Steam plays an important part in people’s
lives. It is important in food preparation, for cooling and heating buildings,
in the pulp and paper industry, and as a source of power for ships. Probably
its most important use is for electricity production.
Steam power plants get their energy from a
variety of sources. These plants need a source of fuel to heat water and
generate steam. Many use fossil fuels such as coal, oil or natural gas for
energy, while others use nuclear energy. Renewable sources of energy for steam
power plants include solid waste, wind, geothermal and biomass.
All these energy sources produce heat,
which heats water to generate steam. In a power plant, the steam turns a
turbine that is built somewhat like a windmill and rotates on a wheel made of
tightly packed metal blades. The turbine is connected by an axle to a
generator, which spins around with the turbine blades. As the generator turns,
it uses the kinetic energy from the turbine to generate electricity.
Gas and air flow measurement
Many power plants use natural gas as the
fuel that is burned to generate steam. This natural gas has to be precisely
measured and efficiently managed to ensure the safe and reliable operation of
the power plant’s heat source. Managing and controlling the optimal fuel-to-air
ratio requires a mass flow calculation — and can reap big dividends if it is
done correctly. This type of precise and efficient management ensures lower
overall fuel costs. Some facilities may require that gas distribution and
billing be tracked.
Another important factor in this context is
the measurement of compressed air. Natural gas must be precisely measured and
managed, so the effective management of compressed air in a power plant can
reap big dividends. Just as in apartment buildings where water flow is at its
peak in the morning and evening, but minimal later at night, so power requirements
in a power facility can vary widely with time of day. This gives thermal
flowmeters an advantage in these types of applications because they typically
have a 100-to-1 turndown and can measure low flows as effectively as high
flows. They are also not subject to clogging, unlike turbine and Pitot tube
How steam is measured ?
As part of the electricity production
process, steam must be measured to maximize boiler efficiency. Differential
pressure (DP) flowmeters dominate steam flow measurement. DP flowmeters rely on
a constriction placed in the flow line that creates reduced pressure in the
line after the constriction. A DP flowmeter requires a means to detect the
difference in upstream versus downstream pressure in the flow line. While this
can be done with a manometer, DP flowmeters use DP transmitters that sense the
difference in pressure, and then use this value to compute flow rate.
Vortex flowmeter advantages
Despite the dominance of DP flowmeters,
some end users are turning to multivariable vortex meters for measuring steam
flow. Vortex flowmeters have some advantages over other types of new-technology
flowmeters when it comes to measuring gas and steam flow. Gas flow measurement
is still a relatively new application for Coriolis meters, and the use of
Coriolis meters to measure steam flow is just beginning to occur. While
ultrasonic meters have been used for a number of years to measure gas flow and
boiler feedwater flow, steam flow is a new application for them.
Magnetic flowmeters cannot be used to
measure gas flow, steam flow or nonconductive liquids such as hydrocarbons.
Multivariable DP flowmeters can be used to measure liquid, gas and steam.
However, most multivariable DP flowmeters have substantially greater pressure
drop than vortex meters because of the presence of a primary element.
One reason vortex flowmeters work well with
steam is that they can handle the high temperatures and pressures associated
with steam flow measurement. For vortex meters, only the bluff body and the
sensors that detect the vortices are in contact with the flow. The transmitter
is usually mounted somewhere away from the pipe.
Vortex flowmeters are well-suited for
measuring steam flow, and they are widely used for this purpose. Steam is the
most difficult fluid to measure. This is because of the high pressure and high
temperature of steam and because the measurement parameters vary with the type
of steam. The main types of steam include wet steam, saturated steam and
superheated steam. Steam is often measured in process plants and for power
generation. In addition to their ability to tolerate high process temperatures
and pressures, vortex meters have wide rangeability. This allows them to
measure steam flow at varying velocities. In process and power plants, steam is
often measured coming from a boiler.
Multivariable vortex flowmeters measure
multiple variables in one instrument. In particular, they measure volumetric
flow, pressure, temperature, mass flow and density. The density of saturated
steam changes with temperature or pressure, while the density of superheated
steam depends on temperature and pressure. Because multivariable flowmeters
measure density accurately, they provide an accurate measurement of mass flow.
With steam flow accuracy of 1.5 percent of reading, they provide the necessary
data for reliable and efficient steam flow management.