LIQUID RING PUMP

Abstract: A two-stage liquid ring pump (10) comprises a suction end casing (12), a first stage body portion (14), first and se­ cond stage centre plates (16) and (18), second stage body portion (20) and discharge end casing (22), these parts being clamped together by bolts (32). End bearing housings (40,42) journal a shaft (48) which is eccentrically mounted with res­ pect to the cylindrical first and second stage pumping cham­ bers (54) and (56) and carries first and second stage impel­ lers (58) and (60). A suction inlet (24) directs fluid into a suc­ tion manifold which connects with suction ports located at both ends of the first stage pumping chamber (54) and the first stage impeller (58) discharges this fluid under pressure from first stage discharge ports such as (72) into discharge manifold (28) which directs the fluid into section stage suc­ tion ports such as (80) located at both ends of the second stage pumping chamber (56). The second stage impeller (60) drives the fluid admitted to second stage pumping chamber (56) through discharge ports such as (88) into discharge ple­ num (86) and thence to discharge outlet (30). For the purpose of reducing noise and vibration in the pump whilst maintain­ ing ease of manufacture of the impellers, the impellers each are formed with a prime number of equally spaced blades and, to appreciably reduce peak noise levels, the two impel­ lers (58) and (60) have different numbers of blades.

Description: "Liquid Ring Pump" This invention concerns improvements in or relating to liquid ring pumps which have been widely used, inter alia, in applications where smooth, non-pulsating gas or vapor removal is desired. Known designs of liquid ring pumps are shown for example in United States of America Patents Nos. 2 940 657 and 3 221 659 issued to H. E. Adams; 3 209 987 issued to I. C. Jennings; and 3 846 o46 issued to Kenneth W. Roe and others, and these have achieved a significant measure of success. In our British Patent Application No. 14912/77 (Belgian Patent No. 853376 ) there is disclosed an advantageous design of liquid ring pump one of the principal features of which is the provision of a prime number of equally angularly spaced blades on the pump impeller for the purpose of reducing noise and vibration. In prior liquid ring pumps it was conventional to provide for example twelve equally spaced blades, and we were the first to appreciate that such an arrangement could give rise to vibrations at multiples and sub-multiples of the rotational blade excitation frequency of the impeller. By providing the impeller with a prime number of equally spaced blades, no sub-harmonic vibrations can be generated and a considerable reduction in noise and vibration is obtained. The object of the present invention is to enable yet a further appreciable reduction in noise and vibration to be obtained. As will become apparent from the following, the present invention resides in the concept of providing different prime numbers of baldes on the different impellers of the sequential or parallel arranged multiple stages of the liquid ring pumps embodying the invention of our previous application mentioned above. According to the present invention therefore there is provided an improved liquid ring pump for gases, liquids and mixtures thereof, comprising a casing defining at least two pump chambers; at least two impellers mounted each for rotation within one of said chambers of said casing, each said impeller having a prime number of radial blades supported thereon at equal angular intervals for pumping said fluids, said impellers having different numbers of blades, whereby the number of excitation frequencies of each said impeller and, hence, noise and vibration of said pump, are reduced and the different numbers of blades for the respective impellers cause different excitation frequencies for said impellers to further reduce vibration and noise of the pump, and at least one suction port and at least one exhaust port located adjacent each said impeller for each pump chamber. Preferably, the numbers of said impeller blades for said at least two impellers are selected from the prime numbers 7, 11, 13, 17 and 19 it being preferred for a twoimpeller pump to have 13 blades on one impeller and 17 on the other. In the following, the invention will be explained by reference to embodiments which, other than having different prime numbers of blades on different impellers, are substantially identical to the embodiments described in our previous British Patent Application No. 14912/77 (Belgian Patent No. 853376 ) abovementioned. However, it will not escape those acquainted with the design of liquid ring pumps that the present invention has general application to all manner of specific pump designs and is not restricted to the designs hereinafter described which merely exemplify the invention. In the accompanying drawings, which illustrate exemplary embodiments of the present invention as aforesaid, the showings of the various figures are as follows: Figure 1 shows a perspective view of the exterior of an assembled compound pump embodying the present invention; Figure 2 shows an elevation section taken on line 2-2 of Figure 1, indicating the internal components of the invention; Figure 3 shows a partial, horizontal section taken on line 3-3 of Figure 1; Figure 4 shows an exploded view of the casing sections of a compound pump apparatus according to the invention; Figure 5 shows a view taken along line 5-5 of Figure 2, showing the details of the first stage center plate or manifold according to the invention; Figure 6 shows a view taken along line 6-6 of Figure 2 showing the details of the second stage center plate manifold according to the invention; ; Figure 7 shows an exploded view of the casing sections of a parallel, single stage pump apparatus according to the invention; and Figure 8 shows a simplified, sectional view taken along lines 8-8 of Figure 2. There follows a detailed description of the preferred embodiments of the invention, reference being had to the drawings in which like reference numerals identify like elements of structure in each of the several figures. Figure 1 shows a perspective view of a compound pump embodying the features of the invention. A pump housing or casing 10 comprises a suction end casing 12, a first stage body portion 14, first stage center plate 16, second stage center plate 18, second stage body portion 20 and discharge end casing 22. A suction inlet 24 directs fluids such as gas or vapor into suction end casing 12 and suction manifold 26. Suction manifold 26 connects in parallel the suction ports located at either end of the impeller of the first stage, as shown more clearly in Figures 2 and 3. A discharge manifold 28, formed integrally with the casing sections previously mentioned, directs discharge gases or vapors from the discharge ports of the first stage to suction ports located at either end of the impeller of the second stage. Gases or vapors leaving the discharge port of the second stage are directed into discharge end casing 22 and leave the apparatus via discharge outlet 30. A plurality of tie bolts and nuts 32 are provided to clamp the various casing sections to one another. Finally, an inlet conduit 34 is provided for admitting seal liquid to the interior of casing 10. The views of Figures 2 and 3, taken along lines 2-2 and 3-3 of Figure 1, illustrate the primary interior components of the liquid ring pump. A suction end bearing housing 40 and a discharge end bearing housing 42 support shaft bearings 44 and 46. A shaft 48, mounted for rotation within bearings 44 and 46, passes through seals 50 and 52 located in suction end casing 12 and discharge end casing 22. In the familiar manner for liquid ring pumps, shaft 48 is mounted eccentrically within both the first stage pumping chamber 54 defined by a first stage body portion 14, and the second stage pumping chamber 56 defined by second stage body portion 20. Both chambers 54 and 56 are free of any radial walls or baffles extending toward the centers of body portions 14 and 20; thus, the liquid and gases or vapors being pumped can flow from one end of each chamber to the other without encountering any obstructions other than shaft 48 and its impellers. A first stage impeller 58 having an axial length "L" and a diameter "D" is mounted on shaft 48 for rotation therewith within chamber 54. Also mounted on shaft 48 for rotation within chamber 56 is a second stage impeller 60 having an axial length "L"'and a diameter ''Dl''. Those familiar with liquid ring pump design will appreciate that the pumping capacity of the pump is influenced to a great extent by the axial length and the diameter of the impeller. Together with the pump speed and the thickness of the liquid ring itself, these dimensions control the displacement of the pump to a great extent. Where additional capacity is desired at a given operating speed, the prior art teaches that the impeller diameter may be increased, thereby increasing the volume of the radial displacement chambers between impeller blades. However, this also increases the tangential speed of the tips of the longer impeller blades, with an attendant increase in friction which must be overcome by applying more power to the shaft to maintain speed. Of course, the housing diameter also becomes larger. In prior art pumps1 attempts have been made to increase pump capacity by axially lengthening the impeller without changing impeller diameter. These attempts have been unsuccessful, however, due to undesirable drops in pump efficiency where the length-to-diameter ratio of the impeller exceeded about 1.06. Applicant has discovered that the impeller diameter actually can be reduced to minimize friction at a given speed and the axial length can be increased to maintain displacement with an unexpected improvement in overall pump performance, provided suction1 and preferably discharge, ports are located at both ends of the impeller. Length to diameter ratios greater than 1.06 and preferably in the range of approximately 1.2 to 1.5 have been found to produce lower power consumption due to reduced tip speed, without losing volumetric efficiency. Of course, the use of ratios outside this range is allowable where opposite end suction ports are used. The opposite end suction ports improve the breathing of the pump compared to single end ports so that substantially the entire volume between each pair of impeller blades is effective during pumping. In the prior art devices, an impeller with a length-to-diameter ratio of greater than 1.06 and with a suction port at only one end would be "starved" at the end opposite the single suction port, which reduces volumetric efficiency. While the invention is illustrated for use with a single lobe liquid ring pump, those skilled in the art will realize that the teachings thereof may also be applied to double or other multiple lobe pumps. Continuing in Figures 2 and 3, the flow path for vapors or gases entering the pump is through suction inlet 24 to a first stage inlet plenum 62 and then through a suction port 64 which is located in first stage end plate 65. Inlet flow also proceeds in parallel through integral manifold 26 to parallel first stage inlet plenum 66 which is defined between the first stage center plate 16 and the second stage center plate 18. From plenum 66, flow passes through suction port 68 which is located in first stage center plate 16. Discharge flow from the first stage chamber 54 is into first stage discharge plenum 70 through discharge port 72 also located in first stage end plate 65. The first stage also discharges in parallel to a first stage discharge plenum 74 located between center plates 16 and 18, through a discharge port 76. The flows from plenums 66 and 70 mix in plenum 74 and discharge manifold 28. A portion of the discharge from the first stage flows on through manifold 28 through second stage inlet plenum 78 and through a suction port 80 located in second stage end plate 81. The remainder of the discharge from the first stage passes through plenum 74 which serves as a parallel second stage inlet plenum. A second suction port 84 passes through plate 18 at a location opposite suction port 80. Discharge from the second stage flows through a discharge port 88 located in end plate 81 into a discharge plenum 86, located in discharge end casing 22. Thereafter, the gases or vapors leave the apparatus via discharge outlet 30. The actual sizes and circumferential locations of the opposite end suction and discharge ports are conventionally determined for a particular pump application, depending on factors such as desired suction and discharge pressures, pump operating speed, the fluid to be pumped and related factors familiar to those in the art. Turning now to Figure 4, an exploded view of housing or casing 10 is shown to indicate more specifically the form of specially advantageous flow directing manifolds. Suction end casing 12 includes an interior wall 100 (shown in phantom) which separates plenums 62 and 70. Wall 100 also includes a through bore for shaft 48. First stage end plate 65 includes an interior wall 102 which is congruent with interior wall 100 to separate ports 64 and 72. First stage center plate 16 includes radially extending interior walls 104 and 106 (shown in phantom) which separate ports 68 and 76. Second stage center plate 18 includes radially extending interior walls 108 and 110 which are oriented to be congruent with walls 104 and 106. A circumferential wall segment 112 extends between radial interior walls 108 and 110 to separate plenum 66 from plenum 74. The details of center plates 16 and 18 are discussed hereinafter in detail with regard to Figures 5 and 6. Second stage end plate 81 and discharge end casing 22 include congruent interior walls 114 (in phantom) and 116 similar in function and location to interior walls 100 and 102. Walls 114 and 116 separate plenums 78 and 86 and suction and discharge ports 80 and 88. Suction manifold 26 is defined by integral, radially extending portions of suction end casing 12, first stage end plate 65, first stage body portion 14, first stage center plate 16 and second stage center plate 18. In the assembled pump, these extending portions are joined together in a flow through relationship, as shown in Figure 1. Similarly, discharge manifold 28 is defined by integral, radially extending portions of suction end casing 12, first stage end plate 65, first stage body portion 14, first stage center plate 16, second stage center plate 18, second stage body portion 20, second stage end plate 81 and discharge end casing 22. In the assembled pump, these portions are also joined in flow through relationship. Turning now to Figure 5 first stage center plate 16 comprises a generally flat disc 120 having a central boss 122 surrounding a bore for shaft 48. An axially extending peripheral lip 124 surrounds disc 120 and includes flat mating surface 126 which extends across the thickness of lip 124. Radially extending flanges 128 and 130 are provided which include through passages oriented to form portions of manifolds 26 and 28 in the assembled pump as also shown in Figure 4. Ports 68 and 76 are isolated by radially extending walls 104 and 106 which extend from peripheral lip 124 to boss 122 on either side of suction port 68. Figure 6 shows a view taken along line 6-6 of Figure 2 indicating the geometry of second stage center plate 18. Center plate 18 comprises a generally flat disc 120" having a central boss 122' with a central bore for shaft 48. A peripheral lip 124' is provided which has a flat mating surface 126' extending across the thickness of lip 124. Radially extending walls 108 and 110 and the mating surface of lip 124' are congruent with their counterparts on first stage center plate 16. A seal plate 138 extends from wall 112 to boss 122 to isolate plenum 66 from plenum 74. That is, the suction port 68 is isolated from the suction port 84. Figures 5 and 6 also illustrate interlocking features which permit the use of flat mating end surfaces rather than conventional rabbeted mating joint geometry found on prior art liquid ring pumps. A pair of generally diametrically opposed, radially extending tabs 132/132' and 134/134' are provided which include a bore or other depression of substantial depth. Similar tabs and bores are also provided on the remaining casing sections as shown in Figures 4 and 7. To assemble the pump, dowels 136 are inserted in the bores and tabs of some of the components and the bores of the tabs in the mating surface of the adjacent component are slid over the extending portion of the dowel. The use of this type of joint geometry between casing sections eliminates a substantial number of machining operations during manufacture of the device and also permits the flat joint surfaces to be more easily milled or ground. The capability of milling or grinding these surfaces during manufacture can be very important when the casing sections are coated with an irregular finish such as glass which is sometimes provided for its anti-corrosion properties. Figure 7 shows an exploded view of pump casing 10 similar in most respects to that shown in Figure 4 except that this casing is configured to permit parallel operation of two single stage pumps, rather than a two-stage compound pump such as shown in Figure 4. Casing sections 16, 18, 81 and 22 have been replaced by modified versions 16', 18', 81 e and 22' as indicated. First stage center plate 16' differs from first stage center plate 16 by the optional removal of radial walls 104 and 106 and the necessary addition of an interior wall 140 (shown in phantom) which extends essentially diametrically across the plate to separate ports 68 and 76. Second stage center plate 18' differs from second stage center plate 18 by the optional omission of radially extending walls 108 and 110, circumferential wall section 112 and seal plate 138 and the necessary addition of an interior wall 142 which is congruent with interior wall 140 of center plate 16'. Thus, fluid flowing in through manifold 26 reaches both suction ports 68 and 84. End plate 81' is identical to end plate 81 except for the omission of inlet port 80 and the relocation of the top of interior wall 114 to the other side of manifold 28. End casing 22' is similarly modified to relocate the top of interior wall 116 so as to mate with wall 114 in end plate 81'. The flow through the first and second impellers in this embodiment is completely in parallel, with the first stage having suction ports 64, 68 and exhaust ports 72, 76 located at both ends of impeller 58 and the second stage having suction port 84 located at one end and exhaust port 88 at the other end of impeller 60. Figure 8 shows a schematic view taken along line 8-8 of Figure 2 to illustrate the interior geometry and operational principles of a liquid ring pump according to the present invention. Impeller 58 is mounted on shaft 48 for counter-clockwise motion at an eccentric location in chamber 54, as indicated. When the pump is operating, sealing liquid 144 is thrown to the periphery of body portion 14 by impeller 58 where it forms a moving ring of liquid around a central void. Blades 146 of impeller 58 rotate concentrically about shaft 48 but eccentrically with respect to liquid ring 144. Suction port 64 and discharge port 72 are exposed to the central void, but are separated from each other by the impeller blades and the liquid ring. As the gas or vapor is drawn through suction port 64, it is trapped in the radial displacement chambers between blades 146 and liquid ring 144. During rotation, blades 146 enter deeper into liquid ring 144 as discharge port 72 is approached, thereby compressing the gas or vapor in the familiar manner. As in any piece of rotating machinery, the vibration characteristics of the various components of the device must be adjusted as required to ensure acceptable operating vibration and noise levels. Mechanical imbalances in impeller 58 and shaft 48 can be largely eliminated by careful balancing; however, if the rotational frequency of the machine or any other excitation frequency is within approximately 20% of the natural frequency of the shaft, serious amplification of these vibration and noise levels may occur. These exciting frequencies may also be significant at harmonics or multiples of the rotational frequency and at sub-harmonics thereof. In the case of a machine having an impeller with a plurality of blades, the movement of each blade past a given reference point creates an excitation force. Depending on the number of these blades and their frequency, unacceptable vibration and/or airborne noise may result. For example, assuming an operating speed of 1800 rpm, an impeller having the commonly used prior art number of 12 blades would have a rotatinoal blade excitation frequency of 360 cps. Excitation forces would thus occur at this frequency and at multiples and sub-multiples of it. Multiples of the blade excitation frequency can readily occur; thus, for the assumed frequencies of 360 cps, the harmonic frequencies of 720 cps and 1080 cps may readily be generated. Also, sub-multiples of the blade excitation frequency may occur, applicant has recognized, as the result of "groupings" of the blades. Thus, if the impeller has twelve blades (which is common), and the blades are equally spaced, then each group of four blades, for example, generates a corresponding sub-harmonic and since there are three such groups of four blades in a twelve-bladed impeller, the submultiple frequency for the assumed conditions equals 360/3 or 120 cps. Similarly, each of the two groups of six blades each generates a sub-multiple frequency of 360 180 cps. This undesirable generation of sub-harmonic excitation frequencies may be avoided by spacing the blades at unequal angular intervals provided that blade spacing is selected to avoid the grouping of blades at regular intervals. This expedient is far from desirable, however, because of various factors such as increased cost of manufacture, unequally sized volumes between successive blades etc. Applicant's solution to the problem was to provide the impeller with a prime number of equally spaced blades. With such an arrangement, it is impossible to space the blades at equal intervals with any grouping of multiple successive blades located at equal angular intervals; hence, no sub-harmonic vibrations can occur in response to such a condition, and noise and vibration are then considerably reduced. Thus, to reduce noise and vibration, Applicant's impeller comprised a prime number of blades such as 3, 7, 11, 13, 17 or 19 blades for which only one grouping, i.e. the actual number of blades, exists. A thirteen-blade impeller is preferred in most instances. Fewer blades result in a higher pressure drop between the radial displacement chambers and more leakage; whereas, a very large number of blades reduces the volume available for impeller displacement. In any event, the use of a prime number of blades eliminates some excitation frequencies and helps reduce vibration and noise. The use of a thirteen-blade impeller will reduce the overall effect of the blade frequency by about 25 percent. This much is described in the aforementioned British Patent Application No. 14912/77. Now according to the present invention both of the impellers are provided with a prime number of blades but with the impellers 58 and 60 having different numbers of blades. Thus, for example the impeller 50 may conveniently have 13 blades and the impeller 60 may have 17 blades. As a result, the two impellers will have different excitation frequencies; accordingly, as will be appreciated by those skilled in the art, the peak noise levels of the resultant pump will be appreciably less than if both impellers had the same number of blades.

Claims: CLAIMS: 1. An improved liquid ring pump for gases, liquids and mixtures thereof, comprising: a casing defining at least two pump chambers; at least two impellers mounted each for rotation within one of said chambers of said casing, each said impeller having a prime number of radial blades supported thereon at equal angular intervals for pumping said fluids, said impellers having different numbers of blades, whereby the number of excitation frequencies of each said impeller and, hence, noise and vibration of said pump, are reduced and the different numbers of blades for the respective impellers cause different excitation frequencies for said impellers to further reduce vibration and noise of the pump; and at least one suction port and at least one exhaust port located adjacent each said impeller for each pump chamber. 2. A pump according to claim 1, wherein the numbers of said impeller blades for said at least two impellers is selected fs the prime numbers 7, 11, 13, 17 and 19. 3. A pump according to claim 2, wherein there are 13 blades on one of said at least two impellers and 17 blades on the other.

Inventors: FITCH, JAMES BURR
Applicants: GENERAL SIGNAL CORPORATION
Application year: 1979
Main classification: F04C19
IPC codes: F04C11, F04C7, F01C21, F04C23, F04C29