US3752176A

US3752176A - Fluid flow proportioning device

Info

Publication number: US3752176A
Authority: US
Inventors: W King
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-06-08
Filing date: 1972-02-17
Publication date: 1973-08-14
Anticipated expiration: 1990-08-14

Abstract

A proportioning device for maintaining a constant flow ratio to two separate outlets regardless of changes in the pressure of such outlets, in which the fluid pressure in each outlet is applied to a valve means to adjust the position of the valve means to compensate for pressure changes.

Description

United States Patent [191 King 1451 Aug. 14, 1973 [54] FLUID FLOW PROPORTIONING DEVICE 2,846,850 8/1958 Hall 137/1 18 X 3,011,506 12/1961 Schwartz et a1. 137/118 X [76] wmi'm i 1909 Sunshme 3,099,284 7/1963 Thrap =1 al... 137/118 x Square, lmlsvlew, 76601 3,575,192 4 1971 MacDufl 137/118 x [22] Filed: Feb. 17, 1972 2 pp No: Primary ExaminerWilliam Freeh Assistant Examiner-Leonard Smith Rehud l'l Dam A1torney- Mutray Robinson, Ned L. Conley et al. [63] Continuation-impart of Ser. No. 44,383, June 8, 1970,

Pat. NO. 3,669,572.

[5 7] ABSTRACT :fiigl. l37/l08d6g/J (1:; A proportioning devic for maintaining a constant flow [58] Field s 119 91/9 ratio to two separate outlets regardless of changes in 91/10 24o the pressure of such outlets, in which the fluid pressure in each outlet is applied to a valve means to adjust the 56] Rd: CM position of the valve means to compensate for pressure h UNITED STATES PATENTS c anges 2,799,996 7/1957 Van Meter 137/118 X 3 Claims, 8 Drawing Figures Patented Aug. 14, 1973 3 Sheets-Sheet 1 Patented Aug. 14, 1973 3,752,176

3 Sheets-Sheet 2 Patented Aug. 14, 1973 3,752,176

3 Sheets-Sheet 3 CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of United States application Ser. No. 44,383, filed June 8, 1970, and entitled Constant Flow Pumping System", now US. Pat. No. 3,669,572, issued June 13, 1972.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a proportioning device for proportioning the flow of fluid between a hydraulic accumulator and a fluid powered pump.

2. Description of the Prior Art In certain chemical processes, such as high pressure ethylene polymerization processes, it is necessary to feed liquids into an extremely high pressure container in which the process is taking place. Such a container may, for example, be under a pressure of 30,000 psi or more. High pressure pumping systems have heretofore been devised for pumping such liquids into such a high pressure container.

Heretofore such high pressure pumping systems have consisted of a fluid powered reciprocating pump which has a large power fluid piston on the same rod as two small pumping pistons, so as to make a two cylinder pump, one cylinder taking suction while the other pumps. The power piston is supplied with liquid from a comparatively low pressure source which provides the liquid at a constant flow rate to alternate sides of the power piston. The pressure liquid may, for example, be supplied by a Vickers constant delivery pump, which is a multiple cylinder reciprocating pump, or by a conventional swash plate pump. In one application, the ratio between the areas of the power piston and the pumping pistons is such that if the pumping piston must deliver liquid at 30,000 psi, a power fluid pressure of 600 psi is required.

When the high pressure liquid is being pumped against a back pressure of, for example, 30,000 psi, the liquid is actually compressed a substantial amount, and in addition equipment and pipes carrying the liquid are expanded. Thus at the beginning of each pumping stroke it is necessary to compress the liquid and expand the equipment and pipes containing it enough to reach a pressure of 30,000 psi before any liquid can actually be pumped into the high pressure container. The pistons must therefore move a sufficient distance to compress the liquid and re-expand the equipment and pipes before any flow can begin. As a matter of actual practice in the 30,000 psi system hereinbefore referred to it has been found that the pistons may move from to 12 percent of the total stroke before pressure is built up high enough to start flow again. Thus the operation of such a system may produce an interrupted flow where the high pressure fluid is flowing perhaps 90 percent of the time and is not flowing 10 percent of the time.

It is known in the prior art to use pressure accumulators in hydraulic systems to smooth out pressure surges in the system and to supply additional hydraulic fluid for high demand periods in a cycle. Such systems are shown, for example, in U. S. Patent Nos. 2,881,739 to Huppert, 3,175,354 to Firth et al., 3,192,717 to Lee and 3,205,659 to Hartzell. These prior art applications of accumulators do not, however, provide means by which high pressure fluid flow in a system such as that just described can be substantially continuous without interruption at the end of each stroke.

SUMMARY OF THE INVENTION It is an object of this invention to provide means by which substantially continuous flow of such high pressure fluid can be obtained. It has now been recognized that low pressure of the high pressure liquid at the beginning of each pumping stroke means a reduced force against which the pistons are moving and therefore a reduced back pressure in the hydraulic system providing power fluid to the pump. It is an object of this invention to provide means for maintaining the power fluid pressure at all times.

According to the present invention pressure accum ulator means are connected between the power fluid source and the pump, and means are provided for charging the pressure accumulator with power fluid while it is being supplied to the power end of the pump at the desired pumping pressure and for discharging power fluid from the accumulator into the purnp upon reduction of power fluid pressure when the pump piston reverses direction. The accumulator pressure is maintained above the normal power fluid pressure throughout substantially all the discharge period.

In a preferred embodiment of the invention an adjustable proportioning device is utilized to proportion the amount of power fluid which is supplied to the pump and to the accumulator as required to insure that sufficient power fluid is supplied to the accumulator at a high enough pressure to maintain power fluid pressure when the reciprocating pump piston changes direction.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of one embodiment of the apparatus of this invention;

FIG. 2 is a vertical sectional view of one embodiment of a proportioning valve according to this invention;

FIG. 3 is a horizontal sectional view of the apparatus of FIG. 2, taken at line 3--3 of FIG. 7;

FIG. 4 is another horizontal sectional view of the embodiment of FIG. 2, taken at line 4-4 of FIG. 7;

FIG. 5 is another horizontal sectional view of the embodiment shown in FIG. 2, taken at line 5-5 of FIG.

FIG. 6 is a fragmentary view showing a portion of the apparatus shown in FIG. 2;

FIG. 7 is a plan view of a portion of the apparatus of FIG. 7, taken at line 7-7 of FIG. 2; and

FIG. 8 is a somewhat schematic sectional view of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Looking first at FIG. 1 of the drawing, a fluid powered reciprocating pump indicated generally at 10 is shown mounted upon a base 12. Also mounted upon the base 12 is a constant flow pump 14 which may, for example, be a Vickers constant delivery pump which comprises a plurality of sequentially operating pistons which work together to deliver a liquid at a substantially constant and uniform flow rate regardless of the pressure against which the pump is operating. The pump 14 is connected by means of a coupling 16 to an electric motor 18 provided for driving it. Hydraulic fluid utilized for power fluid which is delivered by the constant delivery pump 14 is supplied to the pump 14 by means of conduits 20 which lead from a reservoir which may be carried within the base 12.

Hydraulic fluid is supplied to the reciprocating pump through a pipe 24, a proportioning valve 26 and a pipe 28.

Also shown in the drawing in FIG. 1 is a hydraulic accumulator 30 connected by means of

pipes

32 and 34 to the proportioning valve 26, and by means of pipe 36, containing a check valve 38 and a dump valve 40, to the pipe 28 leading to the pump 10.

As shown in FIG. 1, the liquid being pumped by pump 10 is supplied through lines 64 and 66 and

check valves

68 and 70, usually from a common source, and the liquid is pumped under high pressure through the

check valves

72, 74 and

lines

76, 78 to a common discharge line 80.

A portion of the power fluid from the pump 14 is diverted to the hydraulic accumulator 30 by means of the proportioning valve 26. FIGS. 2 t0 7 Show one embodiment of a proportioning valve which may be used for this purpose. The valve there shown comprises a housing 82 containing two

parallel bores

84 and 86. Bore 84 contains a shuttle valve 88 which is reduced in diameter at two longitudinally displaced

locations

90 and 92 to provide a fluid passageway across the bore 84 from

ports

94 and 96 to

pipes

32 and 28 respectively.

Bore 86 contains a sleeve member 98 which extends from above the upper edge of port 94 to below the lower edge of port 96 and which is held in place by a dog 100. Telescopically received within the sleeve 98 is a hollow cylindrical slidable valve member 102. Valve member 102 consists of an upper rotatable element 103, and a lower non-rotatable element 105, connected loosely together, as by a connector 107, so that they can rotate relative to one another. Member 103 is closed at the upper end and is provided with a port 104 in its cylindrical wall, positioned adjacent a corresponding port 108 in the sleeve 98.

The connection between members 103 and 105 allows flow from the interior of 105 to the interior of 103, but the ends of 103 and 105 are close enough together that there is substantially no leakage between them. If desired a rotatable seal may be used here. The lower member 105 has a plurality of ports 106 in its cylindrical wall, three being shown, which are aligned circumferentially opposite corresponding ports 110. The dog 100 extends into a slot 116 in the wall of member 105 to prevent it from rotating with respect to the sleeve 98, and thereby maintain the alignment of the

ports

106 and 110. Valve member 102 is preferably a close sliding fit in sleeve 98, but need not be a fluid-tight fit.

The bore 86 is enlarged in diameter at 112 and 114 to provide a fluid passageway from the inside of the sliding valve member 102, through the aligned ports, I

and to the

ports

94 and 96. It will be appreciated that as the sliding valve member 102 moves upwardly the sizes of the openings provided by

ports

104, 108 and 106, 110, for fluid flow therethrough are increased proportionally. The ratio between the flow through the upper ports and that through the lower ports depends upon the relative azimuthal position of upper and lower members 103 and 105.

The valve member 102 is provided with a stem 118 which is slotted at 120 to receive a flat bar 122 attached to a rotatable shaft 124 which is sealingly received within a cover member 126 which covers the upper end of the bore 86. A spring 128 bears upwardly against the cover member 126 and resiliently biases the sliding valve member 102 downwardly.

A pointer 136 mounted on the upper end of shaft 124 is provided to indicate, on a dial 138 mounted on the top of the cover member 126, the relative positions of the ports 104 and 108. Rotation of this pointer causes rotation of the member 103, and thereby changes the sizes of the passageway provided by these ports, and thus the proportion of flow through this passageway as compared to through the passageway provided by

ports

106 and 110.

A fluid passageway 130 is provided between the enlargement 114 around the bore 86 and the space in the bore 86 above the sliding valve member 102 so as to allow liquid to be exhausted from this upper portion of the bore when the sliding valve member is moved upwardly, and to equalize the pressure in the part 96 and the upper end of bore 86.

Similar bypasses

132 and 134 are provided between the ends of bore 84 and the

ports

94 and 96 as shown in the drawing. I

In the operation of the embodiment of the apparatus which has just been described, power fluid is supplied at a uniform flow rate by the constant delivery pump 14 through the line 24 to the proportioning device 26. In the proportioning device the power fluid passes through the bore 86 and the bore of the valve member 102. The flow is divided between the aligned

ports

106, 110, and the ports 104, 108, so that a portion of the power fluid flows through passageway 94 and another portion flows through passageway 96. Pointer 136 has already been set so as to adjust the proportion of flow between

ports

94 and 96 as desired. The power fluid then passes across the bore 84 and into

conduits

32 and 28 respectively.

It will be appreciated that if at any one setting of the pointer 136 the ratio of the flows through

ports

94 and 96 is to be maintained, it is necessary that the pressure in these ports be equal. Equalization of pressure cannot be accomplished by communication between the ports since this would change the flow through the ports. According to this invention, pressure equalization is achieved by restrictions downstream of

ports

94 and 96. Thus passageways 132 and 134 are provided, passageway 132 communicating the pressure of port 96 to the upper end of the shuttle valve 88 and passageway 134 communicating the pressure of port 94 to the lower end of the shuttle valve 88. Thus if the pressure in port 96, for example, were to become greater than the pressure in port 94 the shuttle valve 88 would be moved downwardly and restrict the flow into conduit 32 while at the same time increasing the flow opening into conduit 28. This will, of course, decrease the back pressure on port 96 and increase the back pressure on port 94 until they are equalized.

It is apparent that the pressure in the bore 86 is greater than the pressure in

ports

94 and 96, due to the throttling effect of the

ports

104, 108 and 106, in the valve member 102. Thus the pressure above valve member 102 is less than the pressure below it, so that the valve member 102 will tend to move upwardly against the compression of the spring 128. As the pressure differential increases the upward movement of the valve member is greater so that the ports are opened more. The greater opening of the ports decreases the pressure differential until the upward force due to pressure differential is balanced by the spring, so that the valve member will soon reach an equilibrium position. In other words, liquid being supplied at a constant rate of flow moves the sleeve upwardly until the ports are opened enough to allow this flow rate through the ports. The distance the sleeve moves is a function of spring pressure and back pressure on the upper end of the sleeve, delivered through passage 130. Thus if the pressure in port 96 goes up, this reduces the pressure differential across the sleeve, so that the sleeve must move upwardly more to open the ports enough to achieve the same flow rate at the reduced pressure differential. Of course, on such vertical movement of the valve member the proportion of flow between the

ports

94 and 96 is maintained the same. The passageway 130 provides a fluid relief channel for the spring chamber to allow the valve member to move upwardly.

As is seen in FIG. 1, the liquid from conduit 28 leaving the proportioning device goes to power the pump while the liquid from the proportioning device through conduit 32 goes through conduit 34 to charge the accumulator 30. According to this invention the accumulator is precharged to a pressure near or greater than the desired working pressure of the power fluid being supplied to the pump 10.

Then upon starting the constant flow pump 14 so that fluid flows from the proportioning valve to the accumulator, power fluid will flow in under the piston 144 to charge the accumulator with power fluid. The piston is thereby moved upwardly to compress the gas above it to a still higher pressure. In one example, where a 600 psi pump operating pressure was desired, it was found to be desirable to pressurize the accumulator to a pressure of 700 psi. This may be accomplished at the same time that the constant output pump is providing liquid at 600 psi to the pump 10 because the proportioning valve insures that the proportion of liquid flowing to the accumulator and to the pump 10 is maintained the same regardless of the pressures against which they operate. When the pressure going to the accumulator becomes greater than the pressure going to the pump 10, the shuttle valve 88 in the proportioning valve is moved upwardly to throttle the flow into the pump through line 28 and to reduce the throttling of the flow into the accumulator through line 32. This means that the pres sures in

ports

94 and 96 in the proportioning valve are both increased somewhat. This reduces the pressure differential across the valve member 102, thereby causing it to be moved downwardly so that more pressure drop can increase the back pressure on the constant delivery pump feeding the power fluid into the proportioning valve.

Thus the net effect of the increasing pressure in the accumulator is to increase the pressure of the fluid being fed into the proportioning valve while maintaining the desired flow proportions through the accumulator and to the pump 10.

In an installation in which the pump 10 must pump against a back pressure of 30,000 psi and the effective area of piston 52 is 50 times the area of piston 48, the power fluid supplied to the pump 10 must have a pressure of 600 psi in order to force fluid through the

check valves

72, 74 against the 30,000 psi back pressure. On the return stroke of the piston 48, however, it is drawing liquid into the cylinder, and into the space between the

check valves

68 and 72 at a substantially lower pressure. Thus when the piston 48 again begins its pumping stroke it must first build up the pressure in the cylinder 46 and in the piping between the

check valves

68 and 72 to 30,000 psi before it can force any more liquid through the check valve 72 against the 30,000 psi back pressure. Since all liquids are to some extent compressible, and since the cylinder 46 and the piping between the check valves will expand upon the application of pressure, the piston 48 will move a substantial distance before a 30,000 psi pressure is reached, so that in this portion of the stroke no liquid is pumped through the check valve 72. During this portion of the stroke therefore the power fluid being fed to the pump 10 is also at a lower pressure than the 600 psi required for a 30,000 psi pumped fluid pressure. This means that the pressure in conduit 28 leading to the pump 10, and in conduit 36 connected thereto, is also less than 600 psi.

FIG. 8 shows still another embodiment of the invention, incorporating a proportioning valve and a dump valve which have functions similar to those previously described for the other embodiments, but which operate in a somewhat different manner. In the embodiment shown in FIG. 8 similar reference numerals are utilized for corresponding elements in the previously described embodiments. As there shown, line 24 leads from the constant displacement pump to a proportioning valve 226. The proportioning valve provides liquid to the pump 10 through the line 28, and to the accumulator 30 through the

lines

32 and 34. A line 36 leads from the dump valve 240 to line 28 and a line 151 is provided through which fluid may pass from the accumulator 30 into the dump valve. v

The proportioning valve 226 is similar to the proportioning valve 26 in many respects. It is provided with

parallel bores

284 and 286. Bore 284 contains a shuttle valve 288 which is reduced in diameter intermediate its ends to provide a fluid passageway across the bore 284 from a port 94 to pipe 32. The shuttle valve is biased toward open position by means of a spring 289. A fluid passageway 232 leads from the upper end of the shuttle valve into communication with the fluid entering the proportioning valve from line 24.

Bore 286 contains a sleeve member 98 which extends from below the lower edge of a port 296 in the body of the accumulator, which communicates with the pipe 28, to above the intersection of the port 94. The sleeve 98 is prevented from moving by, for example, a dog 100 like that shown in the embodiment of FIG. 2. Telescopically received within the sleeve 98 is a hollow cylindrical slidable valve member 102, whose construction may be identical to that shown in FIG. 6. A fluid conduit allows fluid communication between the port 296 and the bore 286 above the valve member 102. A spring 128 biases the valve member 102 downwardly against the pressure of fluid flowing in through the line 24.

The dump valve 240 shown in FIG. 8 comprises a body member 260 having

ports

264 and 266 into which

conduits

151 and 36 are respectively connected. A conduit 265 leads from the conduit 36 to a pilot port 267 in the bottom of the dump valve, and is also connected through a conduit 269 to a dump valve actuator 270. Conduit 271 is also connected to the line 36 and leads to the dump valve actuator 270. As shown, a check valve 273 allows flow of fluid through line 271 into the dump valve actuator but prevents flow from the actuator back into line 271, and a check valve 275 allows flow of fluid from the actuator into line 269 but prevents flow in the opposite direction. A pressure relief valve 274 is mounted on the dump valve actuator.

The dump valve is provided with a cylindrical bore 277 which contains a valve spool 279. A spring 280 biases the valve spool downwardly in a direction to close the port 266 and thereby prevent the flow of fluid from line 151 to line 36. An adjusting screw 281 is provided to adjust the compression of the spring 280.

It will be seen that when the constant delivery pump is started the valve 102 in the proportioning valve is in a closed position. However, the pressure of fluid flowing into the proportioning valve forces the valve member 102 upwardly against the compression of the spring 128 to open the ports therethrough, similarly as in the embodiment shown in FIG. 2 of the drawing.

As in the previously described embodiment the valve member 102 is moved upwardly until the pressure dif ferential across the valve provides an upward force equal to the downward force provided by the spring 128. Thus the pressure differential between the

lines

24 and 28 is determined by the load of the spring 128.

Since the proportioning valve is being supplied by a constant volume pump, it is necessary that the valve member 102 move upwardly enough to allow all of the fluid being supplied to flow through it. Thus the amount of upward movement is determined by the flow rate of the fluid flowing therethrough.

As in the previously described embodiment the proportion between the flow rate through port 94 and that through port 296 is maintained the same for any particular setting of the valve member 102 with respect to sleeve 98. The pressure differential between the interior of the valve 102 and the bore 284 is determined by the force of the spring 289 bearing upon the shuttle valve 288.

If the flow proportion is adjusted to allow a greater proportion of flow through the line 32, this will tend to decrease the pressure drop across the opening in valve member 102 and thereby increase the pressure in port 94 and in the bore 284. This increased pressure will cause the shuttle valve 288 to move upwardly and thereby provide a greater opening into line 32 to allow the greater flow rate therethrough.

The functioning of the proportioning valve is thereby made independent of pressure variations within the-accumulator. The slide valve 288 insures that the pressure differential between the bore inside the valve member 102 and the bore 284 is maintained constant at an amount to provide a force to balance that imposed by the spring 289. Thus, as the pressure in the accumulator increases the flow rate to the accumulator tends to decrease, but the pressure within the bore 284 increases to move the shuttle valve 288 upwardly, opening the passageway into conduit 32 and allowing the flow rate to remain constant.

During the stroke of the piston in the pump fluid flows from the proportioning valve through line 36 and line 271 through the check valve 273 to charge the dump valve actuator 270. A bleed line 285 applies this same pressure to the top of the shuttle valve 279, and this pressure is applied to the bottom of the shuttle valve through the

conduits

265 and 267.

At the end of the stroke of the piston in'pump l0, pressure in line 36 is reduced, thereby reducing the pressure in the bleed line 285 and in

lines

269 and 267.

lit

The accumulated fluid in the dump valve actuator 270 is thus released through the check valve 275 and line 269 to impinge upon the bottom of the shuttle valve 279 and cause it to move upwardly to open the dump valve. Accumulated pressure fluid in the accumulator 30 is thereby allowed to pass through the dump valve into line 36 as previously described for the other embodiments, to maintain the pressure in pump 10. lt will be seen that as soon as this fluid begins to pass through line 36 the pressure in bleed line 285 is quickly restored to cause the shuttle valve 279 to move downwardly. The fluid beneath the shuttle valve 279 is released relatively slowly through the orifice 287 to allow the shuttle valve to close somewhat more slowly than it opened. Alternatively, the line 265 and orifice 287 may be omitted if a small weep hole is provided axially through the piston 279 to allow the piston to move downwardly to closed position.

Although various embodiments of the invention have been shown and described herein, the invention is not limited to the specific embodiments thus disclosed but includes such variations and modifications thereof as will be apparent to those skilled in the art and which may be included within the language of the appended claims.

I claim:

. 1. A proportioning device for'dividing fluid flow between two different conduits, the pressure in one of which is variable with respect to the pressure in the other, comprising a pair of ports, one in communication with each conduit,

means operable to modify the sizes of said ports in response to a change in the ratio of said pressures to equalize the pressures upstream of said ports, passage means connected to conduct fluid from said conduit to said port size modifying means,

a second pair of ports upstream of said equalized pressures, a single inlet in communication with said upstream pair of ports to supply fluid through them, and

means for adjusting the size of one of said upstream pair of ports to control the proportion of flow therethrough.

2. A proportioning device as defined by claim 1 and including means operable in response to a change in the pressure differential across said upstream ports to change their size.

3. A proportioning device for dividing a liquid received at an inlet therein at a constant flow rate between two different outlet conduits at a constant predetermined ratio, wherein the pressure in one conduit is variable with respect to the pressure in the other conduit, comprising a port in communication with said one conduit,

first valve means associated with said port,

passage means connected to conduct fluid from said one conduit to said first valve means, said first valve means being operable to increase or decrease the size of said port in response to an increase or decrease, respectively, in the pressure in said one conduit,

second valve means having a first variable opening establishing communication between said inlet and said port, and a second variable opening establish- 9 10 ing communication between said inlet and said said second valve means being movable in response other condult and to the pressure differential between the inlet and passage means connected to conduct fluid from said inlet and fluid from said other conduit to said second valve means in opposition to each other, 5

the other conduit to vary the size of said openings.

0' t III

Claims

1. A proportioning device for dividing fluid flow between two different conduits, the pressure in one of which is variable with respect to the pressure in the other, comprising a pair of ports, one in communication with each conduit, means operable to modify the sizes of said ports in response to a change in the ratio of said pressures to equalize the pressures upstream of said ports, passage means connected to conduct fluid from said conduit to said port size modifying means, a second pair of ports upstream of said equalized pressures, a single inlet in communication with said upstream pair of ports to supply fluid through them, and means for adjusting the size of one of said upstream pair of ports to control the proportion of flow therethrough.

3. A proportioning device for dividing a liquid received at an inlet therein at a constant flow rate between two different outlet conduits at a constant predetermined ratio, wherein the pressure in one conduit is variable with respect to the pressure in the other conduit, comprising a port in communication with said one conduit, first valve means associated with said port, passage means connected to conduct fluid from said one conduit to said first valve means, said first valve means being operable to increase or decrease the size of said port in response to an increase or decrease, respectively, in the pressure in said one conduit, second valve means having a first variable opening establishing communication between said inlet and said port, and a second variable opening establishing communication between said inlet and said other conduit, and passage means connected to conduct fluid from said inlet and fluid from said other conduit to said second valve means in opposition to each other, said second valve means being movable in response to the pressure differential between the inlet and the other conduit to vary the size of said openings.