A needle valve is a type of valve that is designed to control the volume of liquid that flows in small lines. As the liquid that passes through the valve makes ninety degrees turn passing through an orifice that seats a rod with a cone-shaped tip, the size of the orifice changes depending on how much or how little liquid flow is required.
In one type of normally closed two-port solenoid controlled poppet valve shown in Figure 1, pressure at port A is applied to the back of the poppet via a small hole (orifice X) in the side-wall. Pressure keeps the poppet closed in the manner of a check valve. Energizing the solenoid lifts a plunger unblocking a hole (Y) in the center of the poppet. Imbalance occurs because of the pressure differential across orifice (X) and the poppet lifts permitting flow through the valve from A to B. Note that the solenoid armature (plunger) is permanently surrounded by the hydraulic fluid and hence balanced.
Check valves are available with different spring rates to give particular cracking pressures. The cracking pressure is that at which the check valve just opens. If a specific cracking pressure is essential to the functioning of a circuit, it is usual to show a spring on the check valve symbol. The pressure drop over the check valve depends upon the flow rate; the higher the flow rate, the further the ball or poppet has to move off its seat and so the higher the spring force.
Pressure-compensated flow controls must be used when accurate speed control at varying supply or load pressures is required. The minimum regulated stable flow from a good-quality flow-control valve will be in the region of 0.1 l/min. In any precision flow-control valve application it is essential to have well-filtered fluid (better than 10 μm absolute) to promote efficient control and long life of the valve. The smaller the flow to be controlled, the finer the filtration necessary. Various types of valve adjusting mechanism are available - hand knob, lockable hand knob, lever, DC motor control etc. It must be remembered that whenever a flow-control valve is used in a system, there will always be some pressure drop and associated heat generation
The action of pressure-reducing valves always generates heat energy because of the throttling effect. This heat generation must be taken into account when considering their application. Where two separate pressures are continuously required in a circuit, a two-pump system may prove a better solution than one using pressure-reducing valves. This will depend upon the flow and pressures required.
An important feature of all pressure sequence valves is a separate drain connection from the spring chamber. This is because, unlike a conventional relief valve, a high pressure can occur in the output port during the normal course of operation. Should it be internally drained, any pressure in the output port will be reflected back into the spring chamber causing a malfunction. In fact a sequence valve may be used as a relief valve in any circuit "here excessive back pressures are encountered in the return line. The independently drained pilot makes sequence valves insensitive to downstream back pressure.
A check valve is incorporated in the circuit in Figure 1 to allow free-flow in the reverse direction (i.e. to bypass the counterbalance valve when raising the load). Care must be taken if using a conventional relief valve for this application as at some stage of operation the tank port will be subject to maximum circuit pressure. This is not permissible with many relief valves. The counterbalance valve shown has an integral check valve. A separate drain connection from the spring chamber is unnecessary because the pressure section of the valve is inoperative when the T port is pressurized (flow is through the check valve). When it is counterbalancing the back pressure at T should be kept to a minimum.
The function of a relief valve is to set the maximum pressure in a hydraulic system. Although there are many designs and varieties, they can all be denoted by the general symbol (a) in Figure 2. It is a normally closed valve which partially opens permitting flow to the tank port when the pressure at the inlet port overcomes the spring force. Symbol (a) does however more accurately represent a direct acting valve. If there is no arrow through the spring the valve is pre-set, i.e. non-adjustable. A two-stage relief valve may be shown as Figure 2(b) as this more closely illustrates its operation which will be described later.