"4 Constant flow valves of the type in question must be constructed to allow dosing of small volumes of fluid with great accuracy, even in the presence of significant pressure fluctuations on both the upstream and downstream sides of the valve.

US patent 4.893. 649 describes a constant flow valve comprising two flow control elements. The first flow control element is constituted by a fixed nozzle, a so-called choke valve, of the type where the flow rate is dependent on the pressure drop across the nozzle. The second flow control element is a needle valve designed to maintain a constant pressure drop across the fixed nozzle.

The control principle of the device according to the above mentioned US patent is also found in other known constant flow valves.

According to prior art the hydrostatic pressure in the inlet of the constant flow valve acts on an adjusting piston connected to the valve needle of the needle valve, in a manner such that the needle valve is closed. Fluid flows through the choke valve to a chamber at the opposite side of the adjusting piston, where it exerts a pressure against the adjusting piston. The fluid pressure against the chamber side of the adjusting piston, together with the force from a valve spring, moves the valve needle out from the needle valve seat, to allow fluid to drain from the chamber. The needle valve may then close again. By controlling the force from the valve spring, e. g. through pretensioning, the pressure drop across the choke valve may be controlled.

When connecting a constant flow valve to a relatively high fluid pressure, considerable force will be exerted against the closing side of the piston before sufficient fluid flows through the choke valve to fill up and equalize the pressure.

Thus the valve needle of the needle valve is often damaged by crushing, as the materials used are hard and brittle. It is therefore common for constant flow valves according to prior art to have resilient fixing of the valve needle in the adjusting piston.

The seal of the adjusting piston against the valve housing will during the control of the constant flow valve give rise to frictional forces that may have a significant influence on the accuracy of the constant flow valve. It has also been

found that the resilient fixing of the valve needle in the adjusting piston can lead to inaccurate control, as the relative travel of the valve needle relative to the adjusting piston has a disruptive effect.

The valve spring pretensioning control means of known constant flow valves make the valves more expensive and unnecessarily complicated.

The object of the invention is to remedy the disadvantages of prior art.

The object is achieved in accordance with the invention by the characteristics given in the description below and in the appended claims.

Providing a constant flow valve with an adjustable choke valve will allow a wide control range to be achieved without having to adjust the pretensioning of the needle valve- opening valve spring of the constant flow valve.

The dosing accuracy is improved significantly by the valve needle of the needle valve being displaced by a diaphragm.

The control of the valve needle is then not disturbed by friction that is linked to sealing. Tests have shown that a constant flow valve according to the invention exhibits such a degree of repeat accuracy that the choke valve of the constant flow valve, in a preferred embodiment, is equipped with a vernier scale in order to facilitate the setting of e. g. a previously calibrated flow rate.

Damage to the valve needle is prevented through arranging an elastic material with a relatively small overall height between the valve needle and the diaphragm, e. g. in the form of a so-called 0-ring. The length of the valve needle is adjusted so that when the valve needle is in the closing position, it does not project up to the diaphragm when the diaphragm is fully displaced in the direction of the valve needle. The gap between the valve needle and the diaphragm is taken up by the above elastic material.

The operation of the constant flow valve is explained in the specific part of the patent specification.

The following describes a non-limiting example of a preferred embodiment illustrated in the accompanying drawings, in which: Figure 1 shows a section through a constant flow valve; Figure 2 shows a section through the needle valve of figure 1 on a larger scale, the needle valve being in an open position; and Figure 3 shows the same as figure 2, but here the needle valve is in a closed position.

In the drawings, reference number 1 denotes a constant flow valve comprising an upper housing part 2 and a lower housing part 4, a choke valve 6 and a needle valve 8. A diaphragm 10 is arranged sealingly in a recess 12 between the housing parts 2 and 4, see figures 1 and 2.

The upper housing part 2 is provided with an inlet 14 for the fluid that is to flow through the constant flow valve 1, wherefrom an inlet passage 16 connects the inlet 14 to the inlet chamber 18 of the choke valve 6. A passage 20 runs from the inlet chamber 18 to the recess 12, terminating on the closing pressure side 22 of the diaphragm 10, see figure 3.

A bore 24 runs concentrically with the choke needle 26 of the choke valve 6, from the inlet chamber 18 to the lower housing part 4, where it extends further through the lower housing part 4 to the needle valve 8. A gasket 28 prevents leakage from the bore 24 out between the housing parts 2 and 4.

The choke needle 26 is formed with a conical end portion projecting into the bore 24, and the choke needle 26 is designed to be moved into and out of the bore 24 by means of a control member 30 of a type that is known per se.

The diaphragm 10 is clamped between the upper housing part 2 and the lower housing part 4 by means of bolts (not shown).

Gaskets 32 in the form of 0-rings prevent leakage from the recess 12 out between housing parts 2 and 4, see figure 2.

A needle valve bore 34 runs concentrically with and from the recess 12 into the lower housing part 4 to a shoulder portion 36 from which an outlet passage 38 extends to the outlet 40 of the constant flow valve 1.

A needle valve seat 42 with a through bore 44 is disposed in the needle valve bore 34, abutting the shoulder portion 36 in a sealing manner with the aid of a gasket 46.

A valve needle guide 48 is provided in the needle valve bore 34 and connected to the lower housing part 4 by means of a threaded connection 50. The end face 52 of the valve needle guide 48 facing the upper housing part 2 is substantially level with the recess 12 in the lower housing part 4. The valve needle guide 48 has a through bore 54. Closest to the end face 52, the bore 54 is widened further, so as to form a shoulder 56 in the bore 54. A valve needle/closing body 58 is displaceably arranged in the bore 54 of the valve needle guide 48 with sufficient clearance to form an annulus through which fluid may flow between the valve needle 58 and the bore 54. The valve needle 58 is equipped with an external shoulder 60 and with a conical sealing surface 62 at the end portion facing the needle valve seat 42. An 0-ring 64 is disposed between the opposite end portion 66 of the valve needle 58 and the diaphragm 10.

A helical valve spring 68 extends between the shoulder 56 in the valve guide 48 and the shoulder 60 on the valve needle 58, and is designed to displace the valve needle 58 in a direction out from the needle valve seat 42.

Fluid from the bore 24 may flow into the needle valve 8 through cutouts 70 in the needle valve guide 48.

When pressurised fluid is allowed to flow into the inlet 14 to the constant flow valve 1 and on through the inlet passage 16 to the inlet chamber 18, the closing pressure side 22 of the diaphragm 10 is soon subjected to the full fluid pressure via the passage 20. The valve needle 58 is moved against the needle valve seat 42 via the 0-ring 64 with a relatively great force, see figure 3. However, the end portion 66 of the

valve needle 58 is moved to a level slightly beneath the end face 52 of the valve guide 48 before the sealing surface 62 closes against the needle valve seat 42. Thus the diaphragm 10 is prevented from impacting against the end portion 66 of the valve needle 58, consequently limiting the closing force of the valve needle to the closing force that is transferred via the relatively soft O-ring 64. Thus there is no danger of the valve needle 58 being crushed by the closing force from the diaphragm 10.

Fluid then flows through the choke valve 6 in an annular gap formed between the choke needle 26 and the bore 24, and then on through the bore 24 and the cutout 70 to the needle valve 8.

Some fluid flows past the valve needle 58 and into the recess 12. The fluid pressure against the two sides of the diaphragm 10 is equalized, so that the relatively weak valve spring 68 may displace the valve needle out from the needle valve seat 42, whereby fluid may flow through the bore 44 and the passage 38 to the outlet 40 of the constant flow valve 1.

The delivery of fluid to the needle valve 8 is limited by the choke valve 6. When the volume of fluid drained from the needle valve 8 is larger than that delivered through the choke valve 6, the pressure in the needle valve 8 falls. With that, the force from the fluid pressure differential between the two sides of the diaphragm 10 overcomes the force from the valve spring 68, causing the needle valve 8 to close.

Through being able to adjust the relationship between the dimension of the diaphragm 10 and the force of the valve

spring 68, it is possible to achieve high-accuracy pressure drop control, as the friction that exists between the valve needle guide 48 and the valve needle 58 is negligible. The deflection of the diaphragm 10 is not affected by frictional forces either.

With a constant pressure drop across the choke valve 6, and for a specific fluid at a specific temperature, the fluid flow through the constant flow valve 1 will essentially depend only on the opening of the choke valve 6. Tests have shown that the repeat accuracy of the constant flow valve 1 is very high.