Solving The Wet Gas Flow Measurement Challenge

One of the biggest problems encountered in measuring natural gas flows occurs when liquids flow with the gas. Traditional gas meters are not designed to cope with such wet gas flows. The main issues when dealing with wet gas flows include (1) knowing how wet the gas is, (2) knowing how wet gas affects flow measurement and (3) knowing what systems exist to correctly measure the wet gas flow. Wet gas flow metering is of increasing importance to the natural gas production industry. Therefore, wet gas flow testing facilities are also of increasing importance to the industry. State-of-the-art wet gas flow testing facilities allow the research and development and verification of wet gas flow metering technologies by meter manufacturers, operators and regulatory authorities. This article describes the operation of the wet gas testing facility at Colorado Engineering Experiment Station, Inc. (CEESI).

What is wet gas flow? It is a flowing mixture of gas and liquid where the liquid makes up a relatively small part of the mixture by volume. The liquid can be made up of hydrocarbons and/or free water. The flow conditions dictate how the liquid phase is dispersed throughout the pipe. The description of the physical distribution of the liquid phase with the gas phase is termed the flow pattern (or the flow regime). The flow pattern has a considerable influence on the reaction of most meters to the wet gas flow.

At relatively high pressures and flow rates for horizontal or vertical flow, the flow pattern could be mist, where all the liquid flows in small droplets entrained in the gas. At relatively low pressures and flow rates for horizontal flow, the flow pattern could be stratified, where the liquid flows at the base of the pipe with the gas flowing above. However, in many cases moderate pressures and flow rates produce complicated and transient flow patterns that are difficult to predict theoretically due to the fact that they are influenced by many factors: meter orientation, fluid velocity, liquid properties, pipe size, liquid/gas ratios and others. This inability to predict the flow pattern theoretically drives the need for wet gas flow facilities to replicate actual field conditions in order to test wet gas meter systems.

Several terms are commonly used to describe the relative amounts of liquid and gas in a flowing stream. A qualitative term for describing the amount of liquid with the gas is the “liquid loading.” There are several quantitative terms: gas volume fraction (GVF) is the volume of the gas flow divided by the total volume of fluid flowing. Liquid load is the ratio of liquid-to-gas-mass flowrates. Another term commonly used is the Lockhart-Martinelli parameter. This is a non-dimensional method of describing the relative wetness of a gas. The higher the Lockhart Martinelli parameter the wetter the gas flow.

Industry has found the development of accurate wet gas flow meters a difficult task. Attempting to meter wet gas flow with a gas flow meter can cause many problems.