
• = 14.7 psi.