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85-IGT-56

THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y. 10017 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA.

Copyright © 1985 by ASME

Evaluation of Gas Turbine/Compressor Control System Effectiveness Using Dynamic Simulation K. C. CHUI, N. E. POBANZ, and L. H. CHANG

Bechtel Petroleum, Inc. San Francisco, California

ABSTRACT In centrifugal compressor installations that use a variable speed drive such as gas turbine, speed control is an effective method of controlling capacity. Compressors are normally protected from surging by an antisurge control loop which recycles the discharged gas to the suction of the compressor. In an upset condition which results in flow reduction, the two controllers may interact whereby performance of the surge controller may be In this paper, a dynamic simulation compromised. technique is used to illustrate the interaction. NOMENCLATURE



H

• = =



P

R

T z

Q



N

•

F I Bc C

n k

,

,

Cout Fmax



h

K

,

b • = •

Polytropic head Molecular weight Pressure Gas constant Temperature Average compressibility Volumetric flow Compressor speed Mass flow rate Torque Axial moment of inertia Constants Polytropic coefficient Specific heat ratio Polytropic efficiency Time constants Normalized speed controller output Maximum torque available Laplace operator Delta pressure

Subscripts d t





•

suction discharge turbine

c f sl

compression friction surge line

INTRODUCTION In many centrifugal compressor installations, there is the necessity to provide antisurge protection of the machine as well as the ability to handle different gas flow rates. Generally, these two requirements are met with different control loops. In gas turbine/compressor installations, speed control is an effective method of controlling the gas flow rate (capacity control). The compressor is normally protected from surging by a control loop which recycles the discharged gas back to the suction side of the compressor (antisurge control). In an loops can two control these upset condition, In this paper, a dynamic simulation interact. technique is used to illustrate this interaction. The paper briefly describes the simulation technique, the physical system of interest, the control schemes and the simulated results for several upset situations.

DYNAMIC SIMULATION - AN ENGINEERING TOOL Definition Simulation is the process of creating a model which represents a physical system in a particular area of interest. In computer simulation the model is a mathematical model. The model is a set of equations relating the variables of interest for the physical system. The equations consist of algebraic and differential relationships whose origin comes from laws of physics and empirical data. The model is solved with the aid of a computer, e.g., analog, hybrid or digital. In dynamic computer simulation, both the steady state and the transient behavior are represented in the model.

Presented at the 1985 Beijing International Gas Turbine Symposium and Exposition Beijing, People's Republic of China — September 1-7, 1985

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Why Simulate

system that cannot prevent surging can lead to serious mechanical problems within the compressor or its piping system.

The primary objective of any simulation is to gain knowledge about how a physical system will behave before it has actually been built. This knowledge can then be applied to reduce overall costs of design, construction, start-up and operation of the system of interest. The simulation model can be used to: o o o o

Gas Compression System Figure 1 is a simplified schematic of a gas compression system to handle gas from an oil field. An incoming gas/water/oil mixture is separated and the gas is compressed for distribution to a central compression plant which further increases the pressure before being re-injected back into the oil reservoir.

evaluate/design control systems evaluate system behavior during abnormal upsets establish/verify start-up procedures train start-up and plant operators

Figure 2 shows a typical centrifugal compressor performance map. The family of speed curves is bounded on the left by the surge area and on the right by the stonewall area. During an upset condition that reduces the flow to or from the compression system, the operating point will move from the design point to the left along a speed curve. If the operating point moves into the surge area, the compressor will surge.

How It Is Done Generally, the steps taken to perform a successful simulation study are grouped into five (5) tasks. They are: 1. Definition of System/Mathematical Model Generation 2. Computer Implementation 3. Simulation Model Evaluation 4. System Design Evaluation/Recommendation 5. Documentation/Report Generation

Control System The speed control loop and the antisurge control loop are shown in Figure 1. Speed control is accomplished by throttling fuel gas to the gas turbine.

In Task the 1, boundaries of interest are established. The goals and objectives are clearly defined. The operating modes and limits are identified. The criteria of acceptable performance are documented. The types of upsets of interest are identified. The mathematical representations are prepared and assembled. All pertinent physical data are gathered.

The antisurge control loop is depicted by a flow-delta pressure control scheme which results in a control line that lies at a fixed distance to the right of the surge line as shown in Figure 2. The controller normally keeps the recycle valve closed but must open quickly when the operating point moves toward the surge area. The square of the surge flow is approximately linear with the pressure head across the compressor. ) Therefore:

In Task 2, the mathematical model is converted into computer language. The program is checked out. A simulated steady state operating condition is achieved and compared with known/expected operating conditions.

2 C)sl = K ( Pd - Ps

In Task 3, the simulation model is evaluated at transient conditions. The model behavior is analyzed and validated as representing the system performance. This validation involves comparison of past knowledge from similar systems, review of behavior by specialists, comparison with expected behavior of physical laws and review by personnel with many years of dynamic simulation experience.

(1)

)

For flow through an orifice Q2 0(h

(2)

Therefore, the surge line in term of measurable pressure is

In Task 4, the simulation model is used to perform the activities outlined in the Why Simulate section. A series of test runs are finally performed to document how the actual system is expected to behave. The results are analyzed.

h sl = K ( Pd

-

Ps)

(3)

The control line is set parallel to the right of the surge line by b. Therefore, the set point to the flow controller equals

In Task 5, the conclusions and recommendations regarding system performance are documented and submitted in a final report. The data from the test runs are assembled, edited and presented in report form. The final report also documents important aspects of the simulation activity.

[K(Pd

-

-

b]

(4)

and the other input is the delta pressure measurement of flow through the compressor.

SYSTEM DESCRIPTION The evaluation of a control system to keep a compressor out of surge has gained importance during the design phase of a compression system. A compressor will surge if the pressure developed by the unit is less than that in the downstream piping, resulting in flow reversal. An improperly designed 2

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Computer Simulation System

Figure 6 shows the effect of no speed control of the compressor. The compressor speed rises to a new steady value of 4433 rpm because of reduced flow. The minimum surge margin of the compressor is increased to 1090 ACFM.

The simulation model representing the gas compression system (Figure 1), was adapted from an earlier study on a Bechtel project. A description of the model and the mathematics involved for a compression system have been made before. 2,3 The appendix summarizes the mathematical model of the turbine/compressor. The simulation model also contained mathematical modules for the suction drum vessel, the cooler, control valves, block valve, check valve and the controllers.

Figure 7 is a comparison of the two runs plotted on the compressor performance map. In the upset run with tight speed control, the operating point A moves toward the surge line. The antisurge controller brings it back to the control line and the speed controller brings it to point B which is on the same speed curve as point A. Under these conditions, the speed increase and the discharge pressure increase are minimized at the expense of reduced surge margin.

The simulation model was programmed with Bechtel's Dynamic Analysis Program (DAP) which is a software package (FORTRAN base) developed by Bechte1. 4 It is most suitable for simulation of process and control system analysis. DAP enhances input/output capability, allows analyst interaction and facilitates graphic presentation. The flow chart for this application is illustrated by Figure 3. It consists of a series of subroutine CALLs representing the various modules of the system and the selected integration algorithm.

If the speed controller is de-activated (in MANUAL mode) during the upset, the operating point will move from A to C, which lies on the control line but is at a higher speed than point A. In so doing, the minimum surge margin is increased at the expense of a higher discharge pressure. The speed controller can then be re-activated (in AUTO mode) to bring the operating point from C to B. CONCLUSION

SIMULATION RESULTS

Dynamic simulation is a highly effective design tool to evaluate the design of complex centrifugal compressors systems. It can be used to define the parameters of the antisurge control loop as well as to describe the interaction of this loop with the capacity control loop. With such information, a design can be analyzed before start-up and necessary changes made.

Design Condition The simulation model was used to evaluate the compression system's responses to an equipment failure in the downstream of the compressor. This was simulated by a fast closure of the discharge block valve shown in Figure 1. This upset forces the antisurge controller to open the control valve to prevent the compressor from surging. A flow reduction tends to increase the speed of the turbine/compressor. However, the speed controller will bring the speed back to its set point value. Through this analysis, interaction between the two controllers can be evaluated.

REFERENCES

Figure 4 shows time histories of surge margin from the surge line, the outputs of the two controllers, and the compressor speed. Trace 1 (upper curve) shows the actual cubic feet per minute (ACFM) flow margin from surge. The minimum margin is 920 ACFM. If the trace becomes negative then the compressor is operating in the surge area. The results beyond this point in time do not reflect operation in the surge area. Therefore, this trace should only be used to determine if the compressor has surged or not. Trace 2 shows the output of the antisurge controller in percentage. Trace 3 shows the speed of the compressor in rpm and Trace 4 shows the speed controller output in percentage. The speed controller's setpoint is 4390 rpm.

1.

Staroselsky, N. and Ladin, L., "Improved Surge Control for Centrifugal Compressor." Chemical Engineering, May 21, 1979.

2.

Stanley, R. A. and Bohannan, W. R., "Dynamic Simulation of Centrifugal Compressor Systems," Proceedings of the Sixth Turbomachinery Symposium.

3.

"Tutorial Session on Practical Approach to Surge and Surge Control System," Proceedings of the Texas A&M Twelfth Turbomachinery Symposium, University, 1983.

4.

Carlson, A.M., "DAP - A Bechtel Approach to Dynamic Simulation," Summer Computer Simulation Conference, Denver, Colorado, 1983.

5. Pobanz, N. E., "Application of Dynamic Simulation to Antisurge Control," Proceedings of Control Expo '84, Chicago, Illinois, 1984.

The results reflect acceptable performance of a proportional plus integral (P+I) analog controller and the recycle valve which was specified using dynamic simulation. 5 These will serve as a reference for comparison. Speed Control Figure 5 shows the effect of tighter speed control of the compressor. Deviation of the speed transient is reduced. However, the minimum surge margin is reduced to 650 ACFM.

3

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( Initialization t = 0.0 Set t hold t max

OUT

IN

Compute Controller Outputs

Compute Valve Stem Positions Compressor Flow Rate Calculations

Gas Turbine

SUBROUTINE CALLS

Figure 1 - TYPICAL GAS TURBINE/COMPRESSOR SYSTEM

:4)-

//opy,:,

SURGE AREA

DESIGN POINT

120_

Turbine & Compressor Calculations

100

I ,

/

I I STONEWALL AREA

CONTROL LINE

110

40

BOO

80

I