IL115910A - Turbine flowmeter - Google Patents

Description

TURBINE FLOWMETER The invention described here concerns a turbine flowmeter having an impeller wheel consisting of an impeller-wheel hub with vanes attached, whereby said wheel is mounted between a fixed hub situated upstream of the impeller wheel and a fixed hub situated downstream of the impeller wheel in such a way that it can rotate and move in an axial direction, whereby the outside diameters of the ends of the fixed hubs pointing towards the impeller wheel are in each case larger than the outside diameter of the impeller-wheel hub on the ends pointing towards the fixed hubs, and whereby the axial thrust of the impeller wheel, subjected to an axial throughflow and acting in the direction of flow, is compensated by static pressure differences on both end faces of the impeller-wheel hub.

The flow through a turbine flowmeter sets the impeller wheel in rotation. At the same time, the flowing of the fluid medium causes a hydrodynamic axial thrust to act on the impeller wheel in the direction of the flow and said thrust exerts a load on the thrust bearing on the downstream side of the impeller wheel, thereby causing wear which has a detrimental effect on the service life of this bearing. Furthermore, the measuring accuracy also suffers as a result of this.

A multitude of solutions are known aimed at preventing or minimizing the disadvantages outlined above; said solutions compensate or attempt to compensate the axial thrust force generated by the flow which acts on the impeller wheel.

A turbine flowmeter is described in US-PS 4 449 410 in which an impeller wheel, consisting of a hub with vanes attached, is mounted in a tubular element, through which a fluid can pass, in such a way that it can rotate and move in an axial direction. To achieve this, the shaft of the impeller wheel is mounted in thrust bearings fitted in fixed hubs, one upstream and one downstream of the impeller wheel.

The hubs are provided with graded diameters in the flow path of the fluid to be measured in such a form that the outside diameters of the ends of the_fixed hubs pointing towards the impeller wheel are in each case larger than the outside diameter of the impeller-wheel hub on the ends pointing towards the fixed hubs. Furthermore, however, the inside diameters of the ends of the fixed hubs pointing towards the impeller wheel are also larger than the outside diameter of the ends of the impeller-wheel hub pointing towards the fixed hubs. Thus, in the case of corresponding axial play in the impeller wheel, the impeller-wheel hub can penetrate into the fixed hubs with clearance. The inside walls of the fixed hubs are realized in cylindrical form over this penetration length and a little beyond so that in each case a radial annular gap of constant cross-section is created between the penetrating ends of the impeller-wheel hub and these hollow cylindrical inside wall segments. Moreover, the end of the impeller-wheel hub situated downstream has a larger diameter in comparison to the rest of this hub. Of course, the end of the fixed hub situated downstream which points towards the impeller wheel also has a I I diameter which is larger than the fixed hub situated upstream.

Axial thrust of the impeller wheel in the turbine flowmeter described is compensated as follows: Owing to the abrupt change between the outside diameters of the hub situated upstream and the impeller-wheel hub, the flow stalls at this point, thereby causing an almost equal negative pressure in the cavity of the hub situated upstream in all axial positions of the impeller wheel. At the end of the impeller-wheel hub situated downstream, the enlarged outside diameter of this hub and the enlarged diameter of the hub situated downstream form a relatively large backwater profile which projects considerably into the impeller wheel meridian flow and causes a severe change in the flow direction in all axial positions of the impeller wheel. Due to the aforementioned radial annular gap, the resulting backwater pressure, and hence an overpressure, is transmitted to the side of the impeller-wheel hub pointing downstreamJ This pressure causes an axial thrust in the opposite direction to that of the flow in all positions of the impeller wheel.

In comparison to the state of the art, the turbine flowmeter described above achieves a compensation of the axial thrust with relatively simple constructive means. However, the disadvantages are the relatively high pressure loss owing to the backwater profile projecting considerably into the meridian flow and erratic running of the impeller wheel brought about by the unfavourable throughflow conditions.

Therefore, the object of the present invention is to make available a turbine flowmeter in which the axial thrust force created by the flow onto the impeller wheel is compensated by very simple constructive means which, in addition, can be categorized as more favourable in terms of fluid technics due to the lower pressure losses, and which result in a smoother running of the impeller wheel.

According to the invention, this task is solved with a turbine flowmeter of the aforementioned type in that, over its width, the impeller-wheel hub exhibits a constant outside diameter .which is approx. 4-8% smaller than the ends of the fixed hubs pointing towards the impeller wheel.

Surprisingly, it has come to light that merely the fact that the outside diameter of the impeller-wheel hub is made slightly smaller than the corresponding outside diameters of the fixed hubs situated upstream and downstream respectively, can achieve an effective automatic compensation of the axial thrust without the disadvantages of the known state of the art. This connection had not been recognized hitherto.

The automatic compensation of the axial thrust in a turbine flowmeter designed in accordance with the invention operates as follows: As the fluid flows through the turbine flowmeter, the sudden change in cross-section immediately after the hub situated upstream causes a change in the meridian velocity, i.e. the axial flow velocity, whereby depending on the formation of the boundary layer at the hub situated upstream and irrespective of the size of the change in cross-section, a separation of the boundary layer occurs which forms a dead water zone after the abrupt change in diameter (upstream hub to impeller-wheel hub). In the process, the static pressure does not increase in the area of the axial gap existing between the hub situated upstream and the impeller wheel.

This gap forms a non-choked connection to the fluid in the cavity of the hub situated upstream. The width of the axial gap depends on the operation and can, therefore, influence the dead water zone because additional energy can be transferred from the fluid in the hub cavity into the dead water zone; During this process, as the width of the gap increases, so the axial extension of the dead water zone reduces and the flow again already makes contact with the peripheral surface of the impeller-wheel hub within the impeller wheel itself.

Therefore, the dead water zone is controlled by the axial position of the impeller wheel.

The axial gap between the impeller wheel and the hub situated downstream is vitally important for the creation of the automatic axial thrust compensation. If the impeller wheel is located just before the hub situated downstream (small axial gap), then a backwater pressure zone builds up immediately before the abrupt change in diameter at the hub situated upstream because the backwater streamline is already again in contact with the wall of the impeller-wheel hub. This triggers an increase in pressure in the area of the axial gap situated downstream which in turn generates a thrust force which causes an axial displacement of the impeller wheel in the opposite direction to that of the flow.

The backwater pressure effect decreases as the size of the axial gap situated downstream increases. The more the axial gap situated upstream is shortened, the further the dead water zone situated upstream projects into the area of the impeller-wheel hub. Hence, the meridian flow is deflected by the dead water zone away from the wall of the impeller-wheel hub as the axial gap situated upstream decreases in size. As a result of the flow-related shearing stresses between the streamline near the wall and the fluid in the cavity of the fixed hub situated downstream, an ejector effect occurs in the area of the axial gap situated downstream, creating a negative pressure effect there which displaces the impeller wheel axially in the direction of flow. Thus, a reversal in the direction of the effect of the axial thrust takes place — the axial thrust acts in the direction of flow.

Owing to the interaction of the dead water, zone at the axial gap situated upstream and the gap-related overpressure and negative pressure effect (interaction between backwater or ejector effect) at the axial gap situated downstream, a fully automatic thrust compensation occurs. The impeller wheel is relieved of hydraulic forces at every axial position insofar as stationary flow relationships are present .

The invention is explained in more detail in the following by means of an embodiment example. The associated drawing shows: Fig. 1 a schematic longitudinal section through a turbine flowmeter, and Figs 2 & 3 views according to Fig. 1 for explaining the operating principle of the automatic axial thrust compensation.

Other standard turbine flowmeter components which comply with the state of the art and are not directly crucial to the invention, have not been illustrated in the figures.

A fluid which is to be measured flows through a tubular element 1 in the direction indicated by the arrow 2. An impeller wheel 3, mounted in such a way that it can rotate and move in an axial direction, is located within this tubular element 1 and consists of an impeller-wheel shaft 4, a hub 5 and vanes 6 attached to this.

One hub 7 is situated upstream of the impeller wheel 3 and one hub 8 is situated downstream of the impeller wheel 3, whereby axial gaps 9, 10 remain between the ends of the hubs 7, 8 pointing towards the impeller wheel 3 and the axial end faces of the impeller wheel 3. .

The hubs 7, 8 are in each case connected to the tubular element 1 via fins 11 and 12 respectively which extend radially, i.e. said hubs are mounted stationary within this element 1.

The impeller-wheel shaft 4 extends into the cavities 18, 19 of the hubs 8, 7 and is provided with, holes 13 in its end faces. Spigots 14 fit into these holes 13 and said spigots 14 are arranged at the crowns of the hubs 7, 8 in such a way that they cannot move or rotate. The lengths of the spigots 14 and the depths of the holes 13 are matched to each other in such a way that the impeller wheel 3 can move axially with clearance between the end of the hub 7 pointing downstream and the end of the hub 8 pointing upstream. The pairing of spigot 14 and hole 13 represents a fluid-lubricated- plain bearing which guarantees both axial and simultaneous rotational movement of the impeller wheel 3. This bearing arrangement for the impeller wheel 3 is standard for turbine flowmeters.

The axial extension of the vanes 6 corresponds to the width B of the impeller-wheel hub 5. From Fig. 1 it can be seen that' the impeller-wheel hub 5 has a constant outside diameter Όι over its entire width B. It can also be seen that this outside diameter Di is slightly smaller than the outside diameters D2 and D3 of the ends- of the hubs 7, 8 pointing towards the impeller wheel 3. For the scope of the present invention, Όι is approx. 4-8% smaller than D2 or D3, whereby the percentage deviation between Όι and D2 need not be identical with the deviation between Dx and D3.

Owing to the non-continuous diameter gradations between the end of the hub"7 pointing downstream and the impeller-wheel hub 5, a dead water zone 15 forms immediately after this end of the hub 7 and said zone extends over the axial gap 9 and beyond, up to the wall of the impeller-wheel hub 5. The smaller the axial gap 9, the larger the axial extension of the dead water zone 15 up to the wall of the impeller-wheel hub 5. If the impeller wheel 3 is located directly adjacent to the hub 7 (Fig. 2), i.e. a small axial gap 9, then the dead water zone 15 extends a relatively long distance into the impeller wheel 3. The meridian flow 16 near the wall is then deflected by -the dead water zone 15 away from the wall of the impeller-wheel hub 5, whereupon it either fails to strike, or only strikes very much reduced, the small orifice plate 17 formed by the abrupt change in diameter D]/D3 at the end of the hub 8 pointing upstream.

On account of the flow-related shearing stress between the meridian flow 16 near the wall and the fluid in the cavity 18 of the hub 8, an ejector effect then forms in the area of the axial gap 10 which creates a negative pressure there. The resulting force Fax2 acts in the same direction as the axial thrust force, generated by the flow, which acts on the impeller wheel 3, whereupon the impeller wheel 3 is displaced in the direction of flow by a force resulting from this. Hence, the axial gap 10 decreases and with it the axial extension of the dead water zone 15 up to the wall of the impeller-wheel hub 5 ( Fig. 3 ) .

Due to this displacement, the dead water zone 15 projects less and less into the impeller wheel 3. Further, the increasing size of the axial gap 9 means that additional energy from the fluid in the cavity 19 of the hub 7 enters the dead water zone 15 so that this is not only shortened in a relative sense (in relation to the axial position of the impeller wheel 3) but also in abolute terms. · As the axial movement of the impeller wheel 3 continues in the direction of the flow, the meridian flow 16 near the wall makes contact with the wall of the impeller-wheel hub 5 to an increasing extent due to the shortening of the axial extension of the dead water zone 15 and the associated reduction in the deflection.

If the impeller wheel 3 is located just before the hub 8 (small axial gap 10), then a backwater pressure zone builds . up directly before the abrupt change in diameter O1/D3 at the hub 8 because the meridian flow 16 near the wall, already again making contact with the wall of the impeller-wheel hub 5, strikes the sharp-edged orifice 17. The rise in pressure thus generated in the area of the axial gap 10 generates a thrust force Faxl which brings about an axial displacement of the impeller wheel 3 opposite to the direction of flow.

As a result of the working mechanism described, there is an automatic compensation of the axial thrust force created by the flow and acting on the impeller wheel 3. The bearings 13, 14 are less heavily loaded and the measuring accuracy improved.

Claims (1)

P a e n C l a i m

1. Turbine flowmeter having an impeller wheel (3) consisting of an impeller-wheel hub (5) with vanes (6) attached, whereby said wheel is mounted between a fixed hub (7) situated upstream of the impeller wheel (3) and a fixed hub (8) situated downstream of the impeller wheel (3) in such a way that it can rotate and move in an axial direction, whereby the outside diameters (D2/D3) of the ends of the fixed hubs (7, 8) pointing towards the impeller wheel (3) are in each case larger than the outside diameter (Dj) of the impeller-wheel hub (5) at the ends pointing towards the fixed hubs ( 7 , 8 ) , ■and whereby the axial thrust of the impeller wheel ( 3 ) , subjected to an axial throughflow and acting in the direction of flow, is compensated by static pressure differences on both end faces of the impeller-wheel hub ( 5 ) , c h a r a c t e r i z e d i n t h a t the impeller-wheel hub (5) has a constant outside diameter (Di) over its entire width (B) and that said outside diameter (Dji ) is 4-8% smaller than the outside diameters (D2, D3) of the ends of the hubs (7, 8) pointing towards the impeller wheel ( 3 ) .