Flow Meter Selection Guide ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Flow Meter Selection Guide

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One of the most important measurements made in an industrial plant is flow. How much quantity of a given fluid do you require to accomplish a given task? Or how much quantity of gasoline do you require to sell at the gas pump? Whether you are doing the flow meter selection in-house or a vendor or contractor is doing the selection for you, there are many factors you need to consider to successfully see this process through.
To select the right flow sensor or meter, you need to consider many important factors:

Preliminary Factors:
(1) Are your plant personnel familiar with the proposed flow sensor technology?
(2) Are your plant personnel familiar with the calibration and maintenance of the flow meter?
(3) Can the spare parts of the flow meter be readily sourced?
(4) Compute cost of installation after considering (1) – (3)

Application Requirements:
The basis of a good flow meter selection is a clear and thorough understanding of the requirements of the particular application that the flow sensor is intended for. Some of these application requirements include:
(1) Evaluate the nature of the process fluid and the overall installation
(2) Flow rate information: is it continuous or totalized or both
(3) Is the flow rate needed locally or remotely? If remotely, should the transmission be analog, digital or shared. If shared, what is the minimum data update rate or frequency.

Factors Relating to Flow meter Operation:
To successfully select a flow meter that will be able to operate in the desired manner, we need to be able to analyze the properties and flow characteristics of the process fluid, the piping requirements that will accommodate the flow meter and many other operational factors. These factors which should be considered carefully include:
(1) Fluid and flow characteristics:
     (a) What is the nature of the fluid to be measured?
     (b) Is the fluid a gas or a liquid or a solid(slurry)?
     (c) Is the fluid corrosive?
     (d) Is the fluid conductive or not?
     (e) Does the fluid contain slurry or large solids?
     (f) What is the viscosity of the fluid?
     (g) Will the fluid density or viscosity change?

(2) Expected minimum and maximum pressure and temperature in addition to the normal operating values

(3) Consider the piping and the area where the flow meter is to be installed or located. For flow meter piping, specify its direction, size, material, schedule, flange (pressure rating), accessibility, up or down stream turns, valves, regulators and available straight-pipe run lengths. With respect to the area where the flow meter is installed, we must know whether vibration or magnetic fields are present or possible, if pneumatic or electric power is available, if the area is classified for explosion hazards or if there are other special requirements such as sanitary regulations.

(4) Determine the required flow meter range. To accomplish this, identify the minimum and maximum flows (mass or volumetric depending on the application) that will be measured.

(5) Determine the required flow measurement accuracy. Flow meter accuracy comes in different ways:
     (a) Percentage of Flow Rate (% FR)
     (b) Percentage of Calibrated Span (% CS)
     (c) Percentage of Full Scale units (% FS)
     (d) Percentage of Actual Reading (% AR)
Whatever the accuracy metrics, the accuracy requirements should be separately stated at the minimum, normal and maximum flow rates. Failure to do this, may compromise the performance of your flow meter over its full range.

In cases where the flow meter is being used for commercial purpose (buying or selling), the absolute accuracy is extremely important. For other applications, repeatability may just be far more important than accuracy hence it is advisable for accuracy and repeatability requirement to be established separately for each flow meter application.

Care should be taken to interpret flow meter accuracy correctly. Flow meter accuracy if wrongly interpreted can be misleading. Meter accuracy in percentage flow rate, percentage of calibrated span and percentage of full scale units all vary between minimum and maximum flows of the flow meter. The only constant accuracy metric is percentage of actual reading(% AR). Therefore, in order to make fair comparison in terms of accuracy between ranges of flow meter technologies in consideration for a given application, it is advisable to convert all quoted error statements into the same % actual reading units.

It is also recommended that the user compare flow meter installations on the basis of the total error of the flow loop. Total inaccuracy is calculated by taking the root of the sum of the squares of the component inaccuracies at the desired flow rates.

In accurately prepared flow meter specifications, all accuracy statements are
converted into uniform % actual reading units and these % actual reading requirements are specified separately for minimum, normal, and maximum flows. All flow meter specifications should clearly state both the accuracy and the repeatability of the meter at minimum, normal, and maximum flows.

These flow meter selection guidelines above are the basic minimum you should endeavour to incorporate into any flow meter specification and selection process.

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