Natural gas is transmitted through long distance pipe lines under high pressure. However, this pressure is not suitable for local gas distribution networks supplying customers for use in domestic and industrial gas appliances. Usually, a pressure reduction valve (PRV), ie a throttle valve (also known as Joule-Thomson valve) is used at Pressure Reduction Stations (PRS) to reduce natural gas pressure before supply to a local gas distribution network. This pressure reduction in a PRS by a throttle valve results in reduction of both pressure and temperature of natural gas.
For example, natural gas throttled from 25 barg and 10°C to 3 barg would be cooled by about 6.5°C (ie will be at about 3.5°C) after pressure reduction. It is a normal practice to have provision for heating natural gas at PRS, preferably before throttling, so that its temperature is maintained at an acceptable level after throttling to avoid operational and material integrity problems in local gas distribution network that can be caused by low gas temperature. It is estimated that 22kJ of heat would be required per kg of natural gas to preheat it to 16.5°C before throttling which will leave the gas at 3 barg and 10°C after throttling.