New wet gas MASSter sensor for ST80 Series flow meters

AMS Instrumentation and Calibration introduces a new sensor design for
use with the ST80 Series flow meters to resolve wet gas flow measurement

An innovation from Fluid Components International (FCI), the new
optional wet gas MASSter sensor for ST80 Series flow meters is ideal for
applications in biogas, landfill gas, wastewater treatment digester gas and
rain shielding in power plant and refinery stacks among others.

FCI’s new wet gas (WG) sensor delivers accurate, repeatable gas flow
measurement in the presence of even more moisture and condensation droplets.
Available with the Model ST80, this new WG sensor can be applied for use in
entrained moisture and rain-shielding applications.

The advanced WG sensor configuration features a mechanical design that
shunts moisture, condensation and water droplets away from the thermal flow
sensor, thus maintaining an accurate gas flow measurement while minimising
errors that could occur from a cooling effect on the sensor, potentially causing
a spike or false high reading.

A new Best Practices Guide to Measurement in Moist or Wet Gas explains
how this advanced sensor configuration is designed for use in applications that
have a high level of moisture or condensation present in the gas flow stream,
which cannot otherwise be removed. Moisture in the pipe takes a variety of
forms that can range from a fog to a mist to actual liquid droplets that often
include corrosive components such as hydrogen sulphide (H2S).

The measuring principle of thermal mass flow meters involves heat
transfer caused by gas flow. Any moisture or condensate in the gas stream that
contacts the heated sensor can cause a sudden, momentary change in the heat
transfer, resulting in a spiked or fluctuating reading, which creates
inaccurate or unstable flow measurement. Thermal flow meters using the constant
temperature ∆T (CT) method are particularly reactive to moisture droplets,
while constant power (CP) method meters are less so because their slightly
heated sensor elevates the dew point.

Eliminating the moisture from the gas stream is always the preferred
best practice when possible. Common moisture removal methods include installing
gas dryers or knockout drums or knockout pots upstream from the flow meter. Many
times, however, these techniques are either unfeasible due to crowded equipment
areas or awkward pipe configurations.

The new FCI wet gas MASSter sensor option for the ST80 Series relies on
standard thermal constant power (CP) technology and optimises the sensor head
design and installation to minimise or prevent condensation from contacting the
sensor. The wet gas sensor head mechanically shunts the condensate away and
never allows it to reach the sensors, which eliminates moisture interference
with accurate, repeatable flow measurement.

This solution to wet gas measurement featuring the wet gas MASSter sensor
option for the ST80 Series flow meter offers a number of important advantages. First,
all safety approvals remain in place because this is a mechanical solution. There
is no increase in energy consumption to power the instrument, and there is no
impact on sensor service life. Furthermore, there is no de-rating of the
instrument’s T-rating, and the sensor is safe to touch.

The rugged ST80 Series flow meters with the wet gas MASSter sensor
option are suitable for pipe diameters from 25 to 2500 mm and air/gas
temperatures up to 454¡C. They feature accuracy of ±1% of reading, ±0.5%
of full scale and repeatability of ±0.5% of reading with flow rates up to 305
NMPS and 100:1 turndown.

The ST80 Series outputs and user interfaces
are extensive to ensure interfacing with virtually any control system and/or
set-up or configuration devices. Standard outputs include dual 4-20 mA, NAMUR
NE43 compliant analogue outputs, HART (version 7), Modbus 485 and a USB port.
Foundation Fieldbus or PROFIBUS PA can be optionally added. A best-in-class,
backlit LCD display provides digital and bar graph readouts of the flow rate
and temperature, totalised flow, alarms and diagnostics feedback, and a user
defined label field is also available.

The transmitter enclosure is NEMA 4X/IP67
rated, selectable for NPT or metric conduit port threading and is available in
both aluminium and stainless steel and maybe remotely located up to 1000 feet
(305m) apart from the flow element. In addition to SIL rating, the instrument also carries full global
instrument Div.1/Zone 1 Ex hazardous location approvals of FM, FMc, ATEX, and

For further information, please visit the AMS Instrumentation and
Calibration website
or call 03-9017 8225, or Freecall (NZ) 0800 442 743.

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