Displacer Level Devices

Design Pressure
Set by the flange rating of the chamber or by the maximum working pressure of the
displacer, usually up to 100 PSIG (7 bars, 0.7 MPa) for the flexible disc and up to
600 PSIG (41 bars, 4.1 MPa) for the diaphragm-sealed designs. The flexible shaft
unit can operate up to
1000 PSIG (69 bars, 6.9 MPa); torque-tube designs are available
up to 2500 PSIG (170 bars, 17 MPa); magnetically coupled units can be used up to
6100 PSIG (410 bars, 41 MPa). Verify ratings with the manufacturer.
Design Temperature
Generally in the range of
−50 to 451°F (−45 to 230°C). Inconel®torque tubes canoperate from−350 to 850°F (−212 to 454°C). \For electronic transmitters, the temperature
of the topworks must be kept below 180°F (82°C). If the process temperatureisabove500°F(260°C)orbelow 0°F (−18°c), thermal insulation barriers or torquetube
extensions are usually recommended.
Materials of Construction
Displacers are available in type 316 stainless steel, Monel, polypropylene, or solidTeflon
 The hanger cable assemblies can be obtained in type 316 stainless steel,
Monel, and HastelloyC. The cage (chamber) is usually carbon or stainless steel.
Inaccuracy
Varies widely with application and the instrument, typically 0.5% of full scale.
Range
Standard displacers are available in lengths of 14 to 60 in. (0.35 to 1.5 m). The range
of special units can go up to 60 ft (18 m).
Cost
Displacer-type switches cost from $200 to $500, and a 32-in. (0.81-m) electronic
transmitter costs about $2500; add $500 to $700 for an external steel chamber.
Partial List of Suppliers
ABB Instrumentation Inc. (www.abb.com)
Delta Controls Corp. (www.deltacnt.com)
Dwyer Instruments Inc. (www.dwyer-inst.com)
Endress+ Hauser Systems & Gauging (www.systems.endress.com)
The Foxboro Co. (www.foxboro.com)
Magnetrol International (www.magnetrol.com)
Masoneilan Operations Dresser Flow (www.masoneilan.com)
Norriseal (www.norrisel.com)
Schlumberger Measurement Div. (www.slb.com/rms/measurement)
Siemens Moore Energy & Automation (www.sea-siemens.com)
Yokogawa Corp. of America (www.yca.com)


INTRODUCTION
Archimedes’ (c. 290 to 212 BC) principle states that a body
wholly or partially immersed in a fluid is buoyed up by a
force equal to the weight of the fluid displaced. A level or
a density instrument is sensitive to the apparent weight of
an immersed displacer. If the cross-sectional area of the
displacer and the density of the liquid are constant, then a
unit change in level will result in a reproducible unit change
in the apparent weight of the displacer. The simplest level
device of this type involves a displacer that is heavier than
the process liquid and is suspended from a spring scale.
When the liquid level is below the displacer, the scale
shows the full weight of the displacer. As the level rises,
the apparent weight of the displacer decreases, thereby
yielding a linear and proportional relationship between spring
tension and level. The spring scale can be calibrated as
desired.


This simple device is limited to applications in open tanks.
In practical industrial service, the basic problem is to seal the
process from the spring scale or other force-detecting mechanism.
This seal has to be frictionless and useful over a wide
range of pressures, temperatures, and corrosion conditions.
The variations in the design of this seal provide the basis to
distinguish the types of displacement detectors that are in use
and are discussed below. They are the magnetically coupled
switch, the torque tube, the diaphragm and force bar, the spring
balance, the flexible disc, and the flexible shaft design. Each
of these units operates on Archimedes’ principle but is different
as far as its seals are concerned. All of them can be used to
detect a liquid–vapor interface, a liquid–liquid interface, and,
if the level is constant, the changes in density as well. The
flexible disc unit is available as a pneumatic transmitter, and
the flexible shaft unit is available as a high-gain pneumatic
controller or as a switch. The other designs are available with
integral pneumatic or electronic transmitters or controllers

Elbow Taps

Design Pressure
Limited by piping design class only
Operating Temperature Range
−330 to+1100°F (−200 to+600°C)
Fluids
Liquids, vapors, or gases
Differential Pressure
0- to 10-in water column (0 to 2.5 kPa)
Sizes
0.5 to 20 in (12 to 500 mm)
Inaccuracy
±2 to±10% FS
Cost
Approximately $1000 plus value of elbow and measuring device (usually a
differential-pressure transmitter)
Partial List of Suppliers
Normally fabricated on site

Flow measurement using elbow taps depends on the detection
of the differential pressure developed by centrifugal force as
the direction of fluid flow changes in a pipe elbow. Taps are
located on the inner and outer radii in the plane of the elbow.
The pressure taps are located at either 45°or 22.5°from the
inlet face of the elbow


A SIMPLE FLOWMETER
Elbow taps are easy to implement, because most piping configurations
already contain elbows in which taps can be
located. This guarantees an economical installation and
results in no added pressure loss. The measurement introduces
no obstructions in the line. Accumulation of extraneous
material in the differential-pressure connections can plug the
elbow taps. Therefore, they should be purged if the process
fluid is not clean.
As is the case with other head-type primary flow measurement
devices, the differential pressure developed by a
given flow is precisely repeatable. However, the flow coefficient
of an elbow tap calculated from the physical dimensions
of the pipe is generally considered reliable to only
±5 to±10%. This is quite satisfactory for many flow control applications
where repeatability is the primary consideration. If
absolute accuracy is desired, a more precise flowmeter should
be used, or the elbow tap readings should be calibrated,

preferably in place and using the working fluid. Not enough
data exist to establish precise correction factors for effects of
upstream disturbances, viscosity, and roughness in pipe and
elbow surfaces, and no published standards are available.
Elbow taps develop relatively low differential pressures.
For this reason, they cannot be used for measurement of lowvelocity
streams. Typically, water flowing at an average
velocity of 5 ft/sec (1.5 m/sec), roughly 200 GPM in a 4-in.pipe (45 m3
/h in a 100-mm pipe) through a “short-radius”
elbow with a centerline radius equal to the pipe diameter
develops about 10 in. of water differential pressure (2.5 kPa).
This is approximately the minimum full scale value recommended
for reliable measurement. Taps in long radius pipe
or tube bends do not develop sufficient differential pressure
for good flow measurement at low flow velocities.
In comparison with an elbow installation, an orifice will
generate a head (1.4 to 2.2) higher at the same
flow rate. For example, for
β=0.65, the orifice head developed
will be approximately 6.5 times that of a short-radius elbow


Bibliography
Hauptmann, E. G., Take a second look
at elbow meters for flow
monitoring,
Instrum. Control Syst.,
47–50, 1978.
Moore, D. C., Easy way to measure slurry flowrates,
Chemical Eng.,
96,1972.

Cross-Correlation Flow Metering

Current Applications
Pumped paper pulp, pneumatically conveyed coal dust, cement, grain, plastic granules,
chalk, water flow in nuclear and industrial plants, and animal foodstuffs
Sizes
Practically unlimited
Cost
A 4-in 150 # mass flowmeter with epoxy-resin-lined, enameled steel pipe costs $6000.
If the sensor costs are not considered, the electronic detector alone is around $2000.
Nuclear power plant flow metering installations range from $25,000 to $50,000.
Partial List of Suppliers
Analysis and Measurement Services Corp. (www.ams-corp.com)
Endress
+
Hauser Inc. (www.us.endress.com)
Kajaani Electronics Ltd. (Finland)


The oldest and simplest methods of flow measurement are
the various tagging techniques. Here, a portion of the flowstream
is tagged at some upstream point, and the flow rate
is determined as a measurement of transit time. Variations of
this technique include particle tracking, pulse tracking, and
dye or chemical tracing, including radioactive types. The
advantages of tagging techniques include the ability to measure
the velocity of only one component in a multicomponent
flowstream without requiring calibration or pipeline penetration.
For example, electromagnetic tagging of gas-entrained
particles allows for the determination of their speed through
the detection of their time of passage between two points that
are a fixed distance from each other.
Flow metering based on correlation techniques
is similar
in concept to the tagging or tracing techniques, because
it also detects transit time. As illustrated in Figure 2.5a, any
measurable process variable that is noisy (displays localized
variations in its value) can be used to build a correlation
flowmeter. The only requirement is that the noise pattern must
persist long enough to be seen by both detectors
A
and
B
as
the flowing stream travels down the pipe. Flow velocity is
obtained by dividing the distance (between the identical pair
of detectors) by the transit time. In recent years, the required
electronic computing hardware, with fast pattern recognition
capability, has become available. Consequently, it is feasible
to build on-line flowmeters using this technique.
3
The following process variables display persistent enough
noise patterns (or local fluctuations) that correlation flowmeters
can be built by using an identical pair of these sensors:
Density
Pressure
Temperature

Ultrasonics
Gamma radiation
Capacitive density
Conductivity


Several of the above process variables (such as temperature,
4,5
gamma radiation, and capacitive density
6
) have
been investigated as potential sensors for correlation flowmeters.
One instrument has been developed that uses the
principle of ultrasonic cross-correlation to measure heavywater
flow.
3
Others are available for paper pulp applications
using photometric sensors and for solids flow measurement
utilizing capacitance detectors (Figure 2.23v). For crosscorrelation
flowmeters applied in solids flow applications,
refer to Section 2.23.
When fully developed, correlation flow metering can
extend the ability to measure flow not only into the most
hostile process environments but also into areas of multiphase
flow and into three-dimensional flow vectoring.

BTU Flowmeters for Gaseous Fuels

Sensors Required
Conventional head-type flowmeter, calorimeter, or wobble index detector plus pressure
and temperature measurements
Process Fluids
Gaseous fuels
Applications
Combustion processes are optimized by measuring and controlling fuel gas flow on
the basis of the heat flow (BTU/h) requirement of the process
BTU Flow Range
From 100 BTU/min to very large heat flow rates; limited only by pipe sizes
Inaccuracy
±1.0 to± 2.0% of full scale, depending on the accuracy of the head type flowmeter used
Costs
for the costs of the different types of calorimeters. The cost of
a general-purpose wobble index detector is $10,000 to $12,000. Explosion-proof
designs cost $5000 to $6000 more. For flow, temperature, and pressure transmitter
costs, refer to Chapters 2, 4, and
5.
Partial List of Suppliers
ABB Process Automation—Analytical Div. (www.abb.com/us)
The Foxboro Co. (www.foxboro.com)
Honeywell Industrial Control (www.honeywell.com/acs/cp)
ICS (www.icsadvent.com)
Thermo Onix Process Analyzers (formerly Fluid Data/Amscor) (www.thermoonix.com).



The heat flow rate provided by the burning of a fuel gas can
be measured by detecting its mass flow rate and multiplying
it by its heating value, which can be detected by wobble index
sensors or calorimeters . The burning
of waste gases from a variety of process sources .
and the accurate control of regular fuel gases made it necessary
to measure their heat flows on line and continuously.
In the past, only the volumetric flow of the fuel gas was
measured, and the heating value and the specific gravity of
the gas were assumed to be constants. This approach is not
acceptable for applications involving the burning of waste
gases, because both their composition and heating values are
variable.
In the past, in hazardous areas, the heating value of fuel
gases could not be continuously measured, so specific gravity
measurements were used to estimate their heating value and
to control the combustion process. Today, continuous and
explosion-proof calorimeters are available for
the measurement of the heating value of any fuel gas. These
optimized combustion controls are automatically adjusted for
variations in either the specific gravity or the heating value
of the fuel gas.


The heat flow rate provided by the burning of a fuel gas can
be measured by detecting its mass flow rate and multiplying
it by its heating value, which can be detected by wobble index
sensors or calorimeters. The burning
of waste gases from a variety of process sources
and the accurate control of regular fuel gases made it necessary
to measure their heat flows on line and continuously.
In the past, only the volumetric flow of the fuel gas was
measured, and the heating value and the specific gravity of
the gas were assumed to be constants. This approach is not
acceptable for applications involving the burning of waste
gases, because both their composition and heating values are
variable.
In the past, in hazardous areas, the heating value of fuel
gases could not be continuously measured, so specific gravity
measurements were used to estimate their heating value and
to control the combustion process. Today, continuous and
explosion-proof calorimeters are availablefor
the measurement of the heating value of any fuel gas. These
optimized combustion controls are automatically adjusted for
variations in either the specific gravity or the heating value
of the fuel gas.

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