Description:A) TECHNICAL FIELD
[001] The present invention generally relates to method for surge detection and surge control in a centrifugal compressor. The invention more particularly relates to a method for modulating pressure in the centrifugal compressor to prevent surge in the compressor.

B) BACKGROUND OF INVENTION
[002] The process of surge detection and surge control in a centrifugal compressor is a complex one. It requires specialized knowledge of machinery, controls and process to develop strategies that are capable of high-speed execution and optimized controls. As pressure builds up in a centrifugal compressor, there are high possibilities that the potential energy in the discharge vessel exceeds the capability of the compressor and the flow in the system reverses. As a result, the pressure downstream of the compressor drops, and that allows the compressor to re-establish the forward flow. If the corrective action is not initiated, the surge cycle repeats. Surge can be experienced during operation at low flow rates or. Consequences of surge include rapid oscillations in flow and pressure, exponential rise in the temperature of the gas, process instability, potential machine / process trips and possible damage to the compressor seals, bearings, impellers and shaft depending upon the energy level associated with surge and the compressor design. Surge detection is an inherent feature of the surge control system. It is used to alarm plant operators, engage surge system controller’s adaptive action and if necessary, shut down the compressor to prevent the damage. [Surge detection and surge control systems for centrifugal compressors – Part 1 by Neetin Ghaisas and Todd Reitsma]
[003] In US 4861233A, the applicant uses the function of pressure differential across the centrifugal compressor and across the orifice in the inlet line of the centrifugal compressor detect the surge and varying the base load on the combined output of the base load machine and the centrifugal compressor.
[004] In CN111828364A, the impeller of the compressor and the rotor of the driving motor are on the same axis. Further, the signal to be detected includes the rotational speed signal of the drive motor which decreases the general surge determination and increases the response time to control the surge in centrifugal compressor. Further, the invention does not disclose whether the centrifugal blower or centrifugal compressor is a single stage or multi-stage.
[005] In view of foregoing, there is a need to have a method and system that sould simultaneously detect the surge in centrifugal compressor and control the surge in centrifugal compressor.
[006] The above-mentioned shortcomings, disadvantages and problems are addressed herein, as detailed below.

C) OBJECT OF INVENTION
[007] The primary object of the present invention is to provide a method for detection of surge in centrifugal compressor and simultaneously control the surge in the centrifugal compressor.
[008] Another objective of the present invention is to provide a system for surge control in centrifugal compressor using motor current and the change in pressure inside the centrifugal compressor.
[009] Another objective of the present invention is to provide a system for surge control in the centrifugal compressor by modulating the inlet guide vane for short or long durations as required.
[0010] These and other objects and advantages of the embodiments herein will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

D) SUMMARY OF INVENTION
[0011] Various embodiments of the present invention provide for a method of surge detection in centrifugal compressor and simultaneous control of surge in the centrifugal compressor. The centrifugal compressor comprises a PID (proportional integral derivative) device, current input, pressure in the centrifugal compressor and the discharge pressure, blow off valve, inlet guide vane, main motor and display screen. The PID device contains multiple values of pressure of pre-decided pressure centrifugal compressor and the high limit allowed in the centrifugal compressor. Once the pressure in the centrifugal compressor deviates from the pre-decided level in the centrifugal compressor, the same shall be communicated to the user via SMS and the data is stored on cloud.
[0012] In one embodiment of the present invention, as soon as the pressure in the centrifugal compressor deviates from the pre-decided pressure level in the compressor, the inlet guide vane is modulated to bring the pressure to the pre-decided pressure level itself in the compressor.
[0013] In another embodiment of the present invention, the blow off valve is opened if the pressure in the centrifugal compressor goes near the surge levels to bring the pressure in the centrifugal compressor to the pre-decided level in the said compressor. In cases wherein blow off valve is employed, the modulation of inlet guide vane may or may not be required.
[0014] In yet another embodiment of the present invention, if the motor current is low that the pre-decided percent load setting, the inlet guide vane is open till the pressure in the centrifugal compressor reaches the level of pre-decided pressure in the centrifugal compressor. In such cases, it the inlet guide vane is not to be closed until the pressure in the compressor matches the pre-decided level of pressure in the centrifugal compressor and the surge level in the compressor is under control.
[0015] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

E) BRIEF DESCRIPTION OF DRAWINGS
[0016] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
[0017] FIG. 1 refers to a chart illustrating drop in motor current.
[0018] FIG. 2 refers to a position of pressure point in different zones in different situations of pressure in the centrifugal compressor.

F) DETAILED DESCRIPTION OF THE EMBODIMENTS
[0019] In the following detailed description, a reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. The embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.
[0020] The surge in centrifugal compressor refers to change in pressure in the centrifugal compressor through the axis. The various embodiments of the present invention teaches method of simultaneous surge detection and surge control due to the pressure change in the centrifugal compressor.
[0021] The PID device has different values of pressure set in about the pre-decided level of pressure in the centrifugal compressor and the deviation of pressure allowed in the centrifugal compressor. The PID device detects pressure at any given point of time in the centrifugal compressor and logs data with each small fluctuation in the input motor current and pressure change in the centrifugal compressor and gives a real-time result and update about the pressure level in the centrifugal compressor and any change in the pressure level in the centrifugal compressor.
[0022] The studies conducted for determining the surge in the centrifugal compressor is mentioned below:
Motor set point:

Motor FLC:

[0023] In the present invention, the pressure level would change on change in motor current input. Certain examples illustrating changes in centrifugal compressor and response required to control the surge are as follows:
Example1:
Fig. 1 illustrates example 1 wherein the X-axis (100) corresponds to time period and Y-axis (101) corresponds to current. The lower dotted line (102) is corresponds to 40 A and the upper dotted line (103) corresponds to 60A. The peak line (104) corresponds to 125A. The dip in the peak line illustrated by curve (105) represents dip of the current and the gap represents the time T=100 milliseconds (106) i.e., the time for which the current dips to less than 60% of amplitude and recovers again to the last amplitude in predefined time wherein T=100 milliseconds.
When there is a dip in motor current from full load to less than 60%, i.e. below 40% and recovers again to the actual level, then the surge count increment is by 1. However, if the dip in motor current happens 5 times in 15 seconds, then centrifugal compressor could be in surge. Time for dip in motor current could be of any duration but time for dip in motor current from full amplitude to approx. 40 and back to original level in 100 milli seconds is also sufficient time to cause the surge in centrifugal compressor. In such cases, the blow off valve is to be opened and the inlet guide vane is to be closed to bring the pressure in the centrifugal compressor to the pre-decided pressure level in the centrifugal compressor. At the same time, the auxillary oil pump is to be started and the main motor is to be shut down to prevent any further dip in motor current and further surge in the centrifugal compressor. At this point, the screen would display – “Compressor deep surge”.

Example 2:
In the modified PID control device used here, the device allows input of multiple values. Thus, the PID control device would have the pre-decided pressure level required in the centrifugal compressor as the set pressure and also provide a range of motor current in which the centrifugal compressor shall operate. In the present invention, the centrifugal compressor operates smoothly around the current of 4mA. However, if the motor current fluctuates and the input current reaches 20mA, then the fail-safe logic of the centrifugal compressor is effectuated. The blow off valve will close once the input current reached 20mA. Once the blow off valve closes, the pressure inside the centrifugal system would rise and the PID would reduce the input motor current. On rising of the pressure in the centrifugal compressor and reduction in the input motor current, the blow off valve shall open slightly and the discharge pressure would be observed. On attaining the level of pre-determined pressure in the centrifugal compressor, the blow off valve is closed and input motor current is reduced from 20mA, the blow off valve is slightly opened and the discharge pressure is observed. Once the pressure in the centrifugal compressor is below the margin of safety of pre-determined pressure, the blow off valve is closed. In the event the pressure is maintained at the margin of safety, the blow off valve is opened only enough to maintain the discharge pressure at the margin level. The formula for calculating the same is
Pressure in the centrifugal compressor – the discharge pressure should be greater than equal to the valve offset.
Once the pressure in the centrifugal compressor is equal to pre-determined pressure in the compressor, the centrifugal compressor is auto loaded.
Illustration: If the pre-determined pressure in the centrifugal compressor is 6 bars, the permissible limit is till 6.4 bars. If the pressure in the centrifugal compressor reaches 6.5 bars, the blow off valve is opened till the discharge pressure in the centrifugal compressor reaches below 6.5 bars, i.e. 6.4 bars. If the discharge pressure is 6.5 bars and the blow off valve is opened fully, there may be a considerable difference between the pressure in the centrifugal compressor and the discharge pressure. If the difference between the two is 0.2 bars or over, the compressor will be required to be unloaded.

Example 3:
All illustrations in the present example are considering scenario during and due to the dip in the motor current as explained in Example 1. Reference is provided to Fig. 2 of the drawings wherein the X-axis (201) corresponds to Discharge pressure and Y-axis (200) corresponds to motor current. The dotted lines (203), (204), (205) and (206) corresponds to pressure levels 7.2 bar, 6.5 bar, 6.0 bar and 3.0 bar respectively. The surge line (207) represents a pre-determined line or zone, such that when the pressure point (213) is beyond the surge line (207) towards the Y-axis (201), the centrifugal compressor will have to be operated according to illustration 6 mentioned below. The control line (209) represents a pre-determined line or zone such that when the pressure point (213) is beyond the control line (209), the compressor will have to be operated according to illustrations VI, V and IV mentioned below.
Illustrations:
If the pressure point in the centrifugal compressor appearing on the display is in zone VI or beyond zone S, then:
Illustration 1.a
If the pressure in the centrifugal compressor is less than the pre-determined pressure in the compressor, the inlet guide vane is opened for long pulse, i.e. approx. 1000 millisecond.
Illustration 1b.
If the pressure in the centrifugal compressor is more than the pre-determined pressure in the compressor, the inlet gas valve is closed for long pulse i.e. approx. 500 millisecond.
If the pressure point in the centrifugal compressor appearing on the display is in zone V or zone S, then:
Illustration 2a.
If the pressure in the centrifugal compressor is less than the pre-determined pressure in the compressor, then the inlet guide vane is opened for short pulse, i.e. approx. 200 millisecond.
Illustration 2b.
If the pressure in the centrifugal compressor is more than the pre-determined pressure in the compressor, then the inlet guide vane is closed for short pulse, i.e. approx. 100 millisecond.

If the pressure point in the centrifugal compressor appearing on the display is in zone IV or zone R, then:
Illustration 3a.
If the pressure in the centrifugal compressor is more than the pre-determined pressure in the compressor, then the inlet guide vane is not to be operated.
Illustration 3b.
If the pressure in the centrifugal compressor is less than the pre-determined pressure in the compressor, the inlet guide vane is opened for long pulse, i.e. approx. 1000 millisecond.

If the pressure point in the centrifugal compressor appearing on the display is in zone III or zone Q, then:
Illustration 4a.
If the pressure in the centrifugal compressor is more than the pre-determined pressure in the compressor, then the inlet guide vane is not to be operated.
Illustration 4b.
If the pressure in the centrifugal compressor is less than the pre-determined pressure in the compressor, the inlet guide vane is opened for long pulse, i.e. approx. 1000 millisecond. At this time, the blow off valve will open enough to keep the discharge pressure at 6.5 bars.

If the pressure point in the centrifugal compressor appearing on the display is in zone II or zone P, then:
Illustration 5.
If the pressure point in the centrifugal compressor appearing on the display is in Zone II, there is no need to check the pressure in the centrifugal compressor. The inlet guide vane is to be opened for short pulse, i.e. 200 millisecond so enable the pressure point away from the surge line.
In this case, the inlet guide vane is not to be given close pulse.

If the pressure point in the centrifugal compressor appearing on the display is in zone I or zone beyond P, then:
Illustration 6.
If the pressure point in the centrifugal compressor appearing on the display is in Zone I, then there is no need to check the pressure in the centrifugal compressor. The inlet guide vane is to be opened for long pulse, i.e. 1000 millisecond so enable the pressure point away from the surge line.
In this case, the inlet gas valve is not to be given close pulse.
, Claims:We Claim:

1. A method for surge detection and surge control in a compressor comprising of a PID control device, current input, pressure input, blow off valve, inlet guide vane, an auxiliary oil pump, main motor, display screen
Which is characterized by
Receiving information on the display screen based on parameters entered into the PID control device;
Opening or closing of inlet guide vane depending upon the zone of the pre-decided pressure in the compressor to control the surge and stabilize the compressor;
maintaining the pressure in the compressor in the manner that the pre-decided pressure in the compressor appearing on the displays screen stays in the control zones IV, V or VI.
2. The method of surge detection and surge control in a compressor as claimed in claim 1,
Wherein when the motor current is less and the pressure in the compressor is lower than the pre-decided pressure i.e. zone VI or beyond S, the inlet guide vane is opened for longer duration and when the pressure in the compressor is higher than the pre-decided pressure in the compressor, the inlet guide vane is closed for longer duration.

3. The method of surge detection and surge control in a control in a compressor as claimed in claim 1, wherein, when the motor current is lower than as claimed in claim 2 i.e. Zone V or zone S and the pressure in the compressor is lower than the pre-decided pressure in the compressor, the inlet guide vane is opened for short duration and when the pressure in the compressor is more than the pre-decided compressor pressure, the inlet guide vane is closed for short duration.

4. The method of surge detection and surge control in a control in a compressor as claimed in claim 1, wherein, when the motor current is lower than as claimed in claim 2 i.e. Zone IV or zone R and the pressure in the compressor is higher than the pre-decided pressure in the compressor, the inlet guide vane needs no operation and if the pressure in the compressor is lower than that pre-decided pressure in the compressor, the inlet guide vane is opened for longer duration.

5. The method of surge detection and surge control in a compressor as claimed in claim 1, wherein, when the motor current is lower than as claimed in claim 2 i.e. zone III or zone Q and the pressure in the compressor is higher than the pre-decided pressure in the compressor, the inlet guide vane requires no modulation. If the pressure in the compressor is lower than the pre-decided pressure in the compressor, the inlet guide vane is opened for longer duration. At the same time, the blow-off valve will open to maintain the discharge pressure to the pre-decided pressure set in the compressor.

6. The method of surge detection and surge control in a compressor as claimed in claim 1, wherein, when the motor current is lower than as claimed in claim 2 i.e. zone II or zone P, the inlet guide vane is opened for short duration to control the surge of pressure in the compressor.

7. The method of surge detection and surge control in a compressor as claimed in claim 1, wherein, when the motor current is lower than as claimed in claim 2 i.e. zone I or zone beyond P, the inlet guide vane is opened for longer duration to bring the compressor from the surge zone to the control zone.