"Method and device for emergency brake of a compressor"

The present invention concerns compressors for boosting hydrocarbon transport in pipelines, preferably in a seabed environment. More specifically, the present invention concerns emergency braking to stop such compressors in the event of power failure or in situations with failure in magnetic bearings.

Underwater compressors with magnetic bearings are usually equipped with backup bearings (ball bearings, roller bearings or slide bearings). These backup bearings operate in connection with an emergency stop, i.e. if the magnetic bearings fail, or in connection with power failure. The emergency bearings are dimensioned for a certain number of such emergency stops, for example 3-5 emergency stops, before the compressor must be raised for maintenance. Such maintenance is expensive and may also result in loss of production.

The number of emergency stops before an overhaul may be increased if the emergency bearings are subject to less stress during each emergency stop. One way of achieving this is to brake the compressor in connection with each emergency stop. The brake used should be extremely reliable and should not result in increased wear or create wear products.

One type of brake that can solve the problem is a contact-free magnetic brake, i.e. an eddy current brake.

Since these brakes must act when the magnetic bearings fail or in connection with power failure, they have to use permanent magnets and be designed so that they operate in connection with power failure or an emergency stop signal.

An eddy current brake based on permanent magnets is described in US Patent No. 5,712,520. However, the design is not suitable for compressors. It is intended to brake rotors in a drive system.

Therefore, there remains a need to produce a method and a device for emergency braking for a compressor in situations such as those described above.

In accordance with the present invention, therefore, a method has been arrived at as defined in the attached claim 1. Advantageous embodiments of the method in accordance with the present invention are described in the attached dependent claims 2-6.

In accordance with the present invention, an emergency braking device has also been arrived at as defined in the attached independent claim 7.

Advantageous embodiments of the emergency braking device in accordance with the present invention are described in the attached dependent claims 8-11.

Another aspect of the present invention is presented in the attached independent claim 12, and another aspect is presented in the attached independent claim 13, which concerns a compressor.

In the following, the present invention will be illustrated further by describing a preferred embodiment and variants of it.

Reference is also made to the attached Figure 1 , which shows an outline of the preferred embodiment of an emergency braking device in accordance with the present invention.

In a compressor of the type specified above, it is necessary to arrange one or more brake discs 5 around the compressor's rotary shaft (not shown in Figure 1). Only part of the brake disc 5 is shown in Figure 1 as the compressor's rotary

shaft can be assumed to be perpendicular to the disc 5 shown, in the paper plane. Disc 5 thus rotates into and out of the paper plane.

The brake disc 5 must at least have an annular area 7 that is of conductive material, preferably a metal. The annular area may have any width, for instance from the periphery of the disc right up to the rotary shaft. Thus, the entire brake disc 5 may be made of compact metal or another material that has good electrical conductivity.

In addition, a disc already installed for journal or axial bearings in the compressor may be used as such a brake disc 5, in particular for a compressor on the seabed.

The braking effect is produced by inducing current paths (eddy currents) in the disc 5 by moving a permanent magnet 1 along and under part of the brake disc 5, or at any rate to a position at which interaction is achieved with such a conductive annular area 7 as mentioned above. An electromotoric force will act on these current paths and try to counteract rotations, thus braking the compressor.

The permanent magnet 1 is shown as a horseshoe magnet with an air gap 6 arranged so that, in connection with movement to the left in the figure, the air gap 6 will mainly be filled by the thickness of the brake disc 5. However, there must, of course, be a little clearance above and below to avoid mechanical contact. Moreover, a device with two separate magnets with different poles facing each other, i.e. with the same air gap in between, may replace the horseshoe magnet.

The position shown with unbroken lines for the permanent magnet 1 is called the idle position for the brake, while the position shown with dotted lines is called the braking position. When an abnormal situation occurs, i.e. a power failure or a failure in at least one magnetic bearing in the compressor, it is

important for the switch from idle position to braking position to take place fast so that the backup bearings mentioned above in the compressor are subject to wear for as short a time as possible. An example of a simple mechanism that can produce a fast switch from idle position to braking position without the need for electric power for the switching operation itself is shown to the right in Figure 1.

An electric motor 3 has the function of tensioning a spring 2 (here a spiral spring) between the magnet 1 and a limit stop 8. The tensioning takes place by a cogwheel rotating in mesh with grooves on a piston rod 9. The spring 2 is laid around the piston rod 9 in the example. The motor 3 drives the piston rod 9 and thus the permanent magnet 1 to the right, while the spring 2 is tensioned until the idle position is reached, at which the permanent magnet 1 does not interact with the brake disc 5. A locking device 4 clicks in here and holds the system in the idle position with the tensioned spring 2. The locking device in the case shown is a simple stop pin but may have other designs such as a hook, claw or other design.

The locking device 4 must, however, be easily releasable, for example so that it is released in connection with power failure or by means of an emergency stop signal when a failing magnetic bearing is detected. When, for example, the stop pin 4 is released, the tensioned spring 2 will immediately drive the piston rod 9 and the permanent magnet 1 to the left until a stop detail at the far right of the piston rod 9 strikes the limit stop 8. At this point, the system has reached the braking position shown with dotted lines. (The motor 3 is then either dead or it was disconnected from being in mesh with the piston rod 9 when the tensioning was done so that the motor does not prevent movement to the left in the figure.)

Several of the elements shown in Figure 1 may be replaced by equivalent elements. As stated above, they may be special brake discs 5 or discs that already have a function as supporters of bearings. Moreover, we have mentioned various types of permanent magnet. The actual mechanism that

causes the permanent magnet 1 to mesh with the disc may have various designs. For example, the magnet may be turned into place in a turning movement in the same plane as the plane of the brake disc instead of a linear, or radial, movement such as that produced by the piston rod 9. In this case, the spring will have an adapted design, as will the locking device 4.

The present invention concerns not only compressors on the seabed. It may also be used in offshore compressors or land-based compressors with magnetic bearings and backup bearings, especially where it is important to have a long operating time between each maintenance stoppage.

It should also be noted that the braking device does not need to comprise a single brake disc 5. There may also be two or more. The mechanism for moving one or more permanent magnets 1 may be a common mechanism or several mechanisms may be arranged, one for each brake disc.

Finally, it should be noted that the two principal aspects of the present invention are an emergency braking device in a compressor and a compressor with an emergency braking device. In addition, in the following claims, an aspect is specified with a method for emergency braking for a compressor, and an aspect with an application of an eddy current brake for emergency braking for a compressor.