US3804565A

US3804565A - Screw pumps

Info

Publication number: US3804565A
Application number: US00141156A
Authority: US
Inventors: M Sennet; M Bond
Original assignee: Delaval Turbine California Inc
Current assignee: Delaval Turbine California Inc
Priority date: 1961-09-27
Filing date: 1961-09-27
Publication date: 1974-04-16
Anticipated expiration: 1991-04-16

Abstract

1. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180* with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage.

Description

United States Patent 1 Sennet et al.

[111 3,804,565 [451 Apr. 16, 1974 Primary Examiner-Samuel Feinberg Attorney, Agent, or Firm--Busser., Smith & Harding EXEMPLARY CLAIM l. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180 with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage.

10 Claims, 3 Drawing Figures SCREW PUMPS [75] Inventors: Morgan B. Sennet, Erwinna, Pa.;

Morton K. Bond, Trenton, NJ. [73] Assignee: De Laval Turbine Inc., Trenton,

[22] Filed: Sept. 27, 1961 [211 App]. No.: 141,156

52 us. C1. ..418/201, 415/99 [51] Int. Cl. E% yl6 [58] Field of Search 103/128;230/l43;418/201, 121/70 A, 123/12 BC [56] References Cited 1 UNITED STATES PATENTS 1,306,169 6/1919 Brooks et a1 103/128 2,382,042 8/1945 Etnye 103/128 1,430,894 10/1922 103/128 1,653,534 12/1927 103/128 1,701,166 2/1929 Tifft 103/128 1,991,541 2/1935 Cannizzaro 103/128 2,358,815 9/1944 Lysholm 230/143 2,699,122 l/1955 Erickson 103/126 B FOREIGN PATENTS OR APPLICATIONS 430,601 6/1935 Great Britain 103/128 PATENTEDAPR 16 I9 4 $804,565

INVENTOR.

ATTORNEYS MORGAN B. SEN E a BY MORTON K. B0

SCREW PUMPS This invention relates to screw pumps and has particular reference to the reduction of noise during operation thereof.

The present invention is particularly applicable to both positive and non-positive screw pumps of the types described in the Montelius Pat. Nos. 1,698,802, 1,821,523 and 1,965,557, dated, respectively, Jan. 15, 1929, Sept. 1, 1931 and July 3, 1934, and Sennet Pat. Nos. 2,693,762 and 2,693,763, dated Nov. 9, 1954. It will be evident that the invention is, however, also applicable to other types of screw pumps in connection with which generally similar problems arise. Referring particularly to positive screw pumps, these theoretically would have continuous delivery and, in fact, in practice substantially continuous delivery does take place in the sense of displacement of volume per unit time assuming an incompressible liquid is being pumped. Compared with other pumps these screw pumps operate very silently. Howevensuperimposed on this continuous delivery are pulsations which may be particularly violent (in the sense of absolute magnitude) in the case of high speed pumps operating to deliver at high pressures and therefore involving very large power delivery. The pulsations produced are in the nature of high intensity sound waves (though representing power quite small as a proportion of delivered power), which are propagated through the liquid being delivered and to and through the structure of the pump and its associated piping, etc. Multiple causes apparently contribute to the production of these pulsations and contribute to different degrees in dependence upon the circumstances surrounding operation. One of the causes of pulsation may be readily recognized. Positive screw pumps contain one or more closures in the sense of boundaries of closed chambers which effectively travel axially during operation. These closed chambers open successively to the outlet. Since running clearances must necessarily be provided between the component screws and between the screws and their housing, examination of these will make clear that the resistance to backflow of the pumped liquid varies during a cycle of screw rotation. For example, assume a closed chamber which is approaching but has not yet opened to the outlet. The pressure therein will at this time lie somewhere between the inlet and outlet pres sures of the pump, deviating from a theoretical inlet pressure due to the fact that backflow has occurred to some extent during the progress of this chamber toward the outlet. Nevertheless, prior to opening to the outlet the pressure will ordinarily be considerably less than the outlet pressure, particularly if the clearances are small and the liquid being pumped relatively viscous.

The closed chamber just described is suddenly opened during operation to the outlet. This means that a sudden transient pressure change will occur involving asharp pressure drop beyond the opening, a momentary drop of forward flow velocity, and then an acceleration involving increase of pressure. If the liquid was completely incompressible, and if completely tight closures were provided, there might theoretically be no source for pulsation at this time. But liquids are compressible and, in fact, this is evidenced merely by the fact that they will transmit sound waves. Merely due to inherent compressibility, it is evident that the sudden pressure changes would give a pulsation having, in particular, a steep wave frontQThis situation is made all the worse by the fact that such pumps are ordinarily used to handle oils containing volatile constituents and dis solved gases, and their content of gasesat low pressures is increased in many cases by cavitation at the pump inlet producing separation of the gas or volatile constituents in bubble form. Accordingly, the sudden transient pressure changes which have been described may produce not only what might be considered a true sound wave but an additional shock or disturbance of compression of the separated gas or volatile constituents.

Another factor which in ay be involved to a greater or less degree is that of sudden change at the same time of the resistance to backfiow. It will be evident that the flow path backwardly from the outlet to the inlet incorporates successive restrictions which are provided at the theoretical closure points of the chambers, actual provision for flow resulting from clearances. The number of restrictions is suddenly changed at the time a chamber opens to the outlet, there being a sharp shift of pressure relationships along the closure lines and surfaces resulting, necessarily, in a change of rate of backward flow. Different pressures are also applied to 7 different parts of the rotors as the phase relationship changes during a cycle.

All of the foregoing, and other causes as well, result in the production of pulsations. As already indicated, these from the standpoint of change of volume of flow percentagewise are merely small superpositions on a theoretically constant flow rate. But measured in terms of sound energy the pulsations may be of very high noise level and productiveof not only noisy but actual damaging vibrations in piping and other structures associated with the pump. Involving shock aspects, furthermore, the noise spectrum is very broad. The fundamental frequency is related to the frequency of rotation of the screws. The usual screw arrangement which is used and which has many advantages is that of a single power screw having two threads meshingwith a pair of idler screws each having two threads. In the case of such a pump, the fundamental frequency of the pulsations is twice that of the frequency of revolutions of the power rotor or screw. This would be as expected from consideration of the events occurring during a cycle of operation, there being two traveling chambers opened to the exit during each revolution of the power screw.

Since it is difficult to eliminate the sources of the pulsations or vibrations, the present invention is concerned with a pump structure in which the transmission of pulsations is minimized. In accordance with the invention there are several inter-related but independently useful aspects of construction involved.

First there is the matter of formation of a casing to include a muffler having substantial effectiveness in reducing the transmission of pulsations.

Secondly, in accordance with this inventioma choke arrangement is used which is further effective to reduce pulsations.

The third improvement involved in accordance with the invention results from the provision of an out-of phase arrangement of the screw elements of a double unit pump. g

The objects of the invention incorporating the foregoing matters will become more apparent from the following description, read in conjunction with the accompanying drawing in which:

FIG; 1 is an axial section taken through a screw pump provided in accordance with the invention;

FIG. 2 is a transverse section take on the plane indicated at 22 in FIG. 1 through one of the pump units; and

FIG. 3 is a section similar to FIG. 2 taken on the line indicated at 3-3 of FIG. 1 through the other unit and indicating the out-of phase relationship of the units.

The numeral 2 indicates generally the casing of the pump which comprises a pair of symmetrical units (though these need not necessarily be the same). As illustrated, the pump is of the positive type in which each unit comprises a power screw containing two threads and a pair of idler screws, each containing two threads. The housing is closed at one end by a cover 4 and at its other end by a cover 6 which is provided with a conventional bearing and packing arrangement for the drive shaft 8 which has formed integrally therewith the

power screws

12 and 14 rotating in a central bore of the inner housing 10. A pair of idlers 16 mesh in usual fashion with the power screw 12, and a similar pair of idlers 18 mesh with the power screw 14. These idlers rotate in the usual side bores of the housing 10. As so far described the arrangement is conventional. However, as will be more evident with respect to the sections illustrated in'FIGS. 2 and 3, the phase relationships of the screws of the two units are displaced 90 with respect to the power screw threads and their relationships to corresponding axial positions of the arrays of the two units. What is basically involved in this phase displace ment is with relationship to the axial positions at which opening occurs to the exit passages. Stated in still another way, for each rotation of the common shaft, a closed chamber of one of the units will open to the exit, 90 of revolution later a chamber of the other unit will open to the exit. Ninety degrees following this a chamber of the first unit will open to the exit, etc. As has been indicated above, this opening of a chamber to the exit, and therefore its attainment of the discharge pressure, is to a primary degree the initiation of a pulse. By displacing the phases of such an occurrence by 90, the pulsations, considering both units, occur at double the frequency at which they would occur due to a single unit. A plot of pulsation pressure variations against a cycle reveals that such pulsations roughly follow a sinusoidal form of wave, so that the 90 phase shift provides to some extent a balancing of the pulsations originating from the separate units.

However, there is considerable deviation of the wave of pulsations from sinusoidal form, and accordingly better results from the standpoint of noise reduction may sometimes be effected by providing a relative phase shift displaced from the 90 relationship just discussed. This is a matter of refinement of the operation and depends so largely on the overall construction including associated piping and the like that it may be effectively arrived at empirically only. The phase displacement which is desirable, however, is approximately 90 based on rotation of the power screw shaft. (It may be noted that the 90 relationship is referred to the power screw shaft herein, because of the fact that the shaft has been assumed to contain two threads in a symmetrical relationship; a complete operative cycle, therefore, involves only a half rotation of the shaft, and

with respect to a pulsation cycle the phase shift is or approximately so.)

Continuing with the description of structure, the two

power screws

12 and 14 may be cut with the displaced phase relationship of their threads or, alternatively, the screws may be cut in the usual way and the portions forming 12 and 14 separated, shifted in phase, and then reconnected at the central region 20 whereat liquid is received from the inlet passage 22 which has external connection as at 23 to the liquid supply.

As just indicated the inlet is at the center of the two units which provide discharge at 24 and 26, respectively. This arrangement is highly desirable as compared with an arrangement in which inlet is at the outer ends of the screws and discharge is from the central portion of the array. It is undesirable to have gas in the inlet because of the undesirable results arising from cavitation. If air is admitted at the inlet, the bubbles thereof which are included will ordinarily expand considerably under the suction pressure and consequently the closed chambers will entrap less than the proper amount of liquid. This in turn will result in compression at the outlet with an effective backward flow at the moment of chamber opening which will increase the magnitude of the pulsations. By providing a central inlet as illustrated, the inlet passage is hermetically sealed in contrast with a shaft seal which is liable to permit some leakage. In case of very high pressure operation the sealing at the shaft exit from the casing may involve a conventional type of breakdown arrangement and relief valve such as are conventionally used for packing and sealing against high pressure gradients.

Surrounding the inner housing is the enlarged chamber 28 which receives flow from both of the units and which by reason of its large volume acts as a muffler. This chamber should have a volume of the order of upwards of four times the total volume of the chamber spaces of the pumping units. Velocities are greatly reduced in the muffler with resulting dissipation of the pulsations. A symmetrical arrangement of the muffler is desirably used with respect to the exit passages from the units and also with respect to the exit passage from the muffler. This most effectively combines the two out-of phase pulsations of the individual flows to provide to a considerable extent neutralization of each other.

From the muffler there extends the converging passageway 30 which is defined by the inner surface of a tapered opening in the muffler casing and the outer surface of the domed member 34, the arrangement providing a choke passage 36 leading to the delivery connection 32. The choke in connection with the muffler provides a quite effective damping or attenuation of the pulsations which are residual even after the combination of the out-of phase discharges.

As will be evident, the phase shift arrangement of multiple units in itself provides substantial reduction of pulsation and may be used independently of the muffler and choke arrangement, or in conjunction with a different type of muffler which may be incorporated in the delivery piping.

While the description has been particularly given with respect to a particular screw arrangement of a positive pump it will be evident that the invention may be utilized in the matter of other screw arrangements for both positive and non-positive pumps. In such case, it

will be obvious that to obtain effective partial cancellation of pulsations the phase shifting may be otherwise than described above. In any case what should be used in the way of phase shift will be that which provides for a single cycle of operation a shift of 180 with respect to that cycle, the term cycle as here used referring to that extent of rotation which gives rise to a repetition of a similar event. Such event, for example, may be regarded as the opening of a traveling chamber in the case of a positive pump, or in the case of a non-positive pump the particular occurrence which sharply changes a relationship such as that of extent of a leakage passage, to effect, for example, a sharp increase of freedom of leakage.

It will also be clear that the muffler and/or choke arrangement which hasbeen described may be used even if, for some reason, it is not desired to utilize a phase shift of a pair of pumping units or if a single pumping unit is provided in which such a phase shift may not enter the picture.

It will be evident that various changes in details of construction and operation may be made without departing from the invention as defined by the following claims.

What is claimed is:

1. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inletand outlet openings in free continuous communication with the ends of saidscrew assemblies, said screw assemblies and said housing bores being of types producing theoretically, exceptfor running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screwsofsaid respective assemblies being displaced in phase substantially 180 with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage.

2. A screw pump forliquids comprising a pair of similar screw assemblies each of which includes a plurality of intenneshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for bothof said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect-at least partialcancellation of flow disturbances in said common passage.

3. A screw pump for liquids comprising a pair of similar axially aligned screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge ofpumped liquid, said inlet openings being at the contiguous inner ends of said screw assemblies, and said outlet openings being at the outer ends of said screw assemblies, means providinga common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180 with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage.

4. A screw pump for liquids comprising a pair of similar axially aligned screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, said inlet openings being at the contiguous inner ends of said screw assemblies, and said outlet openings being at the outer ends of said screw assemblies, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage.

5. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies,

and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber.

6. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies,

and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180 with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber surrounding said housing of the screw assemblies.

7. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber.

8. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase with respect to each other from the standpont of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber surrounding said housing of the screw assemblies.

9. A screw pump for liq uids comprising a pairofsimilar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially with respect to each other from the V v standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber, and means providing a choke outlet from said muffler chamber. n

10. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, a common drive means for said screw assemblies operating both at the same speed, said means providing a common passage providing an enlarged muffler chamber, and means providing a choke outlet from said muffler chamber.

Claims

2. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage.

3. A screw pump for liquids comprising a pair of similar axially aligned screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, said inlet openings being at the contiguous inner ends of said screw assemblies, and said outlet openings being at the outer ends of said screw assemblies, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180* with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage.

5. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180* with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber.

6. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180* with respect to each other from the standpoint of repetitive phases of discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber surrounding said housing of the screw assemblies.

8. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, and a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase with respect to each other from the standpont of repetitive phases of discharge discharge operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber surrounding said housing of the screw assemblies.

9. A screw pump for liquids comprising a pair of similar screw assemblies each of which includes a plurality of intermeshing screws and a housing having intersecting bores enclosing said screw assemblies and defining for both of said screw assemblies inlet and outlet openings in free continuous communication with the ends of said screw assemblies, said screw assemblies and said housing bores being of types producing theoretically, except for running clearances, continuous discharge of pumped liquid, means providing a common passage receiving liquid flow from both of said screw assemblies, a common drive means for said screw assemblies operating both at the same speed, the corresponding screws of said respective assemblies being displaced in phase substantially 180* with respect to each other from the standpoint of repetitive phases of operation thereby to effect at least partial cancellation of flow disturbances in said common passage, said means providing a common passage providing an enlarged muffler chamber, and means providing a choke outlet from said muffler chamber.