Measuring the flow of gas accurately and reliably

21st February 2013

Gases are used in literally all industrial fields. Most gases, especially those in the utility services, have been seen and treated as a relatively cheap source of energy. The cost of energy, however, has risen dramatically over the past 10 years with natural gas tripling in price as an example.
Due to this fact utility gases come more and more into the focus of customers to identify potential savings. Energy is not a fixed overhead – it is a variable cost, therefore if you do not measure it you cannot control it.
The three different states of matter can be seen in our daily life. At ambient conditions:

  • Wood and steal are solids.
  • Water and oil are liquids.
  • Air and oxygen are gaseous.

‘Gases’ are per definition in the gaseous state at room temperature and atmospheric pressure (eg oxygen). ‘Steam’ and ‘vapours’ are liquid under the conditions described above, but thermodynamically there is no difference between steam/vapours and gases.
An important physical property of gases is their compressibility (liquids are usually considered incompressible): Closing the valve of an air pump for bicycle tires and depressing the piston will increase the pressure. Another important property of gases is that their volume will increase with increasing temperature – a gas property that is used in hot air balloons.
The simplest description of gases is given by the Ideal Gas Law which results from the combination of these two properties:

(p = pressure, V = volume, m = mass, R = gas constant, T = temperature)

Thus, the volume (m3 or cuft) and density of a gas will change with pressure and temperature – but its mass (kg or lbs) will remain constant (law of conservation of mass). For this reason, mass is a much more relevant term for measuring gases than volume.
Often, gases like natural gas or compressed air are measured in so-called ‘corrected volume’ terms, eg Scf or Nm3. These terms – at the first glance – look like volumetric terms, which they are not: Corrected Volume is defined as mass over the density at reference conditions (eg at 0°C and 1013.25mbar; 1.29kg/m3 for air), thus it is a mass term.
Endress+Hauser offers two families of measuring principles that are able to measure mass flow directly, ie without the need for compensation:

  • Thermal mass flowmeters (Proline t-mass 65I) are based on the thermal dispersion principle and are able to measure dry, clean gases of stable composition with accuracies of up to 1.5percent of measured value (mass). They offer a remarkable turndown of 100:1, ie they can measure even very low flowrates.
  • Coriolis mass flowmeters (eg Proline Promass 80F) also measure mass flow directly measuring the phase shift of a frequency imposed onto the measuring tubes. Accuracies of up to 0.35percent of measured value (mass) can be reached using this type of flowmeter – without the requirement for straight piping in inlet or outlet.

Both meters, in addition to mass flow are able to provide temperature as a separate output signal saving time and money for installing a separate temperature transmitter. By programming the gas reference density both meter types are able to provide corrected volume as an output.
Both DP cells and vortex flowmeters require compensation for measuring gases.
DP flowmeters (eg Cerabar S Evolution Series) measure the pressure differential across an obstruction. Assuming fixed values for pressure and temperature, they are able to output volume flow, mass flow and corrected volume flow based on the differential pressure measured.
Vortex flowmeters measure volume flow directly. Additionally, assuming fixed values for pressure and temperature (eg in Proline Prowirl 72W), they are able to output mass flow and corrected volume flow.
These solutions for measuring mass flow may not be very accurate in cases where temperature and pressure are fluctuating.
Imagine a vortex meter without additional compensation programmed for corrected volume flow of compressed air at 7bar abs and 20°C. If, in reality, however, pressure is varying from 6.5 to 7.5bar abs and temperature from -15 to 35°C the resulting maximum error is in the range of more than 20percent.
An improvement can be obtained using vortex meters with integrated temperature measurement, such as Proline Prowirl73. By measuring the gas temperature, this device is able to compensate for the temperature changes. The calculation of corrected volume is done in the device itself without the requirement of performing calculations externally in a flow computer or PLC.
In our example the error would be reduced to about 8percent (instead of 20percent) using the Proline Prowirl 73. A 12percent gain in accuracy for an application with US$100000 of annual gas cost would be equivalent to US$12000 annually!
This solution is perfectly suitable for gas applications where pressure is relatively stable – but temperature is not. Measurement is done in a single device without the requirement of wiring additional equipment.
Better results in the range between 2 to 3percent of measured value (mass flow) could be obtained by installing a solution that is both pressure and temperature compensated, either:

  • A multivariable vortex meter (Proline Prowirl 73) reading digitally in an external pressure using HART or another bus (Profibus PA or Foundation Fieldbus).
  • A flow computer solution (eg with Endress+Hauser’s RMC621) including a standard vortex meter (eg Proline Prowirl 72) or a DP cell, a temperature and a pressure sensor.
  • Compensation in DCS or PLC including a standard vortex meter (eg Proline Prowirl 72) or a DP cell, a temperature and a pressure sensor. Please ensure that the correct equation is used as experience has shown that this is often not the case leading to errors of 10 to 30percent. In our example this equates to US$10000 to US$30000.

In our example application with US$100000 annual gas cost this improved accuracy (2–3percent compared with 20percent with a non-compensated solution) is equivalent to US$17000 annually. Thus, it will offer a faster pay-back period to install the more accurate solution.
There is not the perfect gas meter. In addition to accuracy issues discussed above, every flow meter comes with its inherent benefits, but also disadvantages.
Thermal Mass Flow Meters: This type of flow meter offers a direct measurement of mass flow at a competitive price. This principle offers turndown ratios (relationship of the maximum flow rate you can measure with a device over the measurable minimum flow) of 100:1 saving a lot of money compared to other measuring principles: To achieve similar results with DP cells a so-called split range installation including at least two DP cells would have to be installed. Because of their capability to measure very low flow rates, they are commonly used for leakage detection. They measure flow with the minimum of pressure drop, ie less than two millibar resulting in lots of potential savings in electricity cost caused by pumping or compression. These devices require long straight runs in the inlet, however. These can be reduced by ordering the device calibrated with a flow conditioner to a minimum of 5 diameters, slightly increasing pressure drop. The use of these devices is not recommended in applications with strong pressure swings (eg from 2 to 10bar), wet gas applications, gases with changing composition and SIL applications according to IEC61508.
Coriolis mass flow meters: Coriolis meters are renowned for their high accuracies at elevated gas pressures (the cost saving potential of accurate devices has been discussed above) and good repeatability. Installation of Endress+Hauser Coriolis meters is easy, time and cost efficient: no piping supports and no straight inlet or outlet runs are required. For safety relevant applications, versions with SIL2 rating are available. This type of flow meter, however, is less suitable for wet gases and has a comparably high pressure loss.
DP flow meters: Differential pressure meters have a long tradition – they were standardised for flow measurement in 1929 and are commonly used for gas measurement today. DP Cells and retractable orifice plates can be replaced and recalibrated under process pressure, the widest variety of materials, line sizes and pressure ratings is available for this type of flow meter. Pitot tubes, compared to orifice plates, offer a reduced pressure loss and are much less subject to wear (the wear of an orifice plate’s sharp edge can easily result in additional uncertainties in the range of 1 to more than 10percent).
Endress+Hauser DP cells are developed according to SIL2 (IEC61511). Differential Pressure installations usually have a limited turndown (3:1 to 6:1). If necessary, this can be more than doubled by using the so-called ‘split range’ functionality of RMC621 flow computers. In addition, this type of flow computer offers a gain in accuracy due to its permanent recalculation of the orifice’s characteristics as a function of process conditions. DP installations, however, do have a greater potential for fugitive emissions and in the case of orifice plates the
long-term metering stability can be affected by wear. As with vortex and thermal mass flow meters, this type of flow meter requires inlet and outlet straight runs.
Vortex Flow Meters: Endress+Hauser Vortex Flow Meters are robust against vibration, extreme temperature changes and ‘water hammer’ in steam lines. The operating principle also allows them to be used on gases constaining particulates and even ‘wet’ gases up to 5percent by volume. With a proven track record for reliability, vortex meters also offer high repeatability, low pressure losses and a life time calibration for fluids that are non-corrosive and
non-abrasive. Compared to more traditional measurement technologies, vortex meters are easier to order, install and commission. Endress+Hauser offers SIL2 vortex meters for safety relevant applications. Care must be taken, when sizing a vortex meter, to ensure suitability at the lower flowrates and that the meter has sufficient upstream and downstream straight lengths of pipework.

Oliver Seifert is with Endress+Hauser Flowtec AG, Reinach, Switzerland.

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