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Title: Heat Quantity Calculation Relating To Water Vapor In Natural Gas
Author: Edgar B. Bowles, Jr., Darin L. George
Source: 2008 International School of Hydrocarbon Measurement
Year Published: 2008
Abstract: Natural gas oftentimes contains some amount of water, in either vapor or liquid form. The solubility of water in natural gas flowing through a pipeline is a function of the pressure and temperature of the flow stream. The amount of water affects the heating (calorific) value per unit volume of natural gas. The more water present in the gas, the less valuable it is as a fuel, since the water does not burn. This water, in vapor form, is sometimes referred to as spectator water and it displaces the hydrocarbon components in a natural gas mixture. The net effect is a reduction in heating value and monetary value per unit volume of gas. The amount of water vapor contained in a natural gas mixture is customarily expressed in terms of the mass of water per unit volume of gas for example, pounds mass of water per million standard cubic feet of natural gas (lbm/MMSCF). If water is present in natural gas that is to be transported, a decision must be made as to whether or not the water should be removed. There is obviously a cost associated with removing and disposing of water extracted from natural gas. Even if the water in left in the gas to avoid the removal costs, there is still a cost, because the water has mass, which requires energy (or compression horsepower) to transport. Another important consideration is that water is one of the constituents that can cause corrosion in the steel pipes used to transport natural gas. Repairing or replacing corroded pipelines can be a significant expense. In addition, corroded pipelines may degrade operational safety and system reliability. There are also issues associated with how water adversely affects the combustion process when natural gas is used as a fuel. As one can see, there are a number of considerations to take into account when deciding how much water to remove from a natural gas flow stream. Gas transportation and delivery contracts and tariffs usually limit the amount of water allowed in transmission- and distribution-grade natural gas streams as one way to help control the amount of water introduced into the natural gas pipeline grid in the United States. This paper explains how to account for water vapor when calculating the heating (or calorific) value of natural gas.

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