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Compressor : Hitachi

Hitachi

Responding to the demand for oil-free products

Image: Centrifugal compressors with magnetic bearings

he demand for oil-free rotating machinery is increasing, including centrifugal compressors for process use.
In the past, oil was essential for the bearings of compressors. However, installing magnetic bearings and non-contact gas seals makes oil-free centrifugal compressors possible, which do not use oil.

Features

Use of non-contact bearings

There is no contact between the body of the compressor and the rotor, so there are no parts that wear.
The bearings can be used semi-permanently without maintenance.

No need for lubricating oil or oil seals

There is no need for lubricating oil or oil seals because non-contact gas seals are used, so there is no need to change oil filters or inspect pumps.
Parts related to lubricating oil and oil seals are responsible for 60-70% of normal compressor breakdowns. These parts aren't used with magnetic bearings, so this means higher reliability and a dramatic reduction in breakdowns.

Internal operating status can be constantly monitored

Internal operations, such as increases in the rotor balance load and changes in labyrinth gaps due to changes in axial thrust force can be accurately and easily monitored.
We use various types of analysis techniques, including high-pressure fluid vibration analysis technology.
We aim for more advanced products through the use of an integrated system of design and manufacturing using 3D CAD.

Higher speeds and smaller size

Magnetic bearings use dry gas seals and rotate without contact, so higher speeds than conventional compressors can be used.
The shaft system, which was limited to the secondary critical speed, can be operated at or above the tertiary critical speed, so a large number of impeller stages can be accommodated per casing, which means that size can be reduced by reducing the number of casings.

Running costs can be greatly reduced

Because there is no contact, mechanical losses at bearings and seals can be reduced to a range of 1/10 to 1/100 of that with conventional bearings.
By also using non-contact gas seals, utility costs such as steam, electrical power, cooling water, process gas, oil, etc. can be reduced.

Principle of magnetic bearings

Radial bearings

A pair of electromagnets provided circumferentially opposite each other with the shaft in the center create a magnetic field between the rotor. This magnetic field attracts the rotor, which is held suspended in the air. However, the magnetic field varies as the surface of the rotor rotates, which generates eddy current losses. In order to minimize these losses, a laminated structural sleeve is press-fitted to the rotor.
The rotor position is detected by 4 or more position sensors, and deviation from the rotor positional standard is transmitted to a control circuit, which controls the strength of the magnetic field to restore the rotor position to its standard position.

Image: 3D cross-section of a radial bearing

Thrust bearings

The operating principle of a thrust bearing is basically the same as that of a radial bearing. However, the electromagnets in a thrust bearing are arranged in the radial direction, so the magnetic field between a thrust collar is always constant, so eddy currents are not generated.
Therefore, high-strength steel alloys are used for the thrust collar, the same as for oil bearings.

Image: 3D cross-section of a thrust bearing

Ancillary bearings

Image: 3D cross-section of ancillary bearings

To prevent contact between the rotor and the stator, ancillary bearings are provided as a back-up. These bearings are fixed to the stator side, and maintain a gap with the rotor that is about 1/2 the radial bearing air gap. As long as the magnetic bearings are operating, the ancillary bearings do not rotate. Also, during power outages, the rotor can be normally suspended for 5-10 minutes by a back-up battery. Operation is stopped during this time, so the ancillary bearings do not take the load of the rotor. In other words, the ancillary bearings are the final protection device. They are provided for safety redundancy to stop the electrical power and the mechanical operations in an emergency.

Control circuits

Image: 3D cross-section of control circuits

The role of the control circuit is to accurately control the position of the rotor by varying the current in the electromagnets based on the signal from the position sensors.

Main applications

These are mainly used in oil refineries and pipeline stations.