Check valves are a simple but important part of a hydraulic system. Simply stated, these valves are used to maintain the direction that fluid flows through a system. And since check valves are zero leakage devices, we can use them to lock hydraulic fluid from the cylinders. This section has been designed to help you understand how the different valves function and the strategy of where they are used in the system.
In-line check valves are classified as directional control valves because they dictate the direction flow can travel in a portion of the circuit. Because of their sealing capability many designs are considered to have zero leakage. The simplest check valve allows free flow in one direction and blocks flow from the opposite direction. This style of check valve is used when flow needs to bypass a pressure valve during return flow, as a bypass around a filter when a filter becomes clogged, or to keep flow from entering a portion of a circuit at an undesirable time.
Because of slight spool leakage on standard directional control valves, we must add a check valve to the circuit if we need to hydraulically lock a cylinder. This type of check valve is referred to as a pilot operated check valve.
Pilot operated check valves may be pilot to open or pilot to close. This is determined by the application.
Unlike a simple check valve, reverse flow is required through the valve to extend or retract the cylinder. This is accomplished by allowing pilot pressure to act on a pilot piston, thus opening the check valve and retracting the cylinder. To extend the cylinder, the check valve allows fluid to flow freely in one direction and blocks flow in the opposite direction.
Pressure Control Valves – A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level. Common schematics for pressure control valves:
The primary concern in fluid power circuits is to either control the rate of flow or the pressure level. One misconception has been that pressure may be controlled with an orifice or flow control device. This is never accomplished with any degree of accuracy. For accurate control of force, six types of pressure controls have been developed.
- Relief valve
- Unloading valve
- Sequence valve
- Reducing valve
- Counterbalance valve
- Brake valve.
By symbol, these valves closely resemble one another. Often only their location in the hydraulic circuit will designate what type of pressure valve they are.
Pressure Sequence Valve
A sequence valve is a normally closed pressure control valve that ensures that one operation will occur before another, based on pressure. In our clamp and drill system we want the clamp cylinder to extend completely before the drill cylinder extends. To accomplish this, we place a sequence valve just before the drill cylinder. We set the cylinder to 500 psi. This will ensure that the drill will not extend before we have reached 500 psi on the clamp cylinder.
Pressure Reducing Valve
A pressure reducing valve is a normally open pressure control valve used to limit pressure in one or more legs of a hydraulic circuit. Reduced pressure results in a reduced force being generated. A pressure reducing valve is the only pressure control valve that is normally open. A normally open pressure control valve has primary and secondary passages connected. Pressure at the bottom of the spool is sensed from the pilot line which is connected to the secondary port. Remember, a pressure reducing valve is normally open.
The illustrated clamp circuit requires that clamp cylinder B apply a lesser force than clamp cylinder A. A pressure reducing valve placed just before the clamp cylinder B will allow flow to go to the cylinder until pressure reaches the setting of the valve.
At this point, the valve begins to close off, limiting any further buildup of pressure. As fluid bleeds to the tank through the valve drain passage, pressure will begin to decay off and the valve will again open. The result is a reduced modulated pressure equal to the setting of the valve.
A High-Low pump system provides a high-volume flow at low pressure and low volume flow at high pressure. These circuits are used to extend and retract the loads at low pressure and high flow, followed by high pressure, low volume flow to do work. Inasmuch as the power required is the product of pressure and flow, a High-Low system allows components and input motors to be kept small which increases operating efficiency by sizing the system to load requirements.
(Hydraulic hp = pressure (PSIG) x flow rate (GPM)/1714)
Consider a High-Low pump circuit that incorporates an 18 GPM pump which unloads at 1000 psi and a 10 GPM pump which relieves at 3000 psi. What is the maximum theoretical input fluid hp required?
- 8.5 hp
- 17.5 hp
- 12.5 hp
- 20 hp
Just prior to unloading, the system will supply 28 GPM (18 GPM + 10 GPM) at 1000 psi. Based on our theoretical input horsepower formula, the required hp=16.3. With the 18 GPM pump unloading we supply only 10 GPM at 3000 psi. Again, using our formula, we calculate 17.5 hp required.
Answer: 17.5 hp (theoretical)
An unloading valve is a remotely piloted, normally closed pressure control valve that directs flow to the tank when pressure at that location reaches a predetermined level. A good example of an unloading valve application would be a High-Low system. A High-Low system may consist of two pumps; one high volume pump, the other a low volume pump. The system is designed to give a rapid approach or return on the work cylinder. The total volume of both pumps is delivered to the work cylinder until the load is contacted.
At this point the system pressure increases, causing the unloading valve to open. The flow from the large volume pump is directed back to the tank at a minimal pressure. The small volume pump continues to deliver flow for the higher-pressure requirement of the work cycle. Both pumps join again for rapid return of the cylinder. This application allows less input horsepower for speed and force requirements
The purpose of a counterbalance valve is to prevent a loaded cylinder, having potential energy, from falling (extending or retracting). Counterbalance valves may be internally piloted, externally piloted, or piloted internally as well as externally, and they may be internally or externally drained. If conditions exist that would interfere with internal draining the valve, it should be externally drained, but usually this is not necessary. Counterbalance valves are equipped with a free reverse flow check valve to allow for retraction of the cylinder.
The simplest counterbalance valve application is to support a constant induced load. In a down acting press application, the counterbalance valve would be installed at the rod end of the cylinder to control return oil flow. This would prevent the press platen from dropping. Pilot pressure to open an internally piloted counterbalance valve would be set approximately 100 psi above the pressure of the rod end of the cylinder caused by the weight of the platen. In order for the platen to be lowered (and powered down), the pressure at the cap end of the cylinder would have to be sufficient to generate 100 additional psi at the rod end of the cylinder. Thus, 100 psi added to the pressure generated by the weight of the platen would open the counterbalance valve and allow the platen to lower smoothly.
Fig. 1-13. Counterbalance Valve in a Press Circuit.
One disadvantage of the counterbalance valve shown in the circuit in Figure 1-13 is that back pressure on the cap side of the cylinder limits the effective force developed by the cylinder. In order to achieve full force from the cylinder, the back pressure must be relieved from the cap side of the cylinder.
This is easily achieved by using a counterbalance valve that includes an external pilot. Counterbalance valves that include an external pilot in addition to the internal pilot are called holding valves, over center valves, load control valves, or motion control valves by some manufacturers. After the cylinder has stalled against the load, the external pilot will fully open the counterbalance valve, allowing the pressure in the cap end of the cylinder to fall to virtually zero psi.
Counterbalance valves may prevent a loaded cylinder from falling. Pilot check valve circuits also hold loaded cylinders in place. Both types of circuits have unique applications. Counterbalance valves may be leak-free. For example, manufacturers commonly give the leakage rates across a counterbalance spool in drops per minute. If a cylinder must be locked in place with a valve that allows no leakage across the spool, the valve must be designed to do so.
Counterbalance valves may also incorporate external piloting for smoother, “non hunting” performance. When the manufacturer utilizes both internal and external pilots you have the best of both worlds. The internal pilot lowers the load with counter pressure, while the external pilot drops all back pressure when performing work.
A counterbalance valve is a normally closed pressure valve used with cylinders to counter a weight or potentially overrunning load. In this circuit, without a counterbalance valve the load would fall uncontrolled or overrun and pump flow would not be able to keep up. To avoid the uncontrolled operation, we place a counterbalance valve just after the cylinder.
The pressure setting of the counterbalance valve is set slightly above the load- induced pressure of 1100 psi. This counters the load. As we extend the cylinder, pressure must slightly rise to drive the load down.
Pressure Relief Valves
Relief valves are normally closed valves which sense pressure upstream of the pressure relief valve. When the pressure reaches the setting of the valve, the valve opens to relieve the over pressure fluid to the reservoir. Figure 1-9a shows a direct acting, or single stage, relief valve.
Fig. 1-9. (a) Direct Acting Pressure Relief,
(b) Pilot Operated Pressure Relief (Simple), and (c) Pilot Operated Pressure Relief (Detailed),
(d) Pressure Reducing, (e) Pressure Reducing Relieving, (f) Unloading Relief, valve symbols
The dashed pilot line connected to the valve envelope at the point at which the inlet line meets the envelope indicates the pilot pressure in sensed internally to the body of the valve. The spring chamber in relief valves are internally drained to the outlet, or secondary port, though that feature is not shown by current ISO 1219-1 symbols. Back pressure in the outlet line of a relief valve acts on the spring side of the poppet or spool, and thus is additive to the pressure setting of the valve. What this means is that if the tank line back pressure increases by 100 psi, the valve will open at 100 psi more than it was set to open, though the differential pressure across the valve does not change.
Figure 1-9b shows the simplified symbol for a pilot operated, or two stage, relief valve, while Figure 1-9c shows the detailed symbol for a pilot operated relief valve. Pilot operated relief valves may be remote piloted, sometimes from the operator’s station. The detailed symbol shown includes a vent port connection allowing a second direct acting relief valve to be connected to this port, thus allowing remote control of the main relief valve.
In addition to a remote pilot relief valve, or as an alternative, a solenoid valve may be connected to the main relief valve in order to vent the main relief valve down to low pressure.
Circuits using fixed displacement pumps must have a pressure relief valve. Not all variable volume pumps are pressure compensated. Therefore, these pumps also require relief valves. Many pressure-compensated pumps have compensators that can fail in an “on stroke” condition, therefore requiring a relief valve as well. The main relief valve in a circuit is generally termed the system relief valve. However, relief valves are also used in branch circuits in order to protect an actuator. These circuit relief valves are usually called cross port relief valves when used with a motor and cylinder relief valves when used to protect a cylinder.
Pressure Control Valves
Pressure relief, pressure reducing, unloading, sequence, counterbalance, and brake valves control the pressure in systems. One of these valves can serve multiple purposes, depending upon where it is located in the circuit, how it is plumbed, how the pilot circuit operates, and whether or not the valve drains internally into the reservoir return line or has an external drain.
Pressure Reducing Valves
The pressure reducing valve schematic shown in Figure 1-9d (on page 39 above), shows normally open valves used to limit the maximum force of actuators in branch circuits. Pressure reducing valves control the force by sensing the pressure at the secondary (outlet) port of the valve. When downstream pressure reaches the pressure setting of the valve, the spool begins to meter flow into the circuit, limiting the downstream pressure to the pressure setting of the valve. Since pressure is defined as resistance to flow, pressure can be controlled by regulating the flow into the circuit. In a typical application, the pressure reducing valve comes into operation when a cylinder in a branch circuit deadheads against the load resistance. The pressure then rises to the pressure setting of the reducing valve.
By controlling the downstream pressure, the valve limits the maximum output force of the actuator in the branch circuit. Because pressure reducing valves sense pressure at the outlet port, they are externally drained. Obstructing the drain of a pressure reducing valve will prevent the valve from operating from the normally open to the closed position.
Figure 1-9e (on page 39) shows the symbol for a pressure reducing-relieving valve, which in addition to reducing downstream pressure, will relieve downstream pressure.
Unloading valves are used with high-low pump circuits and with accumulator circuits to save power when fixed displacement pumps are used. Some manufacturers market an unloading-relief valve which, in addition to the external pilot that is connected downstream of the check valve, includes an internal pilot connection. This version is shown by the symbol in Figure 1-9f above. The main characteristic of an unloading valve is the external pilot line that allows the valve to sense pressure downstream of the check valve used in applications for unloading valves. Several manufacturers offer the unloading valve and check valve in the same body assembly.
In addition, an unloading valve is a remotely piloted, normally closed pressure control valve that directs flow to the tank when pressure at that location reaches a predetermined level. A good example of an unloading valve application would be a High-Low system. A High-Low system may consist of two pumps; one high volume pump, the other a low volume pump. The system is designed to give a rapid approach or return on the work cylinder. The total volume of both pumps is delivered to the work cylinder until the load is contacted.
In a typical accumulator application, shown in Figure 1-10 below, hydraulic oil from the fixed displacement pump will pass through an isolating check valve to fill the accumulator. This type of circuit uses a 3-position directional control valve that has a blocked pressure port in the center envelope. When the accumulator becomes filled, pressure on the accumulator side of the check valve pilots the unloading valve open, unloading the pump to the reservoir at low pressure.
The unloading valve will remain open as long as the accumulator can supply pilot pressure above the setting of the valve. When the pressure downstream of the check valve drops below the pressure setting of the unloading valve, the unloading valve closes, allowing the pump to refill the accumulator.
Fig. 1-10. An Unloading Relief Valve.
The unloading valve shown in Figure 1-10 is a variation on a standard unloading valve as it includes an internal as well as an external pilot, making the valve illustrated an unloading-relief valve. The valve will open upon sensing adequate pilot pressure from either pilot source. There are differential pressure unloading valves which are specifically used in accumulator circuits to open at a higher pressure than they close.
Unloading valves are normally closed, externally piloted, and may be internally or externally drained. An external drain is required if there is back pressure at the outlet port, for example if the fluid is unloaded through a heat exchanger or circuit that creates back pressure that would upset the pressure differential of the valve. An unloading valve has a low pressure drop across the valve when it is in the open state. The valve is held fully open by the pilot signal to unload the pump at low pressure.
Figure 1-11 shows a typical high-low pump circuit. The unloading valve is actuated by rising pressure downstream from the check valve, unloading the high-volume pump at low pressure. When an unloading valve is piloted open by the external pilot, there is a low pressure drop across the valve, as it is being held open by the pilot pressure. If an unloading valve is subject to back pressure, it should be externally drained.
Fig. 1-11. An Unloading Valve in a High-Low Pump Circuit.
Sequence valves shown as S1 and S2 in Figure 1-12 below,
They are used on clamp and work circuits to assure required clamping force is reached in the clamp cylinder before the work portion of the cycle begins. Sequence valves may be internally or externally pilot operated, but they must have an external drain because the outlet port is pressurized. Sequence valves may be equipped with integral reverse free-flow check valves. Sequence valves are normally closed and are pilot operated to open to allow full flow to the actuator. In a typical application, fluid is directed to extend both the clamp and drill cylinders at the same time.
The sequence valve is installed in series with the drill cylinder. The clamp cylinder receives fluid first, with its minimum force determined by the pressure required to open the sequence valve at the drill cylinder, and the area of the clamp cylinder. When the minimum clamping cylinder pressure is reached, the sequence valve opens, and the drill cylinder will advance. The maximum extension force of both cylinders is determined by the pressure setting of the system relief valve, the areas of the cylinders, or by pressure reducing valves, if any are used.
When the directional control valve is reversed to retract the cylinders, some means must be employed to prevent both cylinders from retracting at the same time. This would cause the clamp to relax while the drill was still in the work piece. The proper sequence would be first to retract the drill, and then to retract the clamp.
One method to accomplish the reverse sequence would be to install a second sequence valve at the rod end of the clamp cylinder. This would route flow first to the rod end of the drill cylinder, causing it to retract, followed by the opening of the sequence valve when the pressure rises, allowing flow to the rod side of the clamp cylinder. Both cylinders will now operate in the proper sequence. It should be noticed that the clamp cylinder loses pressure to hold the clamp closed when the directional control valve is shifted to retract both cylinders.
A sequence valve is essentially an externally drained relief valve. As such, it may be used as a relief valve in applications where the back pressure that acts on the tank port of the relief valve varies, causing changes in the opening pressure of the relief valve.