Yokogawa

Advantages of Electromagnetic Flow Meters
Unobstructed flow passage without projecting parts
No moving parts
No additional pressure drop
Essentially flow profile insensitive, only short inlet and outlet sections required
Unaffected by changes in temperature, density, viscosity, concentration and electrical conductivity
Favorable choice of materials for chemically aggressive or abrasive measuring media
Unaffected by contamination and deposits
Especially suitable for hydraulic solids transport
Linear relationship between flow rate and measured variable
Operates in both flow directions (forward and reverse)
Measuring range setting can be optimized
Low maintenance, but still easy to maintain

Disadvantages of Electromagnetic Flow Meters
For liquids only
Lower conductivity limit 0.05 μS/cm
Gas inclusions cause errors

Applications of Electromagnetic Flow Meters
Electromagnetic meters can handle most liquids and slurries, providing that the material being metered is electrically conductive. The flow tube mounts directly in the pipe. Pressure drop across the meter is the same as it is through an equivalent length of pipe because there are no moving parts or obstructions to the flow. The voltmeter can be attached directly to the flow tube or can be mounted remotely and connected to it by a shielded cable.

Electromagnetic flow meters operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor; the magnetic field is created by energized coils outside the flow tube. The amount of voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage, which is measured by the secondary element.

Electromagnetic flow meters have major advantages: They can measure difficult and corrosive liquids and slurries; and they can measure forward as well as reverse flow with equal accuracy. Disadvantages of earlier designs were high power consumption, and the need to obtain a full pipe and no flow to initially set the meter to zero. Recent improvements have eliminated these problems. Pulse-type excitation techniques have reduced power consumption, because excitation occurs only half the time in the unit. Zero settings are no longer required.

Advantages of Ultrasonic Flow Meters
Unobstructed flow passage
No moving parts
No additional pressure drop
Favorable choice of materials for chemically aggressive liquids
A linear relationship between flow rate and measured variable
Low maintenance
Operates in both flow directions (forward and reverse)
Transit time meters unaffected by temperature, density, and concentration
Later installation in the existing pipe is possible with individual elements, but onsite calibration required

Disadvantages of Ultrasonic Flow Meters
Still problematic for liquid and gas measurements
The sound beam must traverse a representative cross-section, therefore flow profile is dependent. Long inlet and outlet sections required
Errors due to deposits
Transit time meters require clean liquids
Doppler meters only for slight contamination or few gas bubbles
Doppler meters are affected by sound velocity changes due to temperature, density, and concentration
Unsuitable for heavily contaminated liquids
Gas bubbles cause errors

Applications of Ultrasonic Flow Meters
Ultrasonic flow meters can be divided into Doppler meters and Time-of-Travel (or Transit) meters. Doppler meters measure the frequency shifts caused by liquid flow. Two transducers are mounted in a case attached to one side of the pipe. A signal of known frequency is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in the liquid, cause the pulse to be reflected to the receiver element. Because the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's velocity.
A portable Doppler meter capable of being operated on AC power or from a rechargeable power pack has recently been developed. The sensing heads are simply clamped to the outside of the pipe, and the instrument is ready to be used. Total weight, including the case, is 22 lb. A set of 4 to 20 millampere output terminals permits the unit to be connected to a strip chart recorder or other remote device.
Time-of-travel meters have transducers mounted on each side of the pipe. The configuration is such that the sound waves traveling between the devices are at a 45 deg. angle to the direction of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured. A time-differential relationship proportional to the flow can be obtained by transmitting the signal alternately in both directions. A limitation of time-of-travel meters is that the liquids being measured must be relatively free of entrained gas or solids to minimize signal scattering and absorption.

Advantages of Differential Pressure Flow Meters
Universally suitable for liquids, gases, and steam
Also usable in extreme situations, e.g. viscosity, due to the variety of versions
Calculations possible for unusual situations
Suitable for extreme temperatures and pressures
Range changes possible
Low-pressure drop for nozzles

Disadvantages of Differential Pressure Flow Meters
Square root relationship between flow rate and differential pressure, therefore smaller span
Affected by pressure and density changes
Pressure drop for orifice plates
Edge sharpness for orifice plates must be assured, therefore no solids or contamination
Very long inlet and outlet sections
Expensive installation requiring differential pressure lines, fittings, and sensors
Installation and maintenance experience advantageous
High maintenance requirements

Applications of Differential Pressure Flow Meters
The use of differential pressure as an inferred measurement of a liquid’s rate of flow is well known. Differential pressure flow meters are, by far, the most common units in use today. These meters, which boast high accuracy, calculate fluid flow by reading pressure loss across a pipe restriction. Estimates are that over 50 percent of all liquid flow measurement applications use this type of unit.

The basic operating principle of differential pressure flow meters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.

Differential pressure flow meters, like most flow meters, have a primary and secondary element. The primary element causes a change in kinetic energy, which creates the differential pressure in the pipe. The unit must be properly matched to the pipe size, flow conditions, and the liquid’s properties. And, the measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.

Advantages of Vortex Flow Meters
No moving parts
Rugged construction
Suitable for liquids, gases, and steam
Easily sterilized
Unaffected by pressure, temperature, and density changes
A linear relationship between flow rate and measured value

Disadvantages of Vortex Flow Meters
Inlet and outlet sections required
Minimum Reynolds number required

Applications of Vortex Flow Meters
Vortex meters make use of a natural phenomenon that occurs when a liquid flows around a bluff object. Eddies or vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing through the meter.
streamlined object is placed in the middle of a flow stream, a series of vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing in the pipeline.
The three major components of the flowmeter are a bluff body strut-mounted across the flowmeter bore, a sensor to detect the presence of the vortex and to generate an electrical impulse, and a signal amplification and conditioning transmitter whose output is proportional to the flow rate. The meter is equally suitable for flow rate or flow totalization measurements. Use for slurries or high viscosity liquids is not recommended.