Flow Measurement during Gas Injection and Withdrawal

Flow Measurement during Gas Injection and Withdrawal

Depending on the underground storage type, pressures can reach considerable levels (250+ bar) during gas injection and withdrawal.

Conventional differential pressure instruments are both lacking in accuracy and reliability. Due to their small turndown ratio, they usually work in a cascading fashion to cover the wide flow range, thus entailing large pressure losses. They are also subject to wear and tear. The tiny pressure taps for orifice plates are particularly exposed to clogging due to the possible formation of hydrates or gas contaminants, resulting in excessive maintenance efforts associated with pipework and therefore partial downtimes of plant sections.

Measuring simply from the outside of the pipe clearly proves to be the better solution.

FLEXIM's acoustic measurement offers high accuracy in both flow directions over an extremely large measurement range. Therefore, one FLUXUS G gas flow meter replaces two or more conventional flow meters. As the clamp-on transducers do not come into direct contact with the gas flowing inside the pipe, they are not affected by solid particles, hydrates or by high moisture contents.

In addition, non-invasive gas flow measurement with the FLUXUS G gas flow meter does not cause any pressure losses and therefore does not negatively affect the energy efficiency of the plant.

Application example:

Since 1978, a german utility company has been operating an underground natural gas reservoir in the northern part of Germany. Gas is injected through nine wells into a 15m thick layer of porous sandstone some 2100m underground and is extracted when required. The gas reservoir is connected to the European gas network by three pipelines. Through these, dry and marketable natural gas with a pressure of about 55 bars is fed into the reservoir. During injection, the gas pressurized up to 275 bars.
The withdrawn gas is vapor-saturated and needs to be dried and depressurized prior to being returned into the gas transmission system.

Input and output measurements at the reservoir are carried out with custody transfer gas flow meters. But, in order to create a balance for the entire reservoir, it is necessary to monitor the bi-directional flow through each individual wellhead. In the past, differential pressure meters were generally used for this purpose. However, they are maintenance-intensive and lack accuracy given the wide flow range.

FLEXIM's non-invasive ultrasonic gas flow measurement does not have these problems:

The simple installation of the clamp-on transducers on the outside of the pipes does not require pipe work nor process interruption. This makes it highly suitable for retrofitting. But even for new installations, clamp-on is the better solution. The measuring procedure is maintenance-free and delivers highly accurate measurements over the entire measuring range. The high moisture content of the natural gas (wet gas) at extraction does not affect the measurement.

Advantages:

  • Accurate and reliable flow metering at thick walled steel pipes (up to 35mm)
  • Reliable measurement for a wide turndown range and bi-directional flows
  • Measurement not affected by high moisture content - absolutely wear resistant, little maintenance work required
  • Long-term stability, no calibration intervals required

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