METHOD FOR STARTING AND OPERATING A PLANT FOR THE LIQUEFACTION OF A GASEOUS PRODUCT

Abstract

A method for starting and operating a plant for the liquefaction of a gaseous product comprising the steps of electrically connecting a variable frequency drive to a motor of a first machine string; increasing the speed of the motor of the first machine string up until a first predefined threshold; electrically disconnecting the variable frequency drive from the motor of the first machine string; electrically connecting the variable frequency drive to a motor of a second machine string; the first predefined threshold is function of said frequency of the power supply grid. The variable frequency drive can be switched during operation of the plant among the strings according to process requirements.

Claims

1. A method for starting and operating a plant for the liquefaction of a gaseous product, said plant comprising at least a first and a second machine string, each string comprising an electrical motor and a load mechanically connected to the electric motor, the plant comprising a variable frequency drive connected to a power supply grid having an its own frequency and associable to the electrical motors, the method comprising the steps of: electrically connecting the variable frequency drive to the motor of the first machine string; increasing the speed of the motor of the first machine string up until a first predefined threshold is reached, wherein the first predefined threshold is function of said frequency of the power supply grid; electrically disconnecting the variable frequency drive from the motor of the first machine string and electrically connecting the motor of the first machine string to the power supply grid; electrically connecting the variable frequency drive to the motor of the second machine string; and regulating the speed of the motor of the second machine string by said variable frequency drive.

2. The method according to claim 1, wherein the speed of the motor of the first machine string is substantially maintained at the first predefined threshold.

3. The method according to claim 1, wherein the step of electrically connecting the power supply grid is performed substantially at the same time of the step of electrically disconnecting the variable frequency drive.

4. The method according to claim 1, wherein the steps of electrically disconnecting the variable frequency drive from the motor of the first machine string and electrically connecting the variable frequency drive to the motor of the second machine string occur almost simultaneously.

5. The method according to claim 2, wherein the step of regulating the speed of the motor of the second machine string is performed after a second predefined threshold has been reached.

6. The method according to claim 1, wherein the variable frequency drive and the power supply grid can be switched between each of the motors.

7. The method according to claim 1, wherein said first predefined threshold is comprised between 1500 rpm and 5000 rpm.

8. The method according to claim 1, wherein said first predefined threshold is equal to 3000 rpm when the power supply grid has a frequency of 50 Hz or to 3600 rpm when the power supply grid has a frequency of 60 Hz.

9. The method according to claim 1, wherein said first predefined threshold is equal to the second predefined threshold.

10. A plant for the liquefaction of a gaseous product comprising at least a first and a second machine string, each string comprising an electrical motor and a load mechanically connected to the electric motor; the plant comprising a variable frequency drive connectable to a power supply grid having an its own frequency and associated to the electrical motors, said variable frequency drive being configured to be electrically connected to each of the motors.

11. The plant according to claim 10, wherein the variable frequency drive is configured to sequentially start-up the motors of the first and a second machine string and to regulate the speed of one of said motors when both motors are started-up.

12. The plant according to claim 10, wherein the plant comprises only one variable frequency drive.

13. The plant according to claim 10, wherein the plant comprises a plurality of said machine strings; a plurality of variable frequency drives, the number of said variable frequency drives being less than the number of said machine strings.

14. The plant according to claim 10, wherein the power supply grid is directly connectable with one or more of said motors.

15. The plant according to claim 10, wherein the variable frequency drive is sized to provide power to a single machine string.

16. The plant according to claim 10, wherein each motor is connected to the load by a shaft; the motor being connected at an intermediate section or at an end section of the shaft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Further details and specific embodiments will refer to the attached drawings, in which:

[0024] FIG. 1 is a schematic view of a plant for the liquefaction of a gaseous product; and

[0025] FIGS. 2a and 2b are schematic representation of respective steps of a method for starting and operating the plant of FIG. 1

[0026] FIG. 3 is an alternative configuration of the plant of FIG. 1.

DETAILED DESCRIPTION

[0027] The following description of exemplary embodiments refer to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of in an embodiment defined by the appended claims.

[0028] Reference throughout the specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases in one embodiment or in an embodiment in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0029] With reference to the attached drawings, with the number 1 is indicated a plant for the liquefaction of a gaseous product.

[0030] The plant 1 comprises at least a first 2a and a second machine string 2b. The strings 2a, 2b each comprise a respective shaft 3 to which is connected a respective load 4. In the following part of the disclosure the machine strings 2a and 2b will be described as comprising substantially the same components. However it is understood that, depending on the specific process used in the plant, the first 2a and the second machine string 2b may comprise components of different kind or, alternatively, of the same kind but sized differently.

[0031] More in detail, a load 4 is attached to the shaft 3. Specifically, the load 4 is a driven machine, usually a compressor 4, which is attached to the shaft 3 in order to process a predetermine amount of refrigerant. The refrigerant can either be propane, ethylene, methane, mixed refrigerant or nitrogen. For example, a compressor of this kind has to process a flow rate comprised between 60000 m.sup.3/h and 500000 m.sup.3/h, depending on the specific application. This flow is compressed from 1.5-5 bar to 15-70 bar. It is to be noted that each string 2a, 2b can comprise any number of compressors 4, all connected to the same shaft 3.

[0032] To drive the compressors 4, each string 2a, 2b comprises an electrical motor 5 attached to the shaft 3. In the embodiment of the invention the motor 5 is the only power source for driving the shaft 3. In other embodiments, not shown in the drawing, another power source may be present, in particular a gas or steam turbine. The motor 5 may be of any kind suitable for the above described purpose which is commercially available. With more detail, in the configuration shown in FIG. 1 the motor 5 is a single end motor. In other words, each motor 5 is connected at an end section of the shaft 3.

[0033] In the more general configuration shown in FIG. 3, the motor 5 is a double end motor. In other words, each motor 5 is connected at an intermediate section of the shaft 3. Thus it is possible to attach a compressor 4 on each side of the motor 5.

[0034] The plant 1 also comprises at least a variable frequency drive 6 associated with the electrical motors 5. The variable frequency drive 6 is configured to be connected to each of the motors 5. In the embodiment shown in the drawings, the plant 1 comprises only a single variable frequency drive 6. In other embodiments, not shown in the drawings, the plant 1 comprises a plurality of machine strings and, thus, a plurality of variable frequency drives 6. In this case, the number of the variable frequency drives 6 is less than the number of machine strings. It is to be noted that, in designing a specific instance of the plant 1, the variable frequency drive 6 is sized to provide power to just a single machine string 2a, 2b. For example, the number of variable frequency drives 6 can be one less of the number of machine strings 2a, 2b in the plant 1.

[0035] The plant 1 is also electrically connectable to a power supply grid 7. Specifically, the motors 5 are able to draw power from the power supply grid 7. The power supply grid 7, being the external electrical grid available on the site of each specific instance of the plant 1, is able to supply only a substantially constant frequency to the motor 5, thus driving it at substantially constant speed.

[0036] In the plant 1, the variable frequency drive (6) is configured to sequentially start-up the motors (5) of the first (2a) and a second machine string (2b) and to regulate the speed of one of said motors when both motors are started-up. The regulation of the speed allows to optimize the efficiency and operating conditions of the load of each string. In particular, when the machine stings are two, the speed of the motors are adjusted so to match the best operating conditions of the specific load/s installed on the respective machine string.

[0037] The above described plant 1 can be started as follows. As shown in FIG. 2a, the variable frequency drive 6 is electrically connected first to the motor 5 of the first machine string 2a. The variable frequency drive 6 is then started, gradually increasing its voltage and/or frequency output. Consequently, the speed of the motor 5 of the first machine string 2a is also increased. This is continued up until a first predefined threshold is achieved. The first predefined threshold is function of the typical frequency of the power supply grid 7.

[0038] A power supply grid 7 is then directly connected to the motor 5 of the first machine string 2a, so that the first predefined threshold of speed is maintained. The power supply grid 7 is also electrically connected to the motor 5 of the second machine string 2b by means the variable frequency drive 6.

[0039] Therefore, powering the motor 5 of the first machine string 2a is then taken over by the power supply grid 7. Thus, the variable frequency drive 6 can be electrically disconnected from the motor 5 of the first machine string 2a.

[0040] With more detail, the step of electrically connecting the power supply grid 7 is performed before the step of electrically disconnecting the variable frequency drive 6 in order to guarantee a supply of energy to the motor 5.

[0041] As is schematically shown in FIG. 2b, the variable frequency drive 6 is then electrically connected to the motor 5 of the second machine string 2b. The speed of the motor 5 of the second machine string 2b is then increased up until a second predefined threshold.

[0042] Indicatively, the first and the second predefined threshold can both be comprised between 1500 rpm and 5000 rpm, in an embodiment equal to 3600 or 3000, depending on the electricity grid which is used to power the motors 5. When the power supply grid has a frequency of 50 Hz the first predefined threshold is about 3000 rpm, while when the power supply grid has a frequency of 60 Hz the first predefined threshold is about 3600 rpm. The exact value of the first and of the second predefined thresholds however depends on other design consideration, which can be different in each instance of the plant 1. Also, the first and the second thresholds can be equal or can be different from each other. The first and the second predefined threshold should, in general, be suitable as a substantially constant speed of operation of the motors 5. The substantially constant speed coming from the substantially constant frequency of the power supply grid (7).

[0043] It is also to be noted that, after the plant 1 has been started, the variable frequency drive 6 can be used to regulate the speed of the motor 5 of the second machine string 2b, in particular when the second predefined threshold has been reached. In other words, the variable frequency drive 6 is used to adapt the capacity of the plant 1 depending on the required load of the compressors 4. When the variable frequency drive 6 is used to regulate the speed of the motor 5 of the second machine string 2b, the speed of the first machine string 2a is substantially maintained at a substantially constant speed, specifically the first predefined threshold.

[0044] Also, the variable frequency drive 6 and the power supply grid 7 can be switched between each of the motors 5. Thus, in principle, the variable frequency drive 6 can be used to regulate the speed of the motor 5 of the first machine string 2a. This capability can be used in case the second machine string 2b stops unexpectedly. In such condition, the variable frequency drives takes over the motor 5 of the first machine string 2a and, as soon as the trip condition is overcome, handles it back to the power supply grid 7 in order to restart the second machine string 2b in the manner described above.

[0045] This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.