Eccentric screw pump with overpressure protection

Abstract

An eccentric screw pump comprising an outer part (2) and an inner part (3) therein, one of the parts (2, 3) being driven rotatably and the other part (2, 3) being able to move eccentrically relative to the other part (3, 2). The screwthreads (5a, 5b) of the outer part (2) extend angularly over less than an entire helix along the axial length (L) of the part, so that during operation pumping chambers that are open to both ends are created, through which sudden pressure relief takes place.

Claims

1. A pump for displacing a medium and comprising: a housing having an intake end and an output end; an outer part in the housing; an inner part inside the outer part, one of the inner and outer parts being driven rotatably about an axis of the one of the inner and outer parts and the other of the inner and outer parts being able to move eccentrically relative to the one of the inner and outer parts; at least one screwthread extending helically and axially on the inner part; and a number of screwthreads on the outer part that is greater by one than a number of screwthreads on the inner part, the ratio of the number of screwthreads in every cross-section being identical to the ratio of the pitches of the screwthreads, and the outer part and the inner part making contact in such a way that the inner and outer parts form pumping chambers that move axially on rotation of the one of the inner and outer parts relative to the other of the inner and outer parts so that the medium to be displaced is displaced from the intake end to the output end, the screwthreads of the outer part extending angularly over less than an entire helix along an axial length of the outer part such that on each revolution of the one of the inner and outer parts about the axis opposite ends of each chamber both open briefly at both the intake end and the output end.

2. The pump according to claim 1, wherein the screwthreads of the outer part extend angularly only through between 75% and 95% of an entire helix along the axial length of the outer part.

3. The pump according to claim 1, wherein the outer part is the other of the inner and outer parts and is eccentrically movable around the axis of the inner part, and the inner part is the one of the inner and outer parts and is rotatably driven.

4. The pump according to claim 1, wherein the outer part is made of an elastomeric material and at one of its axial ends is held on the housing via an elastic support structure.

5. The pump according to claim 1, wherein the axial length of the outer part is selected such that the pump has a predetermined maximum delivery head.

Description

BRIEF DESCRIPTION OF THE INVENTION

(1) Further features and advantages of the invention are described hereafter based on the embodiment shown in FIG. 1. In the drawings:

(2) FIG. 1 is an axial section through an eccentric screw pump according to the invention; and

(3) FIG. 2 is a graph of the dependence of the maximum delivery head on the stator length.

SPECIFIC DESCRIPTION OF THE INVENTION

(4) FIG. 1 shows an eccentric screw pump. It has an intake end 7 and an output end 8. A medium to be pumped is moved from the intake end to the output end during operation of the pump.

(5) The pump comprises an outer part 2 and an inner part 3 therein. Of these parts, the inner part 3 is driven. It forms the rotor of the eccentric screw pump and rotates around an axis 9. During operation, the outer part 2 moves radially eccentrically relative to the inner part 3 on a circular path around the axis 9. However, compared to the rotational movement of the inner part 3, the outer part 2 is stationary and is therefore referred to as the stator. The outer part 2 is approximately cylindrical and, at its end close to the intake end 7, merges in one piece into an outer elastic support structure 1. This support structure 1, along with the outer part 2, is formed from an elastomeric plastic material. The support structure 1 and the outer part 2 together form the stator of the eccentric screw pump, and the support structure 1 is attached to a pump housing 10. The part located between this support structure 1 and the rotor or inner part 3 is thus the inner stator part 2. Since this inner stator part is made of an elastomeric material and at one end integrally merges into the is structure 1 forming the outer stator part, it vibrates quasi freely relative to the rotor and is deformed during eccentric movement thereof around the rotor 3 in such that the eccentricity between the pump rotor 3 and pump stator 2 is compensated for. Such a system is therefore also referred to as a wobble stator.

(6) The inner part or rotor 3 has exactly one outer screwthread 6 extending helically and axially. In contrast, the inner surface of the outer part 2 (stator inner part) has a number of screwthreads 5a, 5b that is higher by one than the inner part 3, namely two screwthreads that also extend helically and axially. It is noted out that FIG. 1 shall be understood purely by way of example and that any arbitrary other number of screwthreads is possible. Moreover, the profiles of the inner part 3 and of the outer part 2, which is to say the outer profile of the inner part 3 and the inner profile of the outer part 2, can be arbitrary, they can in particular have any one of the profile shapes shown in DE 602107.

(7) The ratio of the number of screwthreads 5a, 5b: 6 is 1:2 here. The pitches of the screwthreads of the outer part 2 and inner part 3 are selected so that in every cross-section the ratio of the pitches of the screwthreads 5a, 5b: 6 is identical to the ratio of the number of screwthreads 5a, 5b: 6. It is apparent from FIG. 1 that the pitches of the internal screwthread 5a, 5b of the outer part 2 are considerably larger than the pitch of the external screwthread of the inner part 3. The external screwthread 6 of the inner part 3 has two full turns, which is to say it extends along two full helices axially. Due to the higher pitch, the internal screwthreads 5a, 5b would only have one full turn along the same axial length as the rotor 3.

(8) The outer part 2 and the inner part 3 make contact with each other such that pumping chambers A, B are formed between them that can be moved axially by rotating the inner part 3 relative to the outer part 2, so that the medium to be displaced can be displaced from the intake end 7 to the output end 8.

(9) According to the invention, the screwthreads 5a, 5b of the outer part 2, which is to say of the inner stator part 2, extend through less than an entire helix. This means that the axial length L of the inner stator part 2 is shorter compared to a conventional stator of an eccentric screw pump, in which the internal screwthread extends at least through a full helix. As was already described above, this causes the pumping chambers A, B to no longer be completely closed. If the rotor 3 is rotating into such a position in which a pumping chamber on the intake end 7 is in the process of closing, this pumping chamber has just opened on the output end 8, and medium to be displaced briefly flows back through the pumping chamber, which is now open on both ends, and a small portion of the pressure on the output end 8 is reduced. The return flow is higher, the higher the pressure on the output end. In this way, this effectively prevents damage to the pump and/or to the system connected to the pump when the pump is operating against a closed valve or a clog in the output pipe.

(10) What is interesting is that the measure according to the invention increases the efficiency of the pump, since lower friction losses occur between the inner part 3 and the outer part 2 due to the shortened stator.

(11) As a result of the reduction in pressure, the maximum delivery head of the pump is reduced. This effect is more pronounced the lower the displaced volume rate is. FIG. 2 shows is measured normalized hydraulic characteristic curves for a pump comprising a conventional stator 2 (solid line), in which the stator length L is greater than the screwthread pitch H.sub.st, and for a stator 2 according to the invention having a stator length L smaller than the screwthread pitch H.sub.ast (dotted line). The figures for the delivery head H and the flow rate Q are normalized to the maximum values H.sub.nom and Q.sub.nom. It becomes apparent that shortening the stator length L such that the screwthreads 5a, 5b of the outer part 2 extend around less than an entire helix has hardly an effect relative to the delivery head H at medium, and in particular at higher, flow rates Q.

(12) Insofar it has been shown that an eccentric screw pump does not necessarily have to comprise at least one enclosed pump chamber between the inner part 3 and the outer part 2, which is to say that the screwthreads of the outer part must have at least one turn, as is described in DE 602107. Rather, a stator 2 that is shorted compared to this technical teaching may also be used, which surprisingly results in the described pressure reduction on the pressure end 8, and moreover during operation even results in increased efficiency.