of multi-variable technology.
Multi-variable transmitters are well suited to the measurement of compressible fluids by being able to measure and compensate for density changes caused by pressure and temperature variation, as well as correcting for primary element variables, such as the discharge coefficient. Fully dynamic compensation has led to turndowns of 8:1 over the more traditional and accepted standard of 3:1.
Overall flow accuracy has led to measurements better than 1.3% of rate over this turndown – a far cry from the realms of better than 5% normally associated with DP flow installations. For non-compressible fluids, volumetric differential pressure (DP) measurement has been enhanced by improving accuracy and stability in transmitter technology.
Although turndown is still deemed a limiting factor with DP flow, it is important to look at flow coverage rather than flow turndown as the two can be rather misleading. A flow turndown of 8:1 can cover just under 90% of the actual flow range, with 5:1 covering 80%.
Whilst other flow technologies quote turndowns much higher than those associated with DP flow, it should again be noted that this figure is taken from the meter maximum. DP flow however, can be sized against the actual maximum flowing conditions using the rangeability associated with differing module ranges. To benefit from these larger flow turndowns of other flow technologies, it is often necessary to re-size the line to increase the flow through the meters.
This can lead to additional expense and increased unrecoverable pressure loss into the system.
Thanks to improving transmitter technology, it is now possible to enhance the accuracy of the DP flow secondary measurement. However, many applications suffer from poor installation of the primary element leading to inaccuracies far greater than most transmitters. One of the largest sources of error, high maintenance and relatively high installation costs, are impulse lines. So why do we use them? Either, they are needed to move the transmitter to grade level so the transmitter is accessible for maintenance, or in high temperature applications, they ensure that the transmitter is far enough away from the process so that the measurement cell does not exceed its temperature limits.
To address the issues associated with the use of impulse lines, the new 3051S scaleable pressure transmitter from Rosemount offers the user the ability to close couple the sensor to the process whilst mounting the electronics at grade level, thus removing the need for long impulse lines.
It is based around similar principles to that of most inline flowmeters such as magnetic and vortex meters that rely on remote mounted electronics to access the transmitter remotely from grade level.
It is now possible to mount a remote operator interface to enable configuration and troubleshooting using the embedded diagnostic coverage installed within the sensing module mounted directly onto the pipeline.
Even with direct mounting, short impulse lines, be they integral to the design, are a necessity to transmit the pressure drop to the target measuring cell.
Utilising enhanced diagnostics residing in the new transmitter, it is possible through patented pattern recognition technology to detect if these impulse lines are actually plugging, so enabling the user to take the necessary preventative steps to resolve the issue before actually losing the measurement point.
Rosemount has enhanced transmitter accuracy by using advanced dual capacitance sensor technology, so that it is almost immeasurable with all but the very highest of accuracy calibration equipment.
With transmitter technology now addressing some of the historical limitations associated with DP flow measurement, it is sure to remain as a market leading technology, across a very wide application spectrum, for many years to come.
The technical information in this article was contributed by Gary FeniganUK Instruments Product Manager at Emerson Process Management e-mail: firstname.lastname@example.org