The Ideal Gas Law is an equation that describes the behavior of natural gas in a pipeline system; specifically, the relationship between pressure, temperature, and volume. Engineers and operators use the law when designing a gas system and evaluating operating parameters such as how changes in pressure will affect temperature and/or volume. The law helps determine what physical configurations are optimal and allows operators to predict how the system will behave under changing conditions.

The Ideal Gas Law expresses the relationship between pressure, volume, and temperature using known factors. In the above equation, P is pressure, V is volume, T is temperature, and n and R are constants with values that do not change. Thus, if we know two of the variables, say pressure and volume, we can determine the expected gas temperature. It is important to note that not all the units used in the Ideal Gas Law are typical units used elsewhere in the industry. Typically in using the law, P is measured in psia, V is measured in cubic feet, n is the number of moles of gas measured in units of pound-moles, T is measured in degrees Rankine, and R is the gas constant of 10.7316 cubic feet-psia per lb-moles-degree Rankine.

At gas pressures above around 100 psi, real gas in a pipeline does not exactly follow the Ideal Gas Law. To account for this, an empirically derived compressibility factor Z is added to provide a reasonable accurate adjustment from ideal behavior. The Ideal Gas Law becomes PV = ZnRT. The factor Z varies with temperature, pressure, and specific gravity and adjusts for the fact that the actual density of gas under high pressure is usually greater than the theoretical density assumed in the Ideal Gas Law. To make predicting gas behavior at higher pressures easier for engineers and operators, the industry has developed charts that take into account the Z factor. These are called Mollier charts and are available in natural gas engineering handbooks.

An example of use of the Ideal Gas Law and Mollier charts is an engineer studying the temperature change when gas goes through a compressor station which increases the pressure from 500 to 700 psi. After referring to Mollier charts, the engineer can conclude that within the range of 500 to 700 psi, gas temperature changes about 1 degree Fahrenheit for every 15 psi of pressure increase. So the 200 psi increase, results in a temperature increase of 200 x 1° F / 15 = 13.3 ° F. This allows the engineer to determine whether or not the gas will need to be cooled upon exit from the compressor.