FIELD OF THE INVENTION
Present invention relates to the field of centrifugal compressor surge control. More particularly, it relates to the field of controlling surge in the compressor through anti-surge control logic in tabular approach.
BACKGROUNDOF THE INVENTION
Centrifugal compressors are used to compress any gas medium to a higher discharge pressure as per the process requirement. The Compressor is a machine that converts mechanical energy of the driver (electric motor, steam or gas turbine, etc.) into energy of the gas being processed.
Centrifugal compressors having the Compressor Map represent operation of the compressor in an X-Y plot wherein the Suction Volumetric Flow (Qs) is X-axis, and Polytropic head (Hp) is Y-axis coordinates on the map. The various curves N1, N2,….N6 represent the compressor operation at various speeds on the compressor map.(shown in figure 2).
Further, in a typical single stage centrifugal compressor installation, five operating parameters are continuously monitored for anti -surge control. These parameters are: suction pressure Ps, discharge pressure Pd, suction temperature Ts, discharge temperature Td and suction flow Qs. The suction flow Qs is calculated using the differential pressure hs across a flow orifice in suction. The antisurge controller continuously calculate the location of operating point (OP) with respect to the Surge limit line (SLL) and dynamically opens antisurge valve if the operating point crosses the surge limit line (SLL).

Moreover, the left most top end of the each operating curve in a compressor map, represents the maximum Polytropic head that can be achieved for a given speed. This maximum Polytropic head also represents the minimum suction volumetric flow for given speed. These left most top operating points on the compressor curves are known as surge points. When all surge points are joined for different speeds, the resultant curve is called a surge curve or surge limit line (SLL). When the compressor operating point goes on the left side of this surge curve, the compressor is said to be operating in the surge zone.
For centrifugal compressors, surge is a dangerous phenomenon and operation in surge zone may damage the compressor internals. Hence to avoid compressor surge, Antisurge controller (ASC) and Antisurge control valve (ASCV) are provided.
One of the Prior arts US4949276 is related to the creation of surge limit line slope using function generator f(Hp, Qs^2). But, the present invention utilizes a tabular method to store the slopes values of surge limit line(SLL) in the antisurge controller.
In another prior art US 20180135637, an additional surge control line is created by addition of constant or variable safety margin to the surge limit line. But, the present invention utilizes a method of addition of safety margin to the slope of operating point itself, thus eliminating the need of creating an additional surge control line comprising of multiple coordinates.
The plant operators practically consider the surge control line as the surge limit line (SLL), which is imaginary limit line, and try to always keep the operating point on the right side of the (imaginary)SLL by partial opening of antisurge valve even when the compressor is away from actual surge. This

causes unnecessary recycling of high pressure gas leading to waste of compressor energy. Moreover, the safety margin (SM) is added to the operating point before comparison with surge limit line.
Therefore, present invention solve one or more of the above problems by a method for controlling surge in a typical centrifugal compressor using tabular approach.
OBJECT OF THE INVENTION
It is therefore Principal object of the invention is to provide a method to store the values of slope of surge control line (KsllConst) at multiple points in a tabular format called surge limit line set point table at different pressure ratio value.
Yet another object of the invention is to continuously fetch the slope value of surge limit line (Ksll), corresponding to the pressure ratio (Rc) of the operating point, for comparison with slope of operating point (Kop), for deciding the antisurge controller PID input.
Still another objective of the invention is to continuously compare the operating point slope (Kop) with the corresponding slope of actual surge limit line (Ksll) instead of imaginary surge limit line. This provides the true representation of the operating point (OP) with respect to Actual surge limit line (SLL).
SUMMARYOF THE INVENTION

Accordingly, a solution is provided through a method for controlling surge in a compressor using a tabular approach. The compressor having a surge Limit Line (SLL) setpoint Table comprising the steps of: determining slope of Operating point (kop), determining slope of Surge Limit Line(Ksll), Introducing a safety margin (8) to the KOP, and Comparing said Ksll with KOP for determining opening of a ASCV (4a).The compressor having an Anti-surge Controller (ASC) connected to the Anti surge controller valve.The safety margin is introduced to the operating point before comparison with slope of surge limit line (Ksll). Furthermore, output of the surge table is fed to a surge controller PID for dynamic opening of the ASCV.The slope values are recorded against the pressure ratio corresponding to the slope values.
BRIEF DESCRIPTIONOF THE ACCOMPANYING DRAWINGS
It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods or structure in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
Figure 1 is a schematic illustration of a typical compressor process loop with instrumentation.

Figure 2 is a schematic illustration of the compressor map.
Figure 3 is a schematic illustration of anti-surge controller Block Diagram
Figure 4 is a schematic illustration of Antisurge controller module for implementation of Surge limit line set point table of present invention, Module A.
Figure 5 is a schematic illustration of Antisurge controller module for calculation of slope of operating point and addition of safety margin to the slope of operating point, Module B.
DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS
While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such system, or assembly, or device. In other words, one or more elements in a system or device proceeded by “comprises… a” does not,

without more constraints, preclude the existence of other elements or additional elements in the system or device.
It is well known that the equation for calculating the polytropic Head (Hp) is as follows:
Hp = (Z*Ts*R)*((Rc -1)/))----------equation 1
Mw
It is well known that the equation for calculating the suction volumetric Flow (Qs) is as follows:
Qs = A*√((hs*Zs*Ts*R)/Ps*MW)------------equation 2
The abbreviations used in above equations are summarized below:
a) Z is average gas compressibility, Z= (Zs+Zd)/2
b) Zs is suction gas compressibility.
c) Zd is discharge gas compressibility.
d) Rc is pressure ratio, Rc= Pd/Ps
e) σ is Polytropic Exponent, σ = log(Td/Ts)/log Rc
f) A is Flow Element Constant
g) hs is Differential pressure across flow element at Suction h) Pd is Compressor discharge pressure
i) Ps is Compressor suction pressure j) Td is Compressor discharge temperature

k) Ts is Compressor discharge temperature
1) R is gas constant.
Referring to equation-1 and equation-2 above, the difference between suction gas compressibility Zs and average gas compressibility Z to be very small enough to be ignored for all practical purposes ( Z ~ Zs), the term (Z*Ts*R)/W appear in both the equations. Hence it can be interpreted that if a graph is plotted between polytropic head (Hp) and square of suction flow (Qs2), the impact of change in compressibility, molecular weight and suction temperature will be same on both polytropic head (Hp) and square of suction flow (Qs2).
Moreover, any straight line passing through point (x, y) and origin( 0,0) in a graph is represented by the mathematical equation y = mx, where m is known as slope of the line and it equals y coordinate divided by x coordinate on that line. Similarly, in the embodiment, the compressor map (as shown in figure
2) Hp vs Qs2 , the ratio of Hp and Qs2 values at operating point provides
the slope of the operating point(KOP). Hp represent polytropic head on Y-axis
2 represents suction flow on X-axis of the Compressor map. and Qs
In a preferred embodiment of figure 1, a compressor (2) having a compressor driver (2a) for driving the compressor and pumping the gas at a gas outlet (6). The gas enters through an inlet (5) to which a suction flow orifice (4b) is connected. Excess gas flow is recycled to the source via Anti Surge Controller Valve (ASCV) (4a). In the preferred embodiment an anti-surge controller (hereafter referred as ASC) (4) having suction pressure (4c), suction temperature (4d), discharge temperature (4e) and discharge pressure (4f).

Figure 2 is a schematic illustration of the compressor map. In this example, a
compressor map (7) having a surge limit line (hereafter referred as SLL) of
graph between Polytropic head Hp( in meter unit) and Suction flow Qs(in
m3/hr) is drawn. Points on the graph representing compressor speed (N1, N2,
N3…N6) and an operating point (OP). Slopes of SLL (KsllConst0, KsllConst1,……
KsllConst7 for 6 point characterization of SLL) can be determined directly from
the compressor map (7) in the Hp vs Qs2 plane by dividing the Hp with Qs2.
The slope values determined are recorded against pressure ratio Rc (RcConst0,

RcConst1 … RcConst6 for 6 point characterization of SLL) on these points on
the SLL.
In the preferred embodiment of Figure 4, a surge limit line set point table (9)
having a greater/equal comparator (9a), a logic AND gate (9b),an input selector
(9c) and a Maximum value selector (9d). In the embodiment, the slopes of SLL
(KsllConst0, KsllConst1,…… KsllConst7 for 6 point characterization of SLL)
Rc (
corresponding to different values of pressure ratio RcConst0, RcConst1 …
RcConst6 for 6 point characterization of SLL) are stored as SLL set point table
for surge controlling action. The output of SLL set point table is the SLL slope
value(Ksll) corresponding to pressure ratio Rc, nearest to the operating point
(OP) Pressure ratio Rc. Further, output of the SLL set point table (Ksll) is
compared with Slope of the operating point (KOP) in a Surge Proximity
Comparator (in Figure 3)and is fed to antisurge controller PID to dynamically
determine opening of the ASCV(4a).
Figure 5 is a schematic illustration of Antisurge controller module for calculation of slope of operating point and addition of safety margin to the slope of operating point, Module B. In the preferred embodiment, during compressor operation, KOP is continuously compared with Ksll on the SLL. The

comparison can be done by (i) dividing KOP with corresponding Ksll, or (ii) by subtracting KOP from Ksll.
Referring to the (i) division method: the ratio of slopes (Kop / Ksll) equals 1 if the compressor operating point (OP) reaches surge limit line (SLL). This ratio is less than 1 for all operating points on left of surge limit line (SLL) and greater than 1 if operating point lies towards right of surge limit line.
Referring to the (ii) subtraction method, the difference of numerical values of slopes (Ksll- Kop) equals 0 if the compressor operating point (OP) reaches surge limit line (SLL). This difference is negative (<1) for all operating points are on left of surge limit line and positive (>1) if operating point lies towards right of surge limit line. Alternatively, the difference (Kop- Ksll) can also be used.
The comparison of slopes Kop and Ksll is implemented in Module-C (Surge Proximity, SPC) of antisurge controller, shown in figure 3.
The output of the module-C (SPC) is fed to the antisurge controller PID for actuating the antisurge control valve.
EXAMPLE
A compressor defined by the compressor map (7) having following operating point is assumed:

s.no.
1.
2.
3.
5. Measured parameter values

Discharge pressure: Pd 5.47 kg/m2

Suction pressure: Ps 1.009 kg/m2

Discharge temperature: Td 249.4 degreeC

Flow element DP: h 2915 mmH2O

Based on above current operating point, Pressure ratio ‘Rc’ (Pd / Ps) is calculated to be 5.422. Using Module-B (Figure-5), current operating point slope ‘Kop’ is Calculated as 7.71. For a sample project, the RcConst(0,1,2..) and KsllConst(0,1,2..) values are calculated from the Compressor Map (Figure-2) and stored in Module-A (Figure-4). A tabular representation of these values is as follows:

s.no
1.
2.
3.
4.
5.
6. RcConst(0,1,2…) Ksllconst(0,1,2…)

3.55 10.9

4.30 10.6

5.12 10.4

6.00 10.4

6.90 9.1

7.83 7.64
Based on current operating point ‘Rc= 5.422’, calculated above Ksll value is fetched from Module-B (Figure-5). The value of ‘Ksll’ corresponding to ‘Rc’ of 5.422 is 10.2. Now the ‘Kop’ 7.71 is divided by selected ‘Ksll’ 10.2 in Module-C (shown in figure 3) to get the ratio of slopes of Operating point and corresponding Surge Point. As the operating point moves towards the Surge limit line, the result of Module-C keeps increasing and equals 1 at the surge limit line. The ASC PID (Module-D) Set point ‘PID SP’ is kept = 1(in figure 3). Whenever the value of output from Module-C to the ASC PID reaches near or equal 1, (i.e. difference between ASC PID input and PID SP goes toward 0), the ASC PID output decreases thus opening the Anti Surge valve ‘ASCV’. Similarly, if the output of Module-C reduces, the ASC PID output increases to close the ASCV. The amount & rate of opening of ASCV is decided by the difference between ASC PID input and PID SP.

The operating point (N1,N2…) in compressor map keep on changing based on the compressor operation. Accordingly, Rc & KOP changes. Further Based on change in ‘Rc’, the ‘Ksll’ value selected from Module-A (Figure-5) keeps changing if current value of ‘Rc’ crosses any of the values listed for RcConst(0,1,2…), the corresponding values of KsllConst(0,1,2…) is selected and dynamically assigned to ‘Ksll’. The ration of calculated ‘Kop’ and selected ‘Ksll’ determines the operation (opening / closing) of ASCV. Since both the ‘Kop’ and ‘Ksll’ change dynamically based on values of current operting point, the operation of ASCV is controlled dynamically by ASC PID(8a).

We Claim:
1. A method for controlling surge in a compressor (2) using a tabular
approach, wherein said compressor (2) having a surge Limit Line (SLL)
setpoint Table (9) comprising:
- Determining slope of Operating point (KOP);
- Determining slope of Surge Limit Line(Ksll);
- Introducing a safety margin (8) to the KOP; and,
- Comparing said Ksll with KOP for determining opening of a ASCV (4a).

2. The method as claimed in claim 1, wherein the compressor (2) having an Anti-surge Controller (ASC) (4) connected to the Anti surge controller valve (4a).
3. The method as claimed in claim 1, wherein the safety margin (8) is introduced to the operating point before comparison with the slope of surge limit line (Ksll).
4. The method as claimed in claim 1, wherein output of the surge table (9) is fed to a surge controller PID (8a) for dynamic opening of the ASCV (4a).
5. The method as claimed in claim 1, wherein the slope values are recorded against the pressure ratio corresponding to the slope values.