MAGNETROL 54-241 Thermal Mass Flow Meters Instruction Manual
- June 12, 2024
- MAGNETROL
Table of Contents
54-241 Thermal Mass Flow Meters
Instruction Manual
Technology
There are many well documented flow meter technologies that are essentially
trying to accomplish the same thing: measure fluid flow rate. Some of the
technologies that are entrenched in the market, such as flow meters that
utilize differential pressure as the measurement principle, are well
understood by customers. Thermal mass flow meters however, such as the
Magnetrol® Thermatel® TA2, are a rapidly growing technology that is
continuously evolving.
Most flow meters inherently measure a flow rate at operating temperature and
pressure, rather than the often desired mass flow rate at standard temperature
and pressure (STP). Examples of flow rates at operating conditions are CFM or
m3/h. The conversion to mass flow involves measuring operating temperature and
pressure as well as knowing the STP conditions. This calculation can be
accomplished outside of the flow meter or using a multivariable transmitter.
When the term “mass flow meter” is discussed in the instrumentation world it
is typically describing a Coriolis flow meter. Similar to Coriolis technology,
thermal mass flow meters measure a flow rate at standard conditions (SCFM or
Nm3/h) without the need for external measurements or additional calculations.
In-depth commentary on thermal dispersion technology or the principle behind thermal mass flow meters is readily available, but these conversations should emphasize that it is simply another method of measuring flow (primarily gas flow). It is no more or less complicated than other flow meter technologies and it is not “black magic.” Orifice plates utilize differential pressure to enable flow measurement, turbine meters rely on rotor rotation and thermal mass meters measure flow by heat transfer. Some of the key advantages of using thermal include direct mass flow measurement, high sensitivity at low flow rates, and no moving parts in the line.
Installation Versatility
Installation is key for flow meters to perform as accurately as possible. Each
technology has unique guidelines for positioning and straight run. Owning a
technology that allows for greater flexibility will save on installation and
maintenance costs over the life of the flow meter.
In the case of thermal, the sensors can be provided as inline or insertion
type. With insertion probes it is very common to utilize a compression
fitting. This fitting will thread into a half coupling or threadless off of
the pipe and allow the user to tighten down the probe when the tip is at the
recommended insertion depth. Therefore, the user can select a probe that is
longer than necessary if there is no clearance issue in doing so. Compression
fittings are typically stainless steel, but the ferrule material can either be
the same material or Teflon® . The advantage of the Teflon® ferrules is that
they will not swage onto the probe after tightening, allowing the probe to be
repositioned or relocated at a later time.
Another advantage of insertion probes is the option to be “hot tapped.” Using
a retractable probe assembly, the user can remove the flow meter under process
conditions to prevent shutting down the line. The assembly may include blowout
prevention if there are safety concerns of the probe ejecting under higher
pressures.
Knowing the flow direction during installation is also essential. Some
manufacturers will request this information during the ordering process, but
Magnetrol calibrates with flow going from left to right as a standard and
includes the flow arrow on the probe. If installing the probe with the flow
going from left to right is not ideal for viewing and configuration, the user
can rotate the transmitter housing by loosening the set screw at the base.
There is also a stop screw in the back side to prevent over-rotation. Rotate
the head up to 270° and then tighten the set screw back down. The backlit
display is rotatable in 90° increments as well for easier viewing.
Ease of wiring further simplifies installation of the TA2. Thermal flow meters
have a front and rear compartment. Wiring takes place in the rear compartment.
The TA2 terminals come off of the board and the terminal descriptions are
clearly marked. Selection of input voltage is not necessary as there are two
separate terminals for VAC and VDC inputs that come standard with every model.
Calibration
All thermal mass flow meters must be calibrated to measure accurately. This
involves a flow bench and running multiple flow rates past the sensor over the
specific operating range. As a standard, Magnetrol takes at least ten data
points over the customer flow range. Oftentimes, the user will receive 15–30
data points which increases the accuracy of the curve. Depending on the needs
of the application, the manufacturer may calibrate the unit with the actual
gas it is going into or use some type of equivalency/correlation calibration.
Some manufacturers have the capability of producing multiple calibrations to
allow the flow meter to be accurate in two (or more) gases. It is a simple
procedure to switch from one calibration curve to the next. With the TA2, for
any actual gas calibration the user will also receive an air calibration free
of charge. For example, a unit that is currently calibrated for and measuring
natural gas can be moved and installed into an air line. Another advantage is
that the user can enable an equivalency mode in the unit to adjust that base
air curve for other gases. A thermal flow meter will not measure accurately if
it is being used in a gas for which it is not calibrated or configured to
measure.
Depending on the manufacturer, there may also be ways of checking the
calibration of the flow meter in the field or at the customer site. This saves
time and money since the meter does not have to be sent back to the
manufacturer. The TA2 has a calibration verification procedure that does not
involve purchasing any external hardware to run the tests. It is not just a
configuration check, but a true test of heat transfer.
Turndown
A flow meter’s turndown is the ratio of the maximum flow divided by the minimum flow for which the meter is accurate. Thermal mass flow meters will typically specify a standard 100:1 turndown, although higher turndowns are possible with the additional data points previously discussed. This far surpasses differential pressure technologies that specify less than a 10:1 turndown. As shown in the example curve, thermal flow meters have high sensitivity throughout the flow range, but particularly at low flows. The power curve is related to the sensitivity of the flow meter versus heat transfer. The heat transfer increases with the flow rate.
Outputs/Communication
With today’s thermal mass flow meter the user can select from multiple current
outputs, alarm outputs, and pulse outputs. Two current outputs enable the user
to measure flow rate as well as process temperature from one transmitter.
For communication protocols it is still very advantageous to have HART® .
Using the HART® signal in conjunction with the specific product device type
manager (DTM) allows the user to configure and run diagnostics from their
laptop using PACT ware™ software. Both the DTM and PACT ware™ can be
downloaded from the Magnetrol website.
Many manufacturers are also offering a diverse range of additional
communication protocols, including Foundation fieldbus™ digital communications
for an interconnected network.
Summary
The thermal mass flow market is one of the fastest growing flow meter segments. It will continue to rise due to environmental regulations as well as cost savings associated with energy management. Although the oil and gas industry is the primary driver, these flow meters can be found in many industrial, municipal, and commercial facilities. If you have a boiler, you may very well find a thermal flow meter not far away.
Phone: 630-969-4000
Email: info.magnetrol@ametek.com
www.magnetrol.com
©2020 AMETEK Magnetrol USA, LLC.
Bulletin: 54-241.0
References
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