An important property of natural gas is its pressure. Pressure differentials allow natural gas to be moved through the pipeline network. Pressure must be managed to avoid gas leaks and equipment failure, which can result in dangerous accidents. Each component in the gas delivery system is rated for the maximum allowable operating pressure (MAOP), which determines the maximum safe pressure for that component.
A gas is composed of many molecules that are in constant random motion. These molecules frequently collide with each other and with the walls of any container. As the molecules collide with the walls of a container such as a pipe, the molecules produce a force perpendicular to the wall as shown below. The sum of the forces of all the molecules striking the wall divided by the area of the wall is defined as pressure.
As the number of gas molecules in a space is increased by compressing the gas, the amount of pressure the gas exerts increases.
In Imperial and U.S. units, pressure is measured in pounds per square inch (psi). A vintage unit occasionally used is inches of water column. It derives from an old way of measuring pressure using a device called a manometer. The metric unit for pressure is the pascal, defined as the force of Newton per square meter. The gas industry commonly uses the kilopascal, which is 1,000 pascals. Here are conversions between the different units:
1 psi = 27.7 inches of water column
1 psi = 6.985 kilopascals
Pressure is measured in either absolute or gauge. Absolute pressure is measured relative to a vacuum, while gauge pressure is measured relative to atmospheric pressure.
Here is an example of the difference: