Statistical Determination of Natural Gas Superheat Requirements

The ASME Fuel Specification B133.7M [1] states that a typical margin of 25 to 30° C (45 to 54° F) of superheat is used for natural gas fuel but offers no basis for the estimate. The purpose of this paper is to propose a method for the safe determination of superheat that is less conservative, yet will meet the six sigma requirement of less than 4 defects (condensate formation) in one million opportunities. A drop in the total temperature of natural gas will be experienced as the gas expands in pressure reducing stations and across control valves. If the temperature falls below the hydrocarbon or moisture dew point, condensation will take place and liquids will collect or will be entrained with the gas. The temperature drop is inversely proportional to the pressure drop and is often termed ‘Joule-Thomson cooling’ or ‘J-T cooling’. The rate of cooling is described by the Joule-Thomson coefficient that can be determined by experiment or calculated from the gas composition. Superheating the gas prior to expansion can prevent condensation. The degree of superheat required for hydrocarbons, however, is often greater than the expected temperature loss across the valve as the hydrocarbon dew point may increase as the pressure falls. This paper describes a method for determining the quantity of superheat required for a specific gas composition and develops a general equation in terms of gas supply pressure that will satisfy the needs for the majority of natural gases. The general equation is based on the statistical analysis of superheat requirements for over 230 natural and liquefied natural gas compositions. A similar equation is also presented that describes the superheat requirements to avoid moisture condensation. The two equations can be used to specify the heating requirements upstream of pressure reducing stations or control valves.