Measuring Natural Gas Consumption: A Thermal Alternative to Turbine Wheel Meters and Quantometers

At a glance: Thermal mass flowmeters from CS INSTRUMENTS measure natural gas consumption without any moving parts and output the standarized volume flow directly – for consumption monitoring, cost centre management and energy management. The VA 550 and VA 570 are available, depending on the version, optionally with ATEX approval or DVGW approval for natural gas (up to 16 bar).

When is it worth using a thermal mass flow meter?

In industrial natural gas networks, turbine-wheel meters, piston gas meters and quantometers are most commonly used. If low flow rates are not measured accurately, moving measuring parts require maintenance, or digital consumption data is lacking, a thermal mass flow meter for natural gas is an alternative worth considering.

How turbine-wheel and rotary-piston gas meters work – and where their limitations lie. In a turbine-wheel gas meter, the gas flow strikes a mechanically mounted turbine wheel and drives it; the rotational speed is proportional to the flow velocity and, given the known pipe cross-section, to the volume flow. In a rotary piston gas meter, the pressure difference drives rotors within a measuring chamber; each rotor encloses a precisely defined volume of gas and conveys it on to the consumer. The usable measuring range of mechanical gas meters depends heavily on the design, size, operating pressure and model: conventional gas meters often operate with significantly lower dynamic range than a thermal sensor.

The inherent limitations of both designs lie in their moving parts: Bearings age due to dirt or abrasion and cause measurement deviations; exceeding the measuring range can cause bearing damage; some bearings must be lubricated regularly – something that is easily neglected in practice –; contaminated gas leads to greater deviations, and cleaning is virtually impossible; in the case of rotary piston gas meters, there is also a design-related pressure loss.

Measuring partial load and peak flow: The VA 550 and VA 570 offer a measuring span of up to 1:1000. The actual minimum and maximum flow rates (Qmin/Qmax) remain the decisive factors. With the VA 550 and VA 570, the measuring range can be selected to suit the application (Low Speed, Standard, Max and High Speed). Further advantages of the thermal principle include the negligible pressure loss and the ability to measure even the smallest consumption rates: the VA 550 and VA 570 are very often used in large pipe cross-sections with low gas consumption, where mechanical gas meters reach their limits.

Avoiding rotor wear: The thermal principle measures the heat emitted by a heated sensor – without rotor bearings and with minimal additional pressure loss. The sensors (heater and temperature sensor) are designed for harsh industrial use and housed in a thin, robust stainless steel tube: no bearings, no wear, easily accessible and simple to clean if soiled. The meters are very robust and virtually maintenance-free.

Outputting standard volume directly: Mechanical gas meters are generally actual volume meters; conversion to standard volume is carried out via external volume converters fitted with additional pressure and temperature sensors. To achieve the required accuracy, these sensors must be highly precise and located within or close to the meter – their measurement errors are added to the meter’s accuracy. The VA 550 and VA 570 measure the mass flow rate and, provided the gas parameters are correct and the reference conditions are specified, output the standarized volume flow directly; a separate flow converter with additional pressure and temperature measurement is not required for this. For comparisons, the reference conditions must be the same (e.g. 0 °C; 1,013.25 mbar (a) in accordance with DIN 1343 – also referred to as standard m³/Nm³); for kWh calculations, the calorific value must also be taken into account.

Operating volume or standard volume: why the difference is crucial for natural gas

At higher operating pressures, the standard volume is greater than the operating volume – in proportion to the absolute pressures, with an additional correction for temperature. If an operating volume is incorrectly reported as a standard volume or processed as such, the actual gas consumption is systematically underestimated, and the kWh balance mirrors this error exactly, as it depends linearly on volume:

In addition, there is the temperature factor: there is a difference of around 7 % between a gas temperature of 20 °C and the standard reference temperature of 0 °C. The values are provided for illustrative purposes using an ideal gas; for billing or highly accurate state conversion, factors such as temperature, gas composition and compressibility must be taken into account (DVGW Regulation G 685). The most common source of error in energy balances is not measurement precision, but an unnoticed incorrect volume reference.

Where is thermal natural gas metering particularly worthwhile?

In main and distribution pipes, a point of measurement continuously monitors the total consumption of a building, area or boiler house – for larger existing pipework, the VA 550 is a useful option as an insertion type sensor, as there is no need to install a complete in-line measuring section in the nominal diameter. Directly upstream of individual consumers (boilers, burners, production plant), it becomes clear which consumer requires how much natural gas and when – the basis for load profiles and cost centres. The most informative approach is the combination: main metering plus sub-meters shows total consumption and its distribution.

How are the measured values fed into the energy management system?

With many existing mechanical meters already installed, consumption data is transmitted solely via a pulse output; modern digital state of the art thermal mass flow meters can also offer digital interfaces. Depending on the version ordered, the VA 550 and VA 570 can be integrated directly into the central control system or energy monitoring software via Modbus TCP interface over Ethernet (flow, total consumption, gas temperature); an RTU/TCP gateway is not required. Optional Power over Ethernet combines data transmission and power supply in a single network cable – this reduces the installation effort, particularly where there are multiple points of measurement. Alternatively, Modbus RTU, 4…20 mA, pulse, M-Bus, IO-Link or HART are available.

Advantages during installation, maintenance and recalibration

Retrofitting the VA 550 as an insertion type sensor: The sensor is installed via a suitable process access point or ball valve. A depth scale and defined orientation ensure a reproducible sensor position. The VA 550 is designed for operating pressures up to 50 bar (DVGW version up to 16 bar); installation and removal under pressure are carried out via a ½" ball valve with a retaining ring.

VA 570 as an in-line solution: During cleaning or recalibration, the measuring section remains in the pipe – only the sensor unit is removed. The sensor is sent in for recalibration; replacement sensors can be fitted for the duration of the calibration, ensuring there is no interruption to operations.

Operation: On the VA 550 and VA 570, optical keys allow settings to be adjusted without opening the housing – the operation of both devices is identical. Digital device and status information (Modbus registers) assist with troubleshooting and handover documentation.

Practical example: From consumption value to process parameter

In a boiler application, 330 Nm³/h of natural gas and 5,000 kg/h of vapor are measured. The boiler efficiency is calculated from gas consumption, heating value and steam data – 84 per cent in the documented example. In this way, a simple consumption measurement is transformed into a reliable key figure for efficiency and maintenance. (→ Application report: Steam boiler)

Which natural gas flow meter is suitable for the application?

What applies to ATEX and gas installations?

The VA 550 and VA 570 are available with ATEX/IECEx approval, depending on the variant; the Ex marking for these variants, as stated in the data sheet, is: ATEX II 2G Ex db IIC T4 Gb (gas) and II 2D Ex tb IIIC T90 °C Db (dust); IECEx Ex db IIC T4 Gb / Ex tb IIIC T90 °C Db. The Ex version is rated for a medium temperature of up to 120 °C and has an IP 65 protection rating. The EU-type examination certificate is available in the download section. It is the operator’s responsibility to verify suitability for the specific explosion-protected area. CS INSTRUMENTS provides technical data sheets and Ex certificates for this purpose, but does not offer advice regarding zone classification or suitability for a specific installation.