Digester gas consists of methane, CO2 and other trace elements and requires mass flow measurement to properly quantify production in order to meet environmental regulations.
This gas can be used for on-site power (cogeneration), processed into renewable natural gas (RNG), allowing for the captured CO2 to be used for water treatment or sold, or flared to minimize the impact on the environment. Volumetric flow measurements are inadequate. The ability to measure flow velocities as low as 0.25 SFPS [0,08 NMPS] is often required.
Capacity demand can require instrumentation with flow turndown capabilities as high as 100:1. Instruments with no moving parts or small ports that can be fouled by particulates in the gas stream are preferred. When it is not economically feasible to remove water from the biogas, it is important to select an instrument that is not affected by entrained moisture in order to ensure measurement accuracy and repeatability. Gas specific calibrations ensure overall performance of the mass flow meter under actual field conditions. The use of flow conditioners (Vortab VIP) allows for reducing the footprint of the digester room.
One of the configurations we have used for such an application is a ST51A with a packing gland and a ball valve to facilitate on-line flow meter maintenance. The ST51A also comes with HART which was a requirement for the output.
This was being used in a DN250 line size with 65% Methane and 35% Carbon Dioxide with flow rates of up to 750 cubic meters per hour.