Download Magnetic vs Mechanical Flow Meters

Magnetic vs Mechanical Flow Meters
What are the main differences between the
two flow meters?
The primary difference between a mechanical
meter and a magnetic meter is that a mechanical
meter has moving parts and a mag meter doesn’t.
Typical examples of mechanical meters are target
meters, where liquid hits a spring loaded plate, and
turbine meters, where liquid flowing over blades
causes rotation. Positive displacement meters
involve a set amount of material moving through
chambers and being converted to flow rates.
A magnetic (or inductive) flow meter works by
having a conductive liquid flow through a lined
flow tube and of the flow passing through a magnetic field generated as part of the flow tube design causing a voltage to be created. This voltage
increases or decreases based on velocity. So, as
long as the material meets the minimum conductivity level, there are no moving parts.
One of the biggest disadvantages is that mechanical parts wear. As these parts wear, there are a
variety of effects that the wear can have on your
process. When parts wear, they do not work as
well as they did when they were first designed or
installed
Magnetic flow meters, on the other hand, do not
require as much maintenance as mechanical
meters do although any flow measurement device
should be checked intermittently.
Magnetic (or inductive) flow meters are suitable for
the measurement of conductive liquids, typically
liquids with conductivity of 5 microsiemen/cm or
greater. These flow meters are also suitable for liquids with either suspended solids or slurries. This
makes them ideal for water-based products and
most acids and bases as well as slurries like the
ones you find in wastewater treatment, mining, and
pulp and paper applications. Inductive flow
meters are also very flexible when it comes to
viscosity, but they are not suited for hydrocarbon-based products, gases or steam. With hydrocarbon-based products, there is just not enough
conductivity present for a flow meter with this technology to work.
Obviously mechanical meters are not subject to
conductivity restrictions. In most cases, there is an
upper limit for the viscosity tied to the ability of the
meter to physically operate and/or pressure drop
across the mechanical aspects of the meter to allow
a highly viscous liquid to even flow. Mechanical meters are also not suitable for slurries or liquids with
suspended solids as these can cause either wear
and/or build-up on the meter, and will require higher
levels of maintenance or replacement of the device
entirely.
Magmeters are accurate to 0.2% of flow rate with a
5 diameter upstream and a 3 diameter downstream
of straight run. Some mechanical technologies,
such as PD/Oval Gear, have no requirements for
straight run.
A Positive Displacement flow meter offers an interesting compromise as it has only one moving
part. They’re simple with both high resolution open
collector and reed switch outputs as standard. As
each piston rotation passes a known liquid volume,
the inherent repeatability of the Multipulse positive
displacement flowmeter makes it particularly suited
to batching and dispensing duties.
Commonly metered liquids range from non-conductive low viscosity solvents through to extremely
viscous lubricants, chemicals and food bases. Application flexibility is further enhanced as meter performance is independent of flow profile eliminating the
restrictive need for straight pipe runs required with
most alternate metering technologies.
Telephone: 01892 664499
Fax: 01892 663690
www: http://www.pvl.co.uk/
e-mail: [email protected]
So, what type of flow meter is best for your
application?
There are no “universal” flow meters which are suitable
for all applications. To select the proper technology
for an application requires writing a flow specification
which covers the use of the meter. There are usually
trade-offs with each meter type, so knowing the critical
specifications will be important.
What Gas or Liquid will be measured?
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Minimum and maximum flow rates.
What are the accuracy requirements?
The fluid temperature and viscosity.
Fluid compatibility with the materials of construction
The maximum pressure at the location.
What pressure drop is allowable?
Will the meter be mounted in a hazardous location?
Is the fluid flow continuous or intermittent?
What type of output signal or readout do is required?
This list can be used to eliminate the flowmeter technologies that do not apply (Turbine flowmeters don’t
work for viscous fluids. Coriolis meters don’t respond
fast enough for injection flow, etc.). Then a comparison
of the remaining technologies is available for selection
for the correct flowmeter.
Accurate meters are priced based on their capabilities.
It is better to locate the type of meter which fits the
application before trading features for cost savings.
Closely evaluate extreme conditions, such as low flow
rates, high pressure or temperature or the need to
measure over a wide operating range. If these conditions are important, do not consider lower priced alternatives that would be applied outside of their capabilities.
While all measurement technologies have their place,
we encourage you to always weigh the cost of purchase with the overall cost of ownership whenever
selecting a flow measurement technology for your
application.
For further information contact:
David Almond, Head of Sales and Marketing,
PVL Ltd, Unit 9 Lexden Lodge Industrial
Estate, Jarvis Brook, Crowborough, East
Sussex, TN6 2NQ
Telephone: 01892 664499
Fax: 01892 663690
www: http://www.pvl.co.uk/
e-mail: [email protected]
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