Chemical Engineering April 2012 - 56

Engineering Practice
measurements are needed to maintain
process efficiency, detect leaks and
account for steam consumption. The
ability to cost-effectively ensure high
accuracy, repeatability and reliability
is a challenge in outdoor applications
in cold climates. Although vortex
flowmetering technology has enjoyed
great success in recent years, the most
widely used technology for measuring
steam flow remains the differentialpressure
(dP) flowmeter.
The traditional steam dP-flowmeter
is shown in Figure 3. Steam flows
through the " primary element " restriction,
which causes a pressure
drop that is proportional to flowrate.
The orifice plate is the most common
primary element for steam service, although
the averaging pitot tube is usually
preferred in larger lines or when
users wish to minimize permanent
pressure loss through the flowmeter.
The dP transmitter is installed under
the pipe, and columns of condensate
form in each of the impulse lines leading
to the transmitter. These so-called
" wet legs " of condensate are hot where
they contact the steam, and cool by
the time they reach the transmitter.
This prevents the oil in the transmitter
from overheating to past its vapor
pressure, which would cause the oil to
flash and rupture the thin metal diaphragm
in the pressure transmitter.
Users manually fill the wet legs with
condensate prior to startup, and refill
the wet legs after a shutdown.
In outdoor installations in cold climates,
the fluid in the impulse lines
farthest from the process - that is,
nearest the transmitter - can freeze,
particularly during planned or unplanned
process shutdowns. To prevent
this, heat tracing must be used to
heat the impulse lines and maintain
their temperatures above freezing.
This increases cost and complexity.
Figure 3 shows only a single dP
transmitter, which measures volumetric
flow. Since steam is compressible,
the user must compensate for variable
density using a line-pressure and process
temperature measurement [3]. A
lower-cost alternative to multiple devices
is the multivariable " integrated
flowmeter " .
Integrated flowmeters, shown in
Figure 4, include the primary element,
FIGURE 4a and 4b. The integrated dP flowmeter includes the primary element, the
flow computer and the sensors needed to calculate mass and energy, dP, pressure
and process temperature. As shown in Figure 4A (left), the use of a wafer-style orifice
plate as the primary element minimizes cost in smaller lines, and the 4-hole design
minimizes the need for straight pipe. By contrast, in larger lines, the use of an averaging
pitot tube as the primary element (shown in Figure 4B, right) minimizes total
cost and pressure loss
the differential and line-pressure sensors,
the process temperature sensor,
and any isolation valving that may be
required. They arrive pre-assembled,
pre-configured and leak-tested from
the supplier (Figure 2), which minimizes
the overall cost and risk of incorrect
field assembly. An integrated
flowmeter is consistent - for every
line size, the components between the
pipe wall and the pressure transmitter
are virtually identical. Since the heat
dissipating components are the same
in every installation, a given process
temperature should always result in a
given temperature at the transmitter.
Analysis of temperature logs in
operating steam-flow installations -
from both the diagnostic temperature
sensor in the pressure transmitter
and the process temperature sensor
used for density compensation - has
revealed the following:
* Temperature of the fluid at the pressure
sensor is difficult to predict in
a " traditional " installation, in which
the user installs whatever length
and diameter of sensing lines best
fit the installation
* Temperature of the fluid at the
pressure sensor is predictable and
found to be consistently lower than
expected when using an integrated
flowmeter
These field results, and subsequent
56 ChemiCal engineering www.Che.Com april 2012
factory testing, justify the following
new practice for low-to-medium-pressure
steam-flow measurement. Without
risk of damage to the sensors, and
using normal insulation for personnel
protection, the integrated flowmeter
can be directly installed when the
steam temperature does not exceed
205ºC. This temperature corresponds
to a saturated steam pressure of 235
psig, which is suitable for most industrial
applications in the CPI. Higher
temperatures and pressures are possible,
though users should consult
supplier recommendations [4] for
specific applications.
The new best practice for installing
the transmitter above the pipe eliminates
wet legs. This results in a simpler,
lower-cost installation. By eliminating
the risk of wet-leg freezing
during shutdown, the need for heat
tracing is also eliminated (along with
its associated capital and operating
costs). Figure 5 shows installations
of steam flowmeters in a cold climate
(Albany, N.Y.).
Using pressure to infer level
A " bottom-up " pressure measurement
can be used to infer level, because the
pressure exerted by the fluid is proportional
to the fluid height - above the
tap - and the fluid density. Thus, for
a known fluid density, the measured

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Chemical Engineering April 2012

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