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In some cases, the required distance prior to a
temperature sensor cannot be provided by the plant
due to a lack of real estate or specific application
requirements. In these cases, the use of a feed forward
temperature control algorithm is required for control. See
the temperature control methods that are explained later
in this document for further explanation.
The radial location of the temperature sensor can be
a concern for applications that do not pass through a
pipe bend. Long lengths of straight pipe along with large
quantities of water injection can result in spray water
fallout. Thermal sensors located at or near the lower pipe
wall can measure lower temperatures as water fallout
reduces the temperature of the lower boundary layer at
a faster rate than the pipe core. Temperature sensors
must be placed in the middle to upper portion of the pipe
cross-section to ensure the maximum steam temperature
is observed.
As previously noted, the addition of at least one pipe
elbow within a system is extremely beneficial in assisting
with the mixing of spray water with the steam. These
installations must always mount the temperature sensor
downstream of the elbow by at least five pipe diameters.
Close coupling of the sensor with an elbow can result in
poor temperature control.
Pressure Sensor Recommendations
The pressure sensor must be located at a point
downstream where there is a relatively stable flow profile.
The recommended installation point is five pipe diameters
downstream of the valve connection and any pipe
elbows. See Figure 3.
Minimum Pressure Sensor location:
L4 = 5 x Nominal Pipe Diameter
Pipe Size and Selection
Prior to laying out the piping system, the appropriate
pipe sizes must first be determined. This is accomplished
by following velocity limitation guidelines. These
guidelines are usually established by the customer or
by the engineering standards of the design firm. These
guidelines are typically practical for the upstream
design; however, with the addition of spray water some
guidelines become conservative and impractical for the
downstream design of a steam conditioning system.
The ideal environment for water injection is a high
velocity and highly turbulent flow operating between
the ranges of 250 ft./sec and 300 ft./sec. A high velocity
steam jet creates an effect that shears across water
droplets and increases the surface area that is exposed
to steam. By creating more surface area, the diameter
or thickness of the water droplets is decreased. This
drastically decreases the amount of time that is required
for a water droplet to vaporize.
Conversely, a low velocity flow stream does not assist
in breaking down the water droplet. As a result, larger
diameter water droplets are injected into low velocity
applications. Because these larger diameter droplets
carry more mass, it becomes difficult at low velocity for
the steam to suspend the heavier droplet in the flow
stream.
This can result in water fallout and insufficient
evaporation at the temperature sensor location.
