Flow Meters: Proper Selection Minimizes Measurement Setbacks

By Nwaoha Chikezie | July 2009 Vol. 236 No. 7

A flow meter installed in a pipeline.

There are several types of flow meters used to measure the flow profile of a fluid. They are; turbine flow meters, ultrasonic flow meters, magnetic flow meters, Coriolis flow meters and target flow meters.

To prolong the service life and high accuracy of a flow meter, the causes of its problems should be determined. The first step in the troubleshooting process is to make certain it is properly selected and installed (Figure 1).

Figure 1: A simple control loop assembly.

Turbine Flow Meter. These are inferential meters used in measuring both gas and oil flow rate. In a turbine flow meter, a rotor is placed in the flow path. When a fluid flows through a turbine, it causes the turbine rotor to rotate with an angular velocity that is proportional to the flow rate. The meters usually are installed in larger pipe sizes, mostly those exceeding four inches. Turbine flow meters are highly accurate and durable and are restricted only by the fact that they must be applied in clean, non-corrosive services. They have an advantage of measuring higher flow rates when compared to positive displacement meters. In order to minimize installation effects, turbine flow meters should have an integral flow conditioner.

Ultrasonic Flow Meter. These also are inferential meters. Ultrasonic meters are subdivided into two types: Doppler meters and time-of-travel meters. In Doppler flow meters, two transducers are mounted in a case attached to one side of the pipeline, while the time-of-travel flow meters transducers are mounted on each side of the pipeline. Ultrasonic flow meters send a signal of known frequency across the flow stream and measure how the flow modifies it. This value is used to determine the flow rate. The liquid being measured must be relatively free of entrained solids or gas to reduce scattering of signal. Ultrasonic flow meters have an advantage of high rangeability in handling a variety of flow rates and have no moving parts.

Magnetic Flow Meter. The major demerit of magnetic flow meters is that they cannot be applied in hydrocarbon flow measurement due to the low conductivity of hydrocarbons. The meters have an advantage in measuring liquids with solid particles in suspension.

Coriolis Flow Meter. This meter measures flow rates based on the mass of the fluid flowing through the pipe. Coriolis flow meters are composed of one or more vibrating tubes. This flow meter has a sine wave voltage applied to an electromagnetic drive which in turn generates an oscillatory motion of the tube. This motion causes the tubes to vibrate. When the tubes vibrate, it produces an angular rotation about its center. As the fluid accelerates away from the center, a resultant Coriolis force opposes the motion. The resultant force generates the measured sine wave which is measured and converted to the mass flow reading. Coriolis flow meters have many applications such as refineries and gas-processing plants. They have an advantage of giving a direct mass flow measurement, independent of pressure and temperature. They also have a high accuracy and repeatability in their flow measurements.

Target Flow Meter. This is another inferential meter. As implied by its name, it operates by using a target - usually a flat disc with an extension rod - suspended in the flow field inside the pipe (Figure 2). It is connected to a transmitter, either electronic or pneumatic.

Figure 2: The target flow meter is another type of inferential meter.