Factors Affecting the Performance of Ultrasonic Flowmeters

Ultrasonic flowmeters (USMs) have been in commercial use since the 1960s and are widely adopted across various process applications. However, their performance can be significantly impacted by specific process conditions, making proper installation and use essential to harness their benefits.

USMs employ different measurement methods, such as the transit time principle and the Doppler effect principle, but share a common feature: ultrasonic sound waves are transmitted through the fluid and detected by a sensor/transmitter. The path of these sound waves differs based on the meter type. Available in flanged/intrusive and clamp-on non-intrusive variants, USMs are suitable for measuring both gases and liquids. While they can handle two-phase flows, they perform best with single-phase fluids. Certain mixtures or slurries may obstruct ultrasonic signals, leading to reduced performance, which must be considered during meter selection.

Advantages of USMs:

- Robust design with no moving parts, minimizing maintenance costs.  

- Minimal pressure loss due to the absence of flow-obstructing components.  

- Offer diverse output options, including digital and analog, depending on the manufacturer.  

- Provide high accuracy when properly installed, with ongoing advancements improving the accuracy of clamp-on variants.  

- Capable of measuring bi-directional flow.

Disadvantages of USMs:  

- Relatively high upfront cost, often making them more expensive than other flow meter technologies.  

- They have limitations in the fluids they can accurately measure. Heavily contaminated fluids and slurry flows can result in the ultrasonic waves being unable to pass though the fluid, rendering the meter incapable of accurately measuring the flow.

- Reduced performance and accuracy in the presence of two-phase flows, such as liquid and gas.

Operating Principles of Ultrasonic Flowmeters  

- Doppler-Type Ultrasonic Flowmeters:  

These rely on the Doppler effect, which occurs when ultrasonic sound waves reflect off moving particles or bubbles within the fluid and return to the transmitter/receiver. The meter includes an emitting transducer that generates the ultrasonic wave and a receiving transducer to capture the reflected signal. By measuring the frequency shift between the transmitted and received signals, the meter calculates the fluid velocity. This frequency shift is known as the Doppler effect.

- Time-of-Flight (Transit Time) Ultrasonic Flowmeters:

These operate differently by measuring the time it takes for ultrasonic sound waves to travel between a transmitter and receiver through the fluid. During no-flow conditions, the transit time is the same in both directions. When fluid flows, sound waves traveling with the flow move faster, while those moving against the flow are slower. The difference in transit times is analyzed by the meter's signal processor to calculate the flow rate.

Uncertainty  

Measurement devices inherently have a basic level of uncertainty, which varies depending on the design of the ultrasonic flowmeter (USM) and the geometry of the measurement path. This uncertainty can range from less than ±0.25% (k=2) for Class 1 devices to as much as ±5% (k=2) for clamp-on or flare meters.

Calibration  

Prior to installation, USMs are typically calibrated by an accredited flow calibration laboratory. Each laboratory has its own calibration uncertainty. For instance, TÜV SÜD National Engineering Laboratory's primary standard for oil flow measurement is a gravimetric system with an uncertainty of ±0.03% (k=2) for mass and volume.

Process Conditions  

Calibrations are typically performed under relatively low pressures and temperatures, which may not fully reflect real-world industrial conditions, potentially increasing measurement uncertainty. For instance, variations in temperature or pressure can affect the geometry of a USM, which plays a crucial role in velocity calculations.

International standards provide guidance on addressing this issue, including detailed calculations to correct for temperature and pressure effects on the meter due to deviations in operating conditions from calibration settings.

Reynolds Number Effects  

The performance of a USM can be influenced by Reynolds number variations, which alter the velocity profile of the fluid. At low Reynolds numbers, the profile is parabolic or peaked, while at very high Reynolds numbers, it becomes nearly flat.

Reynolds-based profile factor corrections are typically applied to eliminate errors and linearize the meter's performance. If changes in physical properties during operation result in a shift in Reynolds number, this correction method ensures proper meter function, provided the calibration range encompasses the new Reynolds numbers.

Installation Effects  

USMs perform best in fully developed, undisturbed flow with a long straight pipe upstream of the meter, as they are sensitive to installation effects. Single-path meters are the most affected, while dual-path and multipath meters exhibit reduced sensitivity.

In many cases, ideal conditions are unfeasible due to space or cost constraints. Pipe fixtures such as bends and valves can create disturbances, causing asymmetry and swirl in the flow profile, leading to errors. When ideal installation conditions are not achievable, a flow conditioner upstream of the meter can mitigate these effects.

Contaminants  

USMs are designed for single-phase flow and are recommended for such applications. However, this is not always feasible due to conditions like flashing, liquid dropout, waxing, or other fouling issues. When a second phase is present, measurement problems arise not only from its volume or void fraction but also from its distribution within the pipe.  

At higher velocities, the second phase tends to disperse across the entire pipe cross-section, scattering and reflecting ultrasonic signals. This can lead to poor or no signal reception, causing significant negative measurement errors. At lower velocities, stratification may occur, with components separating under gravity. For instance, in a liquid-continuous stream, gas will stratify at the top of the pipe. In such cases, the lower paths of a USM may function correctly, while the upper paths (in the gas section) may fail.  

Contaminants can also clog transducer recesses with fouling deposits, bending the ultrasonic wave's angle of incidence or blocking it entirely. Wax is a common issue in liquid flows, forming in stagnant areas. In gas flows, oils are more typical, but any deposit in the transducer recesses can disrupt the acoustic signal.

Improving Operation  

The impact of process conditions can be effectively minimized with proper operation of the meters. This starts with accurate meter specification and calibration, followed by correct installation and regular maintenance. Adhering to best practice guidelines and procedures is essential to ensure USMs perform optimally.  

Advancements in secondary diagnostics now provide additional confidence in USM measurements. By leveraging data analysis and trending tools, meters can perform self-diagnostics or "health checks" during operation, enhancing efficiency and potentially extending recalibration intervals. These diagnostics can also help identify specific process issues, offering significant benefits. Properly installed and utilized USMs, combined with these capabilities, can deliver reliable performance for many years.