FMT400 thermal mass flowmeters offer the ideal solution for accurate and safe biogas measurement

August 23rd, 2021

David Bowers

Pressure and Process Flow Product Manager UK&IE, Measurement & Analytics

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Volume flow is the most common metric used to determine the flow of liquids; however, it is rarely used for gas due to the vast volume changes that can occur with a change in either the temperature or pressure of the gas. To determine an accurate flow rate for gases a mass flow is preferable. In our Q&A find out how ABB thermal mass flowmeters have increased in accuracy and flexibility.

What are the key benefits to customers?

Utility companies and sewage treatment works are using anaerobic digestion to treat biodegradable waste and sewage sludge to produce biogas. Biogas is a mixture of methane, carbon dioxide and other contaminant gases which is a by-product of the microorganisms breaking down the organic matter contained in the sewage sludge. The methane and any carbon monoxide can be combusted to produce energy. In addition, the sewage sludge is stabilised and it’s dry matter content is reduced. Today a much of the biogas produced in anaerobic digestion plants is from those on municipal wastewater treatment sites, but they are also used for the break down and recycling of food waste.

To run an efficient anaerobic digestion process, the biogas produced must be measured. The rate of digestion can alter dependent on the weather, the time of year and the amount of biomass added. Thermal mass flowmeters can be used to accurately measure the mass and composition of the biogas produced in the process, but also used to help control the process by adding oxygen or air to the digester to ensure the bacteria thrive. Biogas production is one of many uses for thermal mass flowmeters. Perhaps the most common use for these sensors is in determining the efficiency of any combustion engine. The mass flow is a critical measurement when it comes to determining the flow of gases and controlling combustion. Mass flow can also be crucial for the control of many industrial processes, including in the water industry the amount of ozone delivered to water to clean the bacterial from it, or carbonise the water to adjust its pH. In industry they are used to measure the use of compressed air, or in the food and beverage industry the amount of nitrogen used as a preservative in packaged foods. Thermal mass flowmeters are therefore commonly used for measuring flow of gases.

How do they work?

The thermal mass flowmeter works on the principal of heat energy exchanges. As the diagram (right) shows, the heated sensor is placed in a fluid stream, the heat will transfer from the heated sensor to the fluid. The type and quantity of the fluid molecules impinging the surface will determine the amount of heat lost and as such the mass of that fluid flow can be determined.

How accurate are these sensors?

Thermal mass flowmeters work from very low pressures and have an excellent turndown performance of typically 100:1. It is incredibly important to have this low turndown particularly when working with flammable gases such as methane. These need to be operated at a low pressure to reduce the likelihood of a fire. The FMT400 is ATEX rated and therefore particularly well suited to this type of application. The accuracy of a thermal mass flowmeter can be dramatically affected by liquid drops forming on the sensor due to moisture. The specific heat capacity of liquid differs significantly from gas. The sensor needs to be positioned on the upward leg of the pipe so that any liquid in the pipe stays at the bottom. Another method is to use what’s known as a “water trap” to remove liquid from the fluid.

FMT400 Thermal mass flowmeter

What are each of the different thermal mass flowmeters for?

ECO2 FMT200 - is the entry level device designed for air measurement only – Highly capable at measuring flow in compressed air systems and checking for leakage. Compressed air is an expensive commodity and accurate measurement of its use or of leakage in the system can pay for itself.

FMT230 /250 – these devices are ideally suited for OEM. Offering Modbus output, machine makers can write the code to ensure equipment component parts can talk to everything else.

FMT430 and 450 These two devices are for general use across the chemical, pharmaceutical and wastewater industries amongst others. The two variants can measure a multiple number of gases. The 430 measures mass flow for up to two gases, whereas the 450 is capable of measuring up to 8 gas applications. Four are onboard applications and as such the gas type, tube diameter and shape, line pressure and temperature are calibrated, and pre-programmed by the factory, but a further four applications are customisable by the end-user on site. Gas mixtures can be specified via the human machine interface (HMI) or via HART or a laptop using the Field Information Manager (FIM) software. It means one device can meet the needs of lots of different applications. Its flexibility extends to changing the pipe diameter or being able to program the device to specify if the gas flow is through a pipe or a duct. It is revolutionary in that this flexibility affords the requirement of only needing one spare instrument to fulfil multiple applications

Both the FMT400 series and 200 series offers the flexible components and installation has been simplified through clever design. A significant proportion of the cost of purchasing new instruments is in the installation and commissioning of them. Simple solutions such as offering weld on adaptors to reduce pipe alterations, offering pipe components with the correct measure of straight lengths. Thermal Mass Meters are commonly replacing aging orifice plates type meters. In order to reduce installation headaches, the wafer type process connection on ABB’s new thermal mass flowmeter models has been designed to be the same size as the orifice plate which means our wafer pipe component can replace the old orifice plate and be fitted very quickly and cost effectively.

FMT700 is used predominantly for automotive combustion efficiency testing. They are used prolifically in the automotive industry, however more broadly the combustion efficiency principle is as relevant to the combustion engine of a garden strimmer as to a high-end application on testing Formula one racing engines.

Anything else we should know?

Yes, an additional point to note is that the FMT400 and many other instruments are compatible with ABB Ability™ verification for measurement devices. Calibration is important for thermal mass flow meters and it can be costly both in terms of the expertise required to perform the calibration and any downtime incurred by the instrument being offline. The ABB Ability™ SRV500 software allows the instrument to be checked against its original calibration and manufacturing parameters. This data is stored onboard and during verifications the meters performance is checked against this ‘fingerprint’ data. If the verification test results are within the limits set the meter will pass, and a certificate of verification for the instrument will be produced with a clear ‘Pass’ or Fail’ notification.

ABB Ability SRV500 verification software is helping customers to reduces the cost of ownership and therefore increase productivity and is available on all types of instrumentation from Thermal mass flowmeters, pressure transmitters, temperature sensors and positioners.

ABB Thermal mass flowmeters