Methodology and Algorithms for Protecting Centrifugal and Axial Compressors from Surge and Choke

Abstract

This disclosure describes a novel methodology for anti-surge and anti-choke control systems protecting centrifugal and axial compressors. The methodology, based on Buckingham's π-theorem for compressors, presents compressor performance maps in dimensionless rectangular π-term coordinates that are independent of compressor inlet conditions, fluid molecular weight and rotational speed. The full range of compressor operating points from surge to choke is monitored and controlled when surge and choke limits are available. This is accomplished by converting rectangular coordinates presented in π-terms to polar coordinates, and then converting them to a controlled variable used in the closed-loop controllers. The methodology provides control algorithms for variable speed compressors, variable geometry compressors equipped with inlet guide vanes or stator vanes that exhibit displacement of surge and choke limits. The methodology most accurately estimates the location of the operating point relative to its limit in polar coordinates if only the surge or choke limit is available. The presented protection methods are applicable to any known types of dynamic compressors for industrial, commercial, jet engines, turbochargers.

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19. A method for controlling the operation of a centrifugal or axial compressor equipped with automatic control systems that continuously calculate system parameters, said method comprising: reading one or more input signals from one or more sensors; converting a compressor performance map comprising one or more surge points which define a first boundary condition into a compressor flow function in rectangular coordinates of flow Mach number and total pressure ratio; applying said compressor flow function to each of said one or more surge points; converting said one or more surge points of said first boundary condition to polar coordinates at a constant angle; measuring an operating point of the centrifugal or axial compressor via said input signals from said one or more sensors; calculating a control variable in polar coordinates; calculating an error value from a difference between a set point and said control variable in polar coordinates; and sending a control signal to a compressor control mechanism such that said control variable is moved closer to said set point to reduce said error value.

20. The method of claim 19, wherein said compressor flow function comprises a total pressure ratio function; and applying said total pressure ratio function to each of said one or more surge points to define an altered coordinate of flow Mach number.

21. The method of claim 19, wherein said compressor flow function comprises a flow Mach number function; and applying said flow Mach number function to each of said one or more surge points to define an altered coordinate of total pressure ratio.

22. The method of claim 19, wherein said compressor control mechanism comprises a mechanism selected from the group consisting of an anti-surge valve and an outlet valve.

23. A method for controlling the operation of a centrifugal or axial compressor equipped with automatic control systems that continuously calculate system parameters, said method comprising: reading one or more input signals from one or more sensors; converting a compressor performance map comprising one or more choke points which define a second boundary condition into a compressor flow function in rectangular coordinates of flow Mach number and total pressure ratio; applying said compressor flow function to each of said one or more choke points; converting said one or more choke points of said second boundary condition to polar coordinates at a constant angle; measuring an operating point of the centrifugal or axial compressor via said input signals from said one or more sensors; calculating a control variable in polar coordinates; calculating an error value from a difference between a set point and said control variable in polar coordinates; and sending a control signal to a compressor control mechanism such that said control variable is moved closer to said set point to reduce said error value.

24. The method of claim 23, wherein said compressor flow function comprises a total pressure ratio function; and applying said total pressure ratio function to each of said one or more choke points to define an altered coordinate of flow Mach number.

25. The method of claim 23, wherein said compressor flow function comprises a flow Mach number function; and applying said flow Mach number function to each of said one or more choke points to define an altered coordinate of total pressure ratio.

26. The method of claim 23, wherein said compressor control mechanism comprises a mechanism selected from the group consisting of an outlet valve, a variable inlet guide vane controller, and a variable stator vane controller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 schematically shows the input and outlet signals; gas properties at the compressor inlet.

[0026] FIG. 2 shows the different shapes of the compressor performance curves.

[0027] FIG. 3 depicts the operating point in the π-term coordinates between the surge and choke limits on a constant speed line, with rays emanating from the pole indicating possible movements.

[0028] FIG. 4 shows a set, of hypothetical constant speed performance curves with operating point, surge and choke limits.

[0029] FIG. 5 depicts the effect of transformation by plotting polar coordinates r and φ on the rectangular coordinates.

[0030] FIG. 6 represents the correlation between the controlled variable CV (%) and the polytropic efficiency of the compressor.

[0031] FIG. 7 schematically shows two dynamic compressors 14 and 15 in series.

[0032] FIG. 8 shows three sets of four hypothetical constant speed performance curves for three different IGV positions.

[0033] FIG. 9 describes the IGV function.

[0034] FIG. 10 reflects the effect of transforming the horizontal axis using the IGV function.

[0035] FIG. 11 depicts the effect of transformation by plotting polar coordinates r and φ on the rectangular coordinates for variable geometry compressors.

[0036] FIG. 12 shows a set of hypothetical constant speed performance curves with operating point, surge line, and maximum flow points with rays emanating from the pole indicating the polar coordinate α of the surge points.

[0037] FIG. 13 represents the modified compressor map with a constant angular coordinate α.

[0038] FIG. 14 depicts the effect of transforming by plotting polar coordinates r and α on rectangular coordinates with only surge limit available.

[0039] FIG. 15 shows the most common surge line shape in rectangular coordinates when the horizontal axis is the π-term Mach number at outlet of the compressor.

[0040] FIG. 16 depicts a modified surge line in polar coordinates with a constant angular coordinate γ.

[0041] FIG. 17 shows a set of hypothetical performance curves for a constant speed, variable geometry compressor with surge line and maximum flow endpoints.

[0042] FIG. 18 shows the effect of transforming the horizontal axis using the IGV function and the rays emanating from the pole, indicating the polar coordinate α of the surge points.

[0043] FIG. 19 shows a modified surge line with constant polar coordinate a for variable geometry compressors.

[0044] FIG. 20 depicts the effect of transformation by plotting polar coordinates r and α on rectangular coordinates for variable geometry compressors for which only surge limit is available.

DESCRIPTION OF INVENTION

[0045] FIG. 1 shows a schematic diagram of dynamic compressor 1 and most common input signals: measured flow rate 2, static inlet pressure 3, static inlet temperature 4, static output pressure 5, static output temperature 6, rotational speed 7, position of the inlet guide vanes or stator vanes 8; fluid properties 9, 10, 11; calculated axial fluid velocities 12, 13 used in control algorithms.

[0046] There are a number of dimensionless groups (π-terms) that can be obtained from Buckingham's π-theorem applied to compressors, but the most commonly chosen π-terms are Mach number Π.sub.1, and compressor total pressure ratio Π.sub.2. Both of these π-terms are used in present invention. The performance of dynamic compressors may be described by following quantities: [0047] m Fluid mass flow [0048] N Rotor rotational speed, usually measured as revolution per minute (RPM) [0049] V Axial fluid velocity at the compressor inlet or outlet depending on the location of the flow meter [0050] α The speed of sound at the inlet or outlet of the compressor [0051] Mw Fluid molecular weight [0052] k Specific heat ratio [0053] Z Fluid compressibility factor [0054] R.sub.0 Universal gas constant [0055] ρ Density of fluid at the compressor inlet (or outlet) [0056] D Linear dimension of a compressor or piping characteristic [0057] P.sub.t_in Total or stagnation pressure at compressor inlet [0058] T.sub.t_in Total or stagnation temperature at compressor inlet [0059] P.sub.t_out Total or stagnation pressure at compressor outlet [0060] T.sub.t_out Total or stagnation temperature at compressor outlet

[0061] Where:

[00005]$\begin{array}{cc}V=\frac{4.Math.m}{\rho .Math.\pi .Math.D^2}V=V_{\mathrm{in}}\mathrm{and}\rho ={\rho }_{\mathrm{in}}& \left(6\right)\end{array}$

if the flow meter located at the inlet, V=V.sub.out and ρ=ρ.sub.out if the flow meter located at the outlet; π is a mathematical constant of approximately 3.14; D is the diameter of the cross-section area at compressor inlet (D.sub.in.sup.2) or outlet (D.sub.out.sup.2).

[0062] For compressor inlet:

[00006]$\begin{array}{cc}{\rho }_{\mathrm{in}}=\frac{\mathrm{Mw}.Math.P_{t\_\mathrm{in}}}{Z_{\mathrm{in}}.Math.R_0.Math.T_{t\_\mathrm{in}}}& \left(7\right)\end{array}$$\begin{array}{cc}{a}_{\mathrm{in}}=\sqrt{\frac{k_{\mathrm{in}}.Math.R_0.Math.T_{t\_\mathrm{in}}}{\mathrm{Mw}}}& \left(8\right)\end{array}$$\begin{array}{cc}{k}_{\mathrm{in}}=\frac{C_P}{C_v}& \left(9\right)\end{array}$

[0063] Mach number at compressor inlet:

[00007]$\begin{array}{cc}{\Pi }_{1_{\mathrm{\_in}}}=\frac{V_{\mathrm{in}}}{a_{\mathrm{in}}}& \left(10\right)\end{array}$

[0064] Mach number at compressor outlet:

[00008]$\begin{array}{cc}{\Pi }_{1_{\_\mathrm{out}}}=\frac{V_{\mathrm{out}}}{a_{\mathrm{out}}}& \left(11\right)\end{array}$

[0065] Compressor pressure ratio (total to total):

[00009]$\begin{array}{cc}{\Pi }_2=\frac{P_{t\_\mathrm{out}}}{P_{t\_\mathrm{in}}}& \left(12\right)\end{array}$

[0066] And then:

[00010]$\begin{array}{cc}{T}_{t\_\mathrm{in}}=T_{in}.Math.\left(1+\frac{k-1}{2}.Math.{\Pi }_{1\_\mathrm{in}}^2\right)& \left(13\right)\end{array}$

where T.sub.in—static temperature at the compressor inlet in absolute units.

[00011]$\begin{array}{cc}{T}_{t\_\mathrm{out}}=T_{\mathrm{out}}.Math.\left(1+\frac{k-1}{2}.Math.{\Pi }_{1\_\mathrm{out}}^2\right)& \left(14\right)\end{array}$

where T.sub.out—static temperature at the compressor outlet in absolute units.

[0067] For incompressible flow:

[00012]$\begin{array}{cc}{P}_{t\_\mathrm{in}}=P_{in}.Math.\left(1+\frac{k}{2}.Math.{\Pi }_{1\_\mathrm{in}}^2\right)& \left(15\right)\end{array}$

where P.sub.in—static pressure at the compressor inlet in absolute units, Π.sub.1_out—Mach number at compressor inlet ≤0.3.

[00013]$\begin{array}{cc}{P}_{t\_\mathrm{out}}=P_{out}.Math.\left(1+\frac{k}{2}.Math.{\Pi }_{1\_\mathrm{out}}^2\right)& \left(16\right)\end{array}$

where P.sub.out—static pressure at the compressor outlet in absolute units, Π.sub.1_out—Mach number at compressor outlet ≤0.3.

[0068] Whenever the Mach number in the stream exceeds about 0.3, the stream becomes compressible and the density of the fluid can no longer be considered as constant.

[0069] For compressible flow:

[00014]$\begin{array}{cc}{P}_{t\_\mathrm{in}}=P_{in}.Math.{\left(1+\frac{k-1}{2}.Math.{\Pi }_{1\_\mathrm{in}}^2\right)}^{\frac{k}{k-1}}& \left(17\right)\end{array}$

where P.sub.in—static pressure at the compressor inlet in absolute units, Π.sub.1_in—Mach number at compressor inlet >0.3.

[00015]$\begin{array}{cc}{P}_{t\_\mathrm{out}}=P_{out}.Math.{\left(1+\frac{k-1}{2}.Math.{\Pi }_{1\_\mathrm{out}}^2\right)}^{\frac{k}{k-1}}& \left(18\right)\end{array}$

where P.sub.out—static pressure at the compressor outlet in absolute units, Π.sub.1_out—Mach number at compressor outlet >0.3.

[0070] The relationship between the Mach numbers at the inlet and outlet of the compressor, given that Z.sub.in≅Z.sub.out and k.sub.in≅k.sub.out, follows from the equation:

[00016]$\begin{array}{cc}{\Pi }_{1\_\mathrm{in}}=\frac{D_{\mathrm{out}}^2}{D_{\mathrm{in}}^2}.Math.{\Pi }_{1\_\mathrm{out}}.Math.{\left({\Pi }_2\right)}^{\left(1-\frac{n-1}{2.Math.n}\right)}& \left(19\right)\end{array}$

[0071] Where n is the polytropic exponent, which can be calculated using the equation:

[00017]$\begin{array}{cc}n={\left(1-\frac{\left(\frac{T_{t\_\mathrm{out}}}{T_{t\_\mathrm{in}}}\right)}{\left(\frac{P_{t\_\mathrm{out}}}{P_{t\_\mathrm{in}}}\right)}\right)}^{\left(-1\right)}& \left(20\right)\end{array}$

[0072] With a moderate change in friction in the system, n changes insignificantly and can be taken in calculations as a constant.

[0073] In applications where differential pressure meters are used the inlet Mach number Π.sub.1_in can be calculated from the equation:

[00018]$\begin{array}{cc}{\Pi }_{1\_\mathrm{in}}=\frac{4.Math.\mathrm{Const}}{\pi .Math.D_{\mathrm{in}}^2.Math.\sqrt{k_{\mathrm{in}}}}.Math.\sqrt{\frac{\Delta P_{\mathrm{in}}}{P_{\mathrm{in}}}}& \left(21\right)\end{array}$

where ΔP.sub.in—is the pressure drop across of the flow meter at the inlet to the compressor, P.sub.in is the static pressure at the compressor inlet in absolute units, Const is the flow meter constant, π is a mathematical constant of approximately 3.14, D.sub.in—internal diameter of the inlet pipe.

[0074] Typical performance curves of dynamic variable speed compressors without guide vanes are shown in FIG. 2 in π-terms, with shape of the curves varying from slope to horizon for compressors with a nominal compression ratio of about 1.1 to 2.5, intermediate curves for compressors with a nominal compression ratio of 2.5 to 6.0, and relatively straight vertical lines, for compressors with compression ratio higher than 6.0. It should be noted that compressors equipped with guide vanes can have relatively vertical lines at a compression ratio of about 1.1 to 2.5 with the blades closed. FIG. 3 displays a constant speed performance curve in the π-term coordinates: Π.sub.1_in, is the Mach number at compressor inlet, and (Π.sub.2−1) is the compressor pressure ratio (total to total) minus one; operating point between surge point A and choke point B; rays emanating from the zero point, indicating possible movements of the operating point; r.sub.surge is the radial coordinate and φ.sub.surge is the angular coordinate of the surge point; r.sub.choke is the radial coordinate and φ.sub.choke is the angular coordinate of the choke point. The technique of converting the constant speed performance curve from rectangular coordinates to polar coordinates is based on the assumption that the distance from the zero point to the surge point A and the distance from zero point to the choke point B are equal. To equalize two unequal radial coordinates, a polar conversion factor P is entered. The distance from the zero point to the surge point A can then be calculated using the polar conversion factor:

r.sub.surge=√{square root over ((Π.sub.2−1).sub.A.sup.2+(P.Math.Π.sub.1_in).sub.A.sup.2)}  (22)

where (Π.sub.2−1).sub.A and (Π.sub.1_in).sub.A—coordinates of the surge point A.

[0075] The distance from the zero point to the choke point B can also be calculated using the polar conversion factor:

r.sub.choke=√{square root over ((Π.sub.2−1).sub.B.sup.2+(P.Math.Π.sub.1_in).sub.B.sup.2)}  (23)

(Π.sub.2−1).sub.B and (Π.sub.1_in).sub.B—coordinates of the choke point B.

[0076] From the two equations (22) and (23), assuming r.sub.surge=r.sub.choke the polar conversion factor P for the AB constant speed performance curve can be calculated as:

[00019]$\begin{array}{cc}P=\sqrt{\frac{{\left({\Pi }_2-1\right)}_A^2-{\left({\Pi }_2-1\right)}_B^2}{{\left({\Pi }_{1\_\mathrm{in}}\right)}_B^2-{\left({\Pi }_{1\_\mathrm{in}}\right)}_A^2}}& \left(24\right)\end{array}$

[0077] FIG. 4 shows a set of hypothetical constant speed performance curves with points A.sub.1, A.sub.2, A.sub.3 . . . A.sub.n−1, A.sub.n and A.sub.n+1 as surge points; and points B.sub.1, B.sub.2, B.sub.3 . . . . B.sub.n−1, B.sub.n and B.sub.n+1 as choke points. Performance curves may differ from each another, but curves formed by the same compressor, operating in a moderately narrow range with little change in system friction, should be correlated in shape. For this reason, the polar conversion factor P shouldn't change much. However, the polar conversion factor must be calculated for each given curve shown in FIG. 4. Then the arithmetic means or average of polar conversion factors, the sum of the polar conversion factors divided by total number of curves in the set (n+1), must be used to convert a rectangular coordinate system in a polar coordinate system.

[00020]$\begin{array}{cc}{P}_{\mathrm{mean}\_\mathrm{average}}=\frac{P_1+P_2+P_3+.Math.+P_{n-1}+P_n+P_{n+1}}{n+1}& \left(25\right)\end{array}$

[0078] In an imaginary two-dimension polar coordinate system on the plane, each point corresponds to a pair of polar coordinates (r, φ). The operating point, located on the constant speed curve A.sub.nB.sub.n as shown in FIG. 4, has a radial coordinate r.sub.op and an angular coordinate φ.sub.op, which is measured from the vertical axis (Π.sub.2−1). An example of the polar conversion of the constant speed performance curves and positioning of the operating point on the A.sub.nB.sub.n, curve of FIG. 4 is shown in FIG. 5, where the new modified constant speed curves and the operating point on the line A.sub.nB.sub.n are shown in the polar coordinates plotted on the rectangular coordinates with the vertical axis as the radial coordinate r, and the horizontal axis as the angular coordinate φ. The polar coordinates plotted on the rectangular coordinates, as shown in FIG. 5, illustrate the transformation effect, where the compressor performance curves are straightened and can now be approximated by horizontal lines, and the shaded area surrounded by the surge and choke limiting lines defines the dynamically stable area of the compressor operation.

[0079] The equations for calculating of a pair of polar coordinates (r, φ) for each point are shown below:

[00021]$\begin{array}{cc}r=\sqrt{{\left({\Pi }_2-1\right)}^2+{\left(P_{\mathrm{mean}\_\mathrm{average}}.Math.{\Pi }_{1\_\mathrm{in}}\right)}^2}& \left(26\right)\end{array}$$\begin{array}{cc}\phi =\mathrm{ARCTAN}\left(\frac{P_{\mathrm{mean}\_\mathrm{average}}.Math.{\Pi }_{1\_\mathrm{in}}}{\left({\Pi }_2-1\right)}\right)& \left(27\right)\end{array}$

[0080] Where ARCTAN is the inverse mathematical function of the tangent function used to obtain an angle from any of the trigonometric angular relations.

[0081] The functions shown below in tabular form in TABLE 1 with sorted rows and columns of characteristic data represent the polar angles of the surge and choke points as functions of the radial coordinate r as an argument.

TABLE-US-00001 TABLE 1 Radial Polar angle of Polar angle of coordinate surge point choke point 1 (r).sub.1 (φ.sub.surge).sub.1 (φ.sub.choke).sub.1 INPUT (r.sub.op) .fwdarw. 2 (r).sub.2 (φ.sub.surge).sub.2 (φ.sub.choke).sub.2 3 (r).sub.3 (φ.sub.surge).sub.3 (φ.sub.choke).sub.3 n − 1 (r).sub.n−1 (φ.sub.surge).sub.n−1 (φ.sub.choke).sub.n−1 n (r).sub.n (φ.sub.surge).sub.n (φ.sub.choke).sub.n n + 1 (r).sub.n+1 (φ.sub.surge).sub.n+1 (φ.sub.choke).sub.n+1 ↓ ↓ OUT1 (φ.sub.surge) OUT2 (φ.sub.choke)

[0082] The definition of the functions is taken from FIG. 5 when the shaded area is crossed by the horizontal lines r at the indicated surge and choke points used to populate TABLE 1.

[0083] It should be noted that all points other than those inserted in the rows and columns can be considered interpolated values. Linear interpolation is applied to a specific value between the two values listed in the table, which can be achieved by geometric reconstruction of a straight line between two adjacent points in the table.

[0084] The use of table functions is that the input to the table is the radial coordinate of the operating point r.sub.op calculated from to the equation (26), and the outputs are the angular coordinates φ.sub.surge and φ.sub.choke of the surge and choke points. The graphical definition of the functions is shown in FIG. 5 where the shaded area crossed by the horizontal line r.sub.op and the angular coordinates φ.sub.surge and φ.sub.choke are the projections of the surge and choke points onto the horizontal axis φ with the operating point φ.sub.op between them as the calculated value according to the equation (27). Therefore, the controlled variable CV (%) in percent for surge protection is calculated as:

[00022]$\begin{array}{cc}{CV\left(\%\right)=100\%.Math.\frac{{\phi }_{\mathrm{op}}-{\phi }_{\mathrm{surge}}}{{\phi }_{\mathrm{choke}}-{\phi }_{\mathrm{surge}}}.Math.}_{r_{\mathrm{op}}}& \left(28\right)\end{array}$

and for chock protection:

[00023]$\begin{array}{cc}{CV\left(\%\right)=100\%.Math.\frac{{\phi }_{\mathrm{choke}}-{\phi }_{\mathrm{op}}}{{\phi }_{\mathrm{choke}}-{\phi }_{\mathrm{surge}}}.Math.}_{r_{\mathrm{op}}}& \left(29\right)\end{array}$

[0085] The shape of the constant speed performance curves can change from compressor to compressor or as the compressor operating range expands. However, the conversion method represented by equations (26) and (27) is applicable to any shape of performance curve. The compressor performance curves shown in FIG. 2, which have significant shape variations, will still be straightened when plotted in rectangular coordinates r versus φ, and the appearance of the modified constant speed performance curves can be approximated by horizontal lines.

[0086] The generalized correlation between the controlled variable CV (%) in percent and the polytropic efficiency of the compressor η.sub.p in percent covering entire operating range from surge to choke limits, is shown in FIG. 6. The offset, between CV(η.sub.p_max)—the compressor's maximum polytropic efficiency point and CV(η.sub.p_op) of the operating point can be used in PID controllers as a controlled variable to balance the load between the compressors operating in parallel or in series.

[0087] FIG. 7 shows a schematic diagram of two dynamic compressors 14 and 15 installed in series, the second compressor 15 having a side stream inlet flow. The input signals shown in FIG. 7: rotational speed 24, static pressure at the inlet of the first compressor 16, static temperature at the inlet of the first compressor 17, measured flow rate at the inlet of the first compressor 18, calculated mass flow rate of the first compressor 19, static inlet pressure of the second compressor 20, static inlet temperature of the second compressor 21, measured side stream flow rate entering the second compressor 22, calculated mass flow of the side stream entering the second compressor 23, static output pressure 25, static output temperature 26.

[0088] The total mass flow m.sub.total through the second compressor 15 is then calculated as the sum of the mass flow m.sub.1 through the first compressor 14 plus the side stream mass flow m.sub.2 entering between compressors:

m.sub.total=m.sub.1+m.sub.2  (30)

[0089] To protect the second compressor, the Mach number (Π.sub.1_in).sub.2_total for the second stage must be used, which is calculated from the total mass flow m.sub.total, assuming that this mass flow passes through the inlet of the second compressor. For differential pressure meters, taking into account that compressibility factors and specific heat ratios of the first and second compressors are equal Z.sub.1≅Z.sub.2 and k.sub.1≅k.sub.2 the Mach number (Π.sub.1_in).sub.2_total can be calculated as:

[00024]$\begin{array}{cc}{\left({\Pi }_{1\_\mathrm{in}}\right)}_{2\_\mathrm{total}}=\frac{D_1^2}{D_2^2}.Math.{\left({\Pi }_2\right)}_1^{\left(\frac{n-1}{2.Math.n}-1\right)}.Math.{\left({\Pi }_{1\_\mathrm{in}}\right)}_1+{\left({\Pi }_{1\_\mathrm{in}}\right)}_2& \left(31\right)\end{array}$

[0090] Where D.sub.1 is the diameter of the cross-section area at the inlet of the first compressor; D.sub.2—cross-section diameter at the inlet of the second compressor; (Π.sub.2).sub.1—pressure ratio across the first compressor, calculated according to equation (12); n is the polytropic exponent of the first compressor, it can be taken as a constant or calculated by equation (20); (Π.sub.1_in).sub.1—Mach number at the inlet of the first compressor and (Π.sub.1_in).sub.2—Mach number of the side stream of the second compressor, both calculated according to equation (21).

[0091] In many cases, variable geometry compressors with the IGV inlet guide vanes or stator vanes in axial compressors are used. Compressors of this type can have performance drift depending on the blades opening. The effect of IGV opening on the compressor performance is shown in FIG. 8 for three hypothetical sets of constant speed performance curves representing three arbitrary selected IGV opening positions 0%, 50% and 100% in coordinates Π.sub.1_in relative to (Π.sub.2−1). Each set consists of four curves for demonstrative purposes. Lines A.sub.1B.sub.1, A.sub.2B.sub.2, A.sub.3B.sub.3 and A.sub.4B.sub.4 represent constant speed performance curves at 0% IGV position; lines A′.sub.1B′.sub.1, A.sub.2′B′.sub.2, A′.sub.3B′.sub.3 and A′.sub.4B′.sub.4 represent constant speed performance curves at 50% IGV position; and lines A″.sub.1B″.sub.1, A″.sub.2B″.sub.2, A″.sub.3B″.sub.3 and A″.sub.4B″.sub.4 represent constant speed performance curves at 100% IGV position.

[0092] FIG. 8 graphically illustrates a technique for adapting three separate surge limiting lines of three different IGV positions into one common surge line. Points A.sub.3, A′.sub.3 and A″.sub.3 in the FIG. 8 refer to the same compressor speed selected as the assumed design operating speed. The points are chosen as an example to obtain the function ƒ(IGV) of the inlet guide vanes to modify the n-term coordinate of the Mach number. The shift of the surge points A.sub.3, A′.sub.3 and A″.sub.3 to the left while keeping their (Π.sub.2−1) coordinates unchanged, denotes the new positions of the surge points A.sub.3com, A′.sub.3com and A″.sub.3com, which form one common surge line.

[0093] Dividing the coordinates of the surge points A.sub.3com, A′.sub.3com and A″.sub.3com into the values of the coordinates of the surge points A.sub.3, A′.sub.3 and A″.sub.3, respectively, reveals the method for constructing the IGV function:

[00025]$\begin{array}{cc}f\left(IGV\right)=\frac{{\left({\Pi }_{1\_\mathrm{in}}\right)}_{A\_\mathrm{com}}}{{\left({\Pi }_{1\_\mathrm{in}}\right)}_A}& \left(32\right)\end{array}$

[0094] FIG. 9 shows the position of the inlet guide vanes IGV as a percentage relative to the IGV function and depicts the technique, for calculating the IGV function. The function shown below in tabular form in TABLE 2 with two columns of characteristic data for an IGV position as the argument from 0% to 100% and the function ƒ(IGV) of all available surge points of design operating speed to plot the expected common surge line.

TABLE-US-00002 TABLE 2 IGV position function % ƒ(IGV) 1  0% [00026]$\frac{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{0\%\_\mathrm{com}}}{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{0\%}}$ INPUT (IGV) .fwdarw. 2  10% [00027]$\frac{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{10\%\_\mathrm{com}}}{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{10\%}}$  50% [00028]$\frac{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{50\%\_\mathrm{com}}}{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{50\%}}$ i-1  90% [00029]$\frac{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{90\%\_\mathrm{com}}}{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{90\%}}$ i 100% [00030]$\frac{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{100\%\_\mathrm{com}}}{{\left({.Math.}_{1\_\mathrm{in}}\right)}_{100\%}}$ ↓ OUTPUT (ƒ(IGV)

[0095] The result of applying the inlet guide vanes function to three sets of constant speed performance curves for three IGV opening positions in FIG. 8 is presented in FIG. 10, which shows one combined set of all constant speed, performance curves in the ƒ(IGV).Math.Π.sub.1_in if coordinate relative to the (Π.sub.2−1) coordinate. Where all surge points can be approximated by a single surge line, and also all choke points can be aligned to form a choke line.

[0096] The same method of converting constant speed performance curves from rectangular to polar coordinates can now be applied to compressors with IGVs, provided that the π-term coordinate Π.sub.1_in is replaced by the new coordinate ƒ(IGV).Math.Π.sub.1_in. An equal distance statement stating that the distance from the zero point to the surge point A, and the distance from zero point to the choke point B for each performance curve in FIG. 10 is still required for coordinate conversion. To equalize the two unequal radial coordinates of the surge and choke points, it is also necessary to calculate the polar conversion factor P.

[0097] The distance from the zero point to each surge point A can then be calculated as:

r.sub.surge=√{square root over ((Π.sub.2−1).sub.A.sup.2+(P.Math.ƒ(IGV).Math.Π.sub.1_in).sub.A.sup.2)}  (33)

where (Π.sub.2−1).sub.A and (ƒ(IGV).Math.Π.sub.1_in).sub.A—coordinates of the surge points A.

[0098] The distance from the zero point to each choke point B can be calculated as:

r.sub.choke=√{square root over ((Π.sub.2−1).sub.B.sup.2+(P.Math.ƒ(IGV).Math.Π.sub.1_in).sub.B.sup.2)}  (34)

where (Π.sub.2−1).sub.B and (ƒ(IGV).Math.Π.sub.1_in).sub.B—coordinates of the choke points B.

[0099] From the two equations (33) and (34), by assigning r.sub.surge=r.sub.choke, the polar conversion factor P for each constant speed performance curve AB can be calculated as:

[00031]$\begin{array}{cc}P=\sqrt{\frac{{\left({\Pi }_2-1\right)}_A^2-{\left({\Pi }_2-1\right)}_B^2}{{\left(f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}\right)}_B^2-{\left(f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}\right)}_A^2}}& \left(35\right)\end{array}$

[0100] After the polar conversion factors have been calculated for each curve, it is necessary to calculate the arithmetic means or average of the polar conversion factors, the sum of the polar conversion factors divided by the total number of curves in the sets (m+1):

[00032]$\begin{array}{cc}{P}_{\mathrm{mean}\_\mathrm{average}}=\frac{P_1+P_2+P_3+.Math.+P_{m-1}+P_m+P_{m+1}}{m+1}& \left(36\right)\end{array}$

[0101] FIG. 11 illustrates the transformation effect, where the polar coordinates r and φ are plotted again in rectangular coordinates with straightened compressor performance curves, and the shaded area bounded by the surge and choke limiting lines defines the compressor operating area. The operating point in FIG. 11 is defined by the radial coordinate r.sub.op and the angular coordinate φ.sub.op. As before in this invention, each point in the two-dimension polar coordinate system on the plane has a pair of polar coordinates (r, φ), but the equations for calculating the polar coordinates (r, φ) with respect to the IGV function are adjusted as shown below:

[00033]$\begin{array}{cc}r=\sqrt{{\left({\Pi }_2-1\right)}^2+{\left(P_{\mathrm{mean}\_\mathrm{average}}.Math.f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}\right)}^2}& \left(37\right)\end{array}$$\begin{array}{cc}\phi =\mathrm{ARCTAN}\left(\frac{P_{\mathrm{mean}\_\mathrm{average}}.Math.f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}}{\left({\Pi }_2-1\right)}\right)& \left(38\right)\end{array}$

[0102] TABLE 1 can now be filled with surge and choke points taken from FIG. 11. After calculating the radial coordinate r.sub.op of the operating point, the angular coordinates to φ.sub.surge and φ.sub.choke can be obtained from TABLE 1. This is graphically shown in FIG. 11 with the projections of the surge point φ.sub.surge and the choke point φ.sub.choke on the horizontal axis φ, and the calculated angular coordinates φ.sub.op of the operating point between them. The controlled variable CV (%) in percent can then be calculated for surge protection using equation (28), and for choke protection from equation (29).

[0103] A hypothetical compressor map is shown in FIG. 12 in π-term coordinates Π.sub.1_in and Π.sub.2−1) without choke line, where A points are still the surge points and B points are the maximum flow endpoints on each performance curve. In this case, when the full range of compressor operation, defined from surge limit to choke limit, is not available, the controlled variable CV (%) can only be calculated for the surge protection.

[0104] The rays emanating from the zero point in FIG. 12 indicate the angular coordinates of all surge points from the polar angle α_A.sub.1 of the first surge point A.sub.1 to the polar angle α_A.sub.n+1 of the last surge point A.sub.n+1, Then FIG. 13 shows modified compressor map, where each surge point on the surge line now has the same angular coordinate. This is achieved by replacing the π-term Mach number coordinate Π.sub.1_in, with the coordinate (Π.sub.1_in).sub.Corr, which is the corrected Mach number as a function of the π-term (Π.sub.2−1) obtained from surge points by the formula:

(Π.sub.1_in).sub.A=(Π.sub.2−1).sub.A  (39)

[0105] The function shown below in tabular form in TABLE 3 with two columns of characteristic data, where (Π.sub.1_in).sub.A is the argument and (Π.sub.2−1).sub.A is the function derived from FIG. 13 for surge points.

TABLE-US-00003 TABLE 3 π-term Mach (Π.sub.2 − 1) number as function 1 (Π.sub.1_in).sub.A.sub.1 (Π.sub.2 − 1).sub.A.sub.1 INPUT (Π.sub.1_in) .fwdarw. 2 (Π.sub.1_in).sub.A.sub.2 (Π.sub.2 − 1).sub.A.sub.2 3 (Π.sub.1_in).sub.A.sub.3 (Π.sub.2 − 1).sub.A.sub.3 n − 1 (Π.sub.1_in).sub.A.sub.n-1 (Π.sub.2 − 1).sub.A.sub.n-1 n (Π.sub.1_in).sub.A.sub.n (Π.sub.2 − 1).sub.A.sub.n n + 1 (Π.sub.1_in).sub.A.sub.n+1 (Π.sub.2 − 1).sub.A.sub.n+1 ↓ OUTPUT (Π.sub.1_in).sub.Corr

[0106] The same technique of converting constant speed performance curves from rectangular to polar coordinates can now be applied to compressors with only the surge limit line, provided that the n-term coordinate Π.sub.1_in is replaced by the new coordinate (Π.sub.1_in).sub.Corr.

[0107] To equalize the two unequal radial coordinates of the surge and maximum flow endpoint, it is also necessary to calculate the polar conversion factor P.

[0108] The distance from the zero point to each surge point A can then be calculated as:

r.sub.surge=√{square root over ((Π.sub.2−1).sub.A.sup.2+(P.Math.(Π.sub.1_in).sub.Corr).sub.A.sup.2)}  (40)

where (Π.sub.2−1).sub.A and ((Π.sub.1_in).sub.Corr).sub.A—coordinates of the surge points A.

[0109] The distance from the zero point to each maximum flow endpoint B can be calculated as:

r.sub.max_flow=√{square root over ((Π.sub.2−1).sub.B.sup.2+(P.Math.(Π.sub.1_in).sub.Corr).sub.B.sup.2)}  (41)

where (Π.sub.2−1).sub.B and ((Π.sub.1_in).sub.Corr).sub.B—coordinates of the maximum flow points B.

[0110] From the two equations (40) and (41), setting that r.sub.surge=r.sub.max_flow, the polar conversion factor P for each AB constant speed performance curve can be calculated as:

[00034]$\begin{array}{cc}P=\sqrt{\frac{{\left({\Pi }_2-1\right)}_A^2-{\left({\Pi }_2-1\right)}_B^2}{{\left({\left({\Pi }_{1\_\mathrm{in}}\right)}_{\mathrm{Corr}}\right)}_B^2-{\left({\left({\Pi }_{1\_\mathrm{in}}\right)}_{\mathrm{Corr}}\right)}_A^2}}& \left(42\right)\end{array}$

[0111] The arithmetic means or average of the polar conversion factors, the sum of the polar conversion factors divided by the total number of curves can be calculated using the equation (25).

[0112] FIG. 14 illustrates the effect of polar transformation by displaying a radial coordinate r and an angular coordinate α in rectangular coordinates. FIG. 14 shows the straitened performance curves and the shaded area bounded by a surge line on one side, defined as a vertical line with a constant angular coordinate, and a line, connecting the endpoints on the other side.

[0113] The equations for calculating of a pair of polar coordinates (r, α) are shown below:

[00035]$\begin{array}{cc}r=\sqrt{{\left({\Pi }_2-1\right)}^2+{\left(P_{\mathrm{mean}\_\mathrm{average}}.Math.{\left({\Pi }_{1\_\mathrm{in}}\right)}_{\mathrm{Corr}}\right)}^2}& \left(43\right)\end{array}$$\begin{array}{cc}\alpha =\mathrm{ARCTAN}\left(\frac{P_{\mathrm{mean}\_\mathrm{average}}.Math.{\left({\Pi }_{1\_\mathrm{in}}\right)}_{\mathrm{Corr}}}{\left({\Pi }_2-1\right)}\right)& \left(44\right)\end{array}$

[0114] TABLE 4 is populated with surge points and maximum flow endpoints taken from FIG. 14, where the surge points polar angle column is constant. After calculating the radial coordinate r.sub.op of the operating point, the angular coordinates α.sub.const and α.sub.max_flow are obtained from TABLE 4.

TABLE-US-00004 TABLE 4 Radial Polar angle of Polar angle of coordinate surge point maximum flow point 1 (r).sub.1 α.sub.const (α.sub.max_flow).sub.1 INPUT (r.sub.op) .fwdarw. 2 (r).sub.2 α.sub.const (α.sub.max_flow).sub.2 3 (r).sub.3 α.sub.const (α.sub.max_flow).sub.3 n − 1 (r).sub.n-1 α.sub.const (α.sub.max_flow).sub.n-1 n (r).sub.n α.sub.const (α.sub.max_flow).sub.n n + 1 (r).sub.n+1 α.sub.const (α.sub.max_flow).sub.n+1 ↓ ↓ OUT1 (α.sub.const) OUT2 (α.sub.max_flow)

[0115] Graphically it is shown in FIG. 14 with the constant coordinate of the surge points (angular coordinate α.sub.const) and the projection of the maximum flow endpoint (angular coordinates α.sub.max_flow) onto the horizontal a axis with the calculated operating point (angular coordinates α.sub.op) between them.

[0116] The controlled variable CV (%) in percent for the surge protection controller in the case of maximum flow endpoints can be calculated relative to the surge limit as the polar angle of the operating point α.sub.op minus the constant α.sub.const (polar angle of the surge points) divided by the specified operating range up to maximum flow line, defined as subtracting the constant from the polar angle of the maximum flow endpoint α.sub.max_flow:

[00036]$\begin{array}{cc}{CV\left(\%\right)=100\%.Math.\frac{{\alpha }_{\mathrm{op}}-{\alpha }_{\mathrm{const}}}{{\alpha }_{\max \_\mathrm{flow}}-{\alpha }_{\mathrm{const}}}.Math.}_{r_{\mathrm{op}}}& \left(45\right)\end{array}$

[0117] It can be assumed that the hypothetical compressor map, shown in FIG. 12 in π-term coordinates Π.sub.1_in and (Π.sub.2−1) has only surge points A obtained during commissioning. The rays emanating from the zero point in FIG. 12 still indicate the angular coordinates of all surge points, from the polar angle α_A.sub.1 of the surge point A.sub.1 to the polar angle α_A.sub.n+1 of the surge point A.sub.n+1. Similarly, the surge line and surge points may be presented in a corrected Mach number coordinate (Π.sub.1_in).sub.Corr as the function of n-term coordinates (Π.sub.2−1) in FIG. 13, where each surge point on the surge line has the same polar angle. The corrected Mach number coordinate (Π.sub.1_in).sub.Corr still can be found from the equation (38), and TABLE 3 with two columns of characteristic data obtained from surge points is still applicable when only surge points are available. In the absence of performance curves, the calculation of the polar conversion factor P would be impossible. A pair of polar coordinates (r, α) can be calculated from equation (43) and (44) with the parameter P.sub.mean_average equal to one:

[00037]$\begin{array}{cc}r=\sqrt{{\left({\Pi }_2-1\right)}^2+{\left({\left({\Pi }_{1\_\mathrm{in}}\right)}_{\mathrm{Corr}}\right)}^2}& \left(46\right)\end{array}$$\begin{array}{cc}\alpha =\mathrm{ARCTAN}\left(\frac{{\left({\Pi }_{1\_\mathrm{in}}\right)}_{\mathrm{Corr}}}{\left({\Pi }_2-1\right)}\right)& \left(47\right)\end{array}$

[0118] The controlled variable CV (%) in percent for the surge protection controller can be calculated as the polar angle of the operating point α.sub.op minus constant α.sub.const the polar angle of the surge points, divided by the polar angle of the surge points:

[00038]$\begin{array}{cc}CV\left(\%\right)=100\%.Math.\frac{{\alpha }_{\mathrm{op}}-{\alpha }_{\mathrm{const}}}{{\alpha }_{\mathrm{const}}}& \left(48\right)\end{array}$

[0119] If surge points are collected during commissioning with a flow meter located downstream of the compressor, the n-term Mach number is calculated as the Mach number at the outlet of the compressor. FIG. 15 shows surge line and surge points A in the π-term coordinates Π.sub.1_out and (Π.sub.2−1), as they are obtained from field tests without performance curves. Very often, for a Mach number calculated at the outlet of the compressor, starting from a nominal compression ratio of about 4.0 to 5.0 and above, the surge line can become vertical. In this case, there are two ways to calculate the percentage controlled variable CV (%). The first uses equation (19), which links the Mach numbers at the compressor inlet and outlet by changing Π.sub.1_out to Π.sub.1_in.

[0120] The second uses the π-term coordinate Π.sub.1_out, but the π-term coordinate (Π.sub.2−1) is replaced with a new corrected coordinate so that each surge point has the same polar angle. This is achieved by replacing the π-term coordinate (Π.sub.2−1) with the coordinate (Π.sub.2−1).sub.Corr, which is a function of the π-term Mach number Π.sub.1_out obtained from surge points by the formula:

(Π.sub.2−1).sub.A=(Π.sub.1_out).sub.A  (49)

[0121] FIG. 15 shows a surge line with angular coordinates of all surge points from the polar angle γ_A.sub.1 of the first surge point A.sub.1 to the polar angle γ_A.sub.n+1 of the last surge point A.sub.n+1. FIG. 16 shows the modified surge line in rectangular coordinates, but with surge points having the same polar angle or the polar coordinate γ.sub.const. The function shown below in tabular form in TABLE 5 with two columns of characteristic data, where the coordinate (Π.sub.2−1).sub.A is the argument and (Π.sub.1_out).sub.A is the function, are obtained from surge points in FIG. 15.

[0122] In the absence of compressor characteristic curves, the polar radius r can be calculated from the equation below:

r=√{square root over (((Π.sub.2−1).sub.Corr).sup.2+(Π.sub.1_out).sup.2)}  (50)

and the angular coordinate γ can be calculated using the equation:

[00039]$\begin{array}{cc}\gamma =\mathrm{ARCTAN}\left(\frac{{\Pi }_{1\_\mathrm{out}}}{{\left({\Pi }_2-1\right)}_{\mathrm{Corr}}}\right)& \left(51\right)\end{array}$

[0123] The controlled variable CV (%) in percent for the surge protection controller can be calculated as the polar angle of the operating point γ.sub.op, minus constant γ.sub.const the polar angle of the surge points, divided by the polar angle of the surge points:

[00040]$\begin{array}{cc}CV\left(\%\right)=100\%.Math.\frac{{\gamma }_{\mathrm{op}}-{\gamma }_{\mathrm{const}}}{{\gamma }_{\mathrm{const}}}& \left(52\right)\end{array}$

TABLE-US-00005 TABLE 5 (Π.sub.1_out) π-term (Π.sub.2 − 1) as function 1 (Π.sub.2 − 1).sub.A.sub.1 (Π.sub.1_out).sub.A.sub.1 2 (Π.sub.2 − 1).sub.A.sub.2 (Π.sub.1_out).sub.A.sub.2 INPUT (Π.sub.2 − 1) .fwdarw. 3 (Π.sub.2 − 1).sub.A.sub.3 (Π.sub.1_out).sub.A.sub.3 n − 1 (Π.sub.2 − 1).sub.A.sub.n−1 (Π.sub.1_out).sub.A.sub.n−1 n (Π.sub.2 − 1).sub.A.sub.n (Π.sub.1_out).sub.A.sub.n n + 1 (Π.sub.2 − 1).sub.A.sub.n+1 (Π.sub.1_out).sub.A.sub.n+1 ↓ OUTPUT ((Π.sub.2 − 1).sub.Corr)

[0124] The effect of the IGV opening on compressor performance is shown in FIG. 17, similar to that shown in FIG. 8, with the difference that B points represent the endpoints of the maximum flow. Therefore, FIG. 17 represents a case where a hypothetical compressor map is shown in Tc-teen coordinates Π.sub.1_in and (Π.sub.2−1) with surge line but no choke line; with three sets of constant speed performance curves representing three IGV opening positions of 0%, 50% and 100%, and each of them consists of four curves. Likewise, lines A.sub.1B.sub.1, A.sub.2B.sub.2, A.sub.3B.sub.3 and A.sub.4B.sub.4 refer to constant speed performance curves at 0% IGV position; lines A′.sub.1B′.sub.1, A.sub.2′B′.sub.2, A′.sub.3B′.sub.3 and A′.sub.4B′.sub.4 to constant speed performance curves at 50% IGV; and lines A″.sub.1B″.sub.1, A″.sub.2B″.sub.2, A″.sub.3B″.sub.3 and A′.sub.4B″.sub.4 for constant speed performance curves at 100% IGV. Since the surge points are identical to those shown in FIG. 8, and do not change their positions, the IGV function shown in TABLE 2 can be used.

[0125] FIG. 18 shows the result of applying the inlet guide vanes function to three sets of constant speed performance curves with one common surge line in coordinates ƒ(IGV).Math.Π.sub.1_in and (Π.sub.2−1). Again, in the absence of a choke line, the control variable CV (%) can only be calculated for surge protection. The rays emanating from the zero point in FIG. 18 indicate the angular coordinates of all surge points from the polar angle α_A.sub.1 corn of the surge point A.sub.1com to the polar angle α_A″.sub.4com of the surge point A″.sub.4com.

[0126] FIG. 19 shows a modified compressor map, which is a modification of the compressor map shown in FIG. 18, where the π-term coordinate (Π.sub.2−1) is replaced with a new corrected coordinate such that each surge point has the same polar angular. This is achieved in the same way as before, replacing the π-term coordinate (Π.sub.2−1) with the coordinate (Π.sub.2−1).sub.Corr, but as a function of the π-term Mach number ƒ(IGV).Math.Π.sub.1_in of the surge points shown in FIG. 18. The corrected coordinate (Π.sub.2−1).sub.Corr is calculated for each surge point using the formula:

(Π.sub.2−1).sub.A=(ƒ(IGV).Math.Π.sub.1_in).sub.A  (53)

[0127] The same method of converting constant speed performance curves from rectangular to polar coordinates can now be applied to compressors with the IGV and the endpoints of the maximum flow. An equal distance statement for each performance curve that declares the distance from the zero point to the surge point A and from the zero point to the maximum flow endpoint B, as well as the calculation of the polar conversion factor P, are still required for polar conversion.

[0128] The distance from the zero point to each surge point A can then be calculated as:

r.sub.surge=√{square root over ((Π.sub.2−1).sub.Corr).sub.A.sup.2+(P.Math.∫(IGV).Math.Π.sub.1_in).sub.A.sup.2)}  (54)

where ((Π.sub.2−1).sub.Corr).sub.A and (ƒ(IGV).Math.Π.sub.1_in).sub.A—coordinates of the surge points A.

[0129] The distance from the zero point to each maximum flow endpoint B can be calculated as:

r.sub.max_flow=√{square root over ((Π.sub.2−1).sub.Corr).sub.B.sup.2+(P.Math.∫(IGV).Math.Π.sub.1_in).sub.B.sup.2)}  (55)

where ((Π.sub.2−1).sub.Corr).sub.B and (ƒ(IGV).Math.Π.sub.1_in).sub.B—coordinates of the maximum flow points B.

[0130] From the two equations (54) and (55), setting r.sub.surge=r.sub.max_flow, the polar conversion factor P for each AB constant speed performance curve can be calculated as:

[00041]$\begin{array}{cc}P=\sqrt{\frac{{\left({\left({\Pi }_2-1\right)}_{\mathrm{Corr}}\right)}_A^2-{\left({\left({\Pi }_2-1\right)}_{\mathrm{Corr}}\right)}_B^2}{{\left(f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}\right)}_B^2-{\left(f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}\right)}_A^2}}& \left(56\right)\end{array}$

[0131] The arithmetic mean P.sub.mean_average can be calculated from the formula (36) as the sum of the polar conversion factors divided by the total number of curves in the sets. As before, in a two-dimension polar coordinate system on the plane, each point corresponds to a pair of polar coordinates (r, α), but equations for calculating the polar coordinates (r, α) must be adjusted as shown below:

[00042]$\begin{array}{cc}r=\sqrt{{\left({\left({\Pi }_2-1\right)}_{\mathrm{Corr}}\right)}^2+{\left(P_{\mathrm{mean}\_\mathrm{average}}.Math.f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}\right)}^2}& \left(57\right)\end{array}$$\begin{array}{cc}\alpha =\mathrm{ARCTAN}\left(\frac{P_{\mathrm{mean}\_\mathrm{average}}.Math.f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}}{{\left({\Pi }_2-1\right)}_{\mathrm{Corr}}}\right)& \left(58\right)\end{array}$

[0132] FIG. 20 illustrates a polar transformation, where the polar coordinates r and α are plotted in rectangular coordinates, the performance curves are flattened, and the shaded area surrounded by the surge limiting line and the maximum flow line, defines the compressor operating area.

[0133] TABLE 6 is populated with surge points and maximum flow endpoints taken from FIG. 20, where the polar angle of the surge points is constant, and the polar angles of the maximum flow endpoints marked with a symbol (.square-solid.) are defined as the points of intersection of the r coordinates with the maximum flow line at points (B).sub.1, (B).sub.2, (B).sub.3 . . . (B).sub.n−1, (B).sub.n and (B).sub.n+1.

[0134] The radial coordinates r.sub.op and α.sub.op of the operating point can be calculated from equations (57) and (58). The angular coordinates α.sub.const and α.sub.max_flow are obtained from TABLE 6.

TABLE-US-00006 TABLE 6 Radial Polar angle Polar angle of coordinate of surge point maximum flow point 1 (r).sub.1 α.sub.const (α.sub.max_flow).sub.(B)1 2 (r).sub.2 α.sub.const (α.sub.max_flow).sub.(B)2 INPUT (r.sub.op) .fwdarw. 3 (r).sub.3 α.sub.const (α.sub.max_flow).sub.(B)3 n − 1 (r).sub.n-1 α.sub.const (α.sub.max_flow).sub.(B)n−1 N (r).sub.n α.sub.const (α.sub.max_flow).sub.(B)n n + 1 (r).sub.n+1 α.sub.const (α.sub.max_flow).sub.(B)n+1 ↓ ↓ OUT1 (α.sub.const) OUT2 (α.sub.max_flow)

[0135] Graphically it is shown in FIG. 20 with the constant surge point coordinate (angular coordinates α.sub.const) and, the projection of the maximum flow endpoint (angular coordinates α.sub.max_flow) onto the horizontal axis α with the calculated operating point (angular coordinates α.sub.op) between them. The controlled variable CV (%) in percent for the surge protection controller in the case of a variable geometry compressor with surge line and maximum flow endpoints can be calculated relative to the surge limit from equation (45).

[0136] It can now be assumed that only surge points A in FIG. 17 are known. In the same way, the displacement of the surge points A.sub.3, A′.sub.3 and A″.sub.3 to the left with unchanged (Π.sub.2−1) coordinates denote the new positions of the surge points A.sub.3com, A′.sub.3com and A″.sub.3com on the line defined as the expected common surge line. The IGV function can still be calculated by dividing the coordinates of the surge points A.sub.3com, A′.sub.3com and A″.sub.3com by the coordinates of the surge points A.sub.3, A′.sub.3 and A″.sub.3, respectively. And TABLE 2 again can be populated with characteristic data representing the IGV position from 0% to 100%, and a function ƒ(IGV) obtained for all available surge points by forming the expected common surge line.

[0137] In absence of the performance curves the rays emanating from the zero point in FIG. 18 still indicate the angular coordinates of all surge points from the polar angle α_A.sub.1 com of the surge point A.sub.1com to the polar angle α_A″.sub.4com of the surge point A″.sub.4com. Surge points A still have the equal polar angle in the π-term coordinates ƒ(IGV).Math.Π.sub.1_in and (Π.sub.2−1).sub.Corr in FIG. 19. In the case when only surge points are present, the polar radius r can be calculated from equation (57) and the angular coordinate α can be calculated from equation (58), where the parameter P.sub.mean_average is equal to one:

[00043]$\begin{array}{cc}r=\sqrt{{\left({\left({\Pi }_2-1\right)}_{\mathrm{Corr}}\right)}^2+{\left(f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}\right)}^2}& \left(59\right)\end{array}$$\begin{array}{cc}\alpha =\mathrm{ARCTAN}\left(\frac{f\left(IGV\right).Math.{\Pi }_{1\_\mathrm{in}}}{{\left({\Pi }_2-1\right)}_{\mathrm{Corr}}}\right)& \left(60\right)\end{array}$

[0138] And then the controlled variable CV (%) in percent for the surge protection controller can be calculated from equation (48).