November
2016
HYDROCARBON
ENGINEERING
77
the factory for recalibration every time gas
composition changes.
Managing changes in varying pressure
In sampling applications, the pressure at the sample
source can vary from 0 - 3 barg. While variations of
3 barg will not have a major effect on higher flow rates,
pressure can greatly affect low flow rates and zero
flow, in particular. To compensate for these pressure
effects at low flow, the Raptor II OS Engine
dynamically accounts for increased natural convection
occurring as a result of pressure.
Meeting explosion proof requirement for gas
sampling
ATEX, cFMus and IECEx approvals are often required for
sampling devices and are a determining factor for this
application, which makes an industrial flow meter a
necessity. The four-sensor thermal flow meter is the
only ATEX approved thermal mass flow meter that can
measure flows this low (down to 0.01 mps).
Conclusion
While there are many alternatives to getting accurate
flow data in gas sampling applications, advanced
four-sensor thermal flow meters offer the most
accurate flow measurements for the toughest gas
sampling application requirements: high accuracy at
low flows, accuracy when pressures change, and the
ability to meet industrial explosion proof requirements.
For the first time with thermal technology, four-sensor
quad thermal flow meters, such as Sierra’s
QuadraTherm meter, can manage gas composition
changes in the field without losing accuracy. This is
why thermal technology is being readily adopted in
oil and gas applications, in order to quickly comply
with government gas sampling regulations, saving
hundreds of thousands of dollars a day in compliance
costs.
Figure 5.
qMix Gas Composition Smart Interface
Program.
In a unique innovation, all Sierra ‘dry’ sensors
(DrySense
TM
) use no organics and will not shift or
lose accuracy over the life of the meter. The dry
construction of the sensor provides much improved
sensor stability and accuracy (Figure 4).
Application challenges solved with
new technology
With recent advancements, coupled with the
inherent accuracy benefits of thermal mass flow
technology, QuadraTherm thermal meters coupled
with DrySense are now able to solve all the inherent
challenges in gas sampling applications. These
challenges are reviewed below.
Accuracy at low flow rates
In typical gas sampling applications natural gases of
various compositions are being sampled, the piping
is small, typically 0.5 in. - 0.25 in., and the flow
range is very low, ranging from 0 - 10 nlpm or lower.
Four-sensor thermal flow meters are capable of
measuring flow accurately down to 0.01 mps, which
equals approximately 0.1 nlpm in a 0.5 in. pipe.
Previous generations of mass flow meters bottom
out at 10 slpm. Due to this low flow sensitivity, the
QuadraTherm mass flow meter is able to be
calibrated with a full scale of 10 nlpm and a nominal
flow of 1 nlpm.
Metering accuracy in changing gas
compositions
Typically, the gas composition will vary with each
sample point, which makes taking an accurate
representative sample very challenging. With
traditional thermal flow meters, when gas
composition changes the meter must be sent back
to the factory for recalibration, costing engineers
valuable time and money in recalibration.
For the first time, QuadraTherm thermal flow
meters, along with powerful microprocessors, can
run flow measurement algorithms to compute the
mass flow rate of any gas composition. The
microprocessor takes the inputs from the four
sensors and solves the First Law of Thermodynamics
(heat energy in = heat energy out) for each data
point.
This technology now can enable a gas mixing
software app called qMix
TM
, a powerful, user
customisable gas mixing feature (Figure 5). qMix
provides gas flow measurement field flexibility by
allowing end users to create custom gases or gas
mixtures to compensate for gas compositional
changes.
It can be used when the gas composition
changes in the pipe or when moving the meter to
another location with a different gas composition.
The major benefit is that, for the first time, users
can experience the cost benefits of thermal flow
meters versus other technology such as coriolis and
still retain accuracy without returning the meter to
