EP2650543B1 - Radial compressor with a diffuser including passive flow control - Google Patents

Description

The present invention relates to centrifugal compressors. It relates more particularly to the control of the aerodynamic stability of compressors, at low flow.

Compressors generally comprise an electric motor and a compressor unit, for example with several stages, which comprises a set of bladed wheels carried by a shaft. The motor rotates a rotor which is coupled to the compressor shaft. In operation, the handled gas is admitted at the inlet of the compressor and then is transmitted to the successive compression stages of the compressor unit to be delivered at the outlet of the compressor.

The characteristic curve of the compression supplied by a compressor, which represents the discharge pressure as a function of the flow rate, is a function of a series of parameters, such as the suction pressure, the speed of rotation of the rotor, the nature of the gas compressed, ...

It has been observed that, at low flow rates, that is to say at flow rates below the stability range of the compressor, aerodynamic instabilities appear corresponding to an appearance of a separation of the gas flow handled by the compressor. compressor and to the phenomenon known by the term “pumping”.

Such instabilities, downstream of the paddle wheel, are liable to cause vibrations, or even alterations, of the constituent elements of the compressor and limit, in all cases, the operating range of the machine.

This is the reason why it has been proposed, for low pressure applications, to equip the compressors and, in particular the diffusers provided downstream of the wheels, with variable-pitch blades, as regards the bladed diffusers or movable walls, with regard to smooth diffusers. However, such blades are generally moved by mechanical devices which require bores in the body of the rotating machine and, consequently, to provide additional devices for sealing.

Such solutions therefore limit their use to pressures generally below 20 bars.

It has also been proposed to provide compressors with anti-pumping protection devices, ensuring communication between the discharge and the suction, in order to artificially increase the supply flow rate of the compressor.

Such devices require the use of instrumentation making it possible to measure the flow rate passing through the compressor, as well as the suction and discharge pressures, as well as control devices controlling a gas recirculation control valve.

An alternative solution is described in the document US 2007/147987 A1 .

Such solutions are therefore relatively expensive, difficult to implement and liable to be the site of malfunctions.

The aim of the invention is therefore to overcome the drawbacks associated with the solutions according to the state of the art and to reduce the risks of the appearance of pumping phenomena and, in particular, the phenomena of instability of the gases circulating in the diffuser. compressor.

The object of the invention is therefore, according to a first aspect, a diaphragm for a compression stage of a compressor according to claim

Another subject of the invention, according to a second aspect, is a compressor compression stage according to claim 2.

A further subject of the invention is a compressor, comprising a diaphragm per compression stage as defined above.

• la figure 1 est une vue schématique en coupe d'un étage de compression d'un compresseur centrifuge selon l'invention ;
• la figure 2 est une vue de profil de l'étage de compression de la figure 1, montrant les passages de gaz en entrée de l'étage de compression ;
• la figure 3 est une vue de profil de l'étage de compression de la figure 1, montrant les passages de gaz au niveau du diffuseur ; et
• la figure 4 est une courbe caractéristique de compression d'un compresseur illustrant les avantages liés à l'invention.

Other objects, characteristics and advantages of the invention will become apparent on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

• the figure 1 is a schematic sectional view of a compression stage of a centrifugal compressor according to the invention;
• the figure 2 is a side view of the compression stage of the figure 1 , showing the gas passages at the inlet of the compression stage;
• the figure 3 is a side view of the compression stage of the figure 1 , showing the gas passages at the diffuser; and
• the figure 4 is a compression characteristic curve of a compressor illustrating the advantages associated with the invention.

On the figure 1 , there is shown a sectional view of a compression stage of a multistage compressor according to the invention.

In the exemplary embodiment shown, the compressor is a centrifugal compressor and is intended to ensure the compression of a gas taken upstream of the stage and returned downstream at a higher pressure.

Moreover, on figures 1 to 3 , the compression stage is an intermediate stage arranged between two compression stages. But of course, the invention also applies to the input and output stages.

As can be seen, the compression stage, designated by the general reference numeral 1, comprises a bladed wheel 2 driven in rotation by a rotor 3 itself coupled to a shaft (not shown) driven in rotation by an electric motor.

Upstream of the wheel 2, that is to say on the gas intake side, the compression stage comprises a diaphragm 4 intended to communicate the inlet of the compression stage 1 with the outlet d a compression stage immediately placed upstream, as well as a diffuser 5 intended to slow down the speed of the gases issuing from the bladed wheel 2 and thereby increase the pressure at the outlet of stage 1.

Referring also to the figure 2 , on which elements identical to those of the figure 1 bear the same numerical references, the diaphragm 4 has a generally annular shape and constitutes, with the diffuser 5, one of the stator parts of the compression stage.

It has two large faces 6 and 7 mutually opposed, one of which, namely the face designated by the reference 6, is here provided with fixed vanes, such as 8, and the other of which delimits the diffuser 5.

It can also be seen that the diaphragm 4 is provided with a set of through passages, such as 9, extending between the two faces 6 and 7 of the diaphragm so as to ensure communication between the diffuser 5 and the suction zone. of the compression stage located immediately upstream of the wheel 2.

In the exemplary embodiment shown, the number of passages 9 corresponds to that of the fixed blades of the diaphragm 4, so that the passages each open, on the suction side, through an orifice 10 arranged between two fixed blades 8. Although understood, in general, the diaphragm 4 can include any number of such passages greater than one.

The passages are arranged in the thickness of the diaphragm 4 so as to extend in an inclined manner radially outwardly between a zone converging towards the rotor 3, on the suction side, and a radial zone, located in the diffuser, downstream of wheel 2 ( figure 3 ).

It has been observed that the presence of such passages made it possible to compensate for the phenomena of instability causing the pumping of the machine, in particular for diffusers of very small dimensions, or for relatively low flow rates.

In this regard, it is customary to represent the flow rate from the aspirated volume flow rate (Qv) of the front surface of the impeller S for a centrifugal stage and a peripheral speed U, from the following relation: $$\mathrm{\phi }=\frac{\mathit{Qv}}{S\cdot U}$$

It was found that the presence of the passages 9 within the diaphragm 4 made it possible to eliminate the aerodynamic instabilities and in particular the pumping phenomena for relatively low flow rates corresponding to flow coefficients of less than 0.01, by creating a flow of gas from the diffuser to the upstream part of the bladed wheel through the passages, according to the pressure difference between the discharge zone and the suction zone, making it possible to increase the compressor flow rate and thus keep the compressor in its stability range ( figure 4 ).

It will be noted in this regard that the quantity of recirculated gas, from the discharge to the suction, through the passages 9, which depends on the differential pressure on either side of the diaphragm, is therefore greater for low flow rates. It is, on the contrary, less important for relatively higher flow rates for which pumping phenomena are less likely to occur.

In other words, as shown in the figure 4 , in accordance with the invention, by creating this artificial circulation at the inlet of the compression stage, the suction flow rate is artificially increased, so that the operating point of the compressor is pushed back into its stability range.

It will also be noted that the number and size of the gas passages are adapted to the flow coefficient of the centrifugal stage, to the number of rectifying blades of the diaphragm and to the quantity of gas necessary to maintain operating stability.

It will also be noted that the arrangement which has just been described constitutes an entirely passive device which does not require any other control device or measuring means and therefore constitutes a very reliable system. It is also applicable to all types of gas and pressure levels, while remaining particularly advantageous for stages with a low flow coefficient inducing very small diffuser widths which are therefore difficult to use and to control.

The diffuser which has just been described also has a certain number of additional advantages.

It has in fact been observed that no gas leak to the outside was caused by the presence of the passages in the diaphragm, which contributes to the protection of the environment.

The operating principle of the diaphragm remains available for all conditions of higher pressure.

It is also possible to use the technology of centrifugal stages at very low flow rates while maintaining operability in an industrial installation.

Finally, it has been observed that the invention allows stable operation over an extended operating range compared to centrifugal machines according to the state of the art.

Claims (3)

1. A diaphragm for the compression stage of a compressor, characterized in that it comprises a set of at least one through passage (9) extending between two mutually opposite faces (6, 7) of the diaphragm, so as to create a flow of gas between said faces depending on the pressure difference prevailing on either side of the diaphragm and in that it comprises a set of passages arranged regularly in the thickness of the diaphragm (4) so as to extend radially outwardly in an inclined manner between a zone converging toward the rotor (3), on the suction side, and a radial zone, located in the diffuser, downstream of the wheel (2) and each extending, on one of the faces of the diaphragm, between two fixed blades (8) provided on the diaphragm, the number of passages corresponding to the number of fixed blades of the diaphragm.
2. A compression stage of a compressor comprising a diaphragm (4) receiving a gas supplied at the input of said stage, a bladed wheel (2) ensuring a compression of the gas resulting from the diaphragm and an output diffuser (5), characterized in that the diaphragm (4) is a diaphragm according to claim 1.
3. A compressor, characterized in that it comprises one or more diaphragms according to claim 1.

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