There are many types of flow meters with different operating principles in existence today. These meters are used predominantly in the oil and gas industry where measurement is crucial for conducting business between gas suppliers and end-users. Though advancements in flow meter technology have been realized, the real challenge is evident: Different fluids have different properties; as such different principles must be employed for their flow measurement.

This article addresses four of the most common flow meters used in the oil and gas industry with their basic operating principles. The suitability of any of these flow meters depends on the intended application.

## Differential pressure flow meters

According to Bernoulli's equation, which is used to describe fluid flow in pipes, when a constriction is created along a pipe where fluid flows, a pressure drop occurs along with the constriction.

The differential pressure flow meter operates on this principle to measure fluid flow. Bernoulli stated that the pressure drop along the constriction is proportional to the square of the flow rate. Thus, the upstream and downstream pressures of the flow meter are routed to a transmitter that measures the pressure difference and calculates the fluid flow.

One of the disadvantages of using the differential pressure flow meter is the non-linear relationship that exists between flow and differential pressure. Because of Bernoulli's square relationship between pressure and fluid flow, 1% of full-scale differential pressure is produced for a 10% full-scale flow. Therefore, the flow meter’s accuracy at 10% full-scale flow is largely dependent on the transmitter being accurate over a range of differential pressures (100:1). Consequently, it is easy to tell that the flow meter’s accuracy is degraded at low differential pressures.

Nevertheless, these flow meters are affordable and can be optimized to suit different kinds of fluids and at varying flow regimes. For instance, the laminar differential pressure-based flow meter measures the volumetric flow rate through the pressure drop created within a laminar flow element. This element converts turbulent flow into a more streamlined (laminar) flow.

## Thermal flow meters

Thermal flow meters utilize the thermal properties of fluid for fluid flow measurements through the use of heaters and a thermal sensor. A measured amount of heat is applied to the heater of the sensor as fluid flows in a pipe. Heat transfer occurs between the heater and the fluid, and the rate of fluid flow is dependent on the amount of heat absorbed by the fluid.

The heat transfer property of fluids is more pronounced in a turbulent (high-speed, disorderly flow) regime as opposed to laminar (low-speed, organized flow). Thus, as flow increases, more heat is lost to the fluid, and measurements can be obtained easily.

This information might prompt the following question: “How are these flow measurements accurate since fluid thermal properties vary with temperature and pressure?"

Although fluid properties vary with temperature and pressure, these variations are usually negligible in most applications. It is best to utilize thermal flow meters not only when the thermal properties of the fluid are known, but also when these properties are relatively constant. It is for this reason that thermal flow meters are suitable for measuring the flow rates of gases.

## Coriolis flow meters

When French physicist Gaspard Gustave Coriolis investigated the energy yield of machines such as water wheels in 1835, only a few people envisioned the widespread use of his research work in fluid flow measurement today.

Coriolis flow meters operate solely on the principle of the acceleration caused by mass deviating from its center of rotation. The Coriolis flow meter is designed with exciters and a unique tube that oscillates uniformly. Sensors are located at the inlet and outlet of this tube to register the basic oscillations precisely.

As fluid flows through the tube, the inlet and outlet sections of the tube oscillate in different directions at the same time. The sensors pick up this change in terms of phase shift, and the fluid flow rate is estimated. Therefore, the higher the fluid flow, the greater the deflection of the oscillating tube.

The Coriolis flow meter is highly accurate and is significantly used for custody transfer. Although it is suitable in areas where little maintenance is done, its purchasing cost is significantly high. Nevertheless, its application does not stop with fluid flow measurement. It can also be used to estimate the density of fluid since the oscillating frequency of the tube is directly proportional to the density of the fluid. For instance, the tube will oscillate more times in a second if a less dense fluid such as water is flowing as opposed to a denser fluid such as honey.

## Ultrasonic flow meters

Ultrasonic flow meters operate on the principle of propagation of sound waves in fluids to measure fluid flow. Pairs of sensors are fitted across from each other in a pipe transporting the fluid, with each pair capable of transmitting and receiving ultrasonic signals generated by piezoelectric crystals.

These pairs of sensors are attached both upstream and downstream, and the transit time between sending and receiving signals for each pair is measured. The differences in transit times, as measured by the sensors, are used in estimating the flow rate. When there is no fluid flow, the signal transit times are the same upstream and downstream. The moment the fluid starts to flow, ultrasonic signals are accelerated in the direction of flow, and there exists a difference in transit times. This difference is directly proportional to the flow velocity.

The ultrasonic flow meters have no mechanical parts to wear out. Thus, they are small, easy to install and require less maintenance than other types of flow meters. Because of their sophisticated design, they are also highly accurate and very costly.

Though there still exist numerous types of flow meters, these four are quite common in the oil and gas industry. A basic understanding of their operating principles is crucial to determining their suitability for a particular application.