CONSTANT FLOW VALVE

This invention relates to a constant-flow valve. More particularly, it relates to a constant-flow valve which includes a housing, a sealing piston movable within the housing, a first choke and a second choke, the relative axial position of the piston determining the aperture of the second choke, the upstream side of the first choke communicating with one side of the piston, at which a fluid pressure acting on the piston will seek to move the piston in the closing direction of the second choke, and the downstream side of the first choke communicating with the opposite side of the piston. At least a portion of the piston is in a space separated from fluid flowing through the constant-flow valve.

Prior art constant-flow valves are used in a number of areas to ensure that a fluid flow remains constant even if variations occur in, for example, the pressure, temperature or viscosity of the flowing fluid.

A known principle of the functioning of a constant-flow valve involves that the flowing fluid is passed through two chokes connected in series. The pressure drop across the first choke is kept constant by the pressure drop across the second choke being regulated, usually automatically, in accordance with variations in the total pressure drop across the constant- flow valve. According to this working principle, if the flow rate through the constant-flow valve is to be changed, the

second choke must be adjusted in order for the part of the total pressure drop that occurs in the first choke to be changed. A greater throughput through a constant-flow valve of this kind is conditional on an increased pressure drop across the first choke.

US patent 5 494070 discloses a constant-flow valve of such a construction. The first choke is in a movable sealing piston, in which the input pressure of the fluid acts on one side of the piston, and a spring pushes against the opposite side of the piston. The fluid pressure from the fluid which has flowed through the first choke also acts on the spring side of the piston. A valve body is connected to the piston and corresponds to a valve seat within the housing of the constant-flow valve. Together the valve body and valve seat con- stitute the second choke.

According to US 5 494 070 the throughput is regulated by the valve body being moved relative to the piston, which changes the tension of the spring, whereby the second choke regulates with an altered regulating range in consequence of an altered force from the spring.

Because of the relatively great contact forces occurring between the valve body and valve seat, one or both of these components are made of a ceramic material. Ceramic materials are sensitive to impacts, and it has turned out to be neces- sary to arrange a damping spring between the valve body and the piston to prevent the second choke from being damaged.

Thus, in constant-flow valves of this kind there are two springs connected in series between the housing and valve body. Such a connection may lead to reduced stability in the valve and, thereby, undesired fluctuations in the throughput.

NO patent 319627 discloses a constant-flow valve, in which

the flow path between the first and second chokes is routed outside the piston of the constant-flow valve.

Prior art constant-flow valves are of such designs that the fluid flowing through the constant-flow valve is present in all the cavities of the valve, also at the piston of the constant-flow valve and the sliding surface of the piston facing the housing of the constant-flow valve. It is evident that, for example, "dry" liquids, such as methanol, could counteract the movement of the piston and, thereby, the accuracy of the constant-flow valve.

The invention has for its object to remedy or at least reduce one or more of the drawbacks of the prior art .

The object is achieved in accordance with the invention through the features which are specified in the description below and in the claims that follow.

A constant-flow valve in accordance with the invention includes a housing, a sealing piston movable in the housing, a first choke and a second choke, the relative axial position of the piston determining the aperture of the second choke, the upstream side of the first choke communicating with one side of the piston, at which a pressure acting on the piston will seek to move the piston in the closing direction of the second choke, and the downstream side of the first choke communicating with the opposite side of the piston. The con- stant-flow valve in accordance with the invention is characterized by at least one portion of the piston being in a space separated from a fluid flowing through the constant- flow valve.

Preferably, a first sealing diaphragm is positioned in a man- ner allowing pressure communication between the inlet side of the first choke and one side of the piston, whereas a second

sealing diaphragm is positioned in a manner allowing pressure communication between the outlet side of the first choke and the opposite side of the piston.

By "in a manner allowing pressure communication" is meant here that the diaphragms are influenced by the prevailing fluid pressure and transmit, typically via another fluid, this pressure or this force to the piston.

In another embodiment the piston may be constituted by, or at least include, a diaphragm. In this embodiment one of said diaphragms may replace the piston, at least partially.

The first choke is preferably adjusted by means of a so- called nanoscrew. If desirable, the nanoscrew can be motor- driven .

A constant-flow valve in accordance with the invention reme- dies, to a substantial degree, the conditions causing operational problems in known constant-flow valves. The reason is that sensitive components, such as the piston with the seal and also the spring arrangement, are separated from the fluid flowing through the constant-flow valve. This fluid may be corrosive or have little lubricity.

In what follows, there is described a non-limiting example of a preferred embodiment which is visualized in the accompanying drawings, in which:

Figure 1 shows, in section, a constant-flow valve, in which a second choke is open;

Figure 2 shows, in section, the constant-flow valve of figure 1 in a closed position; and

Figure 3 shows, on a larger scale, a section of figure 2.

In the drawings the reference numeral 1 indicates a constant- flow valve including a housing 2, a piston 4, a first choke 6 and a second choke 8.

The housing 2 includes a valve housing 10 and a lid 12, the lid 12 being connected to the valve housing 10 by means of bolt connections, not shown.

From its portion facing the lid 12, the valve housing 10 is provided with an axial first bore 16 extending in to a first shoulder 18. From the first shoulder 18 a cylinder bore 20 concentric with the first bore 16 extends in to a second shoulder 22. From the second shoulder a second bore 24 extends in to a third shoulder 26.

A through-going sleeve bore 28 extends concentrically with the cylinder bore 20 from the third shoulder 26. The sleeve bore 28 is provided with internal threads 30.

A cylindrical portion 32 of the lid 12 projects sealingly, by means of gaskets 34, into the first bore 16. The cylinder bore 20 forms a cylinder for the piston 4.

From the inlet 36 of the constant-flow valve 1, which is in the lid 12, an inlet bore 38 extends to the first choke 6. The inlet bore 38 communicates with the first bore 16 on the side of the piston 4 facing the lid 12 via a connecting bore 40.

The first choke 6 includes a first choke bore 42 and a corre- sponding choke cone 44. The choke cone 44 projects movably into the first choke bore 42, the choke cone 44 forming an end portion of the spindle 48 of a nanoscrew 46. The nano- screw 46, which is of a design known per se, is arranged to repeatably move the spindle 48 in the axial direction. The

nanoscrew 46 is connected to the lid 12 by means of threads 50.

The flow area of the first choke 6 is adjusted by moving the choke cone 44 by means of the nanoscrew 46 in the first choke 5 bore 42.

A first intermediate channel 52 in the lid 12 and a second intermediate channel 54 and a third intermediate channel 56 in the valve housing 10 connect the first choke 6 with the sleeve bore 28. A gasket 58 sealingly surrounds the first and io second intermediate channels 52, 54 between the valve housing 10 and lid 12.

The third intermediate channel 56 opens into the sleeve bore 28 in the space between the third shoulder 26 and threads 30, and is arranged with a plug 60 which can be removed if meas- i5 uring equipment is to be connected to the third intermediate channel 56.

The piston 4, which movably arranged in a sealing manner in the cylinder bore 20 by means of a seal 62, is provided with a valve body 64 in its portion facing away from the lid 12.

20 A spring or spring package 66 is compressed between the second shoulder 24 and the piston 4.

By means of a gasket 70, a valve sleeve 68 is screwed sealingly into the threads 30 of the sleeve bore 28 and corresponds to the valve body 64.

25 The valve sleeve 68 is formed with a centric axial second choke bore 72 extending therethrough, communicating with the outlet 74 of the constant-flow valve 1.

The valve sleeve 68 forms, together with the valve body 64, the second choke 8. Thus, the relative axial position of the piston 4 determines the aperture of the second choke 8.

A first diaphragm 76 is sealingly clamped between a first stop ring 78, resting on the first shoulder 18, and the lid 12.

An annular second diaphragm 80 seals at its inner portion by means of a gasket 82 clamped between the piston 4 and valve body 64. At its outer portion, the second diaphragm 80 is clamped sealingly by means of a gasket 84 between the second shoulder 22 and a second stop ring 86. A space 88 defined by the cylinder bore 20 and the two diaphragms 76, 80 is filled with a preferably lubricating liquid.

The fluid pressure on the upstream side of the first choke 6 acts, as mentioned, via the connecting channel 40 on the side of the piston 4 facing the lid. The fluid pressure on the downstream side of the first choke acts via the intermediate channels 52, 54, and 56 and also the sleeve bore 28 on the opposite side of the piston 4 together with the force of the spring 66.

The force from the spring 66 seeks to move the piston 4 in the direction towards the lid 12, whereas the resulting force from the through-flowing fluid after the pressure drop across the first choke 6 seeks to move the piston 4 in the opposite direction. Thus, the pressure drop and thereby the volume flow through the constant-flow valve 1 at a given setting of the first choke 6 depends essentially on the relationship between the spring force of the spring 66 and the area of the piston 4.

When fluid is first supplied under pressure to the inlet 36 of the constant-flow valve 1, see figure 1, the piston 4 with

the valve body 64 is moved to a position towards the first stop ring 78. Thus, the second choke 8 is open.

The entering fluid flows immediately through the inlet bore 38 and connecting bore 40 to the first diaphragm 76. The first diaphragm 76 transmits the fluid pressure to the liquid present between the first diaphragm 76 and piston 4. The piston 4 with the valve body 64 is thereby moved relatively fast towards the valve sleeve 68, see figure 2. The second choke 8 is thereby closed.

As fluid is flowing through the first choke 6 and further through the intermediate channels 52, 54 and 56 and the sleeve bore 28, a fluid pressure builds up under the second diaphragm 80 which transmits the fluid pressure to the liquid present between the second diaphragm 80 and piston 4. This fluid pressure works as a force on the side of the piston 4 facing the valve sleeve 68. After a relatively short time, this force together with the force of the spring 66 will overcome the force from the fluid pressure on the side of the piston 4 facing the lid 12 and move the piston 4 and valve body 64 somewhat in the direction away from the valve sleeve 68, whereby fluid can flow out through the second choke bore 72 of the valve sleeve 68 to the outlet 74. The fluid pressure on the spring side of the piston 4 is thereby reduced, and the piston 4 and valve body 64 are moved in the direction towards the valve sleeve 68, whereby the pressure drop across the second choke 8 increases .

Within a relatively short time the constant-flow valve 1 stabilizes the pressure drop across the first choke 6 by the pressure drop of the second choke 8 being adjusted automati- cally, as described above. Any changes in the total pressure drop across the constant-flow valve 1, the pressure drop being caused by, for example, a change in pressure at the out-

let 74, are compensated by an altered pressure drop across the second choke 8, the pressure drop across the first choke 6 being influenced, in the main, only by structural features of the constant-flow valve, as it has been described above.

The pressure drop across the first choke 6 remains unchanged even if the flow area of the first choke 6 is adjusted to change the throughput of the constant-flow valve 1.

The piston 4 together with the seal 62 and spring 66 is not in the fluid flowing through the constant-flow valve 1 and is therefore not subjected to adverse influence such as corrosion and poor lubrication from this fluid.

In an alternative embodiment, not shown, the piston 4 is constituted by a diaphragm.