1. technology and computing

FEBRUARY 2003
Application & Product News for the Process Control Engineer
How to fit a flowmeter,
transmitter to an application
An energy reconciliation project at a West Virginia paper mill required
more accurate steam consumption; choosing the right flow-measurement
device and transmitter among many options was imperative.
Scott Hosier-Carotek,
and Jonathan RoweInvensys/Foxboro
A
s part of an energy balance and
reconciliation program, a West
Virginia paper mill is in the
process of implementing a plan to
increase the number of steam and water
flow rate measurements. This effort is
part of an energy balance and
reconciliation program designed to
produce very significant cost savings in
mill steam production.
To obtain the required steam flow rate
measurements, the mill’s engineering
staff considered several types of
flowmeters and differential pressure/
primary device combinations before
making a final decision.
Understand the application
Applying a multi-variable
transmitter for measuring flow,
pressure, and temperature,
simplifies the installation.
Main steam flow from the mill’s
industrial
boiler
goes into a 12-in.
(300-mm) diameter
header, operating at
a nominal steam
pressure of 1,200
psig (8.3 MPa). At
the
flow
rate
measurement point
the
steam
is
superheated and is
at a nominal operating temperature of
900 °F (480 °C).
Selection of a
flowmeter for an application of this type
requires considering accuracy, ability to
withstand high-pressure/high-temperature
steam, resistance to wear, dimensional
stability with temperature change, flow
turndown range, and cost.
Linear in-line meters, such as vortex
or mass flowmeters, are generally
unsuitable for this application because
of high pressure, high temperature, and
large line size. The likely solution for
such applications is usually a square
root flowmeter, formed by pairing a
differential-producing primary flow
element and a differential pressure
transmitter.
That’s the basic approach mill
engineers chose to pursue.
Select the primary element
Deciding the type of primary flow
element that would provide the most
accurate measurement and be the most
reliable in this application was the next
decision facing mill staff.
Mill engineers turned to Richard W.
Miller’s
“Flow
Measurement
Engineering Handbook” (1996, McGraw
Hill, New York, NY) and found the
following quote, “The flow nozzle has
an elliptical (ASME) or a radius (ISA)
entrance and is generally selected for
steam (vapor) flows at high pipeline
velocities [100 ft/s (30.5 m/s)]. Because
of its rigidity it is dimensionally more
stable at higher temperatures and
velocities than an orifice.” Mr. Miller
went on to say, “When both are sized to
create the same differential at the same
flow rate, the pressure loss of a flow
nozzle is approximately the same as for
an orifice. Standards used for
Measurement points
are seldom
convienent. Attention
to detail during
design and
installation makes
maintenance a lot
easier.
Calculating mass flow
To calculate the mass flow rate of
superheated steam using a
differential producer primary flow
device, such as a flow nozzle, the
following basic flow-rate equation
must be solved.
Mass Flow Rate = N Cd E Y d2
(ph)1/2 , where:
■
N is the unit’s conversion factor;
Cd is the discharge coefficient--a
function of Beta Ratio, type of
primary device, and Reynolds
Number. (The Beta Ratio, ß, is bore
diameter (d) divided by internal
pipe diameter (D). The Reynolds
Number is a function of velocity,
density, and viscosity);
■ E is the velocity of approach
factor, which accounts for change
of velocity through the primary
device and is a function of Beta
Ratio, ß;
■ Y is the gas expansion factor,
accounting for changes of fluid
density through the primary device;
■ d is bore diameter;
■ p is fluid density at operating
pressure and temperature; and
■ h is differential pressure across
the primary device.
■
construction, installation, and accuracy
are defined in ISO 5167 (1991) and
ASME MFC-3M (1995).”
High pressure, high temperature, and
high flow rates exactly described the
mill’s application, thus the engineers
chose a flow nozzle as the primary
element.
Select the transmitter
Selecting the transmitter most capable
of providing an accurate measurement
of superheated steam flow was the one
remaining issue facing mill engineers.
Steam is generally measured and
accounted for on a mass basis using
flow measurement units such as pounds
per hour (lb/hr) or kilograms per hour
(kg/hr). Because the mill also needed an
accurate mass measurement that
accounted for changes in steam pressure
and temperature, it was decided three
independent measurements--differential
pressure (DP) across the flow nozzle,
pressure (P), and temperature (T)-would be required.
The traditional approach would be to
purchase and install three transmitters-one for DP, one for P, and one for T.
A three-transmitter solution to obtain
mass flow incurs high device purchase
costs and high installation and wiring
costs.
Instead, the mill project engineers
decided to use multivariable transmitter
technology.
The
multivariable
transmitter
resembles a traditional DP transmitter,
but includes measurements of DP and P,
and it can accept lead wires from an
external temperature-measuring element.
Hosting the DP, P, and T
measurements, the transmitter can
calculate mass flow rate and provide a
4-20 mA or digital output signal,
representing mass flow rate. (See
“Mass-flow calculation” sidebar.)
Dynamically solving a mass flow
equation also requires using pressure and
temperature values to simultaneously
calculate correction variables contained
in the formula.
For the superheated steam application
at the mill, the dynamic correction
variables are:
■
Discharge coefficient, Cd, which
changes with viscosity;
■ Expansion Factor, Y, which changes
with P, DP, and isentropic exponent;
■ Steam Density, p, which changes with
P and T; and
■ Thermal expansion of the pipe and
flow nozzle, which varies with T.
Each of these dynamic correction
variables are continually calculated in
the multivariable transmitter and used in
the flow rate equation to continually
calculate mass flow using the physical
properties of the steam in accordance
with ASME requirements.
Install it correctly
To calculate superheated steam
density and compensate for thermal
expansion of the primary device and
pipe, the process temperature, T, must
be continuously measured.
The multivariable transmitter accepts
the wiring connections of an external
resistance temperature detector (RTD)
inserted into the pipe to measure steam
temperature. The RTD is inserted into a
thermowell designed to withstand
pipeline pressure and temperature
conditions.
The thermowell and RTD must be
installed in a location where the
measured temperature is representative
of the steam temperature in the pipeline
at the point where pressure is being
measured. This assures density, viscosity,
and isentropic exponent calculations of
the steam are accurate and in accordance
with the ASME steam tables.
Because mill steam temperature
(approximately 900 °F or 480 °C) is
higher than the maximum allowable
temperature rating of the transmitter
body (250 °F or 121 °C), the transmitter
is mounted below the level of the flow
nozzle. The pipes connecting the flow
nozzle taps and the transmitter are filled
with water. This isolates the transmitter
from the high temperature steam and
allows the transmitter to operate at
ambient temperature.
The calculation of steam density by the
multivariable transmitter requires the
transmitter have appropriate equations to
define the relationship of steam density
to pressure and temperature for a range of
operating pressures and temperatures.
Also, to calculate the mass flow rate, the
transmitter must contain constants that
are used to determine the coefficient of
discharge for the selected primary device.
Multivariable transmitters can be used for
many applications in addition to steam,
including almost any liquid, gas, or vapor.
Therefore, the equations in the
transmitter are common regardless of
application and appropriate coefficients
are downloaded to the transmitter as part
of the transmitter configuration process.
Configure it correctly
Facilitation of the configuration setup
is through PC-based software that
allows users to input relevant
application data, such as the fluid being
measured, primary device type, boreand pipe-size, etc.
To ease device configuration, the
software includes a physical properties
table for many common fluids, so the
user only needs to select the name of
the fluid, rather than having to input
data about fluid density variations.
Also, the configuration software
provides selections for the type of
primary device being used, thus
eliminating the need to research
and enter coefficients for each
type of primary device.
For example, the flow
nozzle selections and the
physical
properties
of
saturated and superheated
steam are included in the
configuration software used
by the mill to set up the
multivariable transmitter.
Following simple, yet
critically important steps,
mill engineers sifted through
options to select and apply the
correct measurements for the
application. For accurately
measuring superheated steam,
this turned out to be a flow
nozzle and a multivariable
transmitter. The next problem
may require something different.
In process instrumentation, many
options often are suitable, however, some
options will always provide better overall
results than others. The trick is to take the
time to develop the one best solution.
For more information, call Invensys
Foxboro at 866-746-6477, or visit
www. foxboro.com/m&i
By locating the
transmitter below the
measurement point,
the sensing lines can
be filled with water.
The water isolates
the transmitter from
the super-heated
steam.
304771
Foxboro IMV30 multivariable
transmitter
When used with in-line primary
flow devices, Foxboro’s IMV30
multivariable transmitter
measures mass or
standard volume flow
rate of liquids, gases,
or vapors. Output
signals include 4-20
mA, HART, and
Foxboro FoxCom
digital
communications.
The IMV30 is offered
with a choice of
differential pressure
ranges from 0-0.5 to
0-840 inH2O (0-0.12
to 0-210 kPa) and
pressure ranges from
0-1 to 0-1,500 psia (00.007 to 0-10 MPa).
The transmitter
connects directly to
an external 2, 3, or 4wire RTD without need
for special cables or connectors.
Supplied as standard with 316ss
wetted parts (316Lss for the sensor),
the IMV30 is optionally available in
Hastelloy C process wetted parts for
applications in corrosive fluids.
Transmitter accuracy is +/0.075% of span (4-20 mA) and +/0.05% of span (digital) for
differential and absolute pressure
measurements, for spans from 100%
to 10% of upper range limit. Other
performance specifications,
including ambient temperature
effect, static pressure effect, and
long-term stability have been
optimized to reduce Total Probable
Error (TPE).
PC-based software, called PCMV,
is provided to configure the
transmitter for application-specific
flow rate
Reprinted from Instrumentation & Control, February 2003 by RSiCopyright.
Copyright © Reed Business Information, a division of Reed Elsevier, Inc. All rights reserved.
For reorders call RSiCopyright 651.582.3800. For subscription information call 630.320.7118.
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AR560