Pressure drop is the amount of line pressure that is permanently lost as gas passes through the gas line. This pressure loss is due to the frictional resistance of the components the gas touches. Pipe walls generate some friction, which causes a pressure drop.
Regular Low Pressure Systems
Natural gas appliances generally work at a minimum pressure of 5″ WC and a Maximum pressure of 7″ WC. Low pressure gas is supplied by the utility at 7″ WC and thus pressure drop calculations are required to make sure the appliance gets proper pressure to operate correctly. The tables below depict the pressure loss per foot at different flow rates.
LPG (Propane) gas appliances generally work at a minimum pressure of 10″ WC and a Maximum pressure of 11″ WC. , Low pressure gas is supplied by the regulator at 11″ WC and thus pressure drop calculations are required to make sure the appliance gets proper pressure to operate correctly. The tables below depict the pressure loss per foot at different flow rates.
2PSI Service Pressure
2 PSI Systems work on 2 stages, the first stage starts at the gas meter or LPG tank regulated at 2 psi and ends at the Pounds to Inches regulator at a manifold the end of the “Trunk Line”, the allowable pressure drop on this stage is 1.0 – 1.5 PSI. The pound to inches’ regulator, typically a Maxitrol 325 or similar. For the second stage, gas is delivered at low pressure as in the 2 paragraphs above with the allowable pressure drop for the second stage. 2 PSI schemes allow the use of smaller diameter piping.
The chart links above show the BTU capacity of a pipe size at given length increments. The BTU or flow capacity is determined based on the allowable pressure drop on the gas line through a given length.