Accuracy diagnostics of liquid ultrasonic flowmeters

Paul Boughton

Joshua W Rose looks at the specific diagnostic capabilities of liquid ultrasonic flowmeters and how diagnostic information can contribute to maintaining high accuracy custody transfer measurement.

Liquid ultrasonic flowmeters are not new to the measurement of crude oil. Over the past decade technological improvements have enabled liquid ultrasonic meters to meet the higher accuracy requirements needed for custody transfer measurement.

The transit time principle of measurement has opened the window to allow the observation and measurement of aspects of the flow stream that have never been visible to traditional measurement technologies such as PD meters and turbine meters but are critical to the proper operation of a liquid ultrasonic meter.

Custody transfer liquid ultrasonic flowmeters as defined in API MPMS chapter 5.8 use the well-known principle of transit time measurement to infer the volumetric flow rate through the meter. In transit time measurement a pair of ultrasonic transducers are used to send an ultrasonic sound signal through the flowing product at an angle to the flow stream. The transit time is the time it takes for the signal to travel from one transducer to the other. Because the signal is sent across the flow stream at an angle, the transit time is greater when the signal is sent upstream against the flow stream than when it is sent downstream with the flow stream. The difference in the transit times, or ΔT is used to calculate the linear (axial) velocity of the fluid. The general configuration of this transducer pair is shown in Fig. 1. In this case, the time from transducer B to transducer A (tba) will be greater than the time from transducer A to transducer B (tAB).

The meter is configured to include transducer pairs located on multiple planes spaced across the cross-section of the flow stream. Typical configurations for custody transfer meters can range from four to 18 paths located on different planes across the flow stream. Fig. 2 shows a diagram of a meter that has transducer pairs located on four planes across the flow stream. In this case, the top two planes have two paths per plane for a total of six paths located on four planes. Fig. 3 shows the six path FMC Technologies Smith Meter Ultra6 Liquid Ultrasonic Meter which employs this principle.

Calculating the axial velocity of the fluid on multiple planes allows the velocity profile of the flow stream to be determined empirically.

By employing two crossing paths on a plane, not only can the axial velocity of the fluid be calculated, the transverse component of the velocity can be calculated as well. This allows the presence of swirl and/or crossflow to be determined empirically in addition to the axial velocity profile as shown in Fig. 4.

Liquid ultrasonic diagnostics

As already mentioned, Liquid Ultrasonic Flow Meters (LUFMs) employing the transit time method enable observation and measurement of aspects of the liquid flow stream that are critical to the proper operation of LUFMs but are not visible to other types of traditional metering equipment. These parameters can not only be used to quantify the liquid flow stream, but can be used to set limits to define if there is an aspect of the flow stream that is abnormal and may impede measurement accuracy. Each of these parameters has a limit that can be programmed into the meter's software to trigger an alarm condition. These alarms can turn on an alarm output to a supervisory system to provide instant notification or they can be monitored via communications by reading the values in the appropriate modbus registers.

The Smith Meter Ultra6 uses a Windows-based software program Winscreen to interface with the meter.

This software can be used to monitor current data, program the meter, access historical data and run diagnostics on the meter.

Fig. 5 shows the Measured Values window within Winscreen which displays current data read from the meter.

The Measured Values screen shows the current combined average values of all paths, the current individual path values, a graphic representation of the current axial and transverse flow profiles and a rolling display of flow velocity and velocity of sound (VOS).

The data shown on the Measured Values screen can be grouped into two categories, flow profile parameters and signal parameters.

These parameters are normally listed as a percent from normal, for example, a profile symmetry value of 0 per cent means that the flow is equally balanced between the top two planes and the bottom two planes (a symmetrical profile).

Calibration of meters

A reference profile can be entered into the database as individual limits for each of these parameters when the meter is calibrated.

Whenever the meter is in operation, as long as these parameters are within the programmed limits, the current flow profile matches the reference profile.

If the deviation between the current profile and the reference profile is larger than allowed by the programmed limits, an alarm condition will be triggered.

If an alarm condition is present on one of these parameters, it indicates that something has caused a change to the flow profile.

This change could be caused by many different conditions such as a significant change in fluid viscosity, a damaged flow conditioner or excessive accumulated debris in the strainer upstream of the meter run.

 Like the flow profile parameters, limits can be set on each of these parameters to indicate a deviation from the reference condition. If an alarm condition is present on one of the signal parameters, it indicates that something is causing the ultrasonic signal to degrade as it is transmitted through the product. Some causes of this signal degradation could be entrained air or particulates in the product, a damaged transducer or cabling or debris fouling in the meter. Naturally, any alarm condition needs to be investigated to correct the root cause of the problem. Isolating the cause of the alarm to specific flow profile or signal parameters will assist in quickly identifying the problem.

Winscreen also includes higher level diagnostic functions that can be used to identify the root cause of the problem. One such diagnostic function is the Signal Analyser window which shows the transmitted and received signals for each of the 12 transducers that make up the six paths in the meter. Each of these windows can be view individually and expanded to analyse the signal in detail.

As discussed, proper operation of liquid ultrasonic flow meters is dependent on aspects of the fluid flow stream that have never been visible using traditional metering technology. These flow profile and signal parameters can be used not only to verify proper operation of the meter but also to identify changes in the flow stream that could lead to inaccurate measurement. By properly using the diagnostic capabilities of LUFMs, the user can be assured that highly accurate custody transfer measurement is being achieved.

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Joshua W Rose is Product Marketing Manager,, FMC Technologies, Erie, Pennsylvania USA. * This article was originally presented at the International School of Hydrocarbon Measurement (ISHM).

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