Chemical Engineering February 2020 - 46

Engineering Practice
Taking Advantage of Improvements in
Level Measurement Technology
Level measurement technologies have become more versatile, robust and accurate
thanks to recent developments. Differential pressure and radar level measurement
provide examples
Lydia Miller
Emerson Automation Solutions
L
evel measurement applications
within the chemical
and petrochemical industries
can be extremely challenging.
Level instruments must provide
repeatable, reliable and accurate
measurement of hazardous materials
over wide temperature and pressure
ranges, and with complicating
factors, such as steam, dust, foam,
turbulence and condensation.
Solving the challenges of measuring
the levels of fluids and solids in
chemical processing applications is
as old as the industry itself. Traditional
methods, such as sight glasses
and mechanical float switches or differential
pressure (DP) level systems
with wet- or dry-leg impulse piping,
have all been used extensively and
are still around today. But they can
cause maintenance and reliability
headaches, so many are getting replaced
or augmented by more sophisticated
methods.
DP level systems are reliable
when applied properly, and well
understood, so they continue to
be commonly used. Advances in
technology have solved many of
the difficulties caused by traditional
impulse-piping based systems.
Radar is another option for easy
and reliable level measurement
that has also seen many advancements
in technology to broaden
overall capabilities. DP level and
radar instruments are a mainstay
with producers today because
they can protect people, plants
and products. This article looks at
ways in which DP level and radar
have advanced in recent years to
deliver reliable performance for
critical level measurements.
46
FIGURE 1. Differential pressure (DP) has been a practical level measuring technique for a century or
more, but it is still being improved
Making DP more robust
Using a DP reading to determine
liquid depth is a proven and reliable
practice (Figure 1). Because of this,
DP level continues to be one of the
most common methods used to
make level measurements. Internal
tank structures, active agitation and
corrosive or viscous processes that
may cause challenges for alternative
level technologies are ideal applications
for DP level technology, which
delivers a repeatable, stable and accurate
measurement provided it is
installed correctly and product density
characteristics are understood.
DP challenges. But using DP to
measure fluid levels is not without
its challenges. Most problems relate
to the necessity of mounting the
head space pressure tap some distance
above the tank bottom, and
then connecting it to a DP transmitter
using impulse lines. Since those
impulse lines are connected directly
to the process, they are often a
source of headaches for any kind of
DP application, because they can
plug from accumulated debris, fill
with the wrong product (gas in liquid
lines and vice versa), and are
subject to freezing and other environmental
effects.
Remote seal systems. Remote diaphragm
seal systems that use capillary
tubes are one solution to some
of the DP level challenges. A capillary
tube is small-diameter, oil-filled tube
that is provided as part of a DP level
remote-seal system. It connects the
remote diaphragm seals back to the
pressure transmitter, which eliminates
the need for impulse piping.
This solves the main problems associated
with plugged impulse lines -
condensation in a dry leg or evaporation
in a wet - but it introduces
new trade-offs.
Balanced systems. There are two
ways to implement remote-seal capillary
tube systems. One creates a
balanced system using two capillaries
that are even in length on both
the high and low side to keep temperature
effects from creating errors
between the two readings. Unfortunately,
with balanced systems, the
difference in the height between the
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2020

http://WWW.CHEMENGONLINE.COM

Chemical Engineering February 2020

Table of Contents for the Digital Edition of Chemical Engineering February 2020