PRESSURE CONTROL VALVES:

The safe and efficient operation of fluid power systems, system components, and related equipment requires a means of controlling pressure. A pressure control valve may limit or regulate pressure, create a particular pressure condition required for control, or cause actuators to operate in a specific order. All pure pressure control valves in a condition approaching hydraulic balance. Usually the balance is very simple: pressure is effective on one side or end of a ball, poppet, or spool and is opposed by a spring. In operation, a valve takes a position where hydraulic pressure balances a spring force. Since spring force varies with compression, distance and pressure also can vary. Pressure control valves are said to be infinite positioning. This means they can take a position anywhere between two finite flow positions which changes a large volume of flow to a small volume, or pass no flow.

Most pressure controlled valves are normally closed (NC). This means that flow to a valve’s inlet port is blocked from an outlet port until there is enough pressure to cause an unbalanced operation. In normally open valves (NO), free flow occurs through the valves until they begin to operate in balance. Flow is partially restricted or cut-off. Pressure override is a characteristic of normally closed pressure controls when they are operating in balance. Because the force of a compression spring increases as it lowers, pressure when the valve first cracks is less than when they are passing a large volume or full flow. The difference between a full flow and cracking pressure is called override.

PRESSURE RELIEF VALVES: Relief valves are the most common type of pressure control valves.

The relief valves function may vary, depending on a system’s needs. They can provide over load protection for circuit components or limit the force or torque exerted by a linear actuator or rotary motor.

The internal design of all the relief valves is basically similar. The valves consist of two sections: a body section containing a piston that is retained on its seat by a spring(s), depending on the model, and a cover or pilot valve section that hydraulically controls a body piston movement. The adjusting screw adjusts this control within the range of valves.

Simple relief valves:This valve is installed so that one port is connected to the pressure line

or the inlet and the other port to the reservoir. The valve is held on its seat by

thrust of the spring, which can be changed by turning the adjusting screw. When the pressure at the valve’s inlet is insufficient to overcome spring force, the ball remains on its seat and the valve is closed, preventing flow through it. When the pressure at the valve’s inlet exceeds the adjustable spring force, the ball is forced off its seat and the valve is opened. Liquid flows from the pressure line through the valve to the reservoir. This diversion of low prevents further pressure increase in the pressure line. When pressure decreases below the valve’s setting, the spring reseats the ball and the valve is again closed.

The pressure at which valve first begins to pass flow is the cracking pressure of a valve. The pressure at which a valve passes its full rated capacity is the full flow pressure of a valve. Because of spring rate, a full flow pressure is higher than a cracking pressure. This condition is referred to as pressure override. A disadvantage of a simple pressure relief valve is its relatively high pressure override at its rated capacity.

Simple relief valves have a tendency to open and close rapidly as they hunt above and below the set pressure, causing pressure pulsations and undesirable vibrations and produce a noisy chatter. Because of the unsatisfactory performance of the simple relief valve in some applications, compound relief valves were developed.

Compound Relief Valve:A compound relief valve operates in two stages. They are

designed to accommodate higher pressures than direct acting relief valves at the same flow rate capacity. The first stage includes the main spool which is normally closed and kept in position by non-adjustable spring. The pilot stage is located in upper valve body and contains pressure limiting poppet, which is held against seat by an adjustable spring. The relief pressure of the valve is set by turning the adjusting screw of pilot valve. The lower body contains port connections. The balanced piston in the lower part of the body accomplishes full diversion of the flow. Figure below shows a compound relief valve. Pressure at the inlet port acts on both sides of the piston, through an orifice, that is drilled through the large land (passage C). Passage C is used to keep the piston in hydraulic balance when the valve’s inlet pressure is less than its setting (diagram A). The valve setting is determined by an adjusted thrust of spring 3 against poppet 4. When the pressure at the valve’s inlet reaches the valve’s setting, the poppet 4 is forced of its seat. This limits the pressure in the upper chamber. The restricted flow through the orifice into the upper chamber results in an increase in pressure in the lower chamber. This causes an imbalance in hydraulic forces, which tends to raise the piston off its seat. When the pressure difference between upper and lower chamber increases the large piston lifts off its seat to permit flow directly to the tank.

If there is a flow increase through the valve, the piston lifts off its seat. However this compresses only the light spring (main piston spring) and hence very little override occurs.

COUNTERBALANCE VALVE;The counterbalance valve is normally located in the line between a

directional control valve and the outlet of a vertically mounted actuating cylinder which supports weight or must be held in position for a period of time. This valve serves as a hydraulic resistance to the actuating cylinder. For example, counterbalance valves are used in some hydraulic operated

forklifts. The valve offers resistance to the flow from the actuating cylinder when the fork is lowered. It also helps to support the fork in the up position.

1. Adjustment Screw 2. Internal Drain3. Spring 4. Spool5. Pressure inlet or reverse flow outlet 6. Pilot passage7. Check Valve 8. Discharge outlet or reverse

free flow inletOne type of counterbalance valve is explained in figure. The valve

element is a balanced spool (4). The spool consists of two pistons permanently fixed on either end of shaft. The inner surface areas of pistons are equal; therefore, pressure acts equally on both areas regardless of the position of the valve and has no effect on the movement of the valve- hence, the term balanced. The shaft area between the two pistons provides the area for the fluid to flow when the valve is fully open. A small piston (9) is attached to the bottom of the spool valve.

When the valve is in the closed position, the top piston of the spool valve blocks the discharge port (8). With the valve in this position, fluid flowing from the actuating unit enters the inlet port (5). The fluid cannot flow through the valve because the discharge port 8 is blocked. However, Fluid will flow through the pilot passage (6) to the small pilot piston. As the pressure increases; it acts on the pilot piston until it overcomes the preset pressure of spring 3. This forces the spool (4) up and allows the fluid to flow

around the shaft of the valve spool and out discharge port (8). Figure the valve in this position. During the reverse flow, the fluid enters the port 8. The spring (3) forces valve spool 4 to the closed position. The fluid pressure overcomes the spring tension of the check valve (7). The check valve opens and allows free flow around the shaft of the valve spool and out through port 5.

The operating pressure of the valve can be adjusted by turning the adjustment screw (1), which increases or decreases the tension of the spring. This adjustment depends on the weight that the valve must support. It is normal for a small amount fluid to leak around the piston of spool valve and into the area around the spring. An accumulation would cause additional pressure on top of the spool valve. This would require additional pressure to open the valve. The drain (2) provides a passage for this fluid to flow to port 8.

SEQUENCE VALVE:Sequence valve controls the operating sequence between two branches

of a circuit. The valves are commonly used to regulate an operating sequence of two separates work cylinders so that one cylinder begins when the other completes stoking. Sequence valves used in this manner ensure that there is minimum pressure equal to its setting on the first cylinder during the subsequent operations at lower pressure.

An example of the use of a sequence valve is in an aircraft landing gear actuating system. In a landing gear actuating system, the landing gear doors must open before the landing gear starts to extend. Conversely, the landing gear must be completely retracted before the doors close. A sequence valve installed in each landing gear actuating line performs this function.

Figure shows the operation of a pressure controlled sequence valve. Fluid enters the valve at inlet port C, flows freely past the piston 1, and enters the primary circuit through port D. When pressure of the liquid flowing through the valve is below the valve’s setting, the force acting upward on piston 1 is less than the downward force of the spring 2. The piston is held down and the valve will remain in closed position.

When the primary actuating unit completes its operation, pressure in the line to the actuating unit increases sufficiently to overcome the force of the spring 2. Hence piston 1 rises and the valve is opens. The fluid entering the valve takes the path of least resistance and flows to the secondary circuit through port E.

Mechanically operated sequence valves:The mechanically operated sequence valve is operated by a plunger

that extends through the body of the valve. The valve is mounted so that the plunger will be operated by the primary circuit. A check valve, either a ball or a poppet, is installed between the fluid ports in the body. It can be unseated by either the plunger or fluid pressure.

Port A and the actuator of the primary unit are connected by a common line. Port B is connected by a line to the actuator of the secondary unit. When the fluid under pressure flows to the primary unit, it also flows into the sequence valve through port A to the seated check valve in the sequence valve. In order to operate the secondary unit, the fluid must flow through the sequence valve. The valve is located so that the primary unit depresses the plunger as it completes its operation. The plunger unseats the check valve and allows the fluid to flow through the valve, out port B, and to the secondary unit.

This type of sequence valve permits flow in the opposite direction. Fluid enters the port B and flows to the check valve. Although this is return flow from the actuating unit, the fluid overcomes spring tension, unseats the check valve, and flows out through port A.