Using Frequency Converters in Automatic Process Control Systems of Crude Oil Distillation Plants

One of the most important tasks in oil refinery management is cost reduction, particularly the technological component of processing costs.
Application of frequency converters is one way to achieve this goal. An automatic process control system of the AT-2 crude oil distillation plant at Rosneft's Tuapsinsky Refinery is a prime example of such systems.
Frequency converters are used to control the following components:
Air coolers at the egress of benzene vapor from towers К-1, К-2 (see Fig. a);
Pumps for fuel oil pumping from tower К-2 (see Fig. b);
Pumps for supply of diesel oil circulating reflux (DCR) to tower К-2 (see Fig. c).
Frequency converters are controlled by the standard current signal 4-20мА control system.
The following problems are registered during air cooler operation without frequency converters:
Temperature fluctuation after using the air coolers;
Pressure fluctuation in the towers depending on the time of day and seasonal ambient temperature changes;
Necessity of manual redistribution of flows between sections in winter to prevent thawing of heat exchanger pipes;
Continuous level of power consumption irrespective of the ambient air temperature.
Application of frequency converters helped to significantly improve the quality of air cooler control by means of regulation of the fan rotation speed by eliminating negative factors which affected the operation of air coolers due to ambient temperature fluctuations. The dynamics of air cooler power consumption has also changed. In summer, at day time, when the ambient air temperature reached 30 C, the electric power consumption was virtually maximal. However, at night the power consumption decreased by 10-15 percent and in winter by 15-25 percent.
Two options of frequency converter control circuits were used for the pumps.
For pumps used to pump fuel oil, the control signal from the level regulator in tower К-2 came directly to the frequency converter control circuit and changed the speed of the pump's electric motor depending on the level. The quality of level control improved considerably. The fuel oil consumption fluctuation became smooth and low. Also, during unit operation at a decreased load, power consumption by the pump decreased by 15-20 percent.
Pumps used for supply of diesel oil circulating reflux (DCR) had a large productivity margin, and frequency converter control directly from the flow regulator was impossible as the pump capacity decreased below the reference value, which would result in pump stopage. Instead, a control signal was delivered to the pumps’ frequency converters maintaining pump power consumption at 60-65 percent of its maximum value, while a signal from the DCR flow regulator came to the regulating valve at the pump discharge.
Application of frequency converters brought the following results:
The stability of process conditions in the K-1 and K-2 columns was improved.
The electric power consumption was reduced.
The starting current value was reduced since a smooth electric motor acceleration mode has been provided in frequency converters.
Operating costs were reduced, as was the possibility of emergency situations due to a decrease of the electric motor load. As a consequence, a longer service life for bearings and other rotating components has been ensured.
It became possible to remove the control valve from the control circuit.

Source: www.krug2000.ru

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