Flow Meters: Proper Selection Minimizes Measurement Setbacks

July 2009 Vol. 236 No. 7

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.

Target flow meters are used for the measurement of cryogenic and corrosive liquids and cooling and process water. The have an advantage of being able to measure bi-directional and sporadic flows.

Common Challenges

Liquid Carry Over. When measuring gas flow, care must be taken to ensure that there are no entrained liquids (Figure 3). In gas flow measurements, free flowing and entrained liquids distort the flow profile inside the meter tube and result in measurement errors. Liquids cause both high and low measurement readings and must be dealt with as soon as possible. The more free liquid removed from a gas stream, the more accurate the measurement. Make sure the gas/liquid separators are working properly and in good condition. Also, make sure the gas temperature is within the required range.

Dirt. A big threat to good flow measurement is dirt in the fluid flow stream caused by corrosion or inadequate separation. For instance, erosion-corrosion produces pipe scrapes, debris which blocks the flow meter filter efficiency, thereby restricting normal flow, and this alters the accuracies in the coefficient of discharge and the gas flowing profile. To prevent errors in flow measurement a filter of proper micron size should be installed upstream of the flow meter.

Improper Flow Meter Installation. Process industry professionals claim that more than 70% of flow meters installed do not perform satisfactorily. And, improper meter selection accounts for more than 80% of these problems. During meter selection, the most important measure is to ascertain the function the flow meter is to perform. To prevent improper flow meter installation, there are basic questions to be asked and answered. They are:

  • Is it a slurry, clean or viscous fluid?
  • Is the fluid electrically conductive?
  • Is the required flow measurement for accounting or process control?
  • Is the flow meter actuated by a remote signal or by local indication?
  • Have you considered the process-operating temperatures, flow rates and pressures?

Troubleshooting

In order to prolong the service life and high accuracy of flow meters, the causes of its problems should be determined. The first step in troubleshooting a flow meter problem is to make sure it is properly selected and installed. Table 1 provides guidance in troubleshooting four common flow meter problems.


Table 1: Guidance for troubleshooting four common flow meter problems.

Author

Nwaoha Chikezie has a bachelor’s degree in petroleum engineering from the Federal University of Technology, Owerri, Nigeria. He has completed a term as a student trainee operator with Port Harcourt Refining Company in Nigeria and is working on several research projects involving flow systems design, including an initiative with the Caribbean African Student Exchange Initiative (CASEI). chikezienwaoha@yahoo.com, +234-703-135-3749.

References

Hudgeons, K.A.,”Insuring Orifice Meter Accuracy,” Pipeline & Gas Technology, vol. 6, No. 6, pp. 44-46 (July 2007).

Nwaoha, C., “Extending Control Valve Life by Proper Selection and Maintenance,” Pipeline & Gas Journal, Vol. 235, No. 11, pp. 78-80 (November 2008).

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