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Articles about Custody Transfer
measurement is primarily concerned with measuring the flow of liquids and
gases. Steam, which is water
vapor, can be considered as a form of gas.
Steam is also an important flow measurement.
many forms. Types of gas
include natural gas, fuel gas, atmospheric gas, compressed natural gas, and
many others. Individual gases
that are especially important include hydrogen, oxygen, nitrogen, and
carbon dioxide. Air is a gas,
although it is a mixture of gases that include nitrogen, oxygen, argon, and
many other gases. Just as
water is an essential element of life, so life as we know it would not be
possible without the air we breathe.
gas plays an especially important role in the flowmeter industry.
Natural gas is an important source of fuel and energy.
Like coal and petroleum, it is a fossil fuel.
Like air, natural gas is a mixture of gases.
These include methane, ethane, propane, butane, and other alkanes.
Natural gas is found in the ground, along with petroleum.
It is extracted and refined into fuels that supply a significant
portion of the world’s energy supply.
natural gas (CNG) has taken on importance as a source of fuel for
alternative energy vehicles. Vehicles
powered by natural gas are highly efficient, and emit significantly lower
amounts of carbon monoxide, nitrogen oxide, and pollutants harmful to the
ozone layer than gasoline powered vehicles.
CNG powered vehicles are fueled at filling stations that are very
much like gasoline stations, and several companies have created flowmeters
specifically designed for these CNG filling stations.
measurement can also be divided into three broad categories: industrial,
commercial, and residential. Industrial gas flow measurement includes flow
measurement that occurs in manufacturing and process plants, including
chemical plants and refineries. Commercial
gas flow measurement occurs at businesses and commercial buildings such as
restaurants, office buildings, and apartment complexes.
This is a form of utility measurement, since these flowmeters
typically measure the amount of natural gas used by the business or in the
commercial building. Residential
gas flow measurement refers to flowmeters that measure the amount of gas
used at individual homes and apartments.
vs. Industrial. Meters
used to measure gas or water entering a building or plant for the purpose
of billing the plant for their use of gas or water are considered to be
utility meters. These meters
are typically sold to a gas or water utility company and installed at the
building or plant by the utility company.
Meters used within a building or plant for internal allocation
purposes of gas, water, or other liquids, are considered to be industrial
meters. They are typically
sold to the owners of the plant or building itself, as opposed to a utility
displacement (PD) flowmeters are widely used for utility measurements of
gas flow. One main type is the
diaphragm meter. Diaphragm
meters have several diaphragms that capture the fluid as it passes through
the meter. Differential
pressure across the meter causes one diaphragm to expand and one to
contract. A rotating crank
mechanism helps produce a smooth flow of gas through the meter.
This mechanism is connected via gearing to the index, which
registers the amount of fluid that passes through the meter.
type of PD meter for gas flow measurement is the rotary meter.
Rotary flowmeters have one or more rotors that are used to trap the
fluid. With each rotation of
the rotors, a specific amount of fluid is captured.
Flowrate is proportional to the rotational velocity of the rotors.
Rotary meters are used for both liquid and gas applications.
Rotary meters are used for industrial applications.
flowmeters have a rotor that spins in proportion to flowrate.
There are many types of turbine meters, but many of those used for
gas flow are called axial meters. Axial
turbine meters have a rotor that revolves around the axis of flow.
Most flowmeters for oil measurement and for measuring industrial
liquids and gases are axial flowmeters.
Axial meters differ according to the number of blades and the shape
of the rotors. Axial meters
for liquids have a different design from axial meters for gas applications.
Like PD meters, turbine meters are used as a billing meter to
measure the amount of gas used at commercial buildings and industrial
The use of
ultrasonic flowmeters to measure natural gas flow gained momentum following
the publication of AGA-9 in June 1998.
This report from the American Gas Association gives criteria for
using ultrasonic flowmeters for custody transfer of natural gas.
The AGA had previously issued reports on differential pressure (DP)
flowmeters (AGA-3) and turbine flowmeters (AGA-7).
Since the publication of AGA-9, the AGA has also issued a report on
the use of Coriolis flowmeters (AGA-11).
The use of
ultrasonic flowmeters is continuing to grow, both for custody transfer and
process gas measurement. Unlike
PD and turbine meters, ultrasonic flowmeters do not have moving parts.
And pressure drop is much reduced with an ultrasonic meter when
compared to PD, turbine, and DP meters.
Installation of ultrasonic meters is relatively straightforward, and
maintenance requirements are low.
flowmeters come as both inline and clamp-on configurations.
Some have one ultrasonic beam, while those with higher accuracy use
multiple beams. These are
known as multipath ultrasonic flowmeters.
Meters used for custody transfer purposes are inline multipath
meters. Most of these custody
transfer meters use four, five, or six paths, depending on manufacturer, to
make a highly accurate measurement. Manufacturers
include Instromet, Emerson Daniel, and FMC Measurement Solutions.
flowmeters consist of a differential pressure transmitter, together with a
primary element. The primary
element places a constriction in the flowstream, and the DP transmitter
measures the difference in pressure upstream and downstream of the
constriction. The transmitter
or a flow computer then computes flow, using Bernoulli’s theorem.
primary elements include orifice plates, venturis, flow nozzles, pitot
tubes, wedges, and others. Venturis
are especially suited to high-speed flows.
Orifice plates are still the most widely used type of primary
elements. Their disadvantages
are the amount of pressure drop caused, and the fact that they can be
knocked out of position by impurities in the flowstream.
Orifice plates are also subject to wear over time.
flowmeters are used to measure the flow of liquid, gas, and steam.
Like ultrasonic and turbine meters, they are used for the custody
transfer of natural gas. In
many cases, end-users buy their pressure transmitters and primary elements
from different suppliers. However,
several vendors have integrated the pressure transmitter with the primary
element to form a complete flowmeter.
These have the advantage that they can be calibrated with the
primary element and DP transmitter already in place.
flowmeters are the most highly accurate meter.
However, liquid flow measurement still predominates for Coriolis
meters because gases are less dense than liquids, and the measurement is
somewhat more difficult. A
number of suppliers have brought out Coriolis meters for gas flow
measurement, however, and this is a growing area for Coriolis.
One application that Coriolis has come to excel in is in measuring
compressed natural gas (CNG) for alternative fuel vehicles.
Here they compete primarily with turbine flowmeters.
flowmeters are used almost exclusively to measure gas flow.
Thermal flowmeters typically inject heat into the flowstream and
then measure how quickly it dissipates.
This value is proportional to mass flow.
Two methods used are called constant current and constant
flowmeters grew out of the use of hot-wire anemometers for research
applications. Early companies
to develop thermal flowmeters include Sierra Instruments, Kurz Instruments,
and Fluid Components International (FCI).
Thermal flowmeters excel at measuring gas at low flowrates.
Measuring low flows is a difficulty for some meters, including
vortex, but this is where thermal flowmeters shine.
Accuracy levels are improving for thermal flowmeters, as suppliers
introduce product improvements.
application where thermal flowmeters are widely used is in the measurement
of stack flows. Gas flow has
to be measured in smoke stacks to conform to Environmental Protection
Agency (EPA) reporting requirements. Insertion
thermal flowmeters are used to measure the flow of sulfur dioxide (SO2),
nitrogen oxide (NOx), and other industrial pollutants.
Because of the large size of the stacks, insertion thermal meters
that use multiple measuring points are used for these applications.
Other flowmeters used for smoke stack applications are DP meters
with averaging pitot tubes, and ultrasonic flowmeters.
Mass Flow Controllers
controllers not only measure flow; they also control it.
They differ from thermal flowmeters in that most divert a small
portion of the flow into a parallel channel, and then measure the flow
through that parallel channel. The
flowmeter then performs a calculation to determine mass flow through the
entire flowmeter. Most also
contain an integrated valve that is used to control flow.
A setpoint is determined, usually by the user, and the valve is
adjusted so that flow reaches that setpoint.
flow controllers use thermal methods to determine flowrate in the parallel
flow path, though some use a differential pressure principle.
And some mass flow controllers are sold without the valve, meaning
they are functioning as flowmeters rather than controllers.
Mass flow controllers are widely used in the semiconductor industry,
but many have industrial applications.
It is important for a mass flow controller to know what gas is being
measured, in order to insure an accurate measurement.
Mass flow controllers can also be used for to measure liquid flow.
variable area flowmeters are also used to measure gas flow.
Vortex flowmeters are one of the few types of meters, besides DP,
that can accurately measure the flow of liquid, steam, and gas.
However, vortex meters especially excel at measuring steam flow,
since they can handle the high temperatures involved.
areas meters can measure the flow of both liquid and gas, and they also are
used for a limited amount of steam flow measurement.
Variable area meters rely on a float that rises in proportion to
flowrate. They are primarily a
low-cost alternative where a visual indication of flow is sufficient.
While most still must be read visually, some variable area meters
have been manufactured with transmitters.