Selecting the right flow measuring instrument for a given flow application can be a challenging task given the numerous measurement techniques available. In this article, we look at some of the key pointers that can help you in the decision making process. What is discussed here isn’t exhaustive in itself but should act as some of the remainders to be taken into account when you are choosing a flow instrument.
Volumetric Flow
When using differential creating devices, there is bound to be an overall pressure loss across the device. The orifice plate has the highest loss, which is irrecoverable, and the venturi tube has the lowest. Differential pressure cells are required when such devices are used, and due considerations should be given to the mounting of these instruments, especially the heads formed by the mounting arrangement, which if not balanced out will affect the accuracy of the measurement.
Differential creating devices and DP cell combination are increasingly being replaced by vortex flowmeters. Vortex flowmeters are now appropriate for measuring steam flow. Unlike the DP cell, the vortex flowmeter produces an output that is directly proportional to the measurement.
Electromagnetic flowmeters have virtually obstruction-less flow characteristics, but they do require the fluid to be electrically conductive for a measurement to be made. If particles are entrained within the flowing liquid, care of the electrodes must be considered. Particles of coagulated grease can be found in waste water through not erosive, are nonconductive and soon cover the electrodes, resulting in a disrupted measurement. Considerations should be given to providing a tuber liner and electrodes of special material for long-term stability. Additionally, do not mount electromagnetic flowmeter tubes in positions that allow them to drain because burnout of the coil could follow.
Related: The Principle of Operation of a Vortex Flowmeter
Mass Flow
Coriolis flowmeters are instrumental for direct liquid mass measurement, but the following two key points should be considered: The head (pressure) loss involved can be relatively high and the amount of entrained gas in the flowing process fluid can affect the measurement. If above certain defined limits, the measurements is indeterminate; one should check with the manufacturer for relevant details. The mounting of Coriolis meters should avoid plant-induced vibrations in the instrument, and the measuring tube should be self-draining.
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Related: The Principle of Operation of Coriolis Mass Flowmeter
Compensating Flow Measurement
Temperature variation in a liquid affects the accuracy because it influences the fluid density. Mass measurements in process lines that are above the sizes offered by the manufacturers of Coriolis flowmeters still have to use the method of the volumetric flow of fluid and multiply it with the fluid density. However, when the computations are being carried out, ensure that the volumetric flow of the fluid is corrected for pressure and temperature variations. When the fluid is a liquid, pressure only needs to be considered. In this case fluid density will always be a separate measurement.
Mounting Considerations
Mounting of the measuring instrument is important especially when the fluid is a gas and pressure differential methods are used. Make sure the impulse piping (connections between the primary device and the measuring instrument) contains fluid of one kind only – either all liquids or all gas, but no combinations. Otherwise, errors caused by the compressibility of the trapped gas are inevitable. Pressure and temperature variation will affect the measurement. To obtain the best accuracy in gas flow measurement, corrections for both pressure and temperature variation should therefore be made.
In all liquid and gas flow measurements, the runs of straight piping upstream and downstream with respect to the primary device must be maintained – ideally 15 to 20 pipe-diameters upstream (minimum 10 pipe-diameters) and 7 to 10 pipe-diameters downstream (minimum 5 pipe-diameters). In difficult circumstances where the upstream dimensions are not feasible, a flow straightener should be used. The straight runs of pipe help avoid turbulence, for it affects the measurement accuracy.
Public water utilities have to resort to venturi flumes because of the very large volumes they process. Ensure the stilling wells (measuring points) are free from obstruction and make sure that any floats that are used can move freely (vertically) and avoid any sideways movement.
Related: How to Connect a DP (Differential Pressure) Flow Sensor to a DP Transmitter
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