The present invention relates to a flow control valve, and in particular to a flow control valve having improved flow control characteristics resulting in improved output flow rates and pressures. The valve is particularly suited to pumps which pump fluids, predominantly containing liquids.
In a known pump assembly, the discharge from the pumping element flows to a valve arrangement having a valve element nominally floating in a bore. The valve arrangement includes a cavity into which the discharge from the pumping element flows. The cavity includes a discharge orifice through which fluid flows from the cavity with a pressure drop being established across the discharge orifice. The cavity has a further separate opening into the bore in which the valve element is "floating". On the downstream side of the discharge orifice, a passage runs to the side of the bore, distant from the cavity, relative to the valve element. Thus, the valve element is balanced with two pressures, the pumping pressure (of the fluid in the cavity) acting on one end and the discharge pressure (determined by the discharge orifice) applied to the other end. A biasing member in the form of a spring is also provided to bias the valve element towards the cavity, whilst the pressure difference across the valve element acts to bias the valve element away from the cavity. In this way the valve arrangement is located in a bore which communicates across the discharge orifice of the cavity which in turn means that the valve element is separate from the discharge orifice.
In this arrangement, if the pumping pressure exceeds the discharge pressure by more than a predetermined amount, the valve element becomes sufficiently displaced from its position of zero displacement (i.e. its position when there is no pressure difference across the valve element) to open a channel to a spill port which directs fluid from the cavity back to the inlet of the pump. The difficulty with this arrangement is that there is insufficient control of the discharge flow rate, from the discharge orifice, with respect to pump speed and pressure. In fact, with such an arrangement, the discharge flow rate tends to increase with both pump speed and pressure.
The disadvantage described above with the conventional valve arrangement can be mitigated to an extent by forming a small recess in the front of the valve element and by suitable shaping of the spill port. Additionally, the valve arrangement may be improved by providing a "needle" (a rod having a profiled shape) on the end of the valve element which moves in the discharge orifice formed in a discharge plug aligned with the valve element, such that the annular area between the needle and the circumference of the discharge orifice varies with the movement of the valve element in the bore. The problems with this arrangement are that the needle position is fixed relative to the position of the valve element, the needle is expensive to manufacture and the region within the bore where the discharge pressure acts has an almost stagnant pool of fluid, where problems of contamination can develop. The valve element must also be further increased in complexity because of the need to incorporate within it a pressure relief valve, which opens at a high pressure setting (e.g. approximately 50-70 bars) to relieve pressure back to the pump inlet in the event of overloading the output.