TECHNICAL FIELD

The present invention relates to a multistage control valve.

BACKGROUND ART State of the art of multistage trim for control valves has many possible configurations. One of the most common types is composed by concentric drilled hollow cylinders.

This type of trim has two main solutions: a) in a first solution, shown in the attached figures l a-b, the stroke is divided into separate axial levels. A central plug slides inside the most inner cylinder from one axial end, corresponding to the valve completely closed position, to the opposite axial end, corresponding to the valve completely open position, of the trim. Along its stroke the plug progressively open in series the axial levels of the valve, from the first to the last; b) a second solution is described in EP 1408265. In this case, the central plug slides inside the most inner cylinder progressively opening a plurality of holes provided along the inner cylinder, an annular plenum being provided between the inner cylinder and a second outer cylinder provided with a second plurality of holes. The annular plenum communicates with all the holes of the inner and outer cylinders. Passing through the holes of the inner cylinder, a gas expands in the annular plenum and then exit the trim by passing through the holes in the outer cylinder.

Disadvantageous features of these solutions are, respectively: a) to keep the velocity of the fluid under certain limits as the gas expands trough the trim, the size of the holes must be increased going from the inlet to the outlet cylinders. This increases the noise at the last stage, corresponding to the most outer cylinder, of the trim; b) as the plug slides the distribution of the pressure drop among the stages is changed. In particular, at low valve openings, the first stage is subject to very high pressure loss and noise generation. In addition the jets upstream of the last stage are not fully deviated and masked.

Similar inconvenient are present also in the solutions described in the documents CA1003727 and US4617963.lt would be therefore desirable to provide an improved valve for which could maintain the advantages and avoid the inconveniences of both solutions above.

SUMMARY

Such an object is achieved through a control valve comprising: a cylindrical hollow trim defining an inner bore extending along an axis between a first opening and a second opening, said first opening defining an inlet of said valve, said trim comprising: a first outer cylinder extending along said axis and comprising a first plurality of radial through holes defining an outlet of said trim, a second cylinder, housed in said first cylinder and coaxial therewith, said second cylinder comprising a second plurality of radial through holes, a plug sized to fit inside the bore and slidably movable along said axis between a closed position proximal to said first opening and a complete open position proximal to said second opening, through a plurality of partially open positions, wherein in said complete open position all the holes of said first and second plurality of through holes are in communication with said inner bore and wherein in each of said partially open positions only a respective portion of holes of said first and second plurality of through holes are in communication with said inner bore, characterized in that: said holes of said first plurality are smaller in diameter than the holes of said second plurality, said holes of said first plurality are in greater number than the holes of said second plurality, for each hole of one of said first and second cylinders the projection along a radial direction on the other of said first and second cylinders is spaced from each hole of the other of said first and second plurality .

The above solution permits to achieve the following advantages: lower noise than the first prior art (figures l a-b) solution at all flow regimes. lower noise than the second prior art (EP 1408265) solution at partial valve opening.

BRIEF DESCRIPTION OF THE DRAWINGS

Other object feature and advantages of the present invention will become evident from the following description of the embodiments of the invention taken in conjunction with the following drawings, wherein:

Figures l a-b are, respectively, an assonometric and a sectional view of a component of a valve according to the prior art;

Figure 2 is a sectional assonometric view of a valve according to the present invention;

Figure 3a-b are, respectively, two sectional views of two different embodiments of a component of the valve in Figure 2.

Figures 4 and 5 are detailed views of the component in Figure 3a.

DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS OF THE INVENTION

With reference to first embodiment of the present invention (figures 2, 3a, 4 and 5), a control multistage valve 1 comprises a cylindrical hollow trim 2 extending along a longitudinal axis Y and a plug 6 sized to fit inside the bore 5 and slidably movable along the axis Y. The hollow trim 2 defines an inner bore 5 oriented along the axis Y between a first circular opening 3 and a second circular opening 4. The first opening 3 defines an inlet of the valve 1. The trim 2 comprises a first outer cylinder 1 1 extending along the axis Y and comprising a first plurality 14 of radial through holes defining an outlet of the valve 1. The trim 2 also comprises a second cylinder 12, housed in the first cylinder 1 1 and coaxial with it. The second cylinder 12 comprises a second plurality 15 of radial through holes.

The plug 6 slides between a closed position proximal to the first opening 3 and a complete open position proximal to the second opening 4, through a plurality of partially open positions. In the complete open position all the holes of the first and second plurality 14, 15 of through holes are in communication with the inner bore 5. In each of said partially open positions only a respective portion of holes of the first and second plurality 14, 15 of through holes are in communication with said inner bore 5. The trim 2 further comprises a plurality of annular partition walls 20 orthogonal to the axis Y and extending through an annular interspace 1 1 a from one to the other of said first and second cylinder 1 1 , 12 in order to divide said trim 2 into a plurality of levels (nine levels 2a-i) in the embodiments of the attache figure 2-4) axially distributed along the axis Y between said first and second openings 3, 4. Each level 2a-i comprises respective portions of the first and second cylinder 1 1 , 12 and respective portions of the first and second plurality of holes 13, 14. Each of the levels 2a-i defines one of the partially open positions.

In the first embodiment of the present invention, the valve 1 comprises two stages and the trim 2 comprises exclusively the first and the second cylinder 1 1 , 12. When the valve 1 is fully or partially open, i.e. when the plug 6 is remote from the first opening 3, a fluid flows from the first opening 3 to at least part of the levels 2a-i by reducing its energy pressure. Dividing the trim 2 in several levels a more efficient reduction of pressure is achieved as well as a reduction of noise. The fluid passes first through the second inner cylinder 12 which constitutes a first stage of pressure drop, then through the annular interspace 1 1 a and finally through the first outer cylinder 1 1 which constitutes a second and last stage of pressure drop.

The holes of the first and second plurality 13, 14 are dimensioned and positioned in such a way that: a) the first plurality 14 are smaller in diameter than the holes of the second plurality 15, b) the holes of the first plurality 14 are in greater number than the holes of said second plurality, c) for each hole 14, 15 of one of said first and second cylinders 1 1 , 12 the projection along a radial direction on the other of the first and second cylinders 12, 1 1 is spaced from each hole 15, 14 of the other of said first and second plurality of holes.

For the above configuration the holes of the first plurality 14 is neither partially aligned, along a radial direction orthogonal to the axis Y, with the holes of the second plurality 15.

The holes of the first and second plurality 13 , 14 are further dimensioned and positioned in such a way that:

- the distance 1 between the centres of two adjacent holes 14 of the first plurality is comprised between 1.3d and 3d, where d is the average diameter of the holes of the first plurality 14. In particular, according to a possible embodiment of the present invention, the holes of the first plurality 14 have all the same diameter d; in each level 2a-i the total cross-sectional area A of the holes of the first plurality 14 is comprised between L IB and 3B, where B is the total cross- sectional area B of the holes of the second plurality of holes 15.

In a second embodiment of the present invention (figure 3b), the valve 1 comprises three stages and the trim 2 comprises also a third cylinder 13 housed in the second cylinder 12 and coaxial with it. The third cylinder 13 comprises a third plurality 16 of radial through holes. Analogously to the first and second cylinder 1 1 , 12, also the holes of the couple of adjacent cylinders constituted by the second and third cylinder 12, 13 are disposed in such a way that for each hole of one of the second and third cylinders the projection along a radial direction on the other of the second and third cylinders is spaced from each hole of the other of said second and third plurality of holes. The trim of the second embodiment comprises a second annular interspace 12a from one to the other of said second and third cylinder 12, 13. In such second embodiment, fluid flows from the first opening 3 to at least part of the levels 2a-i by reducing its energy pressure. The fluid passes first through the third cylinder 13, which constitutes a first stage of pressure drop, and then through the second annular interspace 12a. From the second annular interspace 12a the fluid passes to the second inner cylinder 12 which constitutes a second stage of pressure drop, then through the annular interspace 1 1 a and finally through the first outer cylinder 1 1 which constitutes a third and last stage of pressure drop.

According to other embodiments (not shown) the valve 1 comprises four or more stages and the trim 2 comprises four or more cylinders each corresponding to a respective stage of pressure drop.

In general, for all the embodiments, it however necessary that the holes of the first and second plurality 13, 14 are further dimensioned and positioned in such a way that the above conditions a), b) and c) are fulfilled.