Ultrasonic Flow Meters in Gas Flow Measurement – Application limitations & Best Practices ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Ultrasonic Flow Meters in Gas Flow Measurement – Application limitations & Best Practices

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All flow measurement technologies have their individual limitations that impact on flow measurement accuracy. It is therefore important for engineers and technicians who use any flow measurement technology to consider the limitations of the flow meter proposed for use in a particular application before installation.
Below are the key factors which affect flow measurement accuracies in Ultrasonic meters used in custody applications for gas measurement:
1. Noise
2. Accumulation of Dirts and Liquids
3. Profile Distortions

Noise in Ultrasonic Flow Measurement
Ultrasonic flow measurement depends on accurate transit time measurement of sonic pulses. Noise inside the pipe work especially from fittings – valves, tees etc- can interfere with the detection of sonic pulses if the noise is of coincident frequency with the meter’s transducers and drown out the sonic pulses if it is sufficiently high in amplitude. Once pulses are drowned out, detection and therefore pulse transit time measurement becomes impossible and flow measurement practically stops.

Best Practices that Reduce Meter Errors due to Noise
1. Install Ultrasonic meters upstream of regulating devices
2. Locate the noise attenuating elements between meter and the noise source
3. Consult the meter manufacturer for meters of alternative frequency
that are less susceptible to noise interference and/or their meter’s response to noise so that a potential installation can be analyzed for possible impact based on valve type, flow and pressure drop and then make recommendation for attenuators that mitigate against possible interference.

Accumulation of Dirts and Liquids
Just like other flow meter technologies, accumulation of dirts and liquids can impact the performance and accuracy of Ultrasonic flow meters.
In Ultrasonic meters, transducer blockage and compromise of dimensional integrity (diameter and path changes) are common application problems encountered every now and then.
With regard to diameter changes, recall that in Ultrasonic meters, flow Q = A*V. Where A is the cross-sectional area and V is the measured velocity. A 1% change In area equates to a 1% change in calculated flow. Therefore there is a 1:1 relationship between diameter error and measurement error.

Path length changes due to trash build-up on transducer faces also causes measurement errors. However, these dirt-induced errors caused by path length changes are easily detected using speed of sound comparison. Detection of dirt build-up (which causes diameter reduction) in the meter causes larger measurement errors but is often harder to detect than for a dirt induced path length change.
Even though the diagnostic indicators of the meter might flag a “dirty meter”, it is usually necessary to make a visual inspection and then clean the meter internals to eliminate the diameter reduction.

Best Practices that Reduce Meter Errors due to Accumulation of Dirts and Liquids
1. Install a flow meter after carrying out a site assessment of the possibility for liquids and dirt accumulation. Consider addition of inlet separators, filters and drains on the meter run to either prevent contamination from occurring or to provide mechanism to drain liquids from the meter run.
2. Consider installing meter run with a mild slope to discourage accumulation of liquids down the length of the meter run and through the measuring section as liquids tend to accumulate at the lower end of the meter run which would be suitable for locating a drain.
3. Install inspection ports or even tees with uni-bolt closures to allow for visual inspection with a Boroscope and in the case of inspection tees/caps, to permit for meter run cleaning.
4. Adopt a regular program of diagnostic and visual inspection to detect dirt build-up and avoid measurement errors to diametrical reduction.

Flow Profile Distortions
In Ultrasonic meters, resolution of path velocities into a representative bulk or average fluid velocity is essential to accurate calculation of flow at line conditions. It is necessary to ensure the flow profile is consistent with that found during flow calibration in the manufacturer’s laboratory. In addition, the flow profile should be symmetrical or laminar so that the individual path weighing factors applied by the meter manufacturer retain their validity. Any flow profile distortions introduce errors into the flow measurement process.

Path velocities should be mapped during flow calibration and subsequent field inspections should compare the as-found flow profile to that documented when the meter factor was established during calibration.

In Ultrasonic flow meter applications, flow profile distortions can occur due to meter run obstructions, debris accumulation or surface roughness changes on the pipe wall and protrusions installed upstream of the meter i.e thermo wells, sampling probes. However, the most common source of profile disturbance in meter applications is standard pipe work such as tees, elbows and headers. These elements generate swirl, asymmetry or a combination of the two.

Best Practices that Reduce Meter Errors due to Flow Profile Distortions
1. Design meter runs that minimize profile distortions –use long runs of straight pipe upstream of the meter. Alternatively, include elements such as flow conditioners that normalize flow profile. However, if a flow conditioner is used, the meter must be calibrated with the flow conditioner installed in the same position in the meter run relative to the meter at both the flow lab and field installations.
2. Select a meter design that employs a multi-path design that properly characterizes the flow profile and can report whether swirl and/or asymmetry are present via meter diagnostics.
3. During initial meter start-up, map the flow profile again to check that the translation of the flow-calibrated meter system – meter run, meter and flow conditioner if used – to the field is valid. If the profile in the field is different from that measured at the flow calibration lab, then there is a shift in meter factor and this will inevitably introduce flow measurement errors.

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