This invention relates to the control and reduction of fluid pressure in control valves, especially but not exclusively severe service valves for use in power industries.
The most widely used technology in severe service valves utilises pressure letdown chambers consisting of one or more flow passages containing multiple orifice opening, labyrinths, or abrupt angular turn passageways resulting in a staged pressure reduction. Alternatively, flow restrictions can be afforded by physically reducing the flow passage area through which the fluid passes. As the fluid flows through these physical restrictions the velocity is locally increased at the restriction outlets generating turbulence which dissipates energy and reduces the pressure.
In the afore-mentioned control valves, dissipation of the energy of the flowing fluid is effected by frictional drag through smooth round passages, or by successive abrupt restrictions and expansions through tortuous passages. Some of these arrangements have noise problems associated with them, and since they rely largely on frictional losses, performance is quite susceptible to viscosity changes which are an inherent result of temperature changes. Another problem associated with these technologies is that the energy dissipated through the physical flow restrictions can result in physical damage or erosion to valve components if not controlled in a careful manner.
An alternative technology is disclosed in U.S. Pat. Nos. 3,941,350 and 3,780,767, namely high resistance vortex chamber passage trims, which offers some noise reduction. In vortex designs the pressure reduction occurs by accelerating the fluid in the vortex. As the fluid spirals towards the outlets it accelerates and the pressure drops. In the above existing vortex pressure reduction designs the fluid enters the vortex at full pressure and only accelerates once inside the vortex. Thus, noise still tends to be a problem. This is addressed in UK patent specification No 1 486 440 which discloses vortex chamber pressure reduction utilising a curved inlet flowpath such that the acceleration of the fluid and associated pressure reduction starts to occur before it enters the vortex chamber. However, the proposals therein can still result in noise, albeit to a lesser extent, at the outlet.
It is an object of the present invention to solve or at least mitigate the above-mentioned problems associated with known vortex chamber designs of control valves.
According to a first aspect of the invention there is provided a fluid flow control valve including a plurality of vortex pressure reduction chambers each having an inlet flowpath which opens into its associated vortex chamber, the inlet flowpath having a cross sectional area which decreases along the length of the inlet flowpath to a minimum point where it opens into the vortex chamber thereby, in use, accelerating the fluid flow through the reducing cross sectional area flowpath prior to it entering the vortex chamber, the fluid flow within the vortex chamber taking a generally spiral flow path having both radial and axial components towards an outlet positioned axially at one end of the radial chamber.
The inlet flowpath is preferably curved.
In a preferred arrangement the vortex chamber is substantially cylindrical and the outlet is positioned on, and at one end of, the central axis of the chamber, the outlet being smaller in cross sectional area than the end of the vortex chamber.
In an alternative preferred arrangement the vortex chamber is substantially cylindrical and the outlet is positioned on, and at one end of, the central axis of the chamber, the outlet being substantially equal in cross sectional area to the end of the vortex chamber.
Where the vortex chamber is substantially cylindrical, the inlet flow into the vortex chamber from the inlet flowpath is preferably substantially tangential to the surface of the vortex chamber, thereby minimising any turbulence at the point of entry of the fluid into the vortex chamber, and the outlet is positioned on, and at one end of, the central axis of the chamber. In plan therefore, the inlet flowpath and the vortex chamber preferably resemble a figure “9”.
In an alternative preferred arrangement the vortex chamber is substantially in the shape of a spiral, the spiral reducing from the point of entry of the fluid to the outlet which may preferably be offset from the central axis. Preferably the tail of the spiral continues in curvature to form the inlet flowpath.
In a preferred arrangement the vortex chamber has at least one inlet for the fluid and two outlets for the fluid. In accordance with the this arrangement, therefore, the fluid is introduced under pressure into a vortex chamber via the inlet flowpath and, within the chamber, the fluid flow splits in two and takes a generally spiral flow path having both radial and axial components towards each of two outlets which are preferably opposed to one another, the radial velocity of the fluid increasing with decreasing radius of the spiral flow path. The dual outlets offer significant advantages over the prior art in that the exit velocity is dramatically reduced and so are less likely to reach the extreme levels which cause cavitation.
According to a second aspect of the invention there is provided a trim for a fluid flow control valve according to the first aspect of the invention, the trim being of the stacked disc type and the plurality of vortex chambers being defined by the discs. The vortex chambers in the discs can be of varying dimensions allowing the level of resistance produced by the trim to profile the valve's performance over the entire actuation stroke of the control valve. Preferably, the vortex chambers are located radially on a disc with a central concentric hole, the fluid inlet and fluid outlets being located on, respectively, the inner and outer diameters of the disc. The discs may each consist of one or more elements placed on top of one another with different details machined on or in their surfaces to create the desired geometry of the vortex chambers and the fluid inlet and fluid outlets.
Preferably, the discs are brazed or joined together by other means to form a disc stack control element in the shape of a cylinder with a concentric bore running along its axis and a plurality of fluid inlets and fluid outlets located on the inner and outer surfaces of the disc stack. Preferably, a plug or plunger is movable in the central bore of the disc stack such that it can cover and expose a varying number of fluid flow paths with associated pressure reducing chambers thereby enabling fine control of pressure reduction.
Preferably, the outlets of the vortex chambers in a disc stack are in communication with one another. This communication allows for a further subdivision of the fluid flow up as it passes up the disc stack into vortex chambers above those the inlets of which are exposed by the plug or plunger. Subdividing the fluid flow in this manner reduces the flow's velocity and therefore reduces the noise output. If the valve plug or plunger has lifted to its maximum lift exposing all the vortex inlets of the disc stack, the communication between vortex chambers will become an impingement point for the fluid flow passageways, reducing the communication of fluid but forming an impingement point adding further backpressure or high resistances to the flow. Lastly the communication between the outlets also reduces the potential for erosion in the outlet flow passageway.
In a valve of the invention, two or more dual-outlet vortex chambers may be joined in series with one another, thereby forming a staged velocity control valve, with the two outlets of one vortex chamber being connected to, respectively, the inlets of two of a second series of dual-outlet vortex chambers. This pattern may then be repeated to form as many chambers in series as one desires. Furthermore by varying the ratio of cross sectional area of outlets of one series to inlets of the next it is possible to control the fluid expansion, and therefore the fluid velocity, in the outlet orifice and passageway.
In preferred arrangements, two or three stage passageways provide the majority of the control valve needs in velocity control, but single stage and more then three stage designs can easily be produced with this invention.
Preferably the discs in the stack all use the same vortex chamber design but combining two or more chamber designs can in some cases provide a packaging advantage and a more tailored performance of the disc stack such that the pressure reduction/plunger displacement curve of a valve can be characterised to suit a particular control requirement. Preferably, in a valve of the invention the vortex chambers are used in conjunction with known (e.g. our “DRAG”) valve technology wherein, placed in the outlet flow from the vortex chamber, is a labyrinthine or like flowpath. Preferably this flowpath comprises a series of bends, allowing the fluid to fully expand within the disc which reduces any noise created by the expansion of the fluid as it exits the discs.
Preferably the bends are dimensioned such that the fluid flow passing through them does so at an angle of less than 90 degrees, enabling the fluid to fully expand without any additional pressure drop.
Preferably the outlet path from the vortex chambers is of increasing cross sectional area, aiding the fluid to expand fully and therefore reducing noise as the fluid exits the vortex outlet.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
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Number | Date | Country | Kind |
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0411921.0 | May 2004 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB05/01711 | 5/6/2005 | WO | 00 | 11/27/2006 |