Chemical Engineering November 2011 - 45

Danube refinery
1,000,000
1,200,000
1,400,000
1,600,000
1,800,000
2,000,000
200,000
400,000
600,000
800,000
0.10
0.20
0.30
0.40
Additional profit generated, %
gral-derivative (PID) control output
operates the control valve built in the
loop. The DCS does not display the
true opening value of the control valve,
only what the PID controller needs.
Due to the contamination of the I/P
converter described above, the actual
opening value does not always coincide
with the requested opening value.
This failure can only be identified by
operating a diagnostics system.
If the difference between the true
opening and the requested opening
values is large, it has a fundamental
impact on the operation of the control
loop, too: it will cause control loop oscillation.
Initially, this does not cause
a major problem, as the control algorithm
is capable of compensating
this swaying for a while, but as time
passes, the disturbance in the loop
will grow, thus the amplitude of oscillation
may increase. At present, it is
impossible to exactly screen out every
control-valve defect or disturbance.
This process becomes traceable once
maintenance systems and intelligent
field devices are used. This oscillation
in the control loop then has a profound
influence on the quality of the manufactured
product, too.
The economic impact of diagnostics
on this problem was determined as
follows here:
* Bottom C4 content was taken at
1.5% on the average
* The above quantity could have been
reduced by an average of 0.5%, if a
diagnostics and preventive maintenance
system had been used
* The yield of useful products grows by
the difference in these two amounts
in linear progression
* Considering the mean sales price of
gasoline, the amount of lost profit
can be determined
If diagnostics and preventive maintenance
systems are used, the defects of
0.50
Average
Slovnaft
FIGURE 4.
According to
the calculations,
the average
amount of
profit lost from
inaccurate
control valve
settings is €1
million per
year
control valves can be detected in the
initial period. Thus the control loop in
question remains accurate.
In our case, no diagnostics were
placed at all on the two control loops
considered. The valves,
both in
Slovnaft and in the Danube refinery,
were older than 25 years. Even in
the case of one modern control valve
equipped with advanced diagnostics,
it is possible to report some savings,
due to the improved precision of the
control valve. This effect was also
included when we determined the
amount of lost profit (Figure 4).
According to the calculations, the
amount of average lost profit is obtained
as: €1,000,000/yr.
In this exercise we have proven the
usefulness of field-diagnostic-based
preventive maintenance with historical
data, and we have already realized
the cost savings of diagnostics
and preventive maintenance and have
validated our study.
■
Edited by Rebekkah Marshall
Authors
Gyula Ötvös is a maintenance
engineer in aromatics
and hydrogen production
at the MOL Group Danube
Refinery (gyotvos@mol.hu).
Ötvös is a specialist in control
valves and diagnostic systems
with more than six years experience
as an instrument
and process-automation engineer.
He is a graduate of the
Budapest Polytechnic Kandó
Kálmán Faculty of Electrical Engineering with
a specialty in instrument automation.
József Bartók is a maintenance
engineer in the MOL
Group Danube Refinery
maintenance department
(jbartok@mol.hu). Bartók
is a specialist in diagnostic
systems with more than nine
years experience in process
automation and related fields,
including field operation and
field devices diagnostics. He
is a graduate of the College of
Dunaújváros with a degree in engineering information
technology.
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Chemical Engineering November 2011

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