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Apparatus and method for controlling supply of barrier gas in a compressor module

Apparatus and method for controlling supply of barrier gas in a compressor module description/claims

The invention relates to compressor modules for compressing hydrocarbon gases in a wellstream, and more specifically it relates to a compressor module comprising a pressure housing, a compressor and a motor separated by a sealing element, as disclosed in more detail in the preamble of independent claims 1, 2 and 8. The invention is especially suitable for use in subsea compressor modules.

A compressor module is, in its most basic form, a unit in which a compressor and a motor are connected via a shaft and placed in a common pressure shell. Between the motor and the compressor there is a seal to prevent contamination of the motor. However, there are problems in keeping the gas-filled, electric motor as dry as necessary in order to avoid corrosion and other problems related to the precipitation of hydrocarbon condensates and water in liquid form inside the motor. It is especially important to avoid the presence of water in liquid form together with an H2S or CO2 content which may generate acids and, as a result, accelerated corrosion. These problems are addressed in Norwegian Patents NO 172075 and NO 173197, and also in Norwegian Patent Application 20015199. It is also important to prevent particles from penetrating into and building up inside the motor and magnetic bearings.

It is most common to utilise standard oil-lubricated bearings or the like in compressors and their motors. The inventor has explored the possibilities for reliable use of a high-speed motor and magnetic bearings in integrated compressor modules which have a common pressure shell, by providing a way of protecting the electric motor and magnetic bearings in the compressor module. Magnetic bearings in such compressor modules will have several advantages, particularly during operation. Magnetic bearings are more reliable and less expensive to operate. Of particular importance is the fact that the use of magnetic bearings eliminates lubricating oil and thus potential problems that may occur through: dilution of the lubricating oil by the hydrocarbon gases with which it is in contact, accumulation of hydrocarbon condensates or water in the lubricating oil, or degradation of the lubricating oil over time due to its specific use in compressor modules. The problem encountered when utilising non-encapsulated magnetic bearings in a compressor module is in many respects similar to the problems associated with the use of electromotors: both require a completely dry and inert atmosphere with as low a particle content as practically possible in order to function reliably and problem-free over time. Encapsulated magnetic bearings also exist or are in the process of being developed. It is claimed that these bearings are capable of operating in the untreated wellstream of hydrocarbon gas. However, there are reasons to believe that also in the case of these types of magnetic bearings it is advantage for their long-term functionality and reliability that they should be installed and operated in a dry and particle-free atmosphere.

There is therefore a need for a system and a method for ensuring a completely or nearly completely dry environment for the electric motor and for the magnetic bearings in an integrated compressor module, and which also prevent particles (e.g., from the wellstream flowing through the compressor) from moving from the compressor side into the motor and/or magnetic bearings.

Accordingly, an apparatus is provided for controlling the supply of barrier gas to a compressor module comprising a pressure housing containing an electric motor which via a shaft is drivably connected to a compressor, wherein the shaft is supported by magnetic bearings and an axial seal around the shaft divides the pressure housing into a first compartment comprising the motor and a second compartment comprising the compressor, the second compartment comprising an inlet and an outlet for fluid that is to be compressed, characterised by a line fluid-connected to the first compartment for supply of barrier gas from a reservoir, and a flow restriction means in the line between the reservoir and the first compartment.

In one embodiment of the invention, an apparatus is provided for controlling the supply of barrier gas to a compressor module comprising a pressure housing containing an electric motor which via a shaft is drivably connected to a compressor, wherein the shaft is supported by first magnetic bearings and at least one second magnetic bearing on the opposite side of the compressor in relation to the first magnetic bearings, and an axial seal around said shaft divides the pressure housing into a first compartment comprising the motor and a second compartment comprising the compressor, the second compartment comprising an inlet and an outlet for fluid that is to be compressed, and wherein a second axial seal is fitted around the shaft between the compressor and the said second magnetic bearing thereby forming a third compartment, characterised by a line fluid-connected to the first compartment and a line fluid-connected to the third compartment, the lines being arranged for supply of barrier gas from a reservoir, and respective flow restriction means in the lines between the reservoir and the first and the third compartment respectively.

Furthermore, a method is provided for controlling the supply of barrier gas to a compressor module, characterised by, on the basis of required barrier gas velocity through the seal and the pressure in the second compartment, setting an overpressure in the reservoir, and effecting a controlled supply of barrier gas to at least the first compartment by means of the flow restriction in the feed pipe.

An embodiment of the invention will now be described with reference to the attached drawings, wherein like parts have been given like reference numerals.

FIG. 1 is a schematic diagram of the apparatus according to the invention.

FIG. 2 is a schematic diagram of one embodiment of the barrier gas control device.

FIG. 3 is a schematic diagram of an alternative embodiment of the barrier gas control device.

FIG. 4 is a schematic diagram showing the gas velocity and the mass transport through the axial seal.

FIG. 5 illustrates the barrier gas control device in a basic form.

FIG. 6 illustrates a mechanical barrier gas control device that keeps the mass flow constant when the pressure upstream of the controller, pa, is constant and the temperature is constant, as the temperature will be when the controller is surrounded by seawater.

The compressor module according to the invention comprises an electric motor 1 and a compressor 2, joined together via a shaft 8 and arranged in a common pressure shell 3. FIG. 1 illustrates schematically that a stream of essentially hydrocarbon gas, but which may also contain liquids and particles (hereafter referred to as fluid) is conducted from a reservoir (not shown), preferably via a liquid separator and in a line 17, into the compressor module via an inlet 4. On the upstream side (inlet side) of the compressor 2, the fluid has a suction pressure designated ps, which in a wellstream compression system will fall slowly over time as the reservoir pressure falls. On the downstream side of the compressor, the fluid has a discharge pressure designated pd. When the compressor is in operation, pd is greater than ps. The fluid exits the compressor module via the outlet 5. In cases where the compressor is not in operation and is inactive and the inlet 4 is closed by means of the valve 41 and the outlet is closed by means of the valve 51, the pressure inside the compressor module compartments will equalise and become identical. If the bypass valve 61 is closed, the pressure upstream 51 may build up to become greater than the pressure downstream 61. The maximum limiting case for this pressure build-up, due to leakage through the valves at the wellhead, is the shut-down pressure for the wells (wellhead shut-in pressure, WHSP). When the compressor is not in operation and the bypass valve 61 is open, the pressure upstream 51 and downstream 61 will be equal to the settle-out pressure. If production is then shut down over a longish period of time, and the valves at the gas receiving terminal are closed, the pressure in the transport pipe past the compressor module may build up to WHSP. In both the said cases of an inactive compressor, the pressure inside the compressor module may therefore, due to leakage in either valve 51 or 61 or both, build up to WHSP. To counter the ingress of fluid into the compressor module and the consequences this may have as regards condensation, corrosion and particle build-up when it is inactive, it is therefore advantageous to up-adjust the pressure inside the compressor module to slightly above the pressure in pipes outside the valves 51 and 61, and optionally to slightly above WHSP.

An axial seal 31 is arranged between the compressor 2 and the motor 1 and divides the pressure shell into a first compartment 10 and a second compartment 20. As is generally known, the purpose of the axial seal 31 is to prevent particles, moisture and other contamination from coming into contact with the motor and other electrical equipment. However, the axial seal is not completely tight. In the pursuit for a maximally dry and clean atmosphere in the first compartment 10, it is essential that the pressure in the first compartment 10 (where the motor is located) is higher than the pressure in the second compartment 20 (where the compressor is located) i.e., that inert gas flows from compartment 10 through the seal and into 20. FIG. 1 indicates this by showing that the pressure in the first compartment 10 (and in the optional third compartment, due to the balance piston) is ps+Δp.

The shaft 8 is journalled by magnetic bearings 11, 21 as shown in FIG. 1. If the magnetic bearing 21 is of the encapsulated type as described above, the axial seal 32 is superfluous. The axial seal 32 is thus optional if the magnetic bearing is encapsulated.

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• Utility Patent

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