Fluid control valve and method for subjecting a liquid to a controlled pressure drop

A method and apparatus for subjecting a fluid to a controlled pressure drop. Fluid is supplied under pressure with a tangential velocity component to an annular passageway (17). The fluid swirls around the axis of the passageway while flowing axially along the passageway to an outlet (6). A cross section of at least a part of the passageway (17) is adjustable to enable the flow rate to be adjusted for a given pressure loss or the pressure loss to be adjusted for a given flow rate. The outlet (6) includes a second annular passageway (22) surrounding the first annular passageway so that the first annular passageway leads to, and the second annular passageway leads from, a chamber (21) common to the passageways. The second annular passageway is of increasing outer diameter and cross sectional area in the direction of flow from the chamber to the outlet.

There is frequently a requirement when processing fluids under pressure to 
subject the fluid to a pressure drop in order to achieve a desired fluid 
pressure or flow rate. Conventionally this is achieved by passing the 
liquid through a control valve having a variable orifice across which the 
pressure loss occurs. Although the design of the orifice is varied in 
dependence upon the particular application, in most instances the pressure 
loss is sudden, resulting in very high velocities. This may cause high 
liquid shearing, particularly in the oil industry, where oil and water 
mixtures are processed, since the shearing can result in liquid 
emulsification, making further liquid separation processes more arduous. 
In accordance with the present invention, in a method of subjecting a fluid 
to a controlled pressure drop, the fluid is supplied under pressure with a 
tangential velocity component to an annular passageway, a cross section of 
at least a part of which is adjustable, such that the fluid swirls around 
the axis of the passageway while flowing axially along the passageway to 
an outlet. 
This method, which subjects the fluid to an action similar to that which 
would occur in a hydrocyclone having no overflow outlet, provides a 
gradual pressure drop. The outer diameter of the part of the passageway 
having an adjustable cross section preferably reduces in the axial 
direction of flow, as with a conventional hydrocyclone, in order to 
maintain the angular velocity of the swirl as energy is lost. The 
adjustability of the cross section of the annular passageway enables the 
flow rate to be adjusted for a given pressure loss, or the pressure loss 
to be adjusted for a given flow rate. The gradual nature of the pressure 
drop avoids excessive velocities, or sudden changes in velocity, so that 
liquid shearing will be minimised or eliminated. Furthermore, the swirling 
motion will induce centrifugal forces causing dispersed lighter phase 
fluids, say oil in water, to migrate towards the inner peripheral wall of 
the passageway where they may coalesce and form large, more easily 
separated, droplets. Conversely, if the dense phase is the dispersed 
phase, then this may coalesce on the outer peripheral wall of the annular 
passageway. 
In order to avoid undue turbulence, and possible reemulsification where the 
still swirling liquid passes to and through the outlet, even if the outlet 
is a tangential outlet, the swirl may be dissipated by providing the 
outlet with a second annular passageway of increasing outer diameter and 
cross section in the axial direction of flow. This can be achieved in a 
compact manner if the second annular passageway surrounds the first 
annular passageway, and the axial direction of flow is reversed in passing 
from the first to the second passageway. 
Further benefits of this method are low noise, little cavitation, and a 
wide control range. 
One structure of fluid control valve which is suitable for use in carrying 
out the method of the present invention comprises an outer body having an 
inner wall surface defining a cavity of substantially circular cross 
section which reduces in diameter from one end to the other; and a plug 
located within the wider end of the cavity and having an outer wall 
surface of substantially circular cross section which is spaced radially 
inwardly from the inner wall surface of the outer body to define an 
annular passageway, the plug being axially movable relatively to the outer 
body to vary the width of the annular passageway between the plug and a 
convergent portion of the inner wall surface of the outer body; at least 
one tangential fluid inlet leading into the annular passageway at the end 
of the annular passageway nearer to the wider end of the cavity, and a 
fluid outlet at the narrower end of the cavity. 
The convergent portion of the inner wall surface of the outer body may be 
substantially frustoconical and faces a complementary substantially 
conical portion of the outer wall surface of the plug. 
The plug may have a head portion which progressively obstructs the inlet 
when the plug is moved axially towards the narrower end of the cavity to 
reduce the width of the annular passageway. 
When the outlet is to include a second annular passageway of increasing 
outer diameter and cross section in the axial direction of flow, the 
second annular passageway may surround the first annular passageway and be 
divided therefrom by an annular wall of the outer body, the first annular 
passageway leading to, and the second annular passageway leading from, a 
common chamber.

The illustrated valve has an outer casing 3 integrally formed with a 
tubular inlet portion 4 having a connecting flange 5 and a tubular outlet 
portion 6 having a connecting flange 7. Within the casing 3 is located an 
outer body 8 by means of a cover plate 9 which may be bolted to a flange 
10 of the casing. Within an upper inner cylindrical wall portion 11 of the 
body 8 there slides a cylindrical head 12 of a conical plug 13 having an 
external wall surface 14. The wall surface 14 cooperates with a 
frustoconical inner wall surface 15 of a depending annular wall 16 of the 
body 8. The wall surfaces 14 and 15, which have the same conical included 
angle, define therebetween an annular passageway 17 the radial width and 
cross section of which can be varied by axial movement of the plug 13. A 
stem 18 is attached to the head 12 of the plug and passes through a seal 
19 in the cover plate 9. The plug may therefore be adjusted in position 
via the stem 18, by means of any appropriate motive force, such as a fluid 
cylinder, or a motor or manually driven screw. 
The inlet passage through the tubular portion 4 leads through an involute 
passage 20 tangentially into the upper end of the passageway 17 so that 
fluid entering the valve will swirl around the passageway as it passes 
axially down the passageway. The outlet from the passage 20 is shown 
partly obstructed by the plug head 12 in FIG. 1. 
The lower end of the passageway 17 leads into a cylindrical chamber 21 
formed in the lower portion of the casing 3. A further, divergent, 
passageway 22 leads from the periphery of the chamber 21 back up around 
the passageway 17 between an external frustoconical wall surface 23 of the 
wall 16 and an inner frustoconical wall surface 24 of the casing 3. The 
included conical angles of the wall surfaces 23 and 24 are the same as one 
another and, in fact, the same as those of the wall surfaces 14 and 15. 
At its upper end the second annular passageway 22 is coupled to the outlet 
passage within the tubular portion 6.