US3063379A

US3063379A - Screw pumps

Info

Publication number: US3063379A
Application number: US10398A
Authority: US
Inventors: Montelius Carl Oscar Torsten
Original assignee: Laval Steam Turbine Co
Current assignee: De Laval Steam Turbine Co
Priority date: 1959-02-23
Filing date: 1960-02-23
Publication date: 1962-11-13
Anticipated expiration: 1979-11-13

Description

Nov. 13, 1962 c. o. T. MONTELIUS I 3,063,379

SCREW PUMPS Filed Feb. 23, 1960 FIG. I.

Z G F INVENTOR.

CARL OSCAR TORSTEN MONTELIUS' ATTORNEYS United States Patent SCREW PUMPS Carl Oscar Torsten Montelius, Stockholm, Sweden, as-

signor to De Laval Steam Turbine Company, Trenton,

N.J., a corporation of New Jersey Filed Feb. 23, 1960, Ser. No. 10,398 Claims priority, application Sweden Feb. 23, 1959 1 Claim. (Cl. 103-128) The present invention relates to an improvement in screw pumps of the type which comprises an assemblage of screws including a middle screw with convex thread flanks and one or more side-screws meshing with the middle screw and having concave thread flanks, the threads being of such configuration that they seal against the circumference of cooperating screws, all said screws being sealingly enclosed in a casing formed with intersecting bores for the screws.

In the operation of such a pump, the liquid being pumped is progressed axially from the inlet end or suction side of the screw assemblage to the outlet end or pressure side thereof in essentially closed chambers formed by the thread flanks and bottoms of the screws and the enclosing casing. Such a chamber is formed at the suction side of the screw assemblage when thescrews are rotated and is filled with liquid while it is formed. When the chamber is fully formed, it is closed toward the suction side and on continued rotation of the screws it travels axially along the screw assemblage toward the outlet end of the screw assemblage where it is opened and the liquid is discharged. The screw assemblage may be such that a chamber starts to open toward the outlet end as soon as it is closed toward the inlet end. This can be considered as a limit case whereinthe're is only momentarily a fully closed chamber between the inlet end and the outlet end. In such a ca'se,ho'wever, the inlet end and the outlet end are alwaysseparated' by a closure or seal formed by the screw threads. The volume of the chamber is unchanged while it progresses from the inlet to the outlet end, and provided that. the pump is ideally tight, the liquid in the chamber is throughout this travel subjected 'to the pressure prevailing at the inlet end to be subjected to the higher pressure prevailing at the outlet end only when the chamber is opened at the outlet end. In an actual pump of this type,-of course, a certain amount of leakage will occur on account of unavoidable tolerances in the manufacture which result in a certain amount of play as between the screws and as between each screw and the casing, wherefore a small increase in the pressureon the liquid will occur during travel along the screw assemblage. With a sufiicient accuracy in manufacture, however, this increase in pressure will be so small that substantially the entire increase in pressure occurs'at the outlet end.

If the screws and the casing have a certain minimum length, the above limit case will be obtained, wherein there is constantly a single seal between the inlet end and the outlet end.

This minimumf-length is determined in the following manner: The minimum length (L of the screws to obtain only one seal between the suction side and the pressure side is the highest of the values determined by the two equations and the minimum length of the casing is determined by the equation ICC In these equations:

i is the number of threads of the middle screw i is the number of threads of the side screws 6 is the thread top angle of the middle screw (in degrees) S is the thread lead of the middle screw is the half angle of intersection (in degrees) of the bores of the casing as seen from the center of the middle screw.

It is of interest to note that when these equations are used, the length of the screws will not be equal to the length of the casing. The difference between the two lengths will be equally apportioned to the ends of the screw assemblage. The length of the screws in this connection refers to the efiective length of the threads, i.e. the length in which they mesh with each other. The length of the casing refers to the portion thereof which sealingly surrounds the screws.

If the pump is to operate at a higher pressure, it is necessary to increase the lengths of the screws and the casing, so that there are more seals between the inlet and outlet ends. In such cases, the lengths can be selected so that the number of seals is constant during the operation of the screws. This can be achieved in diiferent ways.

It the number of seals in each thread groove is to be constant, the lengths of the casing and the screws will be increased by an amount equal to AL=(i,1)Z:-S 1v where i, is the number of seals desired. (Here, as well as in connection with Equations V and VI below, the expression thread groove relates to the thread grooves of the screw having the greatest number of threads.) Under certain circumstances, however, it may be advantageous to permit difierent numbers of seals between the inlet and the outlet end in the various thread grooves, the sum of the number of seals in the various thread grooves being constant for all angular positions of'the screws. The amount by which the casing and screw lengths will be increased will then be equal to wherein i is the greatest number of seals in any thread groove in any position of the screws m is the number of thread grooves having i seals in such position 1' is the greatest of i and i In this case the total number of seals will be m+( 1zm1) V1 If the lengths of the casing and the screws are selected in other ways, so that the above equations are not satisfied, then a varying number of seals will be obtained in different angular positions of the screws. Such lengths may be advantageous from other points of view, e.g. to achieve sufficiently large bearing surfaces of the screws. The seals between the pressure side and the suction side are of diflerent types. There are seals between the peripheral surfaces of the screws and the casing, and there are seals between the peripheral surfaces and cores of the screws and between the peripheral surfaces and thread flanks. However, as mentioned above when there is more than one seal, in cases where for some reason or another the screw length is selected so that the above Equations IV, V and VI are not satisfied, the number of seals varies with the angular positions of the screws during each revolution of the middle screw. As above mentioned, on account of unavoidable plays the seals cannot be made perfect in practise, but an internal leakage must always occur. When the number of seals between the pressure side and the suction side varies during each revolution, this means that the internal leakage will also vary during each revolution. Since a positive pump of this type gives a capacity which is equal to a. theoretical quantity per revolution reduced by the internal leakage, it will be understood that the rate of flow through the pump will fluctuate or pulsate slightly due to this variation of the internal leakage. It is desirable, however, to obtain a fiow which is free'from pulsation.

The object of the present invention is to provide a screw pump of the type referred to which has a nonpulsating flow by keeping the number of seals constant for each revolution. It has been found that this can be achieved by providing a shallow circumferential groove in any thread of the middle screw extending from one flank to the other of such thread in such a manner that one seal between the suction side and the pressure side is destroyed in certain angular positions of the revolution of the screw.

The invention will be described more in detail with reference to the accompanying drawings, wherein:

FIGURE 1 shows an embodiment of the invention in elevation, partly in section;

FIGURE 2 shows a cross-section taken along line II--II in FIGURE 1.

The pump illustrated on the drawings is of the type comprising a driven middle screw 1 and two side-

screws

2, 3, the middle screw 1 having two threads with convex flanks and the side screws also having two threads each but with concave flanks and of oppposite hand to the threads of the middle screw, the threads being formed in a manner known per se so as to be in sealing relationship with each other. The screw assemblage comprising the middle screw 1 and the side-

screws

2, 3 is enclosed in a casing 4 which sealingly surrounds the screw assemblage, said casing having at its lower end two symmetrical ports 5 through which the entrance of fluid to the screw assemblage takes place, the fluid being discharged through the open top end of the casing. The casing 4 has an attachment flange 6 for attaching it by means of screws 7 in a pump housing 8 so that the lower end of the casing 4 with the ports 5 is disposed in the inlet chamber 9 of the pump housing, and the upper end is disposed in the discharge chamber 10 of the pump housing.

The middle screw 1 is driven by a motor (not shown) through a driving shaft 11 which is journaled in a bearing 12 attached to the top end of the casing 4 and extends through a' cover -13 attached to the pump housing.

As best seen in FIGURE 2, the casing is formed with a larger central bore for the middle screw 1 and two smaller bores for the side-

screws

2, 3, each of the two latter bores intersecting with the first bore.

The screw pump shown in FIGURES 1 and 2 represents a type which is common in practice and is dimensioned so that the effective length of the casing as well as the effective length of the screws (the distance L in FIGURE 1) is equal to 3D, where D is the external diameter of the middle screw. This represents only an example and is not intended to limit the invention. In a screw assemblage formed in this manner it will be found that during a part of each revolution of the middle screw there is one seal between the suction side and the pressure side, while during another part there are two seals. The leakage will therefore vary during each revolution so that a pulsating flow is obtained. However, by destroying one seal, in accordance with the present invention, so that there is only one seal during each revolution, the leakage is equalized resulting in a nonpulsating flow from the pump. This is. achieved in accordance with this invention by providing a groove 14 in the thread circumference of the middle screw.

The groove 14 may be disposed at either end of (the threaded portion of) the middle screw, but this gives substantially the same result as if the length of the screw had been selected according to the above formula for obtaining a constant number of seals. It is more advantageous to provide the groove intermediate the ends of the threaded portion of the screw, so as to obtain satisfactory bearing surfaces on the middle screw at opposite sides of the groove. The distance (x in FIGURE 1) from one end of the middle screw shall be equal to the minimum length L for obtaining constantly one seal, as determined by Equation I or II above. The distance x may be taken from either end of the screws.

If the groove is disposed at either end of the screw, the width (W) thereof will-be equal to where L the actual effective length of the screws L ==the minimum length of the screws to obtain one seal (according to Equation I or II) n=a positive integer selected so that OW S /i The case, where n can be selected so that W is equal to 0 occurs when the length of the screws is such as to correspond to a constant number of seals in which case no groove is necessary. This case is included above for the sake of completeness to facilitate the understanding of the following.

However, when the groove is disposed intermediate the ends of the screw, as is preferred, the width W must be increased by an amount equal to as compared to W The reason therefor is that in the contacting surfaces between the middle and side screws, the threads of the screws engage each other in an axial direction. Thus, in this case the width will be equal to On account of the axial engagement of the threads with each other, asmentioned above, it will be necessary, even when the groove is to be disposed intermediate the ends of the screws, to determine whether W will be 0, in which case the length of the screws is such as to correspond to a constant number of seals and the groove will be unnecessary. If W 0, the width of the groove will be determined according to Equation VIII.

For screws having a screw length L such that (in which case Equation VII above gives a negative value for W the width can be chosen freely, but should be chosen as small as possible (although of course not so small that the desired function of the groove is not achieved) in order that the bearing surfaces shall be as. large as possible.

The angles l// and 0 referred to in connection with the Equations I, II and III above are indicated in FIGURE 2.

It will be understood that the invention is not restricted to the embodiment shown comprising a driven twothreaded middle screw and two two-threaded side-screws, but is generally applicable to other screw-pumps having a different number of side-screws and a different number of threads, the shape of the threads and the number of side-screws as well as the number of threads of the screws being so selected in relation to each other, in a manner known per se, that in each position of the screws there is at least one seal between the ends of the screw assemblage. It is known to those skilled in the art that this can be realized if the threads obtain a certain mathematically defined geometrical shape, and if the condition is satisfied, where G is the number of threads of the middle screw, 12 is the number of side-screws and g is the number of threads of each side-screw. The invention is applicable to all screw pumps of this type, including such screw pumps of this type where not only the middle screw but also the side-screws are driven.

I claim:

A positive screw pump of the type comprising a screw assemblage including a middle screw with convex thread flanks and at least one side screw with concave thread flanks and a casing enclosing said screw assemblage and having intersecting bores for accommodating said screws, said casing providing inlet and outlet passages, the screws sealing against each other and against the casing to form constant volume closed chambers within which the pumped fluid is conveyed axially along the screw assemblage from its inlet passage to its outlet passage, the length of the screws and a continuously closed length of the bores in the casing being such that there is always at least one closure in the screw assemblage, the last arrangement being such that, without the characteristics specified hereafter the number of closures would vary during rotation of the screws, characterized by the fact that any thread of said middle screw is provided with a shallow circumferential groove at its periphery extending from one flank to the other of said thread, located between the ends of said middle screw and between the ends of said continuously closed length of the bores, which groove has such axial extent and is so located as to interrupt the complete closure of chambers during portions of revolutions of the screws to maintain constant the number of closures existing throughout each revolution of the screws.

References Cited in the file of this patent UNITED STATES PATENTS