Centrifugal pumps are generally used in the transfer of substances in liquid form and mixtures containing mainly liquid, and the pumping effect is created by an impeller rotating inside a stationary pump casing. When the impeller moves the substance to be pumped the rotating movement creates a centrifugal effect in the material and when the material arrives at the discharge opening, which in most cases is tangential, the centrifugal force moves it to the discharge duct. This kind of pumps are disclosed for example in German patent publication no. 25 25 316 and Finnish patent no. 53747.

Prior art pumps have the drawback that pulse-like pressure fluctuations occur in their discharge duct which in certain operation conditions are harmful. Particularly if the pump is used in the pipeline connected to a head box of a paper or a corresponding machine as a fiber suspension feed pump, pressure fluctuations result in wavy deformations in the paper or board web produced and thus degrade the paper quality. There are also other applications of the pump in which a pulseless flow or a flow having as little pulses as possible is pursued. There have been attempts to reduce the pressure pulses for example by designing the tips of the impeller vanes inclined in relation to the periphery, however, this has not removed the pulse disturbances.

For example Finnish patent no. 87009 tries to remove the problem by a structure a characteristic feature of which is that the blade wheel of a centrifugal pump has at least one row oi' blades successive in the direction of the blade wheel periphery, the end of which adjacent the periphery is positioned at an inclined angle in relation to the movement of the blade wheel periphery. Further, it is characteristic of the blade wheel according to the patent mentioned that in order to reduce pressure variations caused by the blades in the discharge conduit of'the pump, the distance between the blades and their angle has been chosen so that

the leading edge of the end of a trailing blade in the movement direction of the blade disc is located at the trailing edge, or in the movement direction mentioned ahead, of the end of the successively leading blade moving in the movement direction mentioned.

Alreadv before the Finnish patent mentioned, i.e. in 1976, the problem of pressure pulses caused by the impeller of a centrifugal pump has been discussed in the name of Jönköpings Mekaniska Werkstads AB (JMW) by G. Ekman in an article entitled "The Pulp Industry Some Difficult Pumping Tasks" Ekman sets out from the idea that a pressure pulse is created in a centriftigal pump when a vane passes the volute cut-water serving as the limit between the spiral and the pressure opening. Ekman suggests as a solution to the pressure pulse problem firstly that the distance between the volute cut-water and the tips of the pump vanes should be as long as possible. The article gives the distance mentioned of 7 - 10 % of the radius of the impeller Secondly, Ekman suggests that since the feed pumps of a head box in most cases are double-sided, i.e. sucking from two opposite sides, which means that also the impeller is double-sided, the vanes of the impeller on the different sides of the impeller should not be positioned opposite to each other but staggered. Then, while the pressure pulse caused by a vane on one side is at its minimum the pressure pulse caused by a vane on the opposite side is at its maximum Further, Ekman suggests that the tips of the vanes are inclined so as to extend the length of the pulses in the peripheral direction. Further still, Ekman suggests that if pressure pulses are to be reduced by a single-suction impeller, a partition wall should be provided in the impeller and the vanes on opposite sides of the wall may then be set staggered and to some extent inclined in relation to the axial direction However, manufacturing a single- suction impeller of this kind by casting and in particular small sizes of impellers of this kind is very difficult and thus very expensive if even possible at all in all sizes.

The Finnish patent no. 87009 mentioned above relates to a corresponding single-suction impeller and suggests that the outer tip of the vane should be inclined so much that the rear edge of the preceding vane is at the same axial level as the leading edge of the following vane.

In other words, the objective is to arrange the increasing and the decreasing pulses one after the other so as to obtain a pulseless flow. However, also impellers of this structure are

practically impossible to manufacture or at least to manufacture them such that they would in small sizes operate with an adequate efficiency.

Finnish patent application no. 953241 in turn discloses a pump employing a single suction impeller which creates smaller pulse disturbances than before and which is more suitable for use for example as a feed pump in connection with a paper machine head box. A second objective of the solution disclosed in the publication is to provide an impeller which is easy to manufacture and solves the problems mentioned above. A characteristic feature of the pump according to the publication is that the impeller vanes and the volute cut-water at the junction of the spiral and the pressure opening serving with the impeller vanes as the means creating the pulses are set in such a position in relation to each other that, while the impeller rotates, at least one valve tip is always passing the volute cut-water whereby the pressure pulse created by the tip of the vane at the volute cut-water is all the time substantially constant and no remarkable pressure fluctuations occur in the discharge duct.

The same pulse problem has been discussed with merit for example in a published EP application, no. 0 648 939, which discloses almost an innumerable number of forms of the outer edge of the impeller vane and configurations of the volute cut-water mentioned.

In spite of all the solutions described above, a fully pulsefree pump has not been developed.

To be precise, probably it never will be possible to develop one, but the approaches described above have not been able to provide apparatus which paper manufacturers could accept without reservations. in other words, irrespective of how well the pulse-reducing solutions described in the above publications have been employed, the feed system of a paper machine head box is still encumbered by harmtully large pulses which can be considered to come from the pump.

The present invention examines the distance between the volute cut-water and the tips of the impeller vanes mentioned above in a new light. The reason for this is of course that inclining the tips of the vanes and the volute cut-water, which has been found to be a good method of reducing pulses, results in practice at least when trying to minimize the pulse effect, in a constructively impossible structure or, because of the complex manufacturing method, at least

in a disproportionately expensive design. The initial distance considered is the distance 7 - 10 % of the impeller radius given by Ekman mentioned above. Studies have indicated among other things that the distance above may be extended manifold without loosing anything essential of other properties ofthe pump.

Thus, an essential feature ofthe pump according to the present invention is that the distance of the outer edges of the impeller vanes from the volute cut-water between the casing and the pressure opening is at least 1/7 of the diameter of the pump impeller. If expressed with the measures Ekman uses (i.e. proportioning to the impeller radius) the distance is at least approx.

14%.

This kind of pumps have been used before but as normal liquid pumps which do not know the problem of pulses. The reason for using the structure described in this kind of liquid pumps designed to be always rotated at a constant operating speed is that pump producers manufac- ture only pumps of certain standard series having certain specific capacities. When a pump is needed which has a higher capacity than that of a certain pump but lower than the capacity of the following larger pump in the series, the impeller of the following larger pump is turned in a lathe so as to obtain the desired lower capacity. This is done knowing that also the efficiency of the pump decreases as generally a pumps is designed to give the maximum efficiency at the maximum diameter of the impeller. In other words, in these pumps the diameter of the impeller is reduced only in order to reduce the capacity of the pump. in this kind of applications the pulse level of the pump has no importance.

The other characteristic features of the pump according to the invention are disclosed in the appended patent claims.

The invention will be described more in detail below by way of example and with reference to the accompanying drawing figures of which Fig. I is a schematic illustration of a centrifugal pump according to the invention partiallv in section at the impeller in a level perpendicular to the axis, and

Fig. 2 is a curve illustrating the change of the pulse level as a function of the distance between the volute cut-water and the outer edge of the impeller vanes.

Figure I illustrates a partial section of a centrifugal pump having a spiral pump casing 12 and an impeller 14 with working vanes 22 and 24 disposed inside the casing on a shaft and rotating with the shaft. The casing naturally has a suction duct (not illustrated) and a discharge duct I 6 in the casing side end of which there is a volute cut-water 18 at a distance s from the impeller 14, more precisely expressed from the outer edge of the vanes, to distribute a part of the flow circulating in the spiral to the discharge duct 16. When the impeller rotates the material to be pumped, such as liquid or pulp suspension arrives parallel with the axis of the pump via the suction duct to the front of the impeller and, while the impeller rotates, moves towards the outer edge of the impeller and further due to the centrifugal force to the discharge duct 16.

Figure 2 illustrates test results studying the dying of pulses as a function of the distance between the volute cut-water and the impeller. In the test performed, the initial position has been a distance of the outer edges of the vanes of 3.3 % of the impeller diameter which is almost seven percent of the impeller radius. When this is compared with the article by Ekman describing the state of the art and giving the distance in question of 7 - 10 %, it may be stated that the tests started from the lower limit of the prior art design value and proceeded increasing the distance up to over 30 % of the radius. The curve in Fig. 2 indicates that in the prior art range the pulse level (the linear scale) is 2.5 - 4.2. The objective of the study was to halve he pulse level of the existing pumps which (i.e. a relative value of 1.25) is obtained by arranging the distance s to be about 10 % of the impeller diameter, more precisely expressed from the outer edge of the impeller vanes. Naturally, the figure also indicates that already a distance of 7 °io gives an essentially lower (about 40 % lower) pulse level than prior art solutions, i.e. with the distance given in the article mentioned.

rurtllel according to the invention the pump is constructed from the beginning in a different way than before. The initial position of the prior art is that the impeller of an existing pump is turned in a lathe to reduce its diameter in order to obtain the desired distance. As a

consequence of this, however, the capacity of the pump decreases dramatically. The initial position ofthe invention is that the form ofthe impeller is kept optimal in view ofthe efficiency and the pump casing is designed to meet the distance requirement mentioned earlier.

Finally, it should be recognized that a pump according to the invention, particularly if constructed the way described above, is especially well suitable to be used as well as a main feed pump of a paper machine head box by means of which the fiber suspension to be distributed from the head box onto the wire section of the paper machine is introduced into the head box, as also as a dilution liquid pump used in modern head boxes by means of which white water or, for example, clean water is introduced into the head box as dilution liquid.

The tasli of the liquid introduced with this kind of a pump is to replace the change of the slice opening by means of which the web thickness is regulated. By introducing more dilution liquid at some point along the length of the head box than at other points the same effect is obtained as by narrowing the slice opening. This kind of pumps are fairly small for example due to the fact that the liquid volumes they pump are very small compared with the ones pumped by the feed pump itself. However, the pulse level requirements of these pumps are the same as those of the main pump. Dilution liquid pumps of this kind, as well as main feed pumps, are speed settable which in practice means that their capacity is regulated by controlling their speed of rotation and thus it is not necessary to change the size of the impeller by turning in a lathe which has been a precondition with the conventional liquid pumps referred to above. Further, dilution liquid pumps most often mainly suck at the end, i. e. they are single-suction pumps, and thus their structure is simple and price relatively low.

The invention has been described above in the specification and drawings schematically and by way of example disclosing only the essential features of the invention. Further, only the feed pump of a paper machine head box has been mentioned as an application, which, although it is one of the main applications of the pump according to the invention, by no means is the only one and of course a solution according to the invention may be applied to all apparatus in which the pulse level ofthe pump must be as low as possible.