Focus on Liquid Flow Measurement

     Controlling the flow rate of liquids is a key control mechanism for any chemical plant.  There are many different types of devices available to measure flow. 

Table 1:  Comparison of Popular Flow Measurement Devices

Head Devices

     Head type devices measure flow by constricting a stream and measuring the resulting pressure drop.  The pressure drop can then be related to a flow.

Orifice Plates

     An orifice plate is a very simple device installed in a straight run of pipe.  The orifice plate contains a hole smaller than the pipe diameter.  The flow constricts, experiences a pressure drop, and then the differential pressure can be related to a flow.

Figure 1:  Orifice Plate Arrangement

     For a discussion of how pressure drop is related to liquid flow for concentric orifices, visit LMNO Engineering.  They have a very good explanation on their website.

     It is also important to note that relating differential pressure to flow across an orifice depends on the location of the pressure taps in relation to the orifice.  In Figure 2 below, the pressure taps are designated as P1 and P2.  "D" is the diameter of the pipe and "d" is the diameter of the orifice.

Figure 2:  Various Tap Positions for Orifice Plates

Venturi

     A venturi tube (also called the Herschel Venturi tube) also measures flow rates by constricting fluids and measuring a differential pressure drop.

     Venturi tubes allow for flow measurement with lower head losses than orifice plates.  Venturi tubes of cast iron cones are most commonly used in pipes with diameters of 4 to 32 inches (10 to 80 cm).  Pipes of up to 10 inches (25 cm) in diameter usually utilize machined venturi constrictions.  Larger diameter pipes (to 48 inches or 1.2 m) usually employ a welded sheet metal convergence.  Venturi accuracy is best for Reynolds numbers between 10⁵ and 10⁶.  Again, for a discussion on relating venturi pressure drops to flows, see LMNO Engineering.  

Target Flowmeters

     A target flowmeter operates just as the name implies.  A small "bullseye" is placed inside the pipe and is connected to a pneumatic transmitter.  Typical applications include flow measurement of steam and outdoor liquids.

     In a target flowmeter, the square of the force exerted on the target is proportional to the volume or mass flow through the pipe.  The force on the target is expressed as:

Rotometers

     Rather than using a constant restriction area and a variable pressure differential, rotometers use a variable restriction and a constant pressure differential to measure flow.  Typically, rotometers are used to measure smaller flows and the reading is usually done locally, although transmission of the readings is possible. 

     The rotometer consists of a float that moves vertically through a slightly tapered tube.  As fluid enters the bottom of the rotometer, the float is forced upward until the force is balanced by gravitational forces.  Most rotometers are made of glass with markings on the outside so that flow readings can be taken visually.  The advantage to rotometers is the simplicity of the device and a constant pressure drop.  Also, rotometers do not require straight pipe runs for installation so they can be installed just about anywhere.

Velocity Devices

     Probably the most common velocity device used for flow measurement is the magnetic flowmeter.  Magnetic flowmeters cause no head loss and they can easily measure liquids with solids in suspension.  By their design, they produce an electrical signal ideal for plant transmission.
     In a magnetic flowmeter, the pipe is lined with a nonconducting material and at least two electrodes are mounted flush with the nonconducting wall.   Electromagnetic coils surround the flow path with a uniform magnetic field.   Faraday's Law dictates that the voltage produced by a conducting fluid flowing through a magnetic field is directly proportional to the velocity of the fluid. 
     The major disadvantage of magnetic flowmeters is that they cannot be used for hydrocarbons due to hydrocarbon's low conductivities.

     Another velocity device, which can be used for hydrocarbons, is called a vortex-shedding meter.  You can read more about these devices here.

Displacement Devices

     The most common displacement flow-measuring device is the turbine meter.  In a turbine meter, a rotor is placed in the flow path.   Usually, the rotor is magnetically coupled so that each rotation produces a pulse.   The spin of the rotor is proportional to the velocity of the fluid.  The turbine meter is highly accurate and durable.  Turbine meters are restricted only by the fact that they must be used in clean, noncorrosive services.

Other Devices

     Another type of device worth mentioning is the Coriolis meter which measures flow rates based on the mass of the fluid.  Many applications, such as a reactor feed stream, are often specified and best measured by mass.  In these applications, using a measuring device based on volume would require corrections for temperature dependent properties such as density and viscosity.  The Coriolis meter gives a direct mass flow measurement, independent of temperature and pressure.  These devices are remarkable accurate as well (typically 0.2 to 0.02 percent of the total flow).  

     The Coriolis meter has a sine wave voltage applied to an electromagnetic drive which produces an oscillating motion of the tube.  The amplitudes are related to the mass flow and the frequency is related to the product density.  The reason that the output amplitude changes with flow may be explained by the Coriolis effect.  The vibration of the tube gives a slight angular rotation about its center.  As the fluid moves away from the center, there is a resultant Coriolis force which opposes the rotational motion.  The flow movement toward the center produces a Coriolis force which aids the tube rotation.  The resultant force produces the measured sine wave which is measured and converted to the mass flow reading. 

References:

LMNO Engineering, website, http://www.lmnoeng.com

Rosaler, Robert C., Handbook of Plant Engineering, McGraw-Hill, New York, 1995, ISBN: 0-07-052164-6

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