AN APPARATUS FOR OPTIMAL LOADSHARING BETWEEN PARALLEL GAS COMPRESSORS

Abstract

A gas compressing system including a plurality of n compressors connected in parallel. Each compressor has a suction line connected to a common suction manifold and a discharge line connected to a common discharge manifold configured to deliver compressed gas to a downstream load. The system also includes a process controller configured to control an average speed of the compressors based upon a discharge pressure in the common discharge manifold or a discharge flow through the common discharge manifold. The system further includes an adaptive load sharing optimizing controller configured to determine the speed of each compressor in the plurality of n compressors. A method of controlling a gas compressing system is also provided.

Claims

1. A gas compressing system, comprising: a plurality of n compressors connected in a parallel pneumatic circuit, each compressor in the plurality of n compressors having a suction line in pneumatic communication with a common suction manifold configured to receive a gas stream from an upstream source and a discharge line in pneumatic communication with a common discharge manifold configured to deliver compressed gas to a downstream load; a process controller configured to control an average speed of the plurality of n compressors based upon an operator set point and at least one parameter selected from a list consisting of a discharge pressure in the common discharge manifold and a discharge flow through the common discharge manifold; an adaptive load sharing optimizing controller configured to control a speed of each compressor in the plurality of n compressors based on suction line pressure and flow and discharge line pressure in the plurality of n compressors and the average speed determined by the process controller; a plurality of n speed demand computation modules configured to produce a speed demand signal for each compressor in the plurality of n compressors,

2. The gas compressing system according to claim 1, wherein the plurality of n compressors include a first compressor driven by a first variable speed drive system and an n.sup.th compressor driven by an n.sup.th variable speed drive system, wherein the adaptive load sharing optimizing controller is in communication with first and n.sup.th speed demand computation modules, which computes first and n.sup.th speed demand signals supplied to the first and the n.sup.th variable speed drive systems, wherein the process controller is configured to calculate an average compressor speed (u.sub.avg) based upon at least one parameter selected from the list consisting of the discharge pressure in the common discharge manifold and the discharge flow through the common discharge manifold, and wherein the adaptive load sharing optimizing controller includes a load split parameter module configured to iteratively calculate a compressor load split parameter λ transmitted to first and the n.sup.th speed demand computation module configured to calculate a first speed signal (u.sub.1) and an n.sup.th speed signal (u.sub.n) transmitted to the first variable speed drive system and the n.sup.th variable speed drive system respectively based upon the average compressor speed (u.sub.avg) and the compressor load split parameter λ.

3. The gas compressing system according to claim 2, wherein λ.sub.i,k=[λ.sub.1,k λ.sub.2,k . . . λ.sub.n−1,k].sup.T for i.sup.th compressor and k.sup.th iteration of the compressor load split parameter λ, where Σ.sub.i=1.sup.n−1λ.sub.i,k=1, and 0<λ.sub.i,k<1.

4. The gas compressing system according to claim 2, wherein u.sub.1=λ.sub.1.Math.u.sub.avg, u.sub.2=λ.sub.2.Math.u.sub.avg, . . . , u.sub.n=(1−λ.sub.1−λ.sub.2 . . . −λ.sub.n−1).Math.u.sub.avg, such that Σ.sub.i=1.sup.n−1λ.sub.i=1 and 0<λ.sub.i<1.

5. The gas compressing system according to claim 2, further comprising a discharge pressure transducer configured to measure the discharge pressure in the common discharge manifold, wherein the process controller is a proportional-integral controller configured to determine the average compressor speed (u.sub.avg) based on a difference between an operator set point and the discharge pressure.

6. The gas compressing system according to claim 2, wherein the process controller determines the average compressor speed (u.sub.avg) according to the following equation: u.sub.avg(t)=K.sub.pe(t)+K.sub.i∫.sub.0.sup.te(t)dt, where K.sub.p is a proportional gain parameter, K.sub.i is an integral gain parameter, and e(t) is a difference between the operator set point and the discharge flow.

7. The gas compressing system according claim 2, wherein the adaptive load sharing optimizing controller includes an implied speed computation module configured to determine a first implied speed signal and an n.sup.th implied speed signal for i.sup.th compressor and k.sup.th iteration from the values of λ.sub.i,k, the average compressor speed (u.sub.avg) a first discharge pressure measured by a first pressure transducer in pneumatic communication with a first discharge line of the first compressor, a first suction flow through a first suction line measured by a first suction flow transducer in pneumatic communication with the first suction line, an n.sup.th discharge pressure measured by an n.sup.th pressure transducer in pneumatic communication with an n.sup.th discharge line of the n.sup.th compressor, an n.sup.th suction flow through an n.sup.th suction line measured by an n.sup.th suction flow transducer in pneumatic communication with the n.sup.th suction line, and an operator set point.

8. The gas compressing system according to claim 2, wherein first and n.sup.th implied speed signals are determined by solving nonlinear algebraic equations of pressure ratios P.sub.1=f.sub.1(q.sub.1, u.sub.1), P.sub.2=f.sub.2(q.sub.2, u.sub.2), . . . , P.sub.n=f.sub.n(q.sub.n, u.sub.n), for q.sub.1, q.sub.2, . . . , q.sub.n using stored performance characteristics of each compressor in the plurality of n compressors, where q.sub.1, q.sub.2, . . . , q.sub.n are the first through n.sup.th suction flows.

9. The gas compressing system according to claim 2, wherein the adaptive load sharing optimizing controller includes a consumed energy computation module configured to determine an energy required each compressor in the plurality of n compressors based upon a current value of λ.sub.i,k, a current first implied speed signal and a current n.sup.th implied speed signal, the first discharge pressure, the first suction flow, the n.sup.th discharge pressure, the n.sup.th suction flow, and the operator set point.

10. The gas compressing system according to claim 9, wherein a total efficiency Q is calculated using pre-computed compressor efficiency curves g where Q.sub.1=g.sub.1(q.sub.1, u.sub.1), Q.sub.2=g.sub.2(q.sub.2, u.sub.2), . . . , Q.sub.n=g.sub.n(q.sub.n, u.sub.n).

11. The gas compressing system according to claim 10, wherein an implied speed computation module computes the implied speed signal for each compressor from k.sup.th iteration of λ denoted as λ.sub.i,k and supplies the implied speed signal to the consumed energy computation module, which provides the energy required for each compressor for a given implied speed, wherein consumed energy computation module determines if the consumed energy at k.sup.th iteration of λ, denoted as λ.sub.i,k, is decreased compared to the previous iteration, a new k+1.sup.th iteration λ.sub.i,k+1 is calculated by the module for λ iteration in accordance with the consumed energy at the k.sup.th decreasing compared to the k−1.sup.th iteration, and no further iterations of λ are calculated by the module for λ iteration in accordance with the consumed energy at the k.sup.th not decreasing compared to the k−1.sup.th iteration.

12. The gas compressing system according to claim 11, wherein the module for λ iteration stops calculating iterations of λ in accordance with exceeding a time threshold for calculating an iteration of λ.

13. A method of controlling a gas compressing system having a plurality of n compressors connected in a parallel pneumatic circuit, wherein each compressor in the plurality of n compressors has a suction port in pneumatic communication with a common suction manifold configured to receive a gas stream from an upstream source and a common discharge manifold configured to deliver compressed gas to a downstream load, the method comprising: controlling a speed of the plurality of n compressors based upon at least one parameter selected from a list consisting of a discharge pressure in the common discharge manifold and a discharge flow through the common discharge manifold wherein the speed of each compressor is based on suction line pressure and flow and discharge line pressure of each compressor, and an operator set point.

14. The method according to claim 13, wherein the plurality of n compressors includes a first compressor driven by a first variable speed drive system and an n.sup.th compressor driven by an n.sup.th variable speed drive system, wherein the method further comprises: calculating an average compressor speed (u.sub.avg) by using a process controller, based upon at least one parameter selected from the list consisting of the discharge pressure in the common discharge manifold and the discharge flow through the common discharge manifold; iteratively calculating a compressor load split parameter λ; calculating a first speed signal (u.sub.1) and an n.sup.th speed signal (u.sub.n), based on the average compressor speed (u.sub.avg) and the compressor load split parameter λ; and transmitting the first speed signal (u.sub.1) and the n.sup.th speed signal (u.sub.n) to the first variable speed drive system and the n.sup.th variable speed drive system respectively.

15. The method according to claim 14, wherein the process controller is a proportional-integral controller, the method further comprising: measuring the discharge pressure in the common discharge manifold; and determining the average compressor speed (u.sub.avg) based on a difference between an operator set point and the discharge pressure.

16. The method according to claim 14, wherein the process controller is a proportional-integral controller, the method further comprising: measuring the discharge flow through the common discharge manifold; and determining the average compressor speed (u.sub.avg) based on a difference between an operator set point and the discharge flow;

17. The method according to claim 14, further comprising: measuring a first discharge pressure in a first discharge line of the first compressor, a first suction flow through a first suction line, an n.sup.th discharge pressure in an n.sup.th discharge line of the n.sup.th compressor, and an n.sup.th suction flow through an n.sup.th suction line; and determining a first implied speed signal and an n.sup.th implied speed signal for i.sup.th compressor and k.sup.th iteration from the values of λ.sub.i,k, the average compressor speed (u.sub.avg), the first discharge pressure, the first suction flow, the n.sup.th discharge pressure, the n.sup.th suction flow, and the operator set point, wherein the first implied speed signal and the n.sup.th speed signal are determined by solving nonlinear algebraic equations of pressure ratios using stored performance characteristics of each compressor in the plurality of n compressors.

18. The method according to claim 14, further comprising: determining an energy required each compressor in the plurality of n compressors based upon a current value of λ.sub.i,k, a current first implied speed signal and a current n.sup.th implied speed signal, the current first discharge pressure, the first suction flow, the n.sup.th discharge pressure, the n.sup.th suction flow, and the operator set point.

19. The method according to claim 18, further comprising: calculating a total efficiency Q using pre-computed efficiency curves.

20. The method according to claim 19, further comprising: determining if energy consumed at a k.sup.th iteration of λ denoted as λ.sub.i,k is decreasing compared to the previous iteration; calculating a new k+1.sup.th iteration of λ denoted as λ.sub.i,k+1 in accordance with the energy consumed at the k.sup.th iteration is decreasing compared to the k−1.sup.th iteration; not calculating any further iterations of λ in accordance with the consumed energy at the k.sup.th not decreasing compared to the k−1.sup.th iteration; and ceasing calculating iterations of λ in accordance with exceeding a time threshold for calculating an iteration of λ.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 shows a schematic diagram of a gas compression system with two dynamic gas compressors connected in parallel according to some embodiments.

[0026] FIG. 2 shows a schematic diagram of an adaptive load sharing optimizing controller of the gas compression system of FIG. 1 according to some embodiments.

[0027] FIG. 3 shows a graph illustrating precomputed discharge pressure vs. flow rate performance characteristics of a compressor at various compressor speeds according to some embodiments.

DETAILED DESCRIPTION

[0028] FIG. 1 shows a schematic diagram of a control system for a gas compression system comprising n dynamic compressors 8, 9 connected in parallel. In the illustrated example, n=2 but n can be any integer greater than 1. The first and second compressors 8,9 are configured to compress a gas entering a common suction manifold 1 and deliver the compressed gas into a common discharge manifold 12 and to a load 13. The first and second compressors 8, 9 are driven by first and second motors 6, 7. The speed of the first and second motors 4,5 and thereby the speed of the first and second compressors 8, 9, can be controlled by first and second variable speed drives 4, 5. Adaptive load sharing optimizing controller 15 controls the speed of the first and second motors 4, 5 via first and second motor speed signals 42, 44. The first and second compressors 8, 9 have first and second suction flow transducers 22, 23 configured to measure the rate of the volume of gas flowing into the first and second compressors 8, 9 through the first and second suction lines 2, 3 from a process upstream to the common suction manifold 1.

[0029] A first suction pressure transducer 26, first temperature transducer 27, first discharge pressure transducer 30, and first discharge temperature transducer 31 are provided for measuring a first suction pressure 45 and a first suction temperature 46 in the first suction line 2 and a first discharge pressure 47 and a first discharge temperature 48 respectively in the first discharge line 10 of the first compressor 8. A second suction pressure transducer 28, a second suction temperature transducer 29, a second discharge pressure transducer 32, and a second discharge pressure temperature transducer 33 are provided for measuring suction pressure 49, suction temperature 50 in the second suction line 3 and second discharge pressure 51 and discharge temperature 52 respectively in the second discharge line 11 of the second compressor 9.

[0030] The control system consists of a common process controller 14 that is configured to control a discharge flow 58 in the common discharge manifold 12 as measured by a flow transducer 35. The process controller 14 may alternatively/also be configured to control discharge pressure 57 in the common discharge manifold 12 which is measured by a pressure transducer 34. The control system also includes a separate adaptive load sharing optimizing controller 15 that optimizes load-sharing between the first and second compressors 8, 9. The control system further includes first and second anti-surge controllers 18, 19 to provide anti-surge protection for the first and second compressors 8, 9.

[0031] The process controller 14 determines a desired compressor speed u.sub.avg based on the operator set point 59 and the discharge pressure 57 or the discharge flow 58. The process controller 14 in one embodiment is implemented as a proportional integral (PI) controller, where the controller output u.sub.avg is determined according to the following equation: u.sub.avg(t)=K.sub.pe(t)+K.sub.i∫.sub.0.sup.te(t)dt, where K.sub.p is a tuning parameter described as the proportional gain constant, K.sub.i is also a tuning parameter described as the integral gain constant, while e(t) is the error variable being the difference between the value of the operator set point 59, which is ether a set discharge flow or a set discharge pressure, and the measured output process variable (either the discharge flow 58 or the discharge pressure 57).

[0032] As illustrated in FIG. 2, an adaptive load sharing optimizing controller 15 includes a load split parameter module 62 for iteratively calculating the compressor load split parameter λ 40 for n parallel compressors The load split parameter module 62 is an optimization module which uses methods of parametric optimization such as the simplex algorithm to generate the values of the compressor load split parameter λ 40. The load split parameter module 62 is an optimization module which uses available algorithms of parametric optimization. In one embodiment the simplex algorithm is used to generate values of the compressor load split parameter λ. The compressor load split parameter λ 40 for i.sup.th compressor and k.sup.th iteration is given as λ.sub.i,k=[λ.sub.1,k λ.sub.2,k . . . λ.sub.n−1,k].sup.T where Σ.sub.i=1.sup.n−1λ.sub.i,k≤1, and 0<λ.sub.i,k<1 for i=1,2, . . . , n−1. In one embodiment,

[00001]${\lambda }_{i,0}=\frac{0.5}{n}$

for n compressors where λ.sub.i,0 represents the initial value of λ.sub.i,k for the compressor load split parameter λ.

[0033] The adaptive load sharing optimizing controller 15 also includes an implied speed computation module 63 which computes the desired speeds u.sub.1, u.sub.2, . . . , u.sub.n for the compressor as follows: u.sub.1=λ.sub.1.Math.u.sub.avg, u.sub.2=λ.sub.2.Math.u.sub.avg, . . . , u.sub.n=(1−A.sub.1−λ.sub.2− . . . −λ.sub.n−1).Math.u.sub.avg such that Σ.sub.i=1.sup.n−1λ.sub.i,k≤1 and 0<λ.sub.i<1. The implied speed is a value varied in the implied speed computation module with the purpose to find an optimal value. Once the optimal value is determined, the compressor load split parameter λ is output from the module. The value of the compressor load split parameter λ is used to produce speed demands for each compressor via the speed demand computation modules 16, 17.

[0034] The speed demands of the compressor system are computed by solving nonlinear algebraic equations of pressure ratios P.sub.1=f.sub.1(q.sub.1, u.sub.1), P.sub.2=f.sub.2(q.sub.2, u.sub.2), . . . , P.sub.n=f.sub.n(q.sub.n, u.sub.n), for q.sub.1, q.sub.2, . . . , q.sub.n using pre-computed performance characteristics of each compressor, where q.sub.1, q.sub.2, . . . , q.sub.n are flows through the first compressor 8, the second compressor 9 . . . , the n.sup.th compressor and u.sub.1, u.sub.2, . . . , u.sub.n are the desired speed of the first compressor 8, the second compressor 9 . . . , the n.sup.th compressor.

[0035] The adaptive load sharing optimizing controller 15 may further include a consumed energy computation module 64 which is used to produce a signal representative 66 of an energy required for each compressor at current value of λ.sub.i,k, current speed demands 65. Both the implied speed computation module 63 and the consumed energy computation module 64 produce their output signals based on current operating points of the first discharge pressure 47 measured by the first discharge pressure transducer 30, the first suction line flow 60 through the first suction line 2 measured by the first suction flow transducer 22, the second discharge pressure 51 measured by the second discharge pressure transducer 32, the second suction line flow 61 through the second suction line 3 measured by the second suction flow transducer 23, and the operator set point 59. The consumed energy computation module 64 computes a signal representative of the energy required for the first and second compressors 8, 9 at current value of λ.sub.i,k, current speed demand and a current operating point of the first discharge pressure 47 of the first compressor 8, the flow through the first suction line 2 measured by the first suction flow transducer 22, the second discharge pressure 51 measured by the second discharge pressure transducer 32, the second suction line flow 61 through the second suction line 3 measured by the second suction flow transducer 23, and the operator set point 59. Total efficiency is calculated using pre-computed efficiency curves Q.sub.1=g.sub.1(q.sub.1, u.sub.1), Q.sub.2=g.sub.2(q.sub.2, u.sub.2), . . . , Q.sub.n=g.sub.n(q.sub.n, u.sub.n).

[0036] Referring now to FIG. 3, performance characteristics curves include multiple curves which represent pre-computed performance of a compressor as a function of discharge pressure, flow rate and speed of the compressor. These curves depict the performance under the standard condition (i.e., a curve indicative of the relationship of the pressure to the flow rate). N.sub.1 in FIG. 3 illustrates the situation where the compressor is running at the slowest speed of the pre-computed performance, the speed of the compressor increases as the operating point moves from N.sub.2 to N.sub.3, to N.sub.4, or to N.sub.5 the situation where the compressor is running at the fastest speed. The surge line indicates the surge limit of the compressor. These performance characteristic curves are used by the implied speed computation module 63 to compute speed demands in the adaptive load sharing optimizing controller 15.

[0037] The process controller 14 also includes first and second anti-surge controllers 18, 19 for the first and second compressors 8, 9, respectively, that are configured to manipulate the set-points for first and second recycle valves 20, 21. A first recirculation line 55 back feeds compressed gas from the first discharge line 10 into the first suction line 2 of the first compressor 8 through the first recycle valve 20 controlled by the first anti-surge controller 18. The second recirculation line 56 feeds the gas into the second suction line 3 of the second compressor 9, which also receives gas from the suction manifold 1. The first and second anti-surge controllers 18, 19 provide first and second recycle valve control signals 53, 54, respectively, to manipulate openings of the first and second recycle valves 20, 21, respectively, by means of actuators and positioners, so that the flow rate through the first or second compressor 8, 9 is increased by means of redirecting some of the gas flow from the first or second discharge lines 10, 11 through the first or second recycle valves 20, 21 to the first or second suction line 2, 3 and increasing the flow rate through the first or second compressor 8, 9.

[0038] While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention is not limited to the disclosed embodiment(s), but that the invention will include all embodiments falling within the scope of the appended claims.

REFERENCE NUMBERS

[0039] 1 Common Suction Manifold [0040] 2 First Suction Line [0041] 3 Second Suction Line [0042] 4 First Variable Speed Drive [0043] 5 Second Variable Speed Drive [0044] 6 First Motor [0045] 7 Second Motor [0046] 8 First Compressor [0047] 9 Second Compressor [0048] 10 First Discharge Line [0049] 11 Second Discharge Line [0050] 12 Common Discharge Manifold [0051] 13 Load [0052] 14 Process Controller [0053] 15 Adaptive Load Sharing Optimizing Controller [0054] 16 First Speed Demand Compensation Module [0055] 17 Second Speed Demand Compensation Module [0056] 18 First Anti-Surge Controller [0057] 19 Second Anti-Surge Controller [0058] 20 First Recycle Valve [0059] 21 Second Recycle Valve [0060] 22 First Suction Flow Transducer [0061] 23 Second Suction Flow Transducer [0062] 24 First Suction Flow Transducer Orifice Plate [0063] 25 Second Suction Flow Transducer Orifice Plate [0064] 26 First Suction Pressure Transducer [0065] 27 First Suction Temperature Transducer [0066] 28 Second Suction Pressure Transducer [0067] 29 Second Suction Temperature Transducer [0068] 30 First Discharge Pressure Transducer [0069] 31 First Discharge Temperature Transducer [0070] 32 Second Discharge Pressure Transducer [0071] 33 Second Discharge Temperature Transducer [0072] 34 Discharge Pressure Transducer [0073] 35 Discharge Flow Transducer [0074] 36 Discharge Flow Transducer Orifice Plate [0075] 37 Flow Set Point/Pressure Set Point [0076] 38 Discharge Flow/Discharge Pressure [0077] 39 Process Control Signal [0078] 40 Compressor Load Split Parameter λ [0079] 41 First Drive Speed Signal [0080] 42 First Motor Speed Signal [0081] 43 Second Drive Speed Signal [0082] 44 Second Motor Speed Signal [0083] 45 First Suction Pressure [0084] 46 First Suction Temperature [0085] 47 First Discharge Pressure [0086] 48 First Discharge Temperature [0087] 49 Second Suction Pressure [0088] 50 Second Suction Temperature [0089] 51 Second Discharge Pressure [0090] 52 Second Discharge Temperature [0091] 53 First Recycle Valve Control Signal [0092] 54 Second Recycle Valve Control Signal [0093] 55 First Recirculation Line [0094] 56 Second Recirculation Line [0095] 57 Discharge Pressure [0096] 58 Discharge Flow [0097] 59 Operator Set Point [0098] 60 First Suction Flow [0099] 61 Second Suction Flow [0100] 62 Compressor Load Split Parameter λ Calculation Module [0101] 63 Implied Speed Computation Module [0102] 64 Consumed Energy Computation Module [0103] 65 Current Speed Demand Signal [0104] 66 Required Energy Signal [0105] u Speed Demand Signal [0106] u.sub.1 First Speed Demand Signal [0107] u.sub.2 Second Speed Demand Signal