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Feeling the pressure

02 November 2020

Dr Chris Mills looks at the challenges of testing and calibrating flowmeters used in high pressure and high temperature applications.

The increasing exploitation of oil reserves where elevated pressure and temperature conditions are encountered presents technical challenges for accurate flow measurement. One immediate challenge is that the pressure and temperature of oil produced from a reservoir may differ considerably from typical calibration laboratory conditions. 

The historical practice for calibrating flow meters in the oil and gas industry has been to attempt to match fluid viscosity followed, if possible, by the fluid temperature and pressure. Matching all parameters has seldom been possible, due to limitations set by the existing calibration facilities. Limitations such as flow rate range, temperature range, pressure range and even the calibration fluid’s physical properties mean that few laboratories can meet all customer requirements.

One limitation of this approach is that temperature and pressure variations are now known to have a potentially substantial effect on the measurement performance of certain flow meter technologies. Moreover, different technologies exhibit diverse sensitivities to temperature and pressure, all of which may be critical to the overall determination of the measurement uncertainty. While increased pressure at service conditions compared to calibration conditions may cause one measurement technology to under-measure the flow rate, another technology may over-measure it. Indeed, there have also been results that show manufacturer specific flow meters of the same technology can exhibit completely different responses to temperature and pressure variations.

Coriolis meters are the most commonly used flowmeter for precision flow measurement of high value products. Manufacturers of the devices already state that their flowmeters have compensation built into the flow computer of the device to allow for the effects of pressure and temperature. However, more work is required due to limited published traceable low measurement uncertainty data. 

It is important to note that while there has been some research into the performance of flowmeters at elevated pressure and temperature, only a small amount of independent and traceable data exists. Furthermore, the independent data that is publicly available only documents the performance of certain meter types and sizes. Meter size and even geometry is believed to be a key factor in the performance of Coriolis flow meters at altering pressures and temperatures. As the fluid pressure increases, the rigidity of the flow tubes increases causing a decrease in Coriolis forces and an under-read of the mass flow. For certain Coriolis designs, as the pressure increases the curved Coriolis tubes stiffen and attempt to straighten to their original tube form. This is known as the Bourdon Effect. The differing performance with respect to pressure means it is not possible to extrapolate the performance of flow meters and flow meter types due to the limited datasets and subsequently claim a performance for the device at different conditions.  

Well-established techniques
The most suitable flow measurement technologies for elevated pressure and temperature may well be those well-established techniques, such as a Coriolis, ultrasonic, turbine or positive displacement devices. Only independent, traceable, low measurement uncertainty research can confirm whether or not that is the case. Likewise, it may be possible that conventional liquid flow meters cannot simply transfer their performance from low pressure and temperature conditions to elevated pressure and temperature service without suitable consideration, characterisation or even modification. 

Furthermore, the performance of devices of the same technology (i.e. Coriolis or ultrasonic flow meters) may not necessarily be similar to one another due to the many and varied difference between designs; flow meter material, sizing effects, and internal corrections all need to be taken into account.

These questions must be answered through planned, comprehensive, and thorough broad-spectrum research. In the future it may even be possible to assure global oil and gas regulators that pressure and temperature corrections are valid and traceable. This could then allow a return to calibration under ambient conditions for certain applications will be possible. This will require a lot of research for pressure and temperature effects to be better understood, and suitable correction strategies developed and implemented. 

Dr Chris Mills is a consultant engineer at TÜV SÜD National Engineering Laboratory.

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