Chemical Engineering October 2014 - 45

Cover Story
electronic signal related to flexing
of the diaphragm. A secondary electronic
package acts on this signal,
producing a standard transmission
signal, such as 4-20 mA. Remote
seals have been developed to widen
the applicability of the DP pressure
transmitter beyond its limitations
of maximum temperature and dirty,
corrosive or abrasive process fluids.
The term " remote seal " refers to
an isolation seal system that protects
a pressure transmitter sensor
and its process connections from
any contact with the measured
process fluid. The seal is called " remote "
because it permits the transmitter
to be located up to some 80
ft from the element connections.
Sometimes the transmitter is closecoupled
to the process pipe or vessel.
Seals in such cases would be
more aptly called isolating seals.
Other names used are diaphragm
seals and chemical seals.
The remote or isolating seal system
constitutes a well-engineered,
useful accessory, usually ordered
in pairs as integral transmitter attachments,
and sized for a specific
application. The system consists of
a flange connection, a stem, and a
seal diaphragm connected through
a capillary to the flanged chamber
of the transmitter. Shown schematically
in Figure 6, the seal has two
main components: a flexible metallic
diaphragm that is exposed to the
process fluid and reflects its static
pressure plus a liquid-filled capillary
tube that connects to the transmitter
body.
Seal systems come with a variety of
diaphragm diameters, materials and
configurations to suit needs of the installation.
In some cases the capillary
is protected with suitable armor.
Once the individual components
are connected, a technician evacuates
the air from the system and fills
it with an incompressible fluid. In
this way when the seal diaphragm
experiences process pressure, it deflects
and exerts a force against the
fill fluid. Since the liquid is incompressible,
this force is transmitted
hydraulically to the sensing diaphragm
within the transmitter body,
causing it to deflect in turn. The deflection
of the sensing diaphragm of
the transmitter is the basis for the
pressure measurement.
Specifying remote seals
Certain considerations affect the
proper dimensioning of a remote
seal system. For one, a full-scale deflection
of the seal diaphragm must
exceed the displacement capacity
within the transmitter; otherwise
the seal element cannot drive the
transmitter to a full-scale measurement.
Additionally, best practice is
to minimize the volume of the capillary
fill cavity between the seal
flange and the primary diaphragm
of the transmitter. This minimizes
the ambient and process temperature
effects on the measurement.
Special flanges are available for
the transmitters that minimize the
cavity volume when connected to a
remote seal.
Response time. Engineers must
consider the response-time issues
when specifying a remote seal. The
time constant for a measurement is
the time required for an instrument
output to reach 63% of the value it
will ultimately reach in response to
a step change in input (pressure in
this case). Normally an instrument
will reach 99.9% of full response
within a time equal to four times
the time constant.
The response time of a DP or gage
pressure transmitter can significantly
increase when connected to
a remote seal. This response time
depends on the following:
* The total length of capillary connecting
the seal element to the
transmitter body: The response
time is directly proportional to
the length of capillary. So the
length of capillary has to be minTransmitter
Diaphragm
Liquid-filled
Capilliary
Seal
Diaphragm
h
LP
HP
FIGURE
4. The interface between two
liquids of different density can be measured
with a DP transmitter
Interface
level
range
LP
DP
HP
Transmitter
FIGURE 5. Changes in a fluid's density
can be monitored with a DP transmitter
Process
Fluid
FIGURE 6. A remote (isolation) seal
configuration enables a broader range of
applications for a DP transmitter
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Chemical Engineering October 2014

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