Flowmeters 101 – Magnetic and Coriolis

Flowmeters 101 – Magnetic and Coriolis Flowmeter

Flowmeters play a vital role in sanitary processing. They are used to measure incoming raw materials, incoming water supply, CIP solutions, ingredients in your formulation, final product production and even waste water leaving the plant. Considering their use in critical applications, ensuring that you are using the right type of meter with the correct level of accuracy for your application can be the difference in the quality of your product and save you thousands of dollars in lost revenue or profit.

In sanitary processing, one will typically find mechanical flowmeters (Positive Displacement, Turbine), electromagnetic
and Coriolis flowmeters.

Magnetic Flowmeters 

Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the
flow of liquid through a pipe. This type of flowmeter works by generating a magnetic
field and channeling that through the liquid in the pipe. Faraday’s Law states that flow of
a conductive liquid through the magnetic field will cause a voltage signal that can be
sensed by electrodes on the tube walls. When the fluid moves faster, more voltage is
generated. The voltage generated is proportional to the movement of the liquid.
Transmitters process the voltage signal to determine liquid flow.
The signals produced by the voltage are linear with the flow. With this, the turndown
ratio is very good without sacrificing accuracy.

Pros and Cons 

Since these flowmeters measure conductivity, obviously the fluids measured need to be conductive – water, acids and
bases. Low conductive liquids, such as deionized water or gases, can cause the flowmeter to turn off and/or measure
zero flow. There is no obstruction in the path of the liquid, therefore no induced pressure drop across the meter. One
other benefit of mag meters is that they can be used on gravity-fed liquids. With gravity-fed liquids, make sure the
orientation of the flowmeter is vertical so that the flowmeter is completely filled with liquid. These flow meters are
sensitive to air bubbles because the meter cannot distinguish entrapped air from the liquid. Air bubbles will cause the
meter to read high.
Mag meters are typically chosen because they have no obstructions, are cost-effective and provide highly accurate
volumetric flow. Additionally, they can handle flow in either direction and are effective at low and high volume rates.

Coriolis Mass Flowmeters 

A Coriolis mass flowmeter operation is based on the principles of motion mechanics. This flowmeter contains a vibrating
tube in which a fluid flow changes in frequency and amplitude. As fluid moves through this tube, it is forced to
accelerate toward the point of peak amplitude vibration. Conversely, a decelerating fluid moves away from the point of
peak amplitude as it exits the tube. The result is a twisting reaction of the tube as flow moves through it. The amount
of twist is proportional to the real mass flow of fluid passing through the tube.

This effect can be experienced when
riding a merry-go-round – when moving
toward the center, a person will “lean
into” the rotation to maintain balance.
Most flowmeters have a split coil design.
During operation, a drive coil stimulates
the tubes to oscillate in opposition (sine
waves). A sensor measures the time
delay between the two sine waves
(Delta T) which is directly proportional to
mass flow rate.

 

Pros and Cons 

These flowmeters are used in a wide range of critical and challenging applications. They can handle low to high flow
rates with very high accuracy. They are highly reliable and have minimal calibration requirements and low maintenance
costs. In addition, fluid density has basically no impact on flow measurement which makes Coriolis meters ideal where
the physical properties are unknown. They have a higher initial cost than other flowmeters. Pressure drop must also be
considered, especially if running high viscosity fluids.

If you want more information, contact us by phone or email. 

Flowmeters 101 – Turbine and PD meters

Flowmeters 101 – Turbine and PD meters

Flowmeters play a vital role in sanitary processing. They are used to measure incoming raw materials, incoming water
supply, CIP solutions, ingredients in your formulation, final product production and even waste water leaving the plant.
Considering their use in critical applications, ensuring that you are using the right type of meter with the correct level of
accuracy for your application can be the difference in the quality of your product and save you thousands of dollars in
lost revenue or profit.
Before we begin, let’s cover a few basics of flow. Both gas and liquid flow can be measured in volumetric or mass flow
rates such as gallons per minute or pounds per minute, respectively. These measurements are related to each other by
the density of the product. In engineering terms, the volumetric flow rate is usually given the symbol 𝑸 and the mass
flow rate is given the symbol ṁ. For a fluid having a density 𝝆, mass and volumetric flow rates are related by ṁ = 𝝆 ∗ 𝑸.
In sanitary processing, one will typically find mechanical flowmeters (Positive Displacement, Turbine), electromagnetic
and Coriolis flowmeters.

Turbine Flowmeters 

Turbine flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster. Shaft rotation can be sensed mechanically or by detecting the movement of the blades. Blade movement is often detected magnetically, with each blade or embedded piece of metal generating a pulse. Turbine flowmeter sensors are typically located external to the flowing stream to avoid material of construction constraints that would result if wetted sensors were used. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid. Transmitters and sensing systems are available to sense flow in both the forward and reverse flow directions.

Pros and Cons 

Turbine flowmeters have a moderate cost and work well in clean, low viscosity fluids at a moderate, steady velocity. They do create some pressure drop. Bearings do wear out over time and accuracy will diminish and eventually fail as the bearings wear. Turbine flowmeters also typically work best in a limited temperature ranges. These meters are less accurate at low flow rates due to bearing/rotor drag.

 

Positive Displacement Flowmeters 

Positive displacement flowmeter technology is the only flow measurement technology that directly measures the volume of the fluid passing through the flowmeter. Positive displacement flowmeters achieve this by repeatedly entrapping fluid in order to measure its flow. This process can be thought of as repeatedly filling a bucket with fluid before dumping the contents downstream. The number of times that the bucket is filled and emptied is indicative of the flow through the flowmeter. Many positive displacement flowmeter geometries are available.


PD flowmeters have the same basic mechanism as a PD pump. Rotors turn to move a fixed amount of liquid through the body of the flowmeter. In most designs, the rotating parts have tight tolerances so these seals can prevent fluid from going through the flowmeter without being measured (slippage). Rotation can be sensed mechanically or by detecting the movement of a rotating part. When more fluid is flowing, the rotating parts turn proportionally faster. The transmitter processes the signal generated by the rotation to determine the flow of the fluid.

Pros and Cons 

One of the main benefits of using a PD flow meter is the high level of accuracy that they offer, the high precision of internal components means that clearances between sealing faces is kept to a minimum. The smaller these clearances are, relates to how high the accuracy will be. Another benefit is the flow meters ability to process a huge range of viscosities and it is not uncommon to experience higher levels of accuracy while processing high viscosity fluids, simply due to the reduction of slippage. When considering and comparing flow meter accuracy, it is important to be aware of both ‘linearity’ i.e. the flow meters ability to accurately measure over the complete turndown ratio, and ‘repeatability’, the ability to remain accurate over a number to cycles. This is another area where PD flow meters excel, repeatability of 0.02% and 0.5% linearity are standard. Similar to a PD pump, PD flowmeters are considered to have low maintenance requirements. The moving parts will wear over time and require maintenance and calibration. They should not be used for products that contain large particles. Another factor to consider is the pressure drop caused by the PD flow meter; although these are minimal, they should also be allowed for in system calculations.

If you want more information, contact us by phone or email.