Gas Physics Pressure and Flow

Topics Covered:

• Molecular Theory of Gases • Definition of Pressure • The Gas Laws • Definition of Flow • Definition of Pressure Drop

Gas Physics Pressure and Flow

Topics Covered:

• Pressure Terminology • Determining the Actual Pressure Drop in a Gas

Line • Units of Pressure • Standard Gas Pressure • Measurement of Gas Pressure

Pressure and Flow

• A gas consists of many molecules and it is the behaviour of the gas molecules that determine the properties of a gas.

• A gas is able to expand uniformly and indefinitely, expansion being limited only by the boundaries of an enclosing container.

• This property of a gas is called diffusion.

Pressure and Flow

• A gas is highly compressible, because of diffusion gas molecules can be readily crowded into a smaller space.

• The pressure of a gas is equal in all directions.

Molecular Theory of Gases

• A gas consists of a very large number of very small particles called molecules that are relatively far apart in an empty space.

• Molecules of a gas are in continuous, random motion colliding with each other and the walls of the container.

• The pressure of a gas is a consequence of the collisions of the gas molecules against the walls.

Definition of Pressure

• The pressure of a gas can be described as the force of the molecules being exerted on the walls of the container.

• Gas pressure is measurable as a force per unit area of the enclosed surface of the container, this pressure is equal in all directions.

Definition of Pressure

Imperial Pressure Formula

• Pressure = 𝑭𝑭𝑭𝑭𝑭𝑨𝑭𝑭𝑨

or Pressure = 𝒍𝒍𝒍𝒍𝒊𝒊𝑭𝒊𝟐

or PSI

Metric Pressure Formula

• Pressure = 𝑭𝑭𝑭𝑭𝑭𝑨𝑭𝑭𝑨

or Pressure = 𝑵𝑭𝑵𝑵𝑭𝒊𝒍𝒎𝑭𝑵𝑭𝑭𝟐

or Pascals

6,895 Pa = 1 psi or 6.895 kpa = 1 psi

The Gas Laws

• The pressure of a gas is determined by two significant factors: volume and temperature.

• The pressure of a gas is determined by the number of molecules of the gas within a given container.

• The more molecules there are per cubic foot of container size, the greater the pressure exerted on the walls of the container by the collision forces.

The Gas Laws

• Reducing the volume of the container will also increase the pressure of the gas, the number of molecules remains the same but the volume has decreased, increasing the pressure.

• Boyle’s law can be stated as the volume of the gas varies inversely to the pressure, provided the temperature of the gas remains constant.

The Gas Laws

• The pressure of a gas is also determined by the temperature of the gas. The higher the temperature of the gas, the greater each molecule’s velocity.

• The increase in the molecule’s velocity will increase the force that the molecules exert on the walls of the container, this is seen as pressure.

The Gas Laws

• Charles’ Law of constant volume states the pressure of a gas varies directly with the temperature, provided that the volume of the gas remains constant.

• Charles Law of constant pressure states the volume of a gas varies directly with the temperature provided that the pressure of the gas remains constant.

The Gas Laws

• The three separate laws, Boyle’s Law and Both Charles’ Laws can be combined to give the General Gas Law equations.

• Combining Boyle’s and Charles’ laws yields

𝑷𝑷 𝒙 𝑽𝑷𝑻𝑷

= 𝑷𝟐 𝒙 𝑽𝟐𝑻𝟐

• Absolute values must be utilized in the formula.

Definition of Flow

• Flow can be thought of as the movement of a gas caused by a pressure differential, or the expansion of a gas along the lines of least resistance.

• The flow of a gas is relative to the pressure difference between two areas. The greater the pressure difference the greater the flow will be.

• When the pressure difference decreases the flow decreases till the flow stops with no differential.

Definition of Flow

• When the flow of a gas is measured against a time period, a flow rate is determined.

• Flow rate measures how much gas has moved past a given point during a specific interval of time.

• Flow is rate is expressed in cubic feet per hour or cubic metres per hour. Or how much gas has moved past a given point in one hour.

Definition of Pressure Drop

• We know the flow is defined as the movement of a gas from a high pressure to a low pressure.

• However, there is resistance to the flow of a gas, just as there is resistance to the flow of electricity.

• As the molecules of a gas move along a pipe system, some of the molecules lose a small amount of their energy due to friction.

Definition of Pressure Drop

• This partial loss in total energy is realized as pressure drop.

• The resistance to the flow of a gas is measured by the pressure drop in the gas line.

• The greater the flow rate, the greater will be the resistance to the flow and the greater the pressure drop.

Pressure drop of a Gas Line Depends on the following;

• Length of the run of gas pipe.

• The size of the gas pipe.

• Quantity of fittings in the gas pipe.

• Quantity of flow rate of gas.

• Specific gravity of the gas.

Pressure Terminology

• Atmospheric pressure is the pressure on the earth’s surface measured in PSIA or KPA

• Gauge pressure is a measurement taken by a pressure gauge in which zero is calibrated at local atmospheric pressure.

• Actual atmospheric pressure varies with weather, latitude, altitude etc. Standard Atmospheric conditions in this book is 14.73 PSIA @ 60°F

Pressure Terminology

• Static pressure refers gas line pressure where no flow is taking place, sometimes referred to as lock up pressure where the gas line is locked up between the gas meter and the burner.

• Working pressure refers to the pressure in a gas line in which flow is taking place within the gas line.

Standard Gas Pressure

• For domestic natural gas appliances the houseline or building line pressure is usually 7” wc

• The average manifold gas pressure, the pressure at the burner orifice is 3.5” wc for natural gas.

• 7” houseline and 3.5” are standard pressures in the gas industry for domestic and light commercial applications.

Measuring Gas Pressure

• There are two basic instruments used to measure gas pressure, the pressure gauge and the fluid filled manometer.

• The fluid manometer is often more reliable than some pressure gauges, as gas pressure is very low and is measured in inches of water column “wc.

Measuring Gas Pressure

• There are two basic pressure gauges used to measure gas pressures.

• The bourdon tube pressure gauge uses the gas forces to expand the bourdon tube moving a system of gears and linkages moving the dial on the face.

• The digital pressure gauge uses a pressure transducer to register a pressure reading.

Measuring Gas Pressure

• The water filled manometer or U-Tube operates under the principle of pressure displacing a column of water.

• One end of the U-Tube is attached to gas pressure, the other end is open to atmosphere.

• The water column is forced down on the pressure side, and lifted up on the open side. The total displacement of water column is the “wc pressure