Chemical Engineering November 2011 - 42

Feature Report
Control Valve Diagnostics
The proof is in the process, as exemplified in this former HART Plant of the Year
winner. This process economic evaluation helps frame the business case
Gyula Ötvös
and József Bartók
MOL Group Refining
he benefits of field-device
diagnostics are often acclaimed
in qualitative
terms, but on a much
lower frequency in quantitative
ones. Yet, an attractive
cost-benefit analysis is necessary
for almost any initiative to
make it off the drawing board
and into a plant. Even more
rare are economic figures that
come directly from the process
plant engineers (as opposed to
a vendor with vested interest
in selling diagnostic equipment
or systems).
Perhaps that state of affairs
T
FIGURE 1. Contaminated instrument air affects the I/P converter
was on the judges' minds when they
selected the MOL petroleum refinery
near Budapest, Hungary to receive
the 2010 HART Plant of the Year
Award. The application, which is the
basis for the analysis here, highlights
two areas where control-valve diagnostics
benefit the bottom line substantially:
Contamination of controlvalve
instrument air and inaccurate
control-valve settings.
In the first part of this article, the
operation and significance of control
valve diagnostics is explained, using
an existing problem of contaminated
instrument air. The newly developed,
diagnostics-based method that solved
the problem, is also briefly described.
Based on the success of the new
method, it was eventually ordered
across the entire operating area of the
Danube refinery.
The second part of the article investigates
the product loss caused by the
inaccuracy of control valve settings
and the economic impact that diagnostics
and preventive maintenance can
have in reducing lost product.
For the purpose of both studies, we
used technical data from the debutanizer
equipment of the fluid catalytic
cracking (FCC) units in the Danube
refinery and in the Bratislava refinery
of Slovnaft, MOL Group.
EFFECTS OF POLLUTED
INSTRUMENT AIR
In many process plants, such as petroleum
refineries, pneumatic control
valves are most often used as intervention
devices. The intervention device
is part of the automatic control
loop; in other words it is the element
executing the command issued by the
control unit. Control valves are incorporated
in pipe armatures or fittings
to regulate the quantity of the flowing
media. This way the control loop
can directly or indirectly influence the
defined parameters of the technology.
The term " pneumatic " refers to the
auxiliary drive (energy) of the intervention
device, that is, where a concerned
device applies energy drawn
from air pressure to move mechanical
elements. To drive these pneumatic
valves, so-called instrument air at 5
bars pressure is used.
42 CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2011
Control valve operation
In the Danube refinery described
here,
intellegent, electro-pneumatic
positioners are used to operate control
valves (Figure 1). The phrase
electro-pneumatic means that the
device generates a pressure value
from an electrical signal. Meanwhile,
the word intelligent describes the information
available through microcontroller
based devices, which have
arisen in the digital age. In fact, the
positioners manufactured in the past
10-15 years all possess intelligent
electronics. This observation will be
important later on.
One of the most important elements
of the positioner is the device converting
the electrical signal into a pressure
signal; in what follows, this element
will be called the I/P converter.
The signals received from the travel
sensor attached to the valve shaft -
a potentiometer in most cases - and
the distributed control system (DCS)
are compared by the electronics of
the positioner, which then sends out
the electrical signal with the proper
magnitude (typically in the 1-2-mA
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