GB2199674A

GB2199674A - Fluid flow control valves

Info

Publication number: GB2199674A
Application number: GB08730113A
Authority: GB
Inventors: Roland Malcolm Scott
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-01-06
Filing date: 1987-12-24
Publication date: 1988-07-13
Anticipated expiration: 2007-12-24
Also published as: GB8730113D0; GB2199674B; GB8700111D0

Abstract

A fluid flow control valve including an annular valve trim (10) formed with a plurality of radial bores (12) and a valve plug (11) which is movable axially within the valve chamber defined by the valve trim (10) to control the rate of fluid flow through the valve, in order to avoid cavitation and to reduce noise and possible internal valve damage at least a significant proportion of the radial bores (12) in the valve trim (10) are so formed that the cross-sectional area of the radially inner end of a bore is less than the cross-sectional area of the radially outer end thereof. Each bore may be formed by drilling a through bore and then counterboring the radially outer end thereof. <IMAGE>

Description

FLUID FLOW CONTROL VALVES Field of the Invention This invention relates to fluid flow control valves of the kind which include an annular valve trim formed with a plurality of radial bores, and a valve plug or closure member contained within the valve chamber defined by said valve trim, the valve plug being movable axially of the valve chamber to control the number of radial bores communicating with an inlet at one end of the valve chamber, and hence to control the rate of fluid flow. Such a control valve is hereinafter referred to as a fluid flow control valve of the kind specified.

Background of the Invention With general technological advances, the service conditions which fluid flow control valves of the kind specified have been expected to withstand, have increased in severity as a result of, for example, the high pressures and high velocities under which fluids are transmitted through pipe lines.

Two main problems have been encountered which have added to the cost of meeting these extreme service conditions. The first of these has been noise and the second has been wear.

Noise levels have been encountered which have exceeded the human ear safety levels and the degree of wear of valve components has been such as to necessitate replacement of the valves to avoid plant malfunction. These two problems are interrelated and accumulative, i.e. as the degree of wear increases so does the noise level, the noise emanating from vibrational effects which serve to increase the rate of wear, and so on.

It is accordingly an object of the invention to provide an improved form of fluid flow control valve of the kind specified, the design of which is such as to reduce the level of noise generated during use of the valve, particularly when the valve is being used to control high flow rates.

Summary of the Invention This invention provides a fluid flow control valve of the kind specified in which at least a significant proportion of the radial bores in the valve trim are so formed that the crosssectional area of the radially inner end of a bore is less than the cross-sectional area of the radially outer end thereof.

It is preferred that all the radial bores in the valve trim should have the variation in cross-sectional area referred to above. In some instances, however, only those of the radial bores closest to the inlet of said valve chamber will have said variation in cross-sectional area and the remainder of the radial bores will be of constant cross-sectional area.

The method of manufacture of the valve trim may be such that the radial bores are formed by drilling, the drilling operation being carried out in two stages, in the first of which a through bore is formed and in the second of which the radially outermost end of the bore is counter-bored. Each radial bore formed in this manner will thus be of stepped configuration.

As an alternative, two counter-boring operations can be carried out so that the radial bore then comprises a radially inner portion of the smallest cross-sectional area, a central portion of intermediate cross-sectional area and a radially outermost portion of the largest cross-sectional area.

As a further alternative, each radial bore may first be drilled and then broached using a tapered broach so that the thus formed radial bore is of progressively increasing cross-section from the radially innermost end to the radially outermost end thereof.

The radial bores of the valve trim are preferably arranged in a plurality of angularly spaced rows parallel to the axis of the valve trim, the radial bores in one row being offset relative to the bores in the adjacent row and with the spacing between adjacent bores in any one row corresponding to the diameter of a bore at the radially inner end thereof.

The angular spacing between adjacent rows of bores and the number of bores in each row are preferably such that the total cross-sectional area of the radially inner ends of the radial bores is approximately equal to the cross-sectional area of the inlet through which fluid flows into the valve chamber.

This arrangement ensures that there is the minimum restriction to fluid flow when the valve plug is in its fully open position.

In an alternative arrangement, a nrestrictive" trim may be provided.

This will be provided with either fewer or smaller bores whose total cross-sectional area is less than the cross-sectional area .of the inlet. Such a restrictive trim will typically be employed during start-up operations.

As explained hereinafter, the particular shaping of the radial bores in the valve trims ensures that the vena contracta pressure does not fall below the vapour -pressure of the fluid, the flow of which is being controlled by the valve. Cavitation, noise and possible internal valve damage are thus avoided.

Further features of the invention, including detailed layouts of the radial bores and manufacturing procedures are illustrated in the accompanying appendices.

Brief Description of the Drawings Figure 1 is a perspective view of a valve trim, Figure 2 is a transverse sectional view of part of the valve trim, Figure 3 illustrates the flow of liquid through one of the bores formed in the valve trim, Figure 4 is a graph illustrating the pressure of the liquid at different positions along its flow path, Figure 5 is a view corresponding to Figure 3 but showing a modified valve design, and Figure 6 is a graph showing the pressure variation for the modified design shown in Figure 5.

Description of the Preferred Embodiments The valve trim 10 shown in Figure 1 is arranged to provide communication between an inlet which is located on the axis of the valve trim and an outlet which leads from a chamber surrounding the valve trim. A valve plug 11 (see Figures 3 and 5) whose external diameter is equal to the internal diameter of the valve trim 10 is movable axially within the interior chamber of the valve trim 10. The valve trim 10 is formed with a series of bores 12 and the valve plug 11 is movable between a position in which all of said bores 12 are closed and a position in which all of said bores 12 are open.

As can be seen from Figure 1, the bores 12 are arranged in a number of rows extending parallel to the axis of the valve trim 10 with the bores in one row arranged so that they are offset from the bores in the adjacent rows. Thus, in Figure 2, the bores 12 shown in full are arranged offset from the bores 12A Indicated in outline. The bores 12 and 12a are arranged to define a closely packed array with an angular spacing between adjacent bores 12 of 22≈. The angular spacing will be so chosen, in relation to the cross-sectional area of each bore 12, 12a, that the total cross-sectional area of the radially inner ends of the bores 12 and 12a is substantially equal to the cross-sectional area of the inlet leading into the valve chamber defined by the valve trim 10.

The positioning of the bores 12 and 12a is such that, as the plug 11 is moved upwardly as viewed in Figure 3, the plug 11 will completely clear one level of bores 12 before it commences to open the succeeding level of bores 12a.

The bores 12 and 12a may have the configuration shown In Figures 2 and 3, i.e. each bore is formed by a radial drilling operation and is then counter-bored from the radially outer end of the bore to a depth of between 1/3 and 1/2 the thickness of the trim 10. Thus, for each bore 12 or 12a, there is a radially innermost portion of one cross-sectional area and a radially outermost portion of a greater cross-sectional area, the two portions being separated by a frusto conical shoulder 14.

As the liquid flows from the interior of the valve chamber through the bores 12 and 12a in the trim 10, there will be a fall in the pressure of the liquid from a value P1 to a value P2 as shown in Figure 4. Figure 4 also shows the varying pressure of the liquid at different points along the flow path. As can be seen, there are two positions at which the liquid pressure falls rapidly, these corresponding to the position of the intermediate shoulder 14 and the outlet end 15 of each of the radial bores 12 and 12a. Figure 4 also indicates the pressure drop which would be obtained if a standard valve trim design were employed, i.e. If each of the bores in the valve trim was of a constant cross-section.

As indicated in Figure 4, if a constant bore cross-section were employed, the vena contracta pressure (PVC) would fall below the vapour pressure (PV) of the liquid and this would result in cavitation, noise and possible internal valve damage.

With, however, the stepped bore design of Figures 2 and 3, the pressure drop characteristics are such that the pressure of the liquid remains above the vapour pressure (PV) and cavitation and noise generation are hence avoided.

As an alternative to the radial bore configuration of Figures 2 and 3, the alternative configuration shown in Figure 5 can be employed. In this instance, each radial bore 12 or 12a is counter-bored twice, initially for 2/3 of the thickness of the trim 10 to provide an internal shoulder 16 and then for 1/3 of the thickness of the trim 10 to provide a second frusto conical shoulder 17. The pressure drop characteristics of the valve trim design of Figure 5 are indicated in Figure 6.

As mentioned above, the total cross-sectional area of the radially inner ends of the radial bores 12 and 12a will be approximately equal to the cross-sectional area of the inlet through which the liquid flows into the valve chamber, thus ensuring that there is the minimum restriction to liquid flow when the valve plug 11 is in its fully open position. For start-up operations, a "restrictive" trim may be provided in which, although the radial bores may have the internal configurations shown in either Figures 2 and 3 or Figure 5, the total number of bores or the dimensions thereof may be such that the total cross-sectional area of the radially inner ends of the bores is considerably less than the cross-sectional area of the inlet.

In order to obtain the maximum rate of flow for a given size of valve trim, the angularly spaced rows of radial bores formed in the valve trim will be disposed in a close-packed array with the spacing between adjacent bores in any one row corresponding substantially to the diameter of a bore at the radially inner end thereof.

Claims

CLAIMS:

1. A fluid flow control valve of the kind specified in which at least a significant proportion of the radial bores in the valve trim are so formed that the cross-sectional area of the radially inner end of a bore is less than the crosssectional area of the radially outer end thereof.

2. A valve according to claim 1, in which all the radial bores in the valve trim are so formed that the cross-sectional area of the radially Inner end of a bore is less than the cross-sectional area of the radially outer end thereof.

3. A valve according to claim 2, in which each radial bore is of stepped configuration.

4. A valve according to any one of the preceding claims in which the radial bores are arranged in a plurality of angularly spaced rows parallel to the axis of the valve trim, the radial bores in one row being offset relative to the bores in the adjacent row and with the spacing between adjacent bores in any one row corresponding to the diameter of a bore at the radially inner end thereof.

5. A valve according to claim 4, in which the angular spacing between adjacent rows of bores and the number of bores in each row are such that the total cross-sectional area of the radially inner ends of the radial bores is approximately equal to the cross-sectional area of the inlet through which fluid flows into the valve chamber.

6. A valve according to claim 2, the trim of which is made by a method in which formation of the radial bores is effected by drilling, the drilling operation for each bore being carried out in two stages, in the first of which a through bore is formed and in the second of which the radially outer most end of the bore is counter-bored.

7. A valve according to claim 6, the method of manufacture of which includes two counter-boring operations.

8. A fluid flow control valve of the kind specified substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

9. A method of manufacturing a fluid flow control valve of the kind specified, the valve trim of which is manufactured substantially as hereinbefore described with reference to the accompanying drawings.