US3016842A - Screw pump - Google Patents

Description

Jan. 16, 1962 Filed June 13, 1960 4III INVENTOR CARL OSCAR TORSTEN MONTELIUS ATToRNgYs Jan. 16, 1962 c. o. T. MoNTELlUs 3,016,842

SCREW PUMP Filed June 13, 1960 2 Sheets-Sheet 2 United States Patent 3,016,842 SCREW PUMP Carl Oscar Torsten Montelius, Stockholm, Sweden, as-

signor to De Laval Steam Turbine Company, Trenton, NJ., a corporation of New Jersey Filed .lune 13, 1960, Ser. No. 35,737 Claims priority, application Sweden Feb. 23, 1959 1 Claim. (Cl. 103128) The present invention relates to an improvement in screw pumps of the type which comprises Aan assemblage of screws including a middle screw with convex thread anks and one or more side-screws meshing with the middle screw and having concave thread anks, 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.

This application is in part a continuation of my application Serial No. 10,514 led February 23, 1960, now abandoned.

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 the screws 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 itis 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 wherein there is only momentarily a fuuly closed chamber between the inlet end and the outlet end. In such a case, however, the inlet end and the outlet end are always separated 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 subected 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 pressure on the liquid will occur during travel along the screw assemblage. With a suicient accuracy in manufacture, however, this increase in pressure will be so small that substantially the entire increase in pressure occurs at the outlet end.

1f 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 minimum length is determined in the following manner: The minimum length (Ls) 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 ICC and the minimum length of the casing is determined by the equation '9i-2% Y LPQ 360 )S In these equations:

(III) i1 is the number of threads of the middle screw i2 is the number of threads of the side screws 01 is the thread top angle of the middle screw (in degrees) S1 is the thread (true) pitch of the middle screw ,b1 is the half angle of intersection (in degrees) of the bores of the casing as seen from the center of the middle screw.

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 different ways. If 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= tF1 Ts1 (IV) where i, is the number of seals desired. (Here, as Well as in connection with Equations V and VI below, the eX- pressions 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 different 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 ALb-lmrntr (afval (V) wherein:

im is the greatest number of seals in any thread groove in any position of the screws m is the number of thread grooves having im seals in such position i12 is the greatest of i1 and i2 In this case the total number of seals Lwill be If the lengths of the casing and the screws are selected in other ways, so that the above equations are not satised, then a varying number of seals will be obtained in dilerent angular positions of the screws. Such lengths may be advantageous from other points of view, e.g. to -achieve suiiiciently large bearing surfaces of the screws.

The seals between the pressure side and the suction;

The t side are of different 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 anks. 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 satislied, 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 toA a theoretical quantity per revolution reduced by the internal leakage, it will be understood that the rate of ow through the pump will fluctuate or pulsate slightly due to this variation of the internal leakage. It is desirable, however, to obtain a ow 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 nonpulsatng ilow by keeping the number of seals constant for each revolution. It has been found that this can be achieved by providing a groove in the bore accommodating the middle screw 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 the casing of the pump of FIGURE 1 in perspective view;

FIGURE 3 shows a cross-section of the casing taken along line III--III in FIGURE 1.

The pump illustrated on the drawings is of the type comprising a driven middle screw 1 and two side-screws Z, 3, the middle screw 1 having two threads with convex anks and the side screws also having two threads each but with concave anks and of opposite 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 10of the pump housing.

The middle screw 1 is driven by a motor (not shown) over 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 FIGURES 2 and 3, 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.

In the. screw pump shown in the drawings, the effective length of the casing (the distance L1 in FIGURE 1) is smaller than the eiective length of the screws (L2 in FIGURE 1). This represents only an example and is not intended to limit the invention. Unless the length of the casing and that of the screwsy arel selected in accordance with Equations I-VI above, it will be found that in a screw assemblage formed in this manner, during a part of each revolution of the middle screw there is one seal more between the suction side and the pressure side than during another part of the revolution. 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, during the first part of the revolution so that there is a constant number of seals during each revolution, the leakage is equalized resulting in a non-pulsating ow from the pump. This is achieved in accordance with this invention by providing a groove 14 in the bore of the middle screw.

The disposition and width of the groove according to this invention is determined by the following equations:

(VII) wherein:

y is the distance of the groove from either end of the casing;

u is the axial width of the groove;

Ls and L1, are the minimum lengths of the screws and casing, respectively, to achieve constantly one seal,

calculated in accordance with Equations I, II and IIIv above;

L1 is the actual length of the casing;

A1 and A2 are the amounts by which the screws extend beyond the ends of the casing withV the proviso, however, that if A1 or A11 exceeds the amount this amount is to be selected for A1 or A2 instead of the actual amount, because the exceeding amount 1s ineffective; if the screws do not extend beyond the A is equal to A1 if y. is measured from the end of the casing at which the excess length of the screws is A1 (i.e. as shown on the drawing) and equal to A3 if y is measured from the opposite end;

n is a coeicient the value of which is selected in a manner explained below;

S1 is the thread true pitch of the middle screw;

i1 is the number of threads of the middle screw.

The coeicient n is to be selected so that usatses the requirement casing at one or both ends, A1 or A2 or both will be stricted to the embodiment shown comprising a driven two-threaded middle screw and two two-threaded sidescrews, 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 G-ng+n=0 is satisfied, where G is the number of threads of the middle screw, n is the number of side-screws and g is the number of threads of each sidescrew. 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 first screw with convex thread flanks and at least one other screw with concave thread flanks intermeshing with said first screw, and a casing enclosing said screw assemblage and having intersecting bores for accommodating said screws, the screws sealing against each other and against the casing to form chambers within which the pumped uid is conveyed axially along the screw assemblage from its inlet end to its outlet end, the length of the screws and a continuously closed length of the bores in the casing being such that during finite portions of each revolution of the rst screw the number of seals is greater than the number of seals during the remainder of each revolution, characterized by the fact that only the bore accommodating the iirst screw is provided with a circumferentially extending groove throughout the circumferential extent of its walls, located between the ends of said closed length of the bores, said groove being located and dimensioned to interrupt the sealing between the chambers during said finite portions of each revolution to maintain constant the number of seals existing during each complete revolution of the first screw.

References Cited in the file of this patent UNITED STATES PATENTS 529,837 Quimby Nov. 27, 1894 1,698,802 Montelius Jan. 15, 1929 2,381,695 Sennet Aug. 7, 1945 2,463,460 Hodgkinson Mar. l, 1949 2,477,002 Paget July 26, 1949 2,519,913 Lysholm Aug. 22, 1950 2,581,451 Sennet Jan. 8, 1952 2,590,560 Montelius Mar. 25, 1952 2,592,476 Sennet Apr. 8, 1952 2,652,192 Chilton Sept. 15, 1953 2,924,181 Sennet Feb. 9, 1960 FOREIGN PATENTS 853,166 France Nov. 18, 1939

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