WO2013048265A1

WO2013048265A1 - Gas density sensor package for measuring polytropic efficiency of a charge gas compressor

Info

Publication number: WO2013048265A1
Application number: PCT/PL2011/000097
Authority: WO
Inventors: Hua Xia; Giorgio Greco; Patrick LUCAS; Vincenzo SANGIORGIO; Kanishk Rastogi; Salim JONES; Rafal JURKOWSKI
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2011-09-29
Filing date: 2011-09-29
Publication date: 2013-04-04
Anticipated expiration: 2014-03-29
Also published as: TW201321604A

Abstract

A gas sensor package for monitoring the polytropic efficiency of a charge gas compressor is provided. The sensor package includes a housing having an inlet and an outlet, both in fluid communication with the charge gas compressor. The sensor package also including a mass flow controller that controls the flow of gas received from the charge gas compressor through the sensor package. A switch that is in selective flow communication with a pair of sensing units, each unit including a fiber gas optical sensor therewithin to measure the temperature drop of heated gas from the switch. The switch selectively flowing gas to the two fiber gas sensors such that the wavelength differential between the temperature measured by each fiber gas sensor is used to determine the effective gas molecular weight, gas density, and associated polytropic efficiency of the charge gas compressor with other measured pressures and temperatures.

Description

GAS DENSITY SENSOR PACKAGE FOR MEASURING POLYTROPIC EFFICIENCY OF A CHARGE GAS COMPRESSOR

FIELD OF THE INVENTION

The present invention relates to systems and methods for monitoring using sensing technologies and, more particularly, to monitoring using fiber optic sensing modules for measuring various operational parameters from charge gas compressor used in ethylene production facilities.

BACKGROUND OF THE INVENTION

It is well known that in an ethylene production unit, fouling is a phenomenon that may significantly limit the performance of the charge gas compressor and affect inter-stage coolers, and therefore the entire operation of the ethylene production unit. For most ethylene production plants, the operation of the charge gas compressor has historically been a critical bottleneck. The compressor often suffers from heavy fouling which requires a dedicated plant stoppage for cleaning purposes. Sometimes these cleanings are required on a yearly basis. Such fouling of the compressor reduces the efficiency of the compressor. Fouling control and prevention are, therefore, very critical processes, and several methods have been used to accomplish this goal, either alone or in combination. Whatever method is used to control fouling, or even when no fouling controls are in place, monitoring the machine performance . and operation conditions of the charge gas compressor is of extreme importance for every ethylene producer, either in planning the production or in determining the maintenance schedule.

Industrial compressors have shown frequent mechanical and thermal anomalies that significantly induce fouling formation and thereby reduce compressor polytropic efficiency. When fouling occurs, the turbine speed is increased in variable speed machines, or recycles are closed in fixed speed machines. Due to loss of the internal energy or heat due to fouling, the polytropic efficiency of a compressor very likely deviates from adiabatic condition. Such a loss of the heat could arise from the fouling formation that actually increases thermal resistance or flux. It becomes critical to continuously monitor the compressor's thermodynamic behavior in steady and transient temperature, pressure, and even vibration.

Compressor polytropic efficiency, n=(k-l)/k*LnP2/Pl)/Ln(T2/Tl), where k=C_p/C_v, the ratio of the constant-pressure specific heat over the constant-volume specific heat; T and P are suction and discharge temperature and pressure. This efficiency mainly depends upon cracked gas composition or k factor, suction/discharge temperatures and pressures. Any change from these parameters will lead to corresponding polytropic efficiency change. If pressure and flow rate remain constant and the temperature change of the gas composition can be monitored, this enables by far a more accurate understanding of the machine condition, plus it allows the possibility of individuating the stages that are more subject to fouling severity. Existing temperature, pressure, and flow sensors have been used as basic system operation indicators. In general, simultaneously measuring both temperature and pressure from a compressor machine can be practically done with conventional pressure gauges and thermocouples.

Although the suction and discharge pressure and temperature are easily measured with existing detection technologies, it is relatively challenging to analyze gas composition to get k factor in real time with gas chromatography (GC) or micro gas chromatography (MGC) because of time-consuming and complicated instrument field calibration. In a normal case, the gas analysis could take five to 10 minutes. In other cases, the gas samples are extracted and sent to remote laboratories for analysis, which may take hours or days. Moreover, three distinct instruments, namely, thermometers or thermocouples, pressure gages, and gas chromatography, are required for compressor performance variation monitoring. Such a method proves difficult for providing online accurate measurement of the compressor efficiency and thereby cannot provide accurate information on the condition of the machine in real time.

A need therefore exists for a gas sensor package that can be fluidly connected to the charge gas compressor for easily determining the polytropic efficiency of each stage of the compressor in real-time. A need also exists for a gas sensor package that can minimize the number of different sensors needed to determine the polytropic efficiency of each stage of the compressor.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention, a charge gas compressor monitoring system for monitoring cracked effective molecular weight and gas density of charge gas compressor is provided. The monitoring system includes a gas sensor package that having a housing, a gas inlet, and a gas outlet, wherein the gas inlet and the gas outlet are in fluid connection with each stage of a compressor. The gas sensor package includes a mass flow controller positioned within the housing, wherein the mass flow controller is in fluid communication with the gas inlet for receiving a gas. A first sensing unit is positioned within the housing, and the first sensing unit is in fluid communication with the mass flow controller. A switch positioned within the housing, and the switch is actuatable between a first operating mode and a second operating mode. A second sensing unit is positioned within the housing, and the second sensing unit in selective fluid communication with the switch and the gas outlet. A first data signal is generatable by the second sensing unit through a first optical fiber. A third sensing unit is positioned within the housing, and the third sensing unit is in selective fluid communication with the switch and the gas outlet. A second data signal is generatable by the third sensing unit through a second optical fiber. The timer is used to control switcher for delivering gas alternatively to second and third gas sensing units. An optical interrogator is operatively connected to the first sensing unit and the second and third sensing units by way of the first and second optical fibers, wherein said the interrogator receives the first and second data signals from the second and third sensing units and relays the first and second data signals. A controller receives the first and second data signals from the optical interrogator, wherein the controller includes a processor for comparing the first and second data signals to determine the cracked gas effective molecular weight and gas density.

In another aspect of the present invention, a gas sensor package for measuring cracked effective molecular weight and gas density of a charge gas compressor is provided. The sensor package includes a mass flow controller in fluid communication with the compressor for extracting a gas from the compressor and controlling flow of the gas. The gas sensor package further includes a heater for heating the gas to a first temperature. Second fiber gas sensor is in alternating fluid communication with the mass flow controller, wherein the second fiber gas sensor generates a first output signal representing a first change in temperature relative to the first temperature. A third fiber gas sensor is also in alternating fluid communication with the mass flow controller, wherein the third fiber gas sensor generates a second output signal representing a second change in temperature relative to the first temperature, wherein the second and third output signals are comparable to determine the cracked effective molecular weight and gas density from a specific compressor stage.

Advantages of the present invention will become more apparent to those skilled in the art from the following description of the embodiments of the invention which have been shown and described by way of illustration. As will be realized, the invention is capable of other and different embodiments, and its details are capable of modification in various respects.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

These and other features of the present invention, and their advantages, are illustrated specifically in embodiments of the invention now to be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a schematic diagram of a charge gas compressor train;

FIG. 2 is a schematic diagram of a charge gas compressor monitoring system;

FIG. 3 is an embodiment of a sensor unit having a fiber Bragg grating sensor in a thermal capacitor-like structure;

FIG.4 is typical fiber sensor spectra from three sensing units;

FIG.5 is a process for differentiating two fiber sensor signals for determining gas molecular weight;

FIG. 6 is a chart illustrating the amount of measured wavelength shift with respect to estimated gas molecular weight and gas density from hydrogen balanced hydrocarbon gas mixture, and

FIG.7 is sensor package installation method in a gas charge compressor. It should be noted that all the drawings are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these figures may have been shown exaggerated or reduced in size for the sake of clarity and convenience in the drawings. The same reference numbers are generally used to refer to corresponding or similar features in the different embodiments. Accordingly, the drawing(s) and description are to be regarded as illustrative in nature and not as restrictive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 , a schematic of an exemplary embodiment of a charge gas compressor monitoring system 10 for monitoring the polytropic efficiency of a compressor train 1 1 for use in an ethylene processing unit for providing a five-stage system is shown. Although a five-stage system is shown and described below, it should be understood that any number of stages can be used in conjunction with the concepts and systems described herein. In typical ethylene processing units, the charge gas compressor train 11 has historically been the critical bottleneck in production. It should also be understood by one of ordinary skill in the art that the concepts for monitoring and controlling a compressor used in conjunction with cracked gas, ethylene production, propylene production, or any other process utilizing a compressor.

Compressor polytropic efficiency depends on gas density, temperatures and pressures at suction and discharge positions. Another parameter that can also be used in conjunction with these parameters to indicate increased fouling and decreased efficiency of a compressor includes vibration of the compressor structure. Because these parameters vary between each of the compressor stages and are dominated by fouling severity, the resulting discharge temperatures, suction pressure, steam turbine speed, and driver horsepower can be increased, while the gas compression ratio and gas throughput can be decreased. Any change of these parameters will affect the efficiency of the compressor. Existing temperature, pressure, and flow sensors have been used as basic system operation indicators, and these sensors are able to provide substantially real-time feedback of the conditions. However, gas density measurement is extremely difficult to measure on a realtime basis. Although gas chromatography (GC) or micro gas chromatography (MGC) can be used to determine gas composition, these processes are time-consuming and require complicated instrument field calibration. As such, analysis of τεβ ΐίβ from these methods may be overly time consuming and does not provide real-time efficiency results of the compressor.

The compressor monitoring system 10, as shown in FIG. 1 includes a plurality of compressor stages 12 having an inter-stage cooler 14 positioned within each compressor stage 12. The compressor stages 12 are fluidly connected in series. A sensor package 16 can be positioned adjacent to both the inlet and outlet of each compressor stage 12. However, it should be understood that any number of sensor packages 16 can be used to determine the polytropic efficiency of each compressor stage 12 or the overall efficiency of the compressor train 11. Each sensor package 16 is operatively connected to an optical interrogator 20 which includes a laser for providing light through an optical fiber 18 to each sensor package 16 as well as a fiber sensor signal processor that receives a reflected wavelength of light from at least one fiber Bragg grating sensor disposed within the sensor package 16. The optical interrogator 20 is operatively connected to a controller 22 which can be located remotely from the optical interrogator 20.

FIG. 2 illustrated an embodiment of a monitoring system 10 configured to measure a gas composition variation induced gas density or molecular weight changes, in which the thermal capacitor-like sensor package 40 is actively controlled by an external power source, and the thermal capacitance change is determined by average gas composition thermal specific heat coefficient and effective gas density. At a constant flow rate, the thermal capacitance variation depends upon the effective gas density or molecular weight. A thermodynamic principle and theory can be used to convert the shift in a light signal wavelength from a fiber Bragg grating sensor formed on an optical fiber to the change in the estimated effective molecular weight(EMW) and gas density of a compressor by

ΕΜΨ(ΐ)=Α+Β*Δλ(ι)+ϋ*Δλ²(ί)+ϋ*Δλ³(ί)^ (1)

where A, a , B, b, C, and c are constants from a standard instrument and calibration process. The EMW and density can be directly converted by

EMW=(R*T/P)*p, (3) where R=8.3145 kJ/kmol.K, T and P are temperature and pressure. . The invented sensing unit is a cylindrical like structure that has a small gas channel with a fiber string located in the middle of structure. The gas molecule will flow through the gas channel and absorb thermal energy that induces channel temperature change. Depending upon the channel geometric size, gas flow rate, and gas inlet pressure, the gas density sensitivity could be effectively enhanced by reducing thermal convective coefficient. A fiber gas sensor has demonstrated a sensitivity of about 0.10 g/mol to effective molecular weight changes, from several gas mixtures (H₂-air, air-N₂, 0₂-C0₂

The sensor package 40 consists of a housing 42 having a gas inlet 44 and a gas outlet 46, wherein the inlet 44 and outlet 46 are in fluid communication with the compressor train 1 1 (FIG. 1). The sensor package 40 is positioned adjacent to the compressor train 1 1 such that compressed gas from one of the compressor stages 12 can be redirected into the sensor package 40 for testing and monitoring the gas. The gas inlet 44 is also in fluid communication with a separate gas line 48 that can be fluidly connected to a source of a known gas such as nitrogen (N₂) or other gas having known gas properties. The gas inlet 44 enters the housing 42 and is fluidly connected to a mass flow controller (MFC) 50 or other similar device configured to control the gas flow rate through the sensor package 40. In one embodiment, the MFC 50 is fluidly connected to a first sensing unit 52a, and the first sensing unit 52a is fluidly connected to a switch 54.

As shown in FIG. 2, the switch 54 is operatively connected to a switch controller 56 which is, in turn, operatively connected to the controller 22. The switch controller 56 is configured to control the operation of the switch 54. In an embodiment, the switch 54 is in selective fluid communication with a second sensing unit 52b and a third sensing unit 52c. It should be understood by one of ordinary skill in the art that although the switch 54 is illustrated as being fluidly connected two only two sensing units 52, the switch 54 may be connected to any number of sensing units 52 for increasing the accuracy of the measured gas characteristic(s). The switch controller 56 is configured to actuate the switch 54 between a first operative mode in which the switch 54 allows the compressed gas from the first sensing unit 52a to flow to the second sensing unit 52b while preventing the compressed gas from flowing to the third sensing unit 52c and a second operative mode in which the switch 54 allows the compressed gas from the first sensing unit 52a to flow to the third sensing unit 52c while preventing the compressed gas from flowing to the second sensing unit 52b. Both the second and third sensing units 52b, 52c are in fluid communication with the gas outlet 46 such that the compressed gas flows from either of the sensing units through the gas outlet 46 and back into the gas stream of the compressor train 11.

Each sensing unit 52a, 52b, 52c includes a heater 58 that is configured to heat the compressed gas as it flows through the sensing unit, as shown in FIG. 2. The heaters 58 are operatively connected to a PID power controller 60 that is configured to provide power to the heaters 58. The PID power controller 60 is also operatively connected to the controller 22. The sensor package 40 is controlled by software that primarily controls the PID power controller 60 settings and keeps feedback control for keeping the gas cell 70 at a substantially constant temperature. The time interval for such a feedback control could be as fast as 10Hz, which can limits the temperature fluctuation, or thermal noise, in the order of 0.1 °C peak-to-peak. In configuring the controller 22, a user can set gas cell 70 temperature, duration of the gas pass through the switch 54 to a specific sensor unit 52b, 52c, and loading of the gas density or concentration calibration table.

Each of the sensing units 52a, 52b, 52c is operatively connected to the optical interrogator 20 which is, in turn, operatively connected to the controller 22. The optical interrogator 20 is connected to each of the sensing units 52a, 52b, 52c by at least one optical fiber 18 that allows a light signal to be sent from the optical interrogator 20 therethrough to the sensing units. A thermocouple (TC) 62 is positioned adjacent to each of the sensing units 52a, 52b, 52c for measuring the localized temperature of the respective sensing unit. The TC 62 provides a feedback to the PID power controller 60.

The gas density of the compressed gas from the compressor train 11 is measured by flowing compressed gas through the sensor package 40, and the thermal capacitance of the compressed gas is changed with averaged gas composition variation. The thermal capacitance change is done mainly by varying the internal temperature which is measured by fiber Bragg grating sensors. The relative wavelength shift produced by each fiber Bragg grating sensor is converted to gas molecular weight, which can then be used to determine the estimated polytropic efficiency of the compressor.

Each sensing unit 52, as shown in FIGS. 2-3, includes an enclosed, sealed cell 70 that forms a shell about at least one optical fiber 18 extending at least partially though the sensing unit 52. The optical fiber 18 is connected to the optical interrogator 20, and the optical interrogator 20 is configured to provide a light signal through the optical fiber 18 to the cell 70. The sensing unit 52 includes a gas inlet 72 and a gas outlet 74, wherein gas from the compressor flows through the sensing unit 52 from the inlet 72 to the outlet 74. At least one fiber gas sensor 76 is formed along the length of the optical fiber 18 within the cell 70. In an embodiment, only one fiber gas sensor 76 is formed on the optical fiber 18. In another embodiment, multiple fiber gas sensors 76 are formed on the optical fiber 18. It should be understood by one of ordinary skill in the art that any number of fiber gas sensors 76 can be formed on the optical fiber 18 within the same sensing unit 52. Each FBG sensor 76 is configured to produce a signal in response to a particular operating parameter of the compressed gas flowing through the sensing unit 52.

In operation, either compressed gas from the compressor train 11 or a known gas such as N₂ enters the sensor package 40 through the gas inlet 44 and into the MFC 50. The MFC 50 controls the flow of gas through the rest of the sensor packet 40, thereby providing a constant pressure and flow rate - two important variables in determining the polytropic efficiency of the compressor train 11. The gas then flows from the MFC 50 to the first sensing unit 52a. Because the temperature of the gas coming from a source external to the sensor package 40 can vary depending upon time of day or season, the heater 58 of the first sensing unit 52a is configured to increase the temperature of the gas. In one embodiment, the heater 58 is operatively connected to wires or other heating elements that are positioned immediately adjacent to the walls of the cell 70 to provide heat energy to the gas as it flows through the first sensing unit 52a. The fiber Bragg grating sensor 76 within the first sensing unit 52a receives a light signal from the optical interrogator 20 and reflects a return wavelength signal to the optical interrogator 20 that can be used to determine a localized measured temperature of the gas within the first sensing unit 52a.

The gas exits the first sensing unit 52a and enters the switch 54. The remotely located switch controller 56 actuates the switch 54 to determine the path that the gas will take as it exits the switch 54. The switch controller 56 actuates the switch 54 so that it directs the gas to the second sensing unit 52b for a period of time and then the switch 54 is actuated again so that it then directs the gas to the third sensing unit 52c. In one embodiment, the time period that the switch 54 allows gas to flow to each of sensing units 52b, 52c is the same between switches. For example, the switch 54 allows the gas to flow to the second sensing unit 52b for five second, then switches such that the gas flows to the third sensing unit 52c for five seconds before it then directs the gas to flow to the second sensing unit 52b for another five seconds, and so on. In another embodiment, the time period that the second and third sensing units 52b, 52c receive gas from the switch 54 may vary. It should be understood by one of ordinary skill in the art that the switch controller 56 can be programmed to provide any switching sequence for directing gas flow from the switch to the second and third sensing units 52b, 52c.

Similar to the first sensing unit 52a, as the gas enters each of the second and third sensor units 52b, 52c, the temperature of the gas is increased by the heater 58 having wires, coils, or other heating elements positioned immediately adjacent to the cell 70 wall. The fiber gas sensor 76 within the sensing unit 52 receives a light signal from the optical interrogator 20 and reflects a return wavelength signal to the optical interrogator 20 that can be used to determine a localized measured temperature of the gas within the sensing unit 52. As the second and third sensing units 52b, 52c continue to provide signals to the optical interrogator 20, those signals or wavelengths are then converted to data that is transmitted to the controller 22. The differential in the sensed wavelengths of the second and third sensing units 52b, 52c is used to calculate the effective molecular weight of the gas, which is then used to calculate an estimated polytropic efficiency of the particular compressor stage 12. The gas then is transferred from the second or third sensing unit 52b, 52c to the gas outlet 46 of the sensor package 40.

Thermal energy that is lost during gas flow into the thermal capacitor sensor package 52 must induce a corresponding temperature dropping in the thermal capacitor package structure, which depends upon the effective gas specific heat capacity, composition variation, and flow rate. A cold gas stream will absorb thermal energy when it enters into the sensor package 52 and induce a thermal gradient from cell 70 wall to the central axis. The FBG sensors 76 installed along the central axis of the sensor package 52 responds to gas composition change by way of measured temperature variations. Since the thermal capacitor cell 70 is actively controlled by the PID power controller 60, high-energy the fiber gas sensors 76 quickly lose its thermal energy to the gas stream by down shifting its Bragg grating resonant wavelength that is proportional to temperature dropping in the cell 70 within a few seconds. A thermodynamic equilibrium or thermal gradient profile is established between the gas and the cell 70 walls, and any variation mainly dominates by gas composition change, or effective gas density or gas molecular weight.

The compressor monitoring system 10 is configured to monitor the cracked gas molecular weight and density and the diagnostics for any kinds of multi-stage compressors. By measuring averaged gas density and molecular weight variation the measured wavelength shift in the fiber gas sensors can be correlated with a polytropic efficiency change. The compressor monitoring system 10 provides a method and sensor package 40 for measuring gas density or effective gas molecule weight that are critical control parameters that represent the performance of a compressor.

Thermal capacitance variation can be used for measuring any bulk gas and its effective density. The thermal capacitor sensor package energy density is rechargeable like a battery so that recalibration can be easily conducted by operating the sensing unit 52 under pulse mode by the switch 54. This merit is of great value for any gas density detection in monitoring compressor polytropic efficiency. A fiber Bragg grating sensor is not required to measure only a single chemical or compound in a gas but can instead measure bulk gas for which the fiber gas sensor is highly responsive with respect to the thermal capacitor energy density variation by the gas medium. The sensor package 40 enables in-field online gas analysis for industrial systems that use compressed gases. The sensor package 40 also enables temperature, pressure, humidity, vibration, strain/displacement to be measured as extended sensing capability. All fiber gas sensors 76 are powered by light and thus avoid electromagnetic interference issues.

The compressor monitoring system 10 utilizes fiber gas sensors 76 as gas density sensing devices that are sealed in a gas cell 70, in which the thermal capacity of the fiber gas sensor 76 is actively controlled by an external power source, such as the optical interrogator 20, and the thermal energy exchange between the compressor gas and fiber gas sensor 76 is determined by gas thermal specific heat capacity and thermal convection effect. At a constant flow rate, the thermal energy variation depends upon the gas composition or gas density under zero-order approximation:

Δλ_Β(ί) = K_T/h · f · Cp · p ^■ [1 + Δ p(t)l p] = λ(0)^■ [1 + Δ p(t)/ p] (3) where /is gas flow rate, p is gas density, determined by effective molecular weight, C_p the constant-pressure specific heat, κ_τ is fiber gas thermal sensitivity, and h is the thermal convective coefficient.

When the fiber gas sensor is thermally activated, it can exchange thermal energy with low thermal energy gas stream. The thermal loss of the gas sensor depends upon the gas specific heat and density. Since increasing the gas mass density will increases the thermal energy loss, the gas density is simply determined by measuring the wavelength shift of the FBG sensor 76. An optical interrogator 20 is used to detect fiber gas sensor 76 with lHz to 10Hz data rate. And a PID controller 60 is used to maintain the cell 70 of the sensing unit 52 in constant temperature with thermocouple-provided feedback control. A solid relay, or switch 54, is used to control the duration of the gas passing to the second sensing module 52b or the third sensing module 52c. The differential response amplitude of the fiber gas sensor 76 in each of the second and third sensing modules 52b, 52c is used to calculate the gas effective molecular weight and density.

FIG.4 has shown reflectance spectra from fiber gas sensors (FGS) that are used from compressor monitoring system 10. The central wavelength of each fiber sensor is determined by its fiber core effective refractive index (n) and grating pitch size (A) by λ=2*η*Λ. The peak seen from each fiber sensor can be tracked by a software in picometer (pm) unit.

FIG.5 further illustrates that the signal process from second and third fiber sensor under timer and switcher control. Since the gas is delivered to each gas cell by timer/switcher control with a pre-setting time, the sensing signal from each sensor will delayed by half period (a). The differentiation of two sensors can give peak-valley like signal (b), where the sensor response amplitude is determined by peak-valley difference, (c). Using a calibration transfer function provided by Eqs. (1) and (2) to get effective molecular weight or gas density (d).

FIG. 6 illustrates exemplary data from a hydrocarbon gas mixture where the hydrogen gas, as balance gas, is used to decrease effective gas molecular weight. The dependence of the wavelength shift versus gas molecular weight could be fitted to a nonlinear polynomial transfer function. For gas purity, quality, and cleanness monitoring, a linear transfer function will be first calibrated and will provide sufficient accuracy. For unknown gas mixture analysis, a cubic polynomial transfer function will ensure reliable accuracy.

The sensor package 40 achieves high accuracy by minimizing the effects of the variables such as pressure, temperature, and flow rate, while providing insensitivity to gas inlet temperature variations. Since the optical interrogator has a peak tracking algorithm for lpm accuracy, the absolute accuracy is therefore mainly defined by the accuracy of calibration and correction applied. The repeatability of measurement is ±lpm or ± (0.010- 0.025) g/mol regardless of density range. The long-term stability of sensor package 40 is mainly governed by the thermal stability of the fiber gas sensors 76. The FBG sensors 76 are based on tetrahedral structured fiber Bragg grating and, being bonded with high- temperature adhesive onto a sensing rake in the gas cell 70, will maintain its thermal response properties for many years. However, corrosion and deposition on the fiber gas sensor 76 surface will degrade the long-term stability, and hence, care should be taken to ensure that the process gas is clean. The possibility of deposition is reduced by the use of coalescing filters but, should deposition take place, the sensing rake has to be replaced.

Instrumented air or nitrogen gas can be used for gas analyzer calibration in the field, and automatic calibration can be done before, after, or during a gas test process, as shown in FIG.7. Gas inlet temperature will vary with ambient conditions which may change from winter to summer or from morning to afternoon. To ensure gas accuracy, the monitoring system 10 includes a pre-heating method to keep constant gas inlet temperature so that the gas composition analysis avoids temperature fluctuation induced errors. On the other hand, to measure gas properties from a high-pressure and temperature gas charge compressor, additional mechanical fittings have to be used as indicated in FIG.7. These mechanical fittings include pressure regulator, flow and pressure indicators, back-pressure regulator, and T-type valves. The exhaust gas has to be recycled back to compressor to avoid pollutants to environment.

While preferred embodiments of the present invention have been described, it should be understood that the present invention is not so limited and modifications may be made without departing from the present invention. The scope of the present invention is defined by the appended claims, and all devices, process, and methods that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.

Claims

CLAIMS:

1. A charge gas compressor monitoring system for monitoring polytropic efficiency of charge gas compressor comprising: a gas sensor package that includes a housing having a gas inlet and a gas outlet, wherein said gas inlet and said gas outlet are in fluid connection with said compressor, said gas sensor package comprising:

a mass flow controller in fluid communication with said gas inlet for receiving a gas; a first sensing unit in fluid communication with said mass flow controller;

a switch positioned in fluid communication with said first sensing unit, said switch being actuatable between a first operating mode and a second operating mode;

a second sensing unit in selective fluid communication with said switch and said gas outlet, wherein a first data signal is generatable in real-time by said second sensing unit through a first optical fiber; and

a third sensing unit in selective fluid communication with said switch and said gas outlet, wherein a second data signal is generatable in real-time by said third sensing unit through a second optical fiber;

an optical interrogator operatively connected to said first sensing unit and said second and third sensing units by way of said first and second optical fibers, wherein said optical interrogator receives said first and second data signals from said second and third sensing units and relaying said first and second data signals;

a controller for receiving said first and second data signals from said optical interrogator, said controller includes a processor for comparing said first and second data signals to determine said polytropic efficiency of said compressor.

2. The charge gas compressor monitoring system of Claim 1, wherein said fiber gas sensor of said second sensing unit and said fiber gas sensor of said third sensing unit measure an operating parameter of said compressor.

3. The charge gas compressor monitoring system of Claim 2, wherein said operating parameter is temperature.

4. The charge gas compressor monitoring system of Claim 1 further comprising a switch controller, wherein said switch controller is operatively connected to said controller and said switch, said switch controller selectively actuates said switch between said first operating mode and said second operating mode.

5. The charge gas compressor monitoring system of Claim 4, wherein said switch is in fluid communication with said second sensing unit when said switch is in said first operating mode, and said switch is in fluid communication with said third sensing unit when said switch is in said second operating mode.

6. The charge gas compressor monitoring system of Claim 1, wherein each of said sensing units is formed of a sealed cell having an inlet and an outlet, said gas flowable between said inlet and said outlet of said cell.

7. The charge gas compressor monitoring system of Claim 6, wherein at least said second and third sensing units contains at least one fiber gas sensor positioned therein for measuring at least one compressor operating parameter.

8. The charge gas compressor monitoring system of Claim 7, wherein said at least one compressor operating parameter is temperature.

9. The charge gas compressor monitoring system of Claim 1, wherein a first fiber gas sensor is formed on said first optical fiber within said second sensing unit, and a second fiber gas sensor is formed on said second optical fiber within said third sensing unit, said first and second fiber gas sensors generate said first and second data signals.

10. A gas sensor package for use in a charge gas compressor monitoring system for monitoring polytropic efficiency of a charge gas compressor, said the gas sensor package comprising:

a mass flow controller in fluid communication with said charge gas compressor, said mass flow controller receiving a gas from said charge gas compressor;

a first sensing unit in fluid communication with said mass flow controller for receiving said gas therefrom;

a switch in fluid communication with said first sensing unit for receiving said gas therefrom;

a second sensing unit in fluid communication with said charge gas compressor, and said second sensing unit in selective fluid communication with said switch; and a third sensing unit in fluid communication with said charge gas compressor, and said third sensing unit in selective fluid communication with said switch; wherein each of said sensing units including an optical sensor for generating a realtime data signal representing a measured operating parameter of said gas.

1 1. The sensor package of Claim 10, wherein said optical sensor is a fiber gas sensor, and said fiber gas sensor generates a data signal in response to said operating parameter of said gas.

12. The sensor package of Claim 10, wherein said operating parameter of said gas is temperature.

13. The sensor package of Claim 10, wherein said switch is actuatable between a first operating mode and a second operating mode such that when said switch is in said first operating mode said switch is in fluid communication with said second sensing unit and when said switch is in said second operating mode said switch is in fluid communication with said third sensing unit.

14. The sensor package of Claim 10 further including a heater operatively connected to each of said sensing units for heating said gas.

15. The sensor package of Claim 14 further including a thermocouple positioned immediately adjacent to each of said sensing units, said thermocouples providing a feedback signal for controlling said heaters.

16. A sensor package for measuring polytropic efficiency of a charge gas compressor comprising:

a mass flow controller in fluid communication with said compressor for receiving a gas from said compressor and controlling flow of said gas;

a heater for heating said gas to a first temperature;

a first fiber gas sensor in alternating fluid communication with said mass flow controller, said first fiber gas sensor generating a first output signal representing a first change in temperature relative to said first temperature; and

a second fiber gas sensor in alternating fluid communication with said mass flow controller, said second fiber gas sensor generating a second output signal representing a second change in temperature relative to said first temperature;

wherein said first and second output signals are comparable to determine said polytropic efficiency in real-time.

17. The sensor package of Claim 16, wherein each fiber Bragg grating sensor is disposed within a separate sealed cell.

18. The sensor package of Claim 16 further comprising a switch fluidly positioned between said mass flow controller and said first and second fiber gas sensors, wherein said switch alternates said flow of gas between said first fiber gas sensor and said second fiber gas sensor.