Are thermal flow meters suitable for biogas ?
Yes, we have 100’s installed on Biogas & Landfill sites in the UK.
The ST51 Thermal Mass Meter is ideally suited to the measurement of dirty, saturated Bio Gas at low flow and low pressure. The ease of installation, negligible pressure drop and mass flow output has proved the ST51 to be an ideal alternative to conventional DP cells and turbine meters. In addition the sensor can be removed “Live” with the optional 1” BSP Ball valve.
What is a mass flow meter ?
A mass flow meter measures the mass (kg) of gas flow in a pipe line. This differs from conventional devices that measure velocity &/or Volume. A flow meter that measures only volumetric flow eg m3/hr will be adversely affected by changes in gas temperature & pressure. A mass flow meter stays accurate despite changes in gas density. It can also, if required, express the flow as a “Normalised” volume referenced to the customer STP ie a fixed reference density.
Do thermal flow meters need Temperature & Pressure compensation?
No, the sensor inherently measures mass. The cooling affect of the gas on the heated element is a function of the gas velocity & density.
How do thermal flow meters cope if the temperature changes ?
Thermal flow meters measure the difference in temperature between an unheated & heated RTD, known as a “Delta T”. The unheated RTD tracks the ambient temperature & so the measurement is independent of temperature changes. Only changes to the mass flow affect the “Delta T”.
Can they measure mixed gas compositions ?
Yes, our FCI thermal mass flow meters can be calibrated using the customer’s actual gas composition. We commonly supply meters calibrated for mixed hydrocarbon flare gas, Syngas & Flue gas.
Can I use a thermal flow meter on a “wet” gas ?
Biogas & Landfill gas is typically classified as “wet” gas because it is usually saturated. Water vapour is just another component of the gas composition. However, if the temperature drops below the dew point the water vapour will condense & this can cause problems for thermal flow meters depending on the installation orientation, please call us for further details.
Can I use thermal meters in an ATEX zone ?
Yes, all our designs are ATEX Exd for zone 1 & 2
Can I use thermal flow meters in a Safety Integrity System ?
Yes, our ST51A / ST80 & ST100 series flow meters are SIL1 compliant.
How many straight pipe lengths do I need for an accurate measurement ?
The overall length of straight pipe required depends on what arrangement of bends & valves are upstream. As an example, if there is a control valve upstream any flow meter would require a minimum of 20 pipe diameters before the measuring point in order to allow the flow profile to develop into a stable form. By comparison, a single 90-degree bend would require 10 diameters. In all cases, we advise a further 5 diameters downstream before the next bend. Please call us for further advice.
What can I do to improve accuracy if I have very limited straight pipe available ?
The larger the pipe diameter, the less likely it is to have a 15 to 20 diameters straight pipe run. In this case, we offer our ST102AA or MT100 averaging system. These devices have multiple sensors controlled by a single transmitter that averages the flow across the sensors. In this way, we can accommodate a skewed/irregular flow profile without a loss in accuracy. An alternative approach is to install a Vortab flow conditioner upstream of the flow meter, please contact us for further details.