Chemical Engineering April 2012 - 57

FIGURE 5a and 5b. In field installations with a transmitter on
top, as shown in Figure 5A (left), the compact orifice integrated
flowmeter is installed between existing flanges and eliminates
heated sensing lines. In this installation, the use of wireless communication
eliminates field wiring. For larger lines (Figure 5B,
right), an averaging pitot tube is used to minimize pressure loss
pressure can be used to infer level.
For dirty or abrasive fluids that might
plug or damage the transmitter, very
high temperatures, or special process
connections, remote seals are used to
isolate the transmitter from the process
and " repeat " the pressure signal.
In a closed tank, the vapor above
the liquid is pressurized, and the level
is proportional to the difference in
pressure between the top and bottom
taps, obtained using a dP transmitter.
The transmitter is typically located
at grade near the low-side (high-pressure)
process connection. The highside
(low-pressure) seal is normally
near the top of the vessel, so it must
be connected to the transmitter via
oil-filled capillaries that hydraulically
repeat the pressure signal from the
seal to the transmitter (Figure 6).
Although the low-side seal is physically
close to the transmitter, users have
traditionally specified equal-length capillaries
- with the excess lower-seal capillary
coiled up at the transmitter. The
rationale for including this extra capillary
is to " balance " the impact of fill-fluid
expansion. An increase in ambient and/
or process temperature causes the fill
fluid to try to expand. Within the fixed
volume of the capillary, this mimics an
increase in pressure and hence inferred
level. In this balanced system, temperature
changes will equally impact both
high- and low-side capillaries, with zero
net error.
Unfortunately, there is another significant
source of error - the head
temperature effect. As ambient or process
temperatures increase, the specific
gravity of the fill fluid decreases.
This causes the force on the transmitter
from the upper seal to decrease,
since head pressure is the product of
specific gravity and height, so the inferred
level is reduced.
There is no effect on the lower seal,
since it is at the same elevation as
the transmitter (that is, the height is
zero). This means that in a balanced
system, the impact of fill-fluid expansion
is zero, while the head temperature
effect is negative.
A better approach is to direct mount
the lower seal with no capillary, as
shown in Figure 6, Since the two effects
act in opposite directions, the
positive effect of expanding fill fluid
will partially counteract the negative
head temperature effect, thereby minimizing
net error. As an additional benefit,
eliminating the lower-seal capillary
speeds response, since changes
in liquid level do not need to travel
through the entire capillary before
being sensed at the transmitter. The
direct-mount installation also reduces
cost, since it eliminates one length
of capillary.
Since fill fluids have known coefficients
of thermal expansion, and
since diaphragms and capillaries have
known stiffness values and volumes, it
is possible for the supplier to quantify
these effects in advance. Given these
application details and expected ambient
and process temperature variation,
suppliers can optimize seal and
capillary design (in terms of fill fluid,
diameter and thickness) so that in
the actual installation, the two effects
will almost exactly cancel each other
out over the widest possible operating
range. Today, this optimization is done
using sophisticated software, producing
a so-called tuned system.
Pressure-based liquid level
One drawback of the tuned system approach
is that it can potentially create
a unique solution for each application,
leading to a large number of unique
spare parts. To simplify matters, most
users choose to sub-optimize individual
applications, balancing the need to
achieve acceptable performance with
maximum commonality. Even so, if
any component of the system is damaged
during installation or operation,
the entire assembly must be removed
and returned to the supplier for repair.
This can be particularly challenging
in cold climates, where working on
devices in the field is uncomfortable
and unsafe. Also, many cold climates
are remote from repair facilities,
and long return shipping times can
lead to days or weeks without a working
measurement.
Another problem observed in cold
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Chemical Engineering April 2012

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