Mario Cesar M. M. Campos, Paulo Sergio B. Rodrigues Petroleo Brasileiro
SA Rio de Janeiro
The control system originally designed for the fluid catalytic cracking unit (FCCU) compressor at Petroleo Brasileiro SA's (Petrobras) Presidente Bernardes refinery in Sao Paulo, Brazil, was inadequate.
The system required almost permanent flow recirculation to prevent surge.
An improved antisurge control strategy was implemented in mid-1990. Since then, the unit has operated without the former surge problems.
BACKGROUND
The FCCU at the Presidente Bernardes refinery employs a two-section centrifugal wet-gas compressor to compress the hydrocarbon gas mixture from the reactor. The original compressor system frequently went into surge despite near-continual flow recirculation.
Some of these surges were followed by a compressor shutdown. This situation was especially undesirable because the gas had to be burned, raising operational costs and polluting the atmosphere.
SYSTEM CHARACTERISTICS
Wet-gas compressors typically are two-section machines (i.e., two casings placed in series, driven by one shaft), with intercooling. The Petrobras system is no different (Fig. 1).
The compressor is turbine-driven, and its suction pressure was controlled by changing the rotation speed. Two other controllers provided independent antisurge protection for each section.
To preserve plant safety, suction pressure fluctuations were limited to a few percent, and the compressor control system had to account for this.
To supply gas flow at satisfactory consumption conditions, pressure was controlled somewhere downstream of the compressor by means of a control valve. The required set point kept the discharge pressure at relatively high levels. This high discharge pressure, combined with the suction-pressure control system, caused the compressor always to be in a potentially dangerous surge situation.
Perhaps the biggest problem in establishing good surge control for this system was in dealing with the large molecular weight changes that are experienced at compressor suction. These changes occur because the compressor is fed not only from the FCCU reactor, but also from other sources having distinct compositions.
Lastly, aside from the aforementioned difficulties arising from the nature of the system, there had been a previous agreement to use only pressure and flow measurements as inputs for the antisurge control strategy.
Furthermore, the flow had to be measured at the discharge side to make use of existing instrumentation. Such constraints were unavoidable because of the short time disposable for the system implementation.
COMPRESSOR CONTROL
In a previous paper, the authors described the equations used to accurately model centrifugal compressor performance (OGJ, Nov. 28, 1988, p. 75).
Centrifugal compressor performance depends mainly on service conditions, that is, suction pressure (PS), suction temperature (TS), discharge pressure (PD), and gas properties, which are determined by the isoentropic exponent, K, and the molecular weight, MW.
These values can be used to calculate the polytropic head (Hp) imposed on the compressor (see Equation 1, Equations box, and Nomenclature). Equation 1 contains the perfect gas constant, R, and a parameter,o/ , defined by Equation 2, in which rp represents the polytropic compression efficiency.
Head, combined with rotation speed (N), defines the operating point on the performance curve, as shown in Fig. 2. The inlet volumetric flow rate, Vs, therefore can be determined from the performance curves.
Surge is the result of an increase in the head imposed by the system beyond the maximum sustainable by the compressor, when running at a given rotation speed. If a line on the compressor map is used to determine the connection between these maxima, then the resulting curve frequently, but not always, resembles a parabolic form. This curve is known as the surge limit line (Fig. 2).
Surge also is related to the minimum stable flow condition for a given rotation speed. If the operating inlet volumetric flow rate (Vs) is greater than this limit, the compressor is in a safe condition.
Because surge can cause serious damage to the compressor, it must be prevented by automatic control. Partial recycle of the compressed flow is the control method universally used for this task. This method has the effect of preventing the head from increasing to an imminent surge condition.
The more the recycle valve is open, the more the compressor moves away from surge. But because bypassing the compressed gas wastes energy, the compressor operating point should be maintained as close as possible to the surge limit line. This, in short, was the problem the Petrobras refinery was experiencing.
CONTROL STRATEGIES
The choice of a compressor control strategy is independent of the centrifugal compressor specifications. These parameters change from one compressor to the next, but in this case, they are as shown in Table 1.
Several types of strategies have been used for antisurge protection. All of them start from a control line depicted on a performance curve, displaced to the right of the surge limit line and as parallel to it as possible (Fig. 3).
Theoretically, as long as recycle is open, the compressor operating point remains on this line. A percent stability margin, S-defined by Equation 3-is illustrated in Fig. 3. The challenge of the antisurge control designer is to adjust the system with a narrow safety margin, without sacrificing control reliability.
Compressor control does not affect the feeds, flow rates, or capacity of the FCCU; it only permits a more stable operation of the unit. The scope of this article, therefore, is limited to the implementation and operation of the improved control strategy.
ORIGINAL SYSTEM
The antisurge controller uses the control line as a means of surge interference. Although the polytropic head vs. inlet flow rate relationship is seen as the best way to represent the control line, it cannot be used directly in this sense.
Because of the complexity of measuring head and volumetric flow rate with the quickness and accuracy needed for this kind of control, another formulation must be considered.
The strategy formerly used in the Presidente Bernardes refinery is found frequently in industrial applications. It assumes that, if suction pressure, suction temperature, and gas composition are invariable, then a given constant differential pressure (dP) across a flow element located in the compressor's discharge pipe is a good indicator to be used for surge prevention.
The resulting control scheme can be seen in Fig. 4.
Considering the relationship given in Equation 4 and plotting the corresponding control line on a head-flow map, a curve shaped like a parabola is produced. And, if the surge limit line has the same conformation, a good compressor system adjustment can be obtained (Fig. 5).
Unfortunately, in the Petrobras plant, process characteristics were causing this strategy to fail. First, the surge limit lines for each compressor section have reversed forms, as seen in Fig. 6, and suffer dramatic influence from gas composition changes.
Moreover, gas composition and intermediate pressure (the suction pressure of the second section) are subject to variations when the process load changes. As a result, while the first section was poorly protected, not even that can be said of the second section.
By virtue of the inlet pressure changes, the second section's control line was assuming an undesirable behavior, as shown in Fig. 7. This can be explained by Equation 4, which shows that, for a lower inlet pressure and the same dP at the discharge pipe, a greater inlet flow rate is produced.
Because, in the Petrobras system, lower pressures are concurrent with lower rotation speed, the slope of the control line becomes inverted, causing the respective recycle valve to open very far from the surge condition.
Under these circumstances, the operator had no other choice than to reset the controller, thus displacing the control line, as shown in Fig. 8. But this did not solve the problem, because for higher rotation speeds, the compressor was left unprotected.
From a dynamic point of view, the new system also has some deficiencies. Because disturbances, in most cases, force compressors into surge conditions very quickly, an antisurge control system must be designed with emphasis on response speed. The full stroke time of recycle valves should not be greater than 2.0 sec.
The original system was very different from this because of not only valve sluggishness, but also control inadequacy. Even though the importance of these aspects cannot be disregarded, the scope of this article is limited to the control strategy.
NEW SYSTEM
The antisurge strategy selected for both compressor stages takes the form of Equation 5, where a and b are constants to be determined. Note that the dP measurement has to be made on the discharge pipe to meet one of the fundamental constraints.
Equation 5 is particularly appropriate to this system for the following reasons, which are illustrated by Fig. 9:
1. It fits properly with the compressor surge limit line. In fact, it was necessary to break down the control line into two segments, each corresponding to Equation 5. This method produced four constants per section, but the result was excellent.
2. The control line built this way adjusts itself satisfactorily to gas composition changes. In this case, the surge control line and surge limit line are subject to similar displacements, forcing each to remain close to the other. Control efficiency is essentially dependent on this feature.
3. The inclusion of the PS term in the left-side member of Equation 5 causes the control system to be independent of suction pressure, avoiding the anomalous condition presented in the past by the second stage.
SYSTEM IMPLEMENTATION
The electronic controllers originally installed in the system were replaced with modern digital equipment. The digital equipment has much higher operational capacity and is capable of receiving at least three analog inputs.
To attain proper operating conditions for the system, the following functions were implemented in each controller:
- Set point evaluation using Equation 5 and observing the difference between the two segments composing the curve
- Strategy devoted to achieving fast opening/slow closing of the recycle valve
- Antireset wind-up device
- Adjustment of static stability margin between 7% and 30%
- Protection scheme against equipment failure.
DYNAMIC SIMULATION
This was Petrobras' first practical job after the development of a dynamic simulation program for compressor systems. Although the specifics of the simulation and implementation processes are beyond the scope of this article, the results are noteworthy.
Dynamic simulation is normally necessary, or at least desirable, in antisurge protection design. Common objectives are the prediction of the controller settings and the evaluation of the minimal stability margin.
In this case, dynamic simulation was particularly important in analyzing the suction vessel pressure oscillations. As mentioned earlier, such oscillations are rigorously limited by safety requirements.
System operation was tested for hard disturbances, such as:
- Instantaneous dropping of 20% of the process load
- Instantaneous dropping of 12% of the disposable shaft power.
As a result of this analysis, static margins of 12% and 11% were recommended for the first and section sections, respectively. Considering the peculiarities of this plant, this can be regarded as very good.
RESULTS
The effectiveness of the antisurge protection system has been proven by two conditions. First, the protected machine cannot go into surge no matter what operational circumstances are encountered. Second, recycle flow must be zero unless the operating point is very close to the surge limit line.
Since the improvement, the system has operated without surge occurrences. Under partial load, recycle remains open without upsetting the system's efficiency, as it did in the past.
Taking energy consumption, machine repairs, and gas burning costs into consideration, the new control system has saved about $300,000/year.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
