Ultrasonic flow meters

The operating principal of ultrasonic meters is shown in (Fig 1). The volume throughput (Q) is equal to the fluid velocity measured (Vm) multiplied by the area (A) or Q = Vm x A, where the fluid velocity measured is proportional to the difference between an ultrasonic signal traveling with the flow (tAB) and against the flow (tBA). The measurement principal is fairly simple but there are number of factors that must be addressed to achieve custody transfer measurement accuracy

A misconception exists that ultrasonic meters are not sensitive to fluid properties. This is not the case. To achieve the level of precision measurement available with other metering technologies, these affects must be addressed. This is especially important with crude oil measurement as the oil is typically highly viscous with high levels of contaminants. On a qualitative level the influences of fluid properties have been addressed by various authors. Knowledge on the quantitative affects of fluid properties on ultrasonic meter accuracy is limited. The influence of fluid properties on the UFM performance may be classified in two main groups:

•  Signal quality affects – the signal attenuation and signal-to-noise ratio (SNR) in the acoustic paths

• Flow profile affects – the robustness of the integration method used to combine the individual acoustic path measurements into a full volumetric flow rate measurement

• The signal quality of an ultrasonic meter in crude oil applications is determined by: viscosity, entrained gas, sediment and water (S&W) and wax content.

• The signal strength, or more precisely, the signalto-noise ratio (SNR), is crucial for the accuracy of the transit time measurements made in the LUFM.

Reduced SNR can mean higher uncertainty of the transit time measurement, resulting in a higher uncertainty of the volumetric flow rate measurement. In the worst case, the signal can’t be discerned from the noise and the measured output is erroneous. Noise is classified as:

• Coherent noise (signal interference) which includes: a. Transducer ‘ringing’ effects; Spool-piece borne signals (acoustic cross talk); c. Liquid borne reflections (transducer ports reflections, pipe wall reflections/reverberation).

• Incoherent noise (‘signals’ with random phase relative to the measurement signal) includes: a. electromagnetic noise (RFI); b. flow noise; c. valve noise; d. structural (pipe) vibrations, etc.

Utrasonic Flow Meters cont.

Metering systems can also have valves, strainers, elbows, tees, and headers upstream of the meter. These elements can distort the flow profile and introduce swirl and crossflow upstream of the meter. Since we are measuring velocity, any change created by these elements will affect the measurement accuracy. Flow conditions are used to minimize these affects but a robust integration method with crossflow compensation is also important to optimize performance.

Proving recommendations In-situ proving at regular intervals is recommended to maintain optimum measurement accuracy. Ultrasonic meters, as previously stated, are like turbine meters in that they infer the volumetric throughput by measuring the velocity over the flow area. For low viscosity products the velocity profile is flat and the flow velocity is nearly constant over the flow area, except for a region near the pipe wall. Therefore, the average stream velocity can be measured at any point except near the pipe wall. As the viscosity increases and/or the flow decreases, the flow profile becomes parabolic . Maximum velocity is at the center of the pipe and the velocity decreases gradually to zero at the pipe wall. To determine the average stream velocity for this type of profile, the stream velocity is measured at several selective points and the velocities are integrated with an algorithm to determine the average velocity. The relationship between velocity and viscosity is defined by Reynolds Number which is the ratio of flow rate to the meter size and the viscosity {Re No ≈ (flow rate)/(meter size x viscosity) } A select few multi-path ultrasonic meters use velocity from four chordal paths  with the VPC to accurately determine the average velocity over the complete flow and viscosity range.

Knowing flow velocity along with the cross sectional area of the pipe you get the volume flow rate. The meter's transmitter is used to process this data and determine the proportional frequency to output based on these measurements. This causes a lag in flow rate indication. Depending on meter manufacture will depend on the latency of the pulses representing that flow rate. Because actual flow rate is constantly changing, these meters have difficulties adjusting to actual flow rate change in a system. If the time it takes to sample the meters pulses is under a few seconds, even in ideal conditions these meters are known to not expected to achieve traditional repeatability requirements(API uncertainty .027%). The industry practice is to use larger prover volumes to achieve repeatability between proving passes(runs), however with most situations this is not feasible because of the foot print needed and life time cost of such large pipe provers. With our small volume provers and turbine master meter setup we are able to achieve the same results as a larger pipe prover, using the master meter method.

Turbine flow meters

An inferred measurement of the flow velocity using a fixed rotor with blades, the flowing fluid is directed into the blades via upstream flow cone. The blades are angled so when the flow contacts them they spin the rotor, this is called the driving force. As the blades spin around the the inside of the pipe they pass over an area where a magnetic pick off coil creates a magnetic field, which is mounted outside of the meter. When the blade passes over the pick off it disrupts the magnetic field producing an alternating current pulse. If the turbine rotor has 5 blades, it will produce 5 pulses for every single rotation of the rotor. Because the rotation of the rotor is proportional to the fluids velocity, and knowing the cross sectional area of the pipe, we can determine volume flow rate.