eFunda: Introduction to Flowmeters
engineering fundamentals Flowmeters: Introduction
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Overview

Flow measurement is essential in many industries such as the oil, power, chemical, food, water, and waste treatment industries. These industries require the determination of the quantity of a fluid, either gas, liquid, or steam, that passes through a check point, either a closed conduit or an open channel, in their daily processing or operating. The quantity to be determined may be volume flow rate, mass flow rate, flow velocity, or other quantities related to the previous three.

The instrument to conduct flow measurement is called flowmeter. The development of a flowmeter involves a wide variety of disciplines including the flow sensors, the sensor and fluid interactions through the use of computation techniques, the transducers and their associated signal processing units, and the assessment of the overall system under ideal, disturbed, harsh, or potentially explosive conditions in both the laboratory and the field.

Categorization
Since flowmeters are integrated instruments that measure different flow quantities by different technologies. Many characteristics can be referred in categorizing flowmeters. Some of the references are listed below:

Common flowmeters that are discussed in this flowmeter section include:

 
  Flowmeter
1.  Coriolis   
2.  Differential Pressure   Elbow 
3.    Flow Nozzle 
4.    Orifice 
5.    Pitot Tube 
6.    Pitot Tube (Averaging) 
7.    Venturi 
8.    Wedge 
9.  Magnetic   
10.  Positive Displacement   Nutating Disc 
11.    Oscillating Piston 
12.    Oval Gear 
13.    Roots 
14.  Target   
15.  Thermal   
16.  Turbine   
17.  Ultrasonic   Doppler 
18.    Transit Time 
19.  Variable Area   Movable Vane 
20.    Rotameter 
21.    Weir, Flume 
22.  Vortex   

Installation

Flowmeters need to be integrated into existing/planning piping system to be useful. There are two types of flowmeter installation methods: inline and insertion. The inline models include connectors to the upstream and downstream pipes while the insertion models insert the sensor probe into the pipes.

Most flowmeters need to be installed at a point where the pipes on both sides remain straight for a certain distance. For inline models, the inner diameter of the pipes have to be the same as the flowmeter's line size. Between the flowmeter and the pipes, there are two types of mostly used connecting methods: flanged and wafer.

Among different types of connection methods, insertion design is more flexible and more economical in larger line sizes while inline design is more confined and usually easier to calibrate. The wafer connection is usually less expansive than flanged connection. However, it may require extra parts to allow the threading with pipes at both ends.

Selection

To select a flowmeter that suits one's application, many factors need to be considered. The most important ones are fluid phase (gas, liquid, steam, etc.) and flow condition (clean, dirty, viscous , abrasive, open channel, ect.) The matching of fluid phase and flowmeter technology can be found in the flowmeter selection page.

The second most important factors are line size and flow rate (They are closely related). This information will further eliminate most submodels in each flowmeter technology.

Other fluid properties that may affect the selection of flowmeters include density (specific gravity), pressure, temperature, viscosity, and electronic conductivity. On the flow part, one needs to pay attention to the state of fluid (pure or mixed) and the status of flow (constant, pulsating, or variable).

Moreover, the environment temperature, the arrangements (e.g., corrosive, explosive, indoor, outdoor), the installation method (insertion, clamped-on, or inline), and the location of the flowmeter also need to be considered, along with other factors which include the maximum allowable pressure drop, the required accuracy, repeatability, and cost (initial set up, maintenance, and training).

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