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.