The company was asked to provide a venturi flow system that could measure the pressure and flow of wet, sour (high-sulphur) gas emerging from wells in the Khuff gasfield. The main challenges were to maintain accuracy across a huge potential flow range and to withstand the effects of the highly-corrosive gas. Haradh encompasses a gas plant capable of delivering 1.5bn cuft/d of sales gas and a gas-oil separation plant capable of stabilizing 300,000brl/d of light crude oil. The gas to Haradh is drawn both from sour wells in the Khuff field and sweet (low-sulphur) wells across a number of other fields.
The flow from the gas wells typically varies from 100-400m cuft/d, requiring meters with a turndown ratio of 25:1 to ensure accurate measurement across all flow ranges. Aramco specified that the accuracy must remain at 1% or better across the entire flow range. ABB’s answer was to adjust the internal design of its regular venturis, while still complying with the ISO5167 2003 standard on differential pressure measurement.
The meters are built from high-molybdenum stainless steel (ASTM A182 F44) to withstand corrosion. To ensure that the anti-corrosion properties of the material remained uncompromised during the construction and welding of the venturis, the meters underwent a series of heat treatment and pickling stages throughout the build process.
Venturi meters measure the flow of fluids as they pass through a conical section set into the pipeline. The pressure difference between the widest and narrowest points of the cone is related to the flowrate. Venturi meters are particularly useful in applications where a low pressure drop is required, for example, in long pipelines.