WO1988009216A1

WO1988009216A1 - Improvements in and relating to separators

Info

Publication number: WO1988009216A1
Application number: PCT/SE1988/000285
Authority: WO
Inventors: Arne Back
Original assignee: Ab Kompositprodukter S.K.-F.M.
Priority date: 1987-05-26
Filing date: 1988-05-26
Publication date: 1988-12-01
Also published as: SE8702213D0; SE8702213L

Abstract

The invention relates to a method for reducing undesirable axial deformation of an end wall (2, 3, 7) of the rotor of a centrifugal separator. This axial deformation is essentially the result of the axial forces that are engendered by liquid in the rotor during centrifugation. The axial deformation of the end wall is reduced by stiffening the end wall in its radial direction by means of a stiffening element (5) which is fitted concentrically around the rotor axle and connected to parts of the end wall so that the stiffening element is able to take-up forces which act radially on the end wall during rotation of the rotor in order to counteract radial deformation of the end wall resulting from axial deformation of said end wall. The invention also relates to an arrangement for use when applying the method.

Description

Improvements in and relating to separators.

The present invention relates to a method in connection with a centrifugal separator of the kind in which the rotor comprises a cylindrical shell and an end wall connected therewith, of reducing undesirable axial deformation of the end wall, this deformation resulting essentially from the axial forces that are engendered by liquid in the rotor durin centrifugation. The invention also relates to an arrangement for use when applying the method.

In order to enable centrifugal separators intended for liquid separation to operate at very high speeds, the separators are εtregthened with outer sleeves made of a plastics material which has been reinforced with carbon fibres. This reduces separator deformation in the radial direction. The sleeves may be fitted so as to pre-stress the rotor.

A liquid separator thus strengthened can operate at very high g-values. At high g-values , however, the hydraulic pressure occurring during centrifugation subjects the end walls of the separator to very high axial loads. The hydrauli pressure is proportional to the square of the radius, and the pressure acting on the end walls in the region of their largest radius may reach ca 100 MPa. The total load acting axially on the end walls of the separator may therefore be in the order of several meganewtons.

As a result of the centrifugal forces acting on the end wall, the end wall is also subjected to radial loads in addition to the aforesaid axial loads. These loads in conjunction with one another result in both radial and axial deformation of the end wall. Excessive axial defor- mation is liable to cause a number of problems. For example, excessive axial deformation can result in excessively high stresses in the juncture between the cylindrical shell of the separator and the end wall, and also in the end wall itself. Furthermore, the rotor may be axially deformed to such an extent as to cause problems with regard to pipe connections and seals.

The stresses occurring in the juncture between the cylindri¬ cal shell and the end wall are also dependent on the dif- ference in radial rigidity between the shell and the end wall. The end wall is normally stiffer in the radial direc¬ tion than the shell, therewith engendering stresses which are liable to limit rotor performance. The radial rigidity of the end wall, and therewith the stresses induced, can be reduced significantly by giving the end wall a curved configuration. When the outer surface of the end wall is convex, said surface will endeavour to move axially in to¬ wards the centre of the separator when no liquid is present during rotation. On the other hand, when the rotor contains liquid the hydraulic pressure occurrent in the liquid in the separator will endeavour to deform the end wall axially in the opposite direction. The resultant direction of end wall deformation depends, inter alia, on the geometric shape of the end wall , the diameter of said end wall and the prevailing density of the liquid, although in the case of large separators thedeformation normally occurs outwardly from the centre of the separator, i.e. the hydraulic forces dominate.

The main object of the present invention is to provide a method and an arrangement for reducing the axial deforma¬ tion of the end wall of a separator rotor, particularly in such case as those in which deformation occurs outwardly from the centre of the rotor. This reduction can be achieved by increasing the wall thickness of the end wall. Alternatively, the reduction can be achieved by providing a juncture between shell and end wall which is highly resistant to bending. These solutions, however, are not sufficiently effectiv to satisfy the increasing demands on performance.

According to the invention the aforesaid object is achieved in a more effective manner, which is based in the realization that when the end wall is pressed axially outwards from the centre of the rotor, the outer surface of the end wall will be subjected to a total radial deformation which is greater than the radial deformation caused solely by the radially acting forces. Consequently, by counteracting this radial deformation a reduction of the axial deformation of the end wall can be obtained. This can be achieved by means of a reinforcement or stif¬ ening which acts in the radial direction .

The main characteristic features of the method of the introductory paragraph are, in accordance with the present invention, thatradial deformation of the end wall resulting from axial deformation of the end wall is counteracted by stiffening the end wall radially by means of a stiffening element which is fitted concentrically around the rotor axle and connected to parts of the end wall located radi¬ ally inwards of the region at which the end wall is con¬ nected to the rotor shell, so that the stiffening element is able to take-up forces which act radially on the end wall during rotation of the rotor.

In addition to reducing the extent of such axial deformation, the stiffening element will also reduce the extent to which stresses occur at the juncture between the end wall and the shell of the separator, and also in the end wall itself. When practicing the invention, the end wall of the irotor may be provided with a flange which extends concentrically around the rotor axle, in which case the stiffening element will have the form of a ring which is fitted around the flange in force transmitting engagement therewith.

Conveniently, the stiffening element is fitted in a manner to prestress the flange and the end wall in a direction which opposes the radially acting deformation forces which occur during centrifugation. The use of a stiffening element will greatly enhance the possibilities of optimizing the whole of the end wall in a manner to maintain deformation and stresses at a desired level.

Calculations have shown that by stiffening the end closure in the aforedescribed manner it is possible to reduce axial deformation of the central part of the end wall to about 35 % of the axial deformation of an end wall with which the juncture between shell and end wall is resistant to bending. In addition hereto, the extent to which stresses are produced in the end wall will also be reduced, so as to increase considerably the useful life of the separator. Alternatively, stiffening of the end wall will enable the separator to be run at higher speeds, while retaining a given useful life span.

Particular advantage is afforded when the stiffening ele¬ ment consists of a fiber-reinforced annulus, preferably made from a plastics material reinforced with carbon fibers.

Carbon-fiber reinforced plastic is a particularly suitable material for use in the present context, because the low density,^- , of such material in combination with a high E-modulus results in a low ratio ^ /E and therewith in small radial deformation caused by centrifugal forces.

When the present invention is practiced on a rotor whose end wall has at least one convex part, the stiffening element is preferably fitted to the convex part.

The invention also relates to an arrangement in centri¬ fugal separators for reducing the extent of axial deforma¬ tion of an end wall of the separator rotor, this ar- rangement being constructed by applying the aforedescribed techniques. The main characteristic features of the εrrangement are set forth in the following apparatus claims.

The invention will now be described in creater detail with reference to the accompanying drawings, in which

Figure 1 illustrates a conventional end closure wall of the rotor of a centrifugal separator;

Figure 2 illustrates a corresponding enc closure wall which has been stiffened in accordance with the invention;

Figure 3 is a sectional view correspcnding to Figure 1 of an end closure wall of convex configuration; and

Figure 4 illustrates the end closure wall of Figure 3 stiffened in accordance with the invention.

The reference 1 in Figure 1 identifies the cylindrical shell of a circular cylindrical rotor in a centrifugal separator, the shell 1 merging with an end wall having a convex part 2 and a concave part 3. The shell 1 is em¬ braced by a stiffening sleeve 4, which suitably comprises a sleeve which is made of a carbon-fiber reinforced plas- tics material and which is prestressed when fitted to the shell.

As recited in the aforegoing, the stiffening sleeve 4 is intended to stiffen the shell 1 in the radial direction, although the provision of such a sleeve is not a prerequi¬ site of the invention.

In order to illustrate the deformations which occur in the case of a separator equipped with a rotor of the kind illustrated in Figure 1, so-called FEM-calculations have been made in regard of the illustrated end wall with the aid of a data processor. The calculations were made in respect of a separator having an outer diameter of 500 mm and an inner diameter of 400 mm. The separator was driven at a speed of 17 000 rprr. while filled with liquid having a density of 1.1. The rotor was assumed to be made of titanium. The deformations suffered by the rotor end wall are shown in broken lines and have been enlarged for the sake of clarity. It will be seen that the end wall is deformed both axially and radially.

As beforementioned, this axial deformation will, inter alia, result in the formation of large stresses in the juncture between the shell 1 and the end wall , and will cause problems with regard to pipe connections and seals.

Figure 2 illustrates a corresponding end wall which, in accordance with the invention, has been stiffened radially with the aid of a stiffening element. In the case of the illustrated embodiment, the stiffening element has the form of a stiffening aniuiluε 5, which is assumed to be made of a plastics material that has been reinforced with carbon fibers. The annulus 5 is fitted against a flange 6 on the convex part of the end wall. The broken lines in Figure 2 illustrate the extent of deformation of the end wall during operation under the same conditions as those recited with reference to Figure 1 and with the same degree of enlargement. It will thus be seen from Figure 2 that the annulus 5 which stiffens the end wall in the radial direction also reduces markedly the extent of deformation in the axial direction of the end wall, such deformation being essentially half that obtained in the Figure 1 illustration. The aforesaid cal- culations showed that the extent of axial deformations of the end wall was reduced from about 3 to about 1.5 mm and that maximum stress was reduced to about 76 % of the orig¬ inal value, even though the annulus 5 was not pretensioned on the flange 6 in this particular case.

Figures 3 and 4 illustrate respectively end walls 7 which are completely convex in shape and which are fitted to a rotor similar to the rotor of the Figure 1 and 2 embodi¬ ments. The same references as those used in Figures 1 and 2 have been used in Figures 3 and 4 to identify identical components. Since the purely convex configuration of the end wall illustrated in Figures 3 and 4 will result in much smaller axial deformation than the end wall configu¬ ration illustrated in Figures 1 and 2, the deformation has been enlarged to a greater extent in Figures 3 and 4 than in Figures 1 and 2, for the sake of clarity and illus¬ tration. Consequently, the radial deformation appears to be greater in Figures 3 and 4 than in Figures 1 and 2, although such is not the case in reality. When using a rotor of the aforesaid kind, radial deformation of the inner radius of the shell 1 is in the order of 0.7 mm in all cases.

The end wall illustrated in Figure 3 lacks the provision of a stiffening element, whereas the end wall illustrated in Figure 4 is provided with a stiffening element in the form of a carbon-fiber reinforced plastics annulus 5 in accordance with the invention, said annulus being fitted around a flange 6 on the end wall 7.

Calculations have shown that under the same conditions as those mentioned above, the extent of axial deformation of the central part of the end wall is also halved in this case. Thus the extent of axial deformation is reduced from about 0.4 mm to about 0.2 mm and the maximum stress is reduced to about 94 % of its original value.

The aforesaid values have been obtained with an imagined application of the present invention in conjunction with known end-wall configurations with no separate optimiza¬ tion of the end walls. Such optimization would be able to provide improvements in the case of all four of the illus¬ trated end-wall configurations. Consequently, since the use of a stiffening annulus on the end wall in accordance with the invention will afford greater possibilities of optimizing the whole of the end closure, the extent to which deformation and the build-up stresses is reduced when practicing the invention would probably be even greater when the end walls are optimized.

The maximum stresses can be reduced still further, by pretensioning the stiffening annulus 5 when fitting the same.

It will be understood that the illustrated examples are only intended to exemplify and show the results that can be obtained when practicing the invention, which is based on the concept of reducing axial deformation of an end wall by stiffening the end wall in the radial direction. Stiffening of the end wall can be achieved in various ways, of which the aforedescribed embodiment of a stiffening element is but one alternative. For example, the stiffening annulus can be replaced with a plate-like element or some corresponding element, provided that it can be suitably fitted to the end wall . Although the small ratio of density to elasticity renders the use of a plastic material which has been reinforced with carbon fibers highly advantageous in the present context, the invention is not limited to such use and other materials may, of course, be used.

Claims

1. In a centrifugal separator of the kind in which the rotor comprises a cylindrical shell and an end wall con¬ nected therewith, a method of reducing undesirable axial deformation of the end wall, this deformation resulting essentially from the axial forces that are engendered by liquid in the rotor during centrifugation, characterized by counteracting radial deformation of the end wall re¬ sulting from axial deformation of the end wall by stiffen¬ ing the end wall radially by means of a stiffening element which is fitted concentrically around the rotor axle and connected to parts of the end wall located radially inwards of the region at which the end wall is connected to the rotor shell, so that the stiffening element is able to take-up forces which act radially on the end wall during rotation of the rotor.

2. A method according to claim 1 , characterized by pro¬ viding the end wall with a flange which extends concen¬ trically around the rotor axle; and by fitting the stif- fening element in the form of an annulus around said flange and in force-transmitting abutment therewith.

3. A method according to claim 2, characterized by fit¬ ting the stiffening annulus in a manner such as to pre- tension the flange and the end closure in a direction opposite to the radial deformation forces acting thereon during centrifugation.

4. A method according to any of claims 1-3, characterized by using for the stiffening element a fiber-reinforced annulus, preferably an annulus which is made of a plastics material reinforced with carbon fibers.

5. A method according to any of claims 1-4 applied to a rotor having an end closure which presents at least one convex part, characterized by fitting the stiffening ele¬ ment to said convex part.

6. In a centrifugal separator of the kind in which the rotor comprises a cylindrical shell and an end wall con¬ nected therewith, an arrangement for reducing undesirable axial deformation of the end wall, this deformation resul¬ ting essentially from the axial forces engendered by liquid in the rotor during centrifugation, characterized in that the arrangement includes a stiffening element (5) which is intended to stiffen the end wall (2, 3, 7) in the radial direction and which is fitted concentrically around the roto axle and so connected with parts of the end wall (2, 3, 7) radially inwards of the region at which the end wall is con¬ nected to the rotor shell as to enable the stiffening ele¬ ment to take-up forces which act radially on the wall duri g rotation of the rotor, in order to counteract radial defor¬ mation of the end wall resulting from axial deformation of said end wall.

7. An arrangement according to claim L , characterized in that the end wall (2, 3, 7) has provided thereon a flange (6) which concentrically encircles the rotor axle; and in that a stiffening annulus (5) is fitted around the flange in force-transmitting abutment therewith.

8. An arrangement according to claim 7, characterized in that the stiffening annulus (5) is fitted in a manner to pre-tenεion the flange (6) and the end vail (2, 3, 7) in a direction opposite to the radial deformation forces acting on the end wall during centrifugation.

9. An arrangement according to any of claims 6-8, characterized in that the stiffening element comprises a fiber reinforced annulus (5) , preferable an annulus which is made of a plastics material reinforced with carbon fibers.