Inline flowmeters, also called wetted or intrusive meters, are installed directly into the pipeline by cutting the pipe and fitting the meter as a spool piece. All types of positive displacement, turbine, Coriolis, and most ultrasonic meters fall into this category. Because the sensing elements are in direct contact with the flowing oil, inline meters typically achieve higher accuracy and are less sensitive to external factors like pipe wall condition or acoustic coupling. They are the preferred choice for permanent installations requiring high precision and regulatory compliance.
Clamp-on flowmeters are non-invasive instruments that attach to the outside of the pipe without any cutting or fluid contact. They work by transmitting ultrasonic signals through the pipe wall and fluid. The primary advantage of clamp-on meters is that they can be installed and removed without shutting down the pipeline or breaking containment, making them ideal for temporary flow checks, line balancing, and diagnostics. They are also useful in situations where the fluid is highly corrosive, toxic, or at extreme temperature or pressure, making wetted installations impractical.
The trade-off between inline and clamp-on meters involves accuracy, cost, and installation complexity. Inline meters generally achieve ±0.1% to ±0.5% accuracy for oil applications, while clamp-on ultrasonic meters typically achieve ±1% to ±3% under ideal conditions. Clamp-on accuracy is highly sensitive to pipe wall thickness, the quality of the acoustic coupling gel, and the presence of scale or deposits inside the pipe. For critical custody transfer or billing applications, inline meters are the standard choice. For operational monitoring, flow surveys, or backup metering, clamp-on meters offer a valuable cost-effective supplement. Many facilities use a combination of both technologies to optimize their measurement capabilities.