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Problems with the selection of Gas Meters for Checking and Monitoring Applications from MWA Technology

Problems with the selection of Gas Meters for Checking and Monitoring Applications

September 19, 2019

Gas Meters – Making the choice between Turbine Quantometers

In this brand new article, Managing Director Martin Wardell looks at the problems when selecting Gas Meters for checking and monitoring applications.

Gas Meters

With the rising cost of gas, legislation and the importance of improving gas utilisation efficiencies has resulted in a significant increase in the demand for gas meters in building services.

Gas Utilities follow a rigorous meter selection process and meters are installed by fully trained engineers who have studied gas meters and are qualified to MET 2.

As you can see from the below guide (fig.1), the preference of Gas Utilities is to install, wherever possible, diaphragm meters which have an excellent turndown ratio and operate accurately without maintenance for over 20 years. Their second choice is to install rotary Displacement RD meters which require maintenance, and their third choice is to install a turbine meter for flow rates above 160m3/h and where there are no small appliances being used.

Guide for Gas Meter Selection

In the building services industry, Turbine Quantometers seem to be the first choice. Generally, such meters DN50 have a turn down ratio of 20:1. Depending on the brand and model, Quantometers can exhibit poor performance at low flow rates and also cause major problems
with high pressure drops across the meters at a given flow rate. Installing a larger diameter meter to minimise the pressure drop results in exacerbating the low flow inaccuracy.

Gas Meters – Allowable Pressure Drop

The downstream outlet pressure of primary meters is generally 21mBar, and according to IGEM UP/2 Installation of gas in Commercial and Industrial Premises, the maximum allowable pressure drop between the incoming supply and appliance shall be no more than 1mbar.

Gas Meters – Diaphragm Meters

The majority of meters for monitoring and checking gas consumption will operate at 21mBar. Whilst diaphragm meters for domestic loads of up to 6m3/h are relatively inexpensive, larger diaphragm meters up to G100 DN100 are not only expensive, but also extremely cumbersome.
Rotary PD meters have excellent rangeability but require maintenance. They can be installed directly into vertical and horizontal pipe runs, but are generally less popular than turbine Quantometers.

Far too often, Quantometers are selected by pipe size, with little attention being paid the range of flows that they are required to register. The loads may even be unknown when the meters are installed. Unless care is taken to determine the range of flow rate expected, it is often found that large diameter Quantometers are installed in pipework leading to kitchens and laboratories – where the diversity of usage is such that the meters often fail to register anything but occasional peak usage.

With modulating boilers often operating in sequence, incorrect selection of the meter will also lead to significant under-recording of consumption. It is far better to install two smaller meters instead of one larger meter. The pressure drop within pipework is critical, because the pressure drop across a meter rises almost exponentially with increasing flow rate. It is vital to specify the range of flow rates to be
metered and ensure that meters are selected to meet the lowest possible specified or required pressure drop. Restrictions in pipework, such as bends and shut-off valves, also add to the overall pressure drop across the system.

We have heard from customers who had installed meters only to find that the meters exhibited too high a pressure drop, and then were told to install “the next size up” with further loss of low end accuracy.

The chart below (fig.2) depicts the pressure drop when tested with air which relates directly to that in gas:

Pressure Drop Testing for Gas Meters

We are strong advocates of selecting low pressure drop turbine meters with good rangeability, such as the TBX electronic meters, the Aichi AS Ultrasonic meters and both Itron MZ and Common CPT Quantometers. As a good rule of thumb, meters for monitoring should be selected to have less than 0.5mBar pressure drop at the maximum load.

If you would like assistance in this matter, contact the metering experts at MWA on 0121 327 7771 or email our expert sales team

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