METHOD FOR REDUCING NOISE AND CAVITATION IN MACHINES AND PRESSURE EXCHANGERS WHICH PRESSURIZE OR DEPRESSURIZE FLUIDS BY MEANS OF THE DISPLACEMENT PRINCIPLE

Abstract: A method for reducing noise and cavitation in machines and pressure exchangers which pressurize or depressurize fluids by means of the displacement principle, wherein at least one of the pressure exchanger's end covers is equipped with connecting channels (14, 17, 18) which substantially increase the inlet and outlet of flow medium in the rotor channels (15, 16) during residence in the depressurization zone (3) or the pressurization zone (4). The method comprises different embodiments such as, for example, a direct channel connection (14) between channel (16) and opposite channel (15).

Description: Method for reducing noise and cavitation in machines and pressure exchangers which pressurize or depressurize fluids by means of the displacement principle The invention relates to a method for reducing noise and cavitation in machines which employs the displacement principle where a limited volume of fluid is subjected to very rapid pressurization to the accompaniment of the generation of noise or depressurization whereby noise is similarly generated, but dramatically augmented by cavitation which also leads to structural damage which shortens the machine's service life. A number of different machines are known, including hydraulic pumps, hydraulic valves, hydraulic actuators, hydraulic motors and pressure exchangers as described in Norwegian patents nos. 161341, 168548, 306272, where the noise level becomes unacceptable if the machines are used at an excessively high rotational frequency or pressure. In practice the last- mentioned machines have been shown to be particularly vulnerable to these operational limitations, since an extremely limited time is available for simultaneous implementation of two processes in the same machine. The object of the invention is primarily to provide above-mentioned machines which are substantially less sensitive to these limitations. The special characteristics of this method according to the invention are presented in the characterising features indicated in the claims. The invention will now be described in more detail with reference to the drawings which schematically illustrate how the invention can preferably be implemented in a pressure exchanger according to the invention. Fig. 1 illustrates an end cover of a pressure exchanger with ports for high and low pressure of conventional design. Fig. 2 shows a cross section through a rotor channel and an end cover in different positions during implementation of a complete course of events during one revolution of the rotor. Fig. 3 is a pressure and leakage diagram for the rotor channel in the pressure exchanger process if the fluid is assumed to be ideal and incompressible and the end covers have symmetrical port openings. Fig. 4 is a pressure and leakage diagram for the same process, but with a real elastic or compressible fluid. Fig. 5 illustrates an example of how the invention can be implemented in the pressure exchanger's end cover. Fig. 6 illustrates another embodiment of the invention in the pressure exchanger's end cover. Figure 1 illustrates all the principal elements in a symmetrical end cover which has a high pressure port 1 and a low pressure port 2. Even though the angular area of the ports is identical in the drawing, this is not a requirement and may be advantageous in combination with different numbers of channels in the rotor. The end cover has two sealing zones, one of which is a depressurization zone 3 and one a pressurization zone 4 between the high pressure side and the low pressure side. Based on the fact that the rotor's channels rotate in a clockwise direction, all the rotor channels will pass from the high pressure port 1 via the depressurization zone 3 to the low pressure port 2 and via the pressurization zone 4 in order once again to be positioned in the high pressure port 1. Furthermore, the depressurization zone 3 has an inlet edge 5 and an outlet edge 6 and correspondingly the pressurization zone 4 has an inlet edge 7 and an outlet edge 8. The angular extension of the sealing zones 3, 4 will at a minimum include a complete rotor channel and its radial wall elements. If the sealing zones have a greater angular extension, the sealing zones will have an additional zone. The depressurization zone 3 has such an additional zone which is marked by a broken line 9, while the pressurization zone 4 has a corresponding area marked by a broken line 10. Figures 2a-d illustrate the cycle for each rotor channel 11 with a trailing or rear channel wall 12 and a leading or forward channel wall 13 while it passes from the high pressure port to the low pressure port. Starting position 2a is when the front edge of the trailing channel wall 12 reaches the inlet edge 5 of the depressurization zone 3 and the channel pressure P2a corresponds to pressure HP in the high pressure zone. In this position the leakage flows are at a maximum, and via the leading channel wall 13 Q l is exposed to maximum flow resistance and pressure difference HP-LP. As the rotor channel's trailing wall 12 takes up position in the depressurization zone 3, the leakage flows decrease and Q2 is exposed to increasing flow resistance until the rotor channel reaches position 2b, where both leakage flows are subjected to equal flow resistance and where the channel pressure P2b corresponds to half the pressure difference between the port openings. It is assumed that both the leakage flows are equally large at all times, since the flow medium is ideal and neither accumulates nor releases flow medium during this course of events. This state remains unaltered until the rotor channel reaches the next position 2c where the front edge of the leading channel wall 13 corresponds with the outlet edge 6. This is the beginning of a state which leads to gradually decreasing pressure in the rotor channel, increasing leakage flow and diminishing flow resistance for the leakage flow Ql until the channel comes into open connection with the low pressure port in position 2d. Figures 2e-h illustrate the cycle for each rotor channel while it moves from the low pressure port to the high pressure port. Starting position 2e is when the front edge of the rotor channel's trailing wall 12 corresponds with the pressurization zone's inlet edge 7 and the channel has the pressure P2e corresponding to the pressure in the low pressure port. In this position leakage flow Q3 is exposed via the leading channel wall 13 to maximum flow resistance and a pressure difference HP - LP. While the rotor channel's trailing wall 12 takes up position in the depressurization zone, leakage flow Q4 is exposed to an increasing flow resistance until the channel reaches position 2f, where both leakage flows have equal flow resistance and the rotor channel has a pressure P2f which corresponds to half of the pressure difference between the ports (HP - LP)/2. This state remains unaltered until the rotor channel reaches the next position 2g where the front edge of leading channel wall 13 corresponds to the outlet edge 8. This marks the start of a state where the pressure gradually increases in the rotor channel and increasing leakage flows Q4, Q3 until the channel is in open connection with the high pressure port in position 2h. Figure 3 illustrates an ideal pressure diagram for the rotor channel during a complete course of events as described in figures 2a-h, based on a rotor with symmetrically opposite channels and symmetrical port openings of equal angular extension. The diagram illustrates two channels which are placed 180 degrees from each other while one channel is pressurized and the other is simultaneously depressurized. It also illustrates the relative magnitude of the leakage flows in the different positions based on an ideal non-compressible flow medium. Under such conditions a leakage flow Q will establish an equilibrium in the gap clearance between the rotor channel's and the end cover's end surfaces and be proportional to Q = Pressure difference / Flow resistance This formula can be used to establish a quantitative analysis of the leakage flows as indicated in the diagram. This illustrates clearly and unambiguously that the pressure in the rotor channel gradually drops to half the pressure difference between the high pressure port and the low pressure port when the rear edge of the trailing channel wall 12 passes the inlet edge 5 of the depressurization zone 3. The leakage flows Q l, Q2 are also reduced gradually to half as soon as the rotor channel's radial wall elements 12, 13 are completely within the depressurization zone 3. The opposite rotor channel moves from the low pressure port to the high pressure port, thereby undergoing a reverse course of events to the former rotor channel and pressure is increased gradually until the pressure reaches half that of the former rotor channel. The leakage flows Q3, Q4 are of maximum value at the beginning, gradually decreasing to half as soon as the rear edge of the trailing channel wall 12 passes the inlet edge 7 of the pressurization zone 4. While the leading channel wall 13 passes the outlet edge 8, the pressure in the channel increases to full high pressure, while the leakage flows Q3, Q4 increase to double the amount. Figure 4 illustrates a pressure diagram for the pressure exchanger process when a real elastic flow medium, e.g. water, is employed. The main difference is that the rotor channel transports a flow medium from the high pressure side which is compressed and contains an extra volume which has to be discharged before the channel is in open connection with the low pressure port, which requires the leakage flows Q l and Q2 to be unequal. The pressure drops very little in the rotor channel on account of the extra volume which is enclosed and gradually discharged, which establishes a continuous high leakage flow Ql and a rapidly decreasing leakage flow Q2 which refills the rotor channel as the pressure difference gradually increases via the channel's trailing wall 12. The flow resistance increases rapidly, with the result that Q2 reaches a very low minimum as soon as the rotor channel's wall elements 12, 13 are within the pressurization zone 4 and only gradually increases thereafter until it reaches the same maximum as in the ideal case. The rotor channel's leading wall 13 is constantly exposed to a high pressure difference and as its front edge passes the outlet edge 6 of the depressurization zone, a dramatic course of events is initiated where the pressure is only gradually lowered and the leakage flow Ql increases rapidly as the flow resistance decreases substantially. During this process there is a great risk that cavitation and an unacceptable noise level may be established. During pressurization the course of events is partly reversed and different. In this case the flow medium is initially exposed to a leakage flow Q3 from the high pressure side which does not immediately lead to rapid pressure increase in the channel, since some of the volume is absorbed by compression and the pressure curve LP - HP is thereby as illustrated in the diagram. This also has the result that the leakage flow Q4 does not reach the same volume, but remains substantially less than Q3 until the rotor channel approximately reaches the high pressure side, where a relatively high pressure difference in combination with a rapidly decreasing flow resistance lead to a considerable increase in the leakage flow Q4. It must be added here that the rotational speed of the rotor entails an increase of the effect of the course of events, since the leakage flows Ql, Q2 which move in the same direction as the channel during depressurization receive higher volume flows, while the leakage flows Q3, Q4 which move in the opposite direction to the rotor channel during pressurization are reduced. This corresponds to experiences from operation where cavitation damage is visible only in the depressurization zone 3. Figure 5 illustrates an embodiment of the invention employed on the end covers of a pressure exchanger. The proposed embodiment consists substantially of various ways of avoiding the high maximum values for the leakage flows Ql and Q4, which are assumed to be the cause of the high noise level and cavitation damages which arise when there is higher pressure and through-flow in the machine. According to the invention, one method will be to equip at least one end cover with a connecting channel 14, which permits transfer of flow medium from opposite channels 15, 16 while both channels have wall elements 12, 13 within the depressurization zone 3 and the pressurization zone 4, with the result that the course of events approximately corresponds to the ideal pressure diagram. Even though each channel is in open communication with the connecting channel 14 when it is in depressurization or pressurization, there is simultaneous connection for only a brief moment to permit pressure balancing or equalization and transfer of flow medium. This takes place when the trailing wall in channel 16 substantially has passed the inlet edge 5 and immediately after the trailing wall in channel 15 has passed the inlet edge 7 or as soon as both channels simultaneously are in sealing engagement with the depressurization zone 3 and the pressurization zone 4. This simultaneous connection via the connecting channel 14 is broken just before the leading wall in channel 15 takes up position in the high pressure port or the leading wall in channel 16 takes up position in the low pressure port. It is also conceivable that the invention may be implemented by separating the corresponding processes, depressurization and pressurization respectively, by equipping at least one end cover with independent connecting channels 17, 18 with low flow resistance, each of which leads to a high pressure port or a low pressure port and results in a substantial increase in the flow into or out of the channels during the above-mentioned state. This may be implemented, for example, by long channels designed with relatively short sealing walls in the end covers, thus permitting high leakage flows, but without the risk of cavitation in the gap clearance at the outlet to the low pressure port. In addition, it is also possible to use nozzles alone or in series as a connection between the channels and port openings. Separation of the processes in this manner may permit further reduction in noise level, since it will be possible to introduce a phase shift which may reduce the resonance of simultaneous opposite events as illustrated in the pressure diagrams in figures 3 and 4. The invention may also be combined with different numbers of rotor channels, different channel sizes, more channels simultaneously in depressurization or pressurization and asymmetrical port openings of different angular extension in order to optimise the effect of this invention.

Claims: PATENT CLAIMS 1. A method for reducing noise and cavitation in machines and pressure exchangers which pressurize or depressurize fluids by means of the displacement principle, characterized in that at least one of the pressure exchanger's end covers is equipped with connecting channels (14, 17, 18) which substantially increase the inlet and outlet of flow medium in the rotor channels (15, 16) during residence in the depressurization zone (3) or the pressurization zone (4). 2. A method for reducing noise and cavitation in machines and pressure exchangers which pressurize or depressurize fluids by means of the displacement principle, according to claim 1 , characterized in that a connecting channel (14) in at least one end cover is arranged for direct communication between opposite channels (15, 16) and provides pressure balancing while the channels are simultaneously in the depressurization zone (3) and the pressurization zone (4). 3. A method for reducing noise and cavitation in machines and pressure exchangers which pressurize or depressurize fluids by means of the displacement principle, according to claim 1, characterized in that at least one end cover is equipped with separate connecting channels (17, 18) with low flow resistance which connect the port openings and the rotor channels (15, 16) during the said phase and permit phase shift of pressurization and depressurization respectively. 4. A method for reducing noise and cavitation in machines and pressure exchangers which pressurize or depressurize fluids by means of the displacement principle, according to claims 1 and 3, characterized in that the connecting channels (17, 18) are very long and have a short sealing surface against the high and low pressure ports respectively, thus permitting high leakage flows. 5. A method for reducing noise and cavitation in machines and pressure exchangers which pressurize or depressurize fluids by means of the displacement principle, according to claims 1 and 3, characterized in that the rotor channels (15, 16) are connected via nozzles with low flow resistance to the high pressure port and the low pressure port respectively during residence in the depressurization zone 3 and the pressurization zone 4, thus providing pressure balancing and thereby reducing the leakage flows Ql and Q4 which are assumed to be the cause of noise and cavitation or a second embodiment with separate connecting channels (17, 18) with low flow resistance which connect the port openings and the rotor channels (15, 16) during the said phase and permit phase shift of pressurization and depressurization respectively.

Inventors: HERMANSTAD RAGNAR A, HERMANSTAD, RAGNAR, A.
Applicants: ENERGY RECOVERY INC, HERMANSTAD RAGNAR A, ENERGY RECOVERY, INC., HERMANSTAD, RAGNAR, A.
Application year: 2001
Main classification: F15B
IPC codes: B01D61, F04B1, F04B11, F04F13, F04F99, F15B21