Flow Control Cartridge

Field of the Invention

This invention relates to fluid flow control devices, particularly fluid flow control cartridges for use in fluid flow control valves.

Background of the Invention

Flow control valves, such as those disclosed in U.S. Patent No. 3,131,716 and U.S. Patent No. 3,256,905, generally employ a ported cylindrical cup axially slidable through a circular opening in a stationary orifice plate. A pressure differential across the cup causes it to slide through the plate opening against the action of a spring. Such movement changes the total combined area of a series of continuous tapered ports exposed to upstream pressure, so that the flow rate remains substantially constant regardless of the pressure differential.

It is also known to use other than symmetrical tapered slots as ports in the cup to achieve the same desired effect. See, for example, U.S. Patent No. 3,752,184. By using other than symmetrical tapered slots, the device overcomes the difficulties of using symmetrical tapered slots such as weakening of the structural strength of the cup, difficulty of construction, and trapping or wedging of foreign objects along the tapered portions of the ports. The flow control valves shown in the aforementioned prior art patents provide devices capable of supplying substantially constant flow rates, but the devices have proven to have certain limitations. For example, the cup shaped pistons with peripheral ports shown in these patents function by severely limiting the amount of total port area that is available for exposure to fluid flow. Additionally, the cups of these prior art devices do not

provide the capability of achieving a relatively high flow rate with only a small differential pressure, because the initial cup position is insufficiently open. Finally, the construction of the valves of these prior art devices is limiting in that once the ports are cut into a cup, there is no capability to manipulate the valve construction so as to affect the flow rate vs. pressure differential curve of the flow control valve.

Summarv of the Invention The fluid flow control cartridge of the present invention includes, among other things, a valve element that slides axially within a housing, the interior surface of the housing having a generally parabolic cross- sectional profile. The initial position of the valve element within the housing is adjustable during assembly to provide a relatively simple means by which the valve can be configured to provide a desired flow rate vs. pressure differential characteristic. The novel design and construction of a fluid flow control cartridge ac- cording to the present invention provides a device that achieves relatively constant fluid flow over a range of differential pressures and a capability of adjusting the flow rate vs. pressure differential characteristic of the valve. These and other advantages are achieved while maintaining a relatively simple design.

Accordingly, a principal object of this invention is to provide an improved fluid flow control cartridge.

Another object of this invention is to provide a fluid flow control cartridge that does not severely limit the amount of total port area that is available for fluid flow.

Another object of this invention is to provide a fluid flow control cartridge that is capable of providing a relatively wide open initial port to achieve relatively high flow rates with a small amount of differential pressure.

Another object of this invention is to provide a fluid flow control cartridge that is adjustable to provide for manipulation of the flow rate vs. pressure differential characteristic of the valve. Another object of this invention is to provide a fluid flow control cartridge having a constant diameter valve element sliding within a parabolic throated opening.

It is also an object of this invention to provide an improved fluid flow control cartridge having a simple design.

The present invention relates to a fluid flow control cartridge which provides a relatively constant fluid flow even though a variety of supplied pressures occur. The device comprises a generally cylindrical housing having an interior surface that defines a throat opening. The shape of the interior surface is generally parabolic in cross- section, such that the throat opening gradually narrows from the inlet end to the outlet end of the housing. An outlet shroud is provided at the outlet end of the housing and an inlet shroud is attached to the inlet end of the housing. The outlet and inlet shrouds support a center shaft that extends along the central axis of the housing, the position of the center shaft relative to the outlet shroud being adjustable. A spring plate is affixed to the center shaft near the outlet end of the housing, and a valve element is axially slidably engaged to the center shaft. A spring is interposed between the spring plate and the valve element to provide a force biasing the valve element toward the inlet end of the housing. As the pressure differential encountered by the flow control cartridge increases, the valve element is pushed against the spring force toward the outlet end of the flow control cartridge. This displacement of the valve element results in a decrease in the annular area between the throat opening and the valve element, which is the port area exposed to fluid flow. The decrease in port area compensates for the increased pressure differential such

that the flow control cartridge provides a relatively constant fluid flow rate over a varying range of differential pressures.

Because the center shaft is adjustably connected to the outlet shroud of the housing, the initial axial position of the valve element is capable of being ad¬ justed. Adjustment of the initial axial position of the valve element changes the flow rate vs. pressure differ¬ ential curve for the flow control cartridge. The flow control cartridge can therefore easily be assembled to provide a flow rate vs. pressure differential curve according to a desired specification.

Brief Description of the Drawings

FIG. 1 is a cross-sectional view of a fluid flow control cartridge in accordance with a preferred form of the present invention.

FIG. 2 is an outlet end view of the fluid flow control cartridge of FIG. 1.

FIG. 3 is an inlet end view of an inlet shroud of the fluid flow control cartridge of FIG. 1.

FIG. 4 is a cross-sectional view of the fluid flow control cartridge of FIG. 1, shown at an adjusted initial gap position.

FIG. 5 is a cross-sectional view of the fluid flow control cartridge of FIG. 1, shown at a further adjusted initial gap position.

FIG. 6 is a graph illustrating the fluid flow rate vs. pressure differential characteristics of the three initial gap positions of the fluid flow control cartridge shown in FIGs. 1, 4 and 5.

Detailed Description of the Preferred Embodiment

Turning now to the drawings, Figs. 1, 4 and 5 show a preferred embodiment of the present invention in the form of a fluid flow control cartridge 10. As shown in Fig. 1, the flow control cartridge 10 comprises a housing 11

having a generally cylindrical shape. The housing 11 has an outlet end 12, an inlet end 13, an exterior surface 14 and an interior surface 15, the interior surface 15 of the housing defining a throat opening 16. The exterior surface 14 of the housing 11 is provided with a protrusion in the form of a shoulder 17 that extends around the circumference of the housing 11 in the vicinity of the outlet end 12. The shoulder 17 is adapted to rest against a shoulder stop within a valve housing (not shown) . Referring now to Figs. 1 and 2, an outlet shroud 18 is provided at the outlet end 12 of the housing. The outlet shroud 18 is provided with three ribs 19, each of which is connected at one end to the outlet end 12 of the housing. As best shown in Fig. 1, each rib 19 has a first portion 20 that extends longitudinally away from the housing 11, and a second portion 21 that extends radially inward, perpendicular to the first portion, toward a central axis of the housing. Each rib 19 is therefore generally "L"- shaped, with the first portion 20 of each rib defining the "leg" of the "L", and the second portion 21 of the rib defining the "foot" of the "L" . Although the outlet shroud 18 is shown .having three ribs 19 in the Figures, one skilled in the art will appreciate that more or fewer ribs may be used. As best shown in Fig. 2, the ends of the second portions 21 of each of the three ribs intersect at a central outlet hub 22. The outlet hub 22 is generally donut-shaped, having a central hole comprising a receptacle 23 having female threads formed therein. As will be more fully discussed below, the female threads of the receptacle 23 are adapted to receive male threads of a center shaft which extends along the central axis of the housing.

An inlet shroud 24 is provided at the inlet end 13 of the housing. The inlet shroud 24 comprises an elongated member having a connector 25 formed at each end. Each connector 25 is adapted to slide into one of two recesses 26 formed at the inlet end 13 of the housing to thereby

secure the inlet shroud 24 to the housing 11. A central inlet hub 27 is formed at a midpoint of the elongated inlet shroud 24. The inlet hub 27 is generally donut- shaped and defines a central hole 28 that is coaxial with the central axis of the housing.

A center shaft 29 extends along the central axis of the housing. The central hole 28 of the inlet hub of the inlet shroud is press-fit onto a first end of the center shaft 29 to hold the center shaft 29 in place. At its second end, the center shaft 29 is provided with male threads to allow the center shaft to be screwed into the receptacle 23, which is retained by the outlet hub 22 of the outlet shroud 18. A tool slot 30 is provided at the second end of the center shaft 29. The tool slot 30 is adapted to receive a screwdriver blade (not shown) or the like to thereby facilitate turning of the center shaft 29 to screw and unscrew the center shaft 29 into and out of the receptacle 23.

A generally disk-shaped spring plate 31 is affixed to the center shaft 29 at a position on the center shaft 29 external of the plane of the outlet end 12 of the housing and radially inward of the outlet shroud 18. The spring plate 31 may be affixed to the center shaft 29 in any number of ways known in the art, but is shown in Fig. 1 held in position by a cotter pin 32 pressed into the center shaft 29. The spring plate 31 is affixed coaxially to the center shaft 29 and is therefore coaxial with the housing 11. The spring plate 31 has a raised central portion 33 and a depressed annular portion to thereby form a peripheral ledge 34. The ledge 34 is adapted to receive one end of a spring, as will be more fully discussed below.

A valve element 35 is slidably attached to the center shaft 29. A main portion 36 of the valve element is generally disk-shaped, having a front surface 37 facing toward the inlet end 13 of the housing, a back surface 38 facing toward the outlet end 12 of the housing, and a

central hole 39 adapted to receive the center shaft 29. The valve element 35 also has a support portion 40 pro¬ vided on the back surface 38 of the main portion, the support portion 40 having a generally cylindrical shape and being adapted to slide on the center shaft 29. The main portion 36 of the valve element has a diameter smaller than the throat opening 16 at any axial point within the housing 11.

A spring 41 is interposed between the spring plate 31 and the valve element 35. The spring 41 rests at a first end against the ledge 34 of the spring plate, and at a second end against the back surface 38 of the valve element. Since the spring plate 31 is fixed to the center shaft 29, the spring 41 thereby provides a force biasing the valve element 35 toward the inlet end 13 of the housing.

A stop 42 is provided on the center shaft 29 in the vicinity of the inlet end 13 of the housing. The valve element 35 is prevented from sliding past the stop 42 on the center shaft 29, and the position of the stop 42 on the center shaft 29 therefore determines the maximum extension of the spring 41 and the extreme position of the valve element 35 toward the inlet end 13 of the housing. In the preferred embodiment, the stop 42 comprises a snap ring inserted into a groove on the center shaft 29.

The space between the interior surface 15 of the housing and the outer periphery of the valve element 35 defines the port area exposed to fluid flow. This port area is represented as the gap 43 shown in Fig. 1. The gap 43 is an annular area of fluid flow around the valve element 35. It has been determined that the flow rate of fluid through the valve is described by the following equation:

Q = 38 * C * A * sqrt(PSID) where Q is the fluid flow rate, the number 38 is a con¬ stant that takes into account various conversion units, C is an orifice coefficient which is determined empirically

through testing, A is the port area (the annular area of fluid flow defined between the outer periphery of the valve element 35 and the interior surface 15) , and PSID is the differential pressure across the valve element. It is also known that the differential pressure is related to the displacement of the valve element according to the following equation based on the basic spring force equation (F=kx) : where R is the radius of the front surface the valve element 35, k is the spring constant of the spring 41, and x is the displacement of the valve element 35 toward the outlet end 12 of the valve against the compression force of the spring 41 (x=0 when the valve element is at the position of the stop) . These two equations can be combined to obtain an equation relating the port area, A, to the axial displacement of the valve element, x:

A = Q / 38 * C * sqrt(k * x / it * R ² ) Since it is desired to maintain a constant fluid flow rate over a wide range of differential pressures, Q is meant to be kept constant. Thus, the equation simplifies to the following:

A = B / sqrt(x) , (x > 0) where B is a constant that takes into account the constant values of the other components of the preceding equations. The foregoing equation expressing the port area, or the gap 43, as a function of the inverse of the square root of the displacement of the valve element is illus¬ trated in Fig. 1 by the cross-sectional profile of the interior surface 15 of the housing 11. Beginning at the inlet end 13 of the housing and proceeding toward the outlet end 12, the throat opening 16 defined by the interior surface 15 is initially large but progressively narrows to a narrowest point at the outlet end 12 of the housing. The throat opening 16 narrows relatively steeply at first, with the narrowing becoming more gradual along the length of the housing 11 as progress is made toward

the outlet end 12. The initially steep but progressively gradual narrowing of the throat opening 16 defines the generally parabolic profile of the interior surface 15. The operation of the flow control cartridge will now be described. In operation, fluid flows through the flow control cartridge in the direction of the arrows F in Figs. 1, 4, and 5. As the pressure differential between the inlet end and outlet end of the flow control cartridge increases, such as would occur if there were an increase in the upstream pressure, the valve element 35 is forced toward the outlet end 12 of the housing against the spring force of the spring 41, and the annular flow area 43 becomes smaller due to the generally parabolic shape of the interior surface 15 of the housing 11. Thus, a relatively constant flow rate is observed through the cartridge despite a change in the pressure differential.

Two useful results follow from the foregoing con¬ struction of the flow control cartridge of the present invention. First, a relatively constant flow rate is obtained even at large differential pressures. Second, due to the adjustability of the center shaft 29 and therefore the initial port area or initial gap 43, it is possible to adjust the flow rate vs. pressure differential curve of the cartridge to a desired specification. These two results are illustrated in Fig. 6. As shown in the graph of Fig. 6, the center shaft adjustment of Fig. 1 provides a relatively larger initial flow rate at a small differential pressure, because the initial gap 43 defining the initial flow area is largest. The center shaft adjustments shown in Figs. 4 and 5 offer progressively lower initial flow rates for a given pressure differential, which correspond to a progressively smaller initial gap. In all of the embodiments, the flow rate is relatively constant over a large range of pressure differentials.

Thus, by adjusting the extent to which the center shaft 29 is screwed into the receptacle 23 during assem-

bly, a flow control cartridge having a desired flow rate vs. pressure differential characteristic can be obtained. This can best be appreciated with an understanding of the assembly procedure of the flow control cartridge, which is as follows:

1. The cotter pin 32 is pressed into the center shaft 29.

2. The spring plate 31 is slid onto the center shaft 29 to a position resting against the cotter pin 32. 3. The spring 41 is slid over the shaft such that the first end of the spring 41 rests against the ledge 34 of the spring plate 31.

4. The valve element 35 is slid over the center shaft 29 to rest against the second end of the spring 41. 5. While pushing the valve element 35 against the spring 41, the stop 42 is attached to the center shaft 29.

6. The sub-assembly assembled in the preceding steps is then inserted into the housing 11, with the threaded end of the center shaft 29 being screwed into the receptacle 23 of the housing 11.

7. This assembly is then placed in a special flow fixture capable of measuring the flow rate through the assembly over a range of differential pressures. With fluid flowing and by varying differential pressure, the assembly is externally adjusted by turning the center shaft 29 by inserting a tool in the tool slot 30 to adjust the extent to which the center shaft is threaded into the receptacle 23. Three such center shaft adjustments are illustrated in Figs. 1, 4 and 5. Once the assembly has been tuned to the proper position to achieve the desired flow rate vs. pressure differential curve, the assembly is removed from the flow fixture.

8. The inlet shroud 24 is press-fit onto the center shaft 29, with the connectors 25 of the inlet shroud engaging the recesses 26 of the housing 11. The inlet shroud 24 prevents further rotation of the center shaft 29

to thereby maintain the position of the center shaft 29 in the flow control cartridge 10.

While the above description contains many specifici¬ ties, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of a preferred embodiment thereof. Other variations are possible.

Accordingly, the scope of the present invention should be determined not by the embodiments illustrated above, but by the appended claims and their legal equivalents.