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Flowmeter is a flow measuring device that measure liner, nonlinear, volumetric, mass flow rate in movable liquid or gas in open or close chamber. Flowmeter is used to measure flow. Flow is measured in every industry. Many types of flow measuring devices are used to measure flow, which are selected on the basis of various parameters. There are many types of flow meters available in the market, which work on different principles. Which type of flowmeter to use for which type of fluid? For this, the data sheet of the flow meter is read and fixed based on the demand of the process. For example, if we want to measure the flow of air, then we can measure both with orifice flowmeter and vortex flow meter, but it will depend on the demand of the process, how much accuracy it needs, because when any instrument is purchased, cost shaving Policy is also adopted. Where low accuracy can work, then a flowmeter with low accuracy is purchased. Because a flow meter with low accuracy comes at a lower price and a flow meter with higher accuracy comes at a higher price.
Various types of flow meters
Mainly three category of flow meter are used in flow measurement that is PD flow meter, mass flow meter and velocity flow meter. PD flow meter direct measure the flow and shows reading locally, mass flowmeter measure flow in mass and velocity flow meter measure velocity flow rate. Types of above flow meters are given below.
1. Positive displacement /volumetric flow meter / PD flow meter
Positive displacement flow meters are unique because they are the only meter to directly measure true quantities. All other types estimate the flow rate by taking some other type of measurement and equating it to the flow rate. With a PD meter, the output signal is directly related to the volume passing through the meter. This includes bi-rotor types (gear, oval gear, and helical gear), notating disc, reciprocating piston and oscillating or rotary piston.
2. Mass flowmeter
The output signal is directly related to the mass passing through the flowmeter.
Thermal and Coriolis flow meters fall into this category.
3. Velocity flowmeter
The output signal is directly related to the velocity passing through the flow meter.
- Electromagnetic flow meter
- Ultrasonic flow meter
- Turbine, Propeller and Pedal Wheel
- vortex shedding flow meter
- target flow meter
- Variable Area and Rotameter flow meter
- orifice plate, open channel, flow nozzle, laminar, venturi and pitot tube
Flowmeter Selection
There are many processes in every industry and each process has a different type of flow meter.
Have you ever thought why this flow meter is installed here, why not another?
If you have thought then you have information about flow meter, if you have not thought then there is no problem, we are here to tell you.
Let’s start-
If Process Engineer demands from you that he needs transmitter in a line. Then what are the things for instrument engineer should keep in mind?
- How to measure the flow that is to be measured, volumetric or totalizer.
- Monitor the value of the flow to be measured locally or remotely
- If the reading is to be viewed remotely, the signal will be either analog or digital.
- Property information of process fluid.
- What is the maximum temperature and pressure range?
- What is Flow Range?
- What is the flow direction?
After knowing all these, the right flow meter should be selected on the basis of the following observation.
There are some steps for selecting a flowmeter.
STEP 1:- Flow Unit
Before specifying a flow meter, it is also advisable to determine whether flow information would be more useful if it were presented in mass or volumetric units. When measuring the flow of compressed material, volumetric flow is not very meaningful unless the density (and sometimes even viscosity) is constant. When the velocity (volumetric flow) of incompressible fluids is measured, the presence of suspended bubbles will cause error; Therefore, air and gas must be removed before the fluid reaches the meter. In other velocity sensors, pipeliners can cause problems (ultrasonic), or the meter can stop working if the Reynolds number is too low (in vortex shedding meters, RD > 20,000 is required).
With these considerations in mind, mass flowmeters, which are insensitive to density, pressure and viscosity variations and not affected by changes in the Reynolds number, should be taken into account. Also less used in the chemical industry are various flows that can measure flow in partially completed pipes and pass large floating or settle able solids.
Step 2:- Process Fluid & Its characteristic
Interacting fluid and its given pressure, temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not?). In addition, all safety or toxicity information must be provided, as well as detailed on the composition of the fluid, bubbles, solids (abrasive or soft, particle size, fibre), tendency to coat, and light transmission properties (opaque, translucent or transparent?), must be provided with the data.
Step 3:- Operating Temperature & Pressure
When selecting a flowmeter, the required minimum and maximum pressure and temperature values should be given in addition to the normal operating values. whether the flow may be reversed, does it not always fill the pipe, may slug flow develop (air-solid-liquid), is there a possibility of aeration or pulsation, may there be sudden temperature changes, or whether the cleaning Special precautions are required during and maintenance, these facts should also be stated.
Step 4:- Accuracy
Is the accuracy of the meter stated in percentages of the full scale or percentage of the reading?
- i) High accuracy: 0.1% for liquid, 0.5% for gas/steam
- ii) Standard accuracy: 0.5% for liquid, 1.5% for gas/steam
iii) Economical accuracy: >1% for liquid, >2% for gas/steam
- iv) What is rangeability or turndown? The turndown ratio is a measure of the rangeability of the equipment and is defined as the full-scale calibrated flow rate divided by the minimum flow rate it is capable of measuring. A flow meter capable of measuring flow rates ranging from 100 to 1000 GPM will have a turndown ratio of 10, or 10:1. A higher turndown ratio generally corresponds to a higher quality, maximum accurate instrument.
Step 5:- Flow meter Installation:-
For installing the flow meter, keeping in mind the information already gathered, what is the line size for installing the flow meter? Whether the process fluid is conductive or not, whether the line is straight or not, what is the flange size of the flow meter? What is the flange rating of a flow meter, what is the direction of flow? Keeping all these things in mind, do it.
For Example-
As the process fluid is conductive then magnetic flowmeter will be used for which some point is important for setting.
- Process fluid must be conductive.
- Downstream line should be bent 90 degree upward.
Step 6:- Best flow Technology Selection
Best fit flow techniques based on fluid media: Liquids, gases and steam can each be measured with many different flow technologies. Once the selection has been narrowed down based on the media phase, other application-specific factors related to the media such as chemical compatibility with the wetted parts of the meter, media viscosity, and media density should be taken into account.
Some flow technologies, such as positive displacement, turbine, differential pressure, have been around for at least a hundred years and have proven to be reliable. However, these technologies also contain mechanical parts that are subject to wear and tear which can increase maintenance and overall cost of ownership. Other new technologies such as ultrasonic may have a higher initial cost, but are very easy to install and have no mechanical parts to wear out. As a result, these devices have a lower overall cost of ownership over the lifetime of the equipment. When selecting a flowmeter, the cost of installation and overall cost of ownership should be evaluated along with the initial purchase price.
Flowmeter properties Table:-
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