Pressure Control Valves

Fluids that are under pressure are not safe or useful unless they are controlled. The focus of this unit is to explain how and where pressure control valves are used. It should be noted that the location of the valve is critical in determining its function as related in a schematic diagram.

Basic Valve Symbol

The above symbol is representative of both hydraulic and pneumatic pressure controls. The system power source determines the fluid nature of the valve. All pressure controls are interpreted the same way. These valves have two ports, one an inlet and the other an outlet, as indicated by the directional arrow. The “dashed” line represents a pressure sensing capability.

In

Out

Hydraulic and Pneumatic Pressure Control Valves

Pictured above are two pressure controls most often discussed in fluid power. On the left is a relief valve used in hydraulics systems and on the left, a pressure regulator used in pneumatics. Their schematic symbols are similar .

Hydraulic and Pneumatic Pressure Control Valves

At a glance the two valve symbols above look the same but they are very different. The one on the left is normally not passing and the one on the right normally passing. Also, the sense lines are located in different ports. You can see that failure to identify these small differences can cause confusion.

Pressure Adjustment

In general pressure control valves that can be adjusted have the arrow through the spring diagonally as shown but if the arrow is not present then the valve is considered not adjustable. Usually, not adjustable valves have a lock nut or some other such device to prevent hand adjustment.

Uses of a Normally Closed (not

passing)Pressure Control Valve

In the circuit above, the pressure control ensures that fluid reaches the actuator at maximum pump flow until a resistance is encountered that exceeds the tension of the spring. When the tension of the spring, due to pressure build up, has been reached, oil will move over the relief valve back to tank. System pressure is equal to the tension of the spring. This is a relief valve function.

Sequence Valve

In the circuit pictured above, a second pressure control valve appears in series between the DCV and one of the actuators. This arrangement ensures that the drill cylinder cannot move until the clamp has fully extended.

Counterbalance Valve

In the circuit above, gravity acts on the platen to generate a pressure on the rod side of the cylinder. If there was nothing to resist the movement of oil from the rod side, the platen could speed the piston away from incoming oil and slam into the work surface below. By using the incompressible nature of oil we can slow a heavy platen by the use of back pressure.

Counterbalance Valve

Back pressure can also be used to slow a motor whose shaft is being driven by the inertia of a flywheel. The back pressure generated is relative to spring tension as is the stopping speed.

Pressure Reducing Valve

In this circuit the clamp cylinder must hold a part that could be damaged by full system pressure. The pressure reducing valve senses pressure downstream and closes when the limit of the spring is reached. Fluid must be able to enter a volume in order for pressure to increase so by cutting off flow to the volume, a cylinder in this case, the pressure and clamping force are controlled.

Drains

Drains are only found in hydraulic pressure control valves. Since hydraulic pressure control valves use an internal member that allows some bypass leakage, a way of releasing that leakage oil is needed. As the diagrams above indicate, there exists an area above the spool that could fill with oil and “lock” up the valve. To prevent the lock up we provide one of two ways for leakage oil to escape the valve. If the valve does not have pressure on the secondary port, it may be drained internally to the secondary port. In the case of all pressure control valves whose secondary port is under pressure, the valve must be drained externally through an additional port. Pressure reducing valves and sequence valves are always externally drained.

Direct and Remote Operation

A valve that senses pressure only from its inlet or out port is said to be directly operated as opposed to one that senses pressure from another location which would be called remotely operated. Usually, remote operated valves are referred to as unloading valves.

Unloading Valve

Unloading valves are used in hydraulic circuits to momentarily “dump” the flow of the pump back to tank during periods of machine idle time. This reduces heat and saves energy since the load on the prime mover is reduced.

Hi-Lo System

The purpose of the Hi-Lo system is to provide momentary high flow for traverse action but then reduce it for feed rate. A ram, for example, with a long stroke could be moved at a high rate of speed to approach a work piece and then slowed to a crawl before contact thus increasing the cycle time of the machine. Both pumps are used for high flow and the large one “dumped” for low flow.

Note position of unloading valve.

Remotely Operated Counterbalance Valve

In a previous slide a counterbalance valve was shown as directly operated. In actual practice a counterbalance valve might be remote operated as above. The reason for remote operation is reduce lost energy as a result of back pressure.

Brake Valve

A real brake valve is more complicated than a simple pressure control. It has two sense lines that compare inlet and outlet pressures. It is assumed by design that when a pressure is present at the inlet port that the motor should turn. The pressure at the inlet causes a “piston” to move against a spool and to hold the valve open for free movement of the motor. The moment the pressure drops at the inlet port the piston allows the spool with its spring to take over and begin to apply back pressure to slow down the motor shaft and its load.

As you may have noticed, the sequence valve, like many others, can only sense pressure from one port. For fluid to get around the valve on return, a check valve is added. The check valve allows normal operation to take place as fluid is moving toward the cap end of the actuator. However, when fluid is leaving the cap end the cylinder is in retraction and the function of the sequence valve is not needed. Without the check valve, fluid would not be able to get around the valve and the cylinder would lock up.

Reverse Flow

Pressure Control Valves in General

1. If a pressure control valve has pressure on its secondary port, it must be externally drained.

2. If the secondary port of a pressure control valve is not under pressure, it will usually be internally drained.

3. Check valves are used to bypass flow around all pressure control valves.

Completed Valve Symbols

In the following slides, schematic valve symbols appear in their completed form. Their simple form was used to introduce them in a non-intimidating manner. In the industry, this is how they really appear. Study each in detail.

Relief Valve

Unloading Valve

Sequence Valve

Counterbalance Valve

Brake Valve

Pressure Reducing Valve

Pressure Regulator

Pressure regulators are normally found at the beginning of branch line circuits. Typically, the pressures required by branch line circuits are much lower than the air pressure at the receiver tank. A venting type regulator is one that can exhaust air from its secondary port if pressure exceeds the spring setting.

Two Stage Pressure Control Valve Operation

At first glance the diagram would appear to show three different valves but the box surrounding the two schematic symbols is a component enclosure meaning that there is only one component and it includes everything seen.

Direct vs. Pilot Operated

A direct acting pressure control valve is one whose sealing member is forced to open solely by the pressure of fluid. Direct acting pressure control valves are common in hydraulics as a relief valve.

Pilot operated pressure control valves are actually two valves made into one. Sometimes called a “two stage” pressure control, pilot operated pressure control valves have a primary and secondary stage. When the primary stage activates

Override-Direct Operated Pressure Control Valves

Override is a characteristic of a valve where the valve will momentarily create a pressure beyond its setting. This is caused by the compression of a spring. High override can damage sensitive components as well as waste energy.

Override-Pilot Operated Pressure Control Valves

Pilot operated valves don’t have as much override because they use a light spring and fluid pressure to hold the valve in its normal state. The moment the fluid is released, the valve opens very fast because the light spring has little resistance.

Direct Acting Pressure Control

In the illustration above, a only spring tension holds the ball against the seat. The adjustment knob, shown at the bottom, is for changing tension on the spring.

Pilot Operated Pressure Control

In the illustration above both fluid and spring force hold the main poppet(main stage) closed. After the pilot relief poppet opens from system pressure, oil begins to vent from the spring side creating a pressure differential and reducing the total force holding down on the main poppet. When the pressure differential is great enough the main poppet opens quickly. It is this rapid opening capability that makes this design more efficient than the direct acting type.

Pilot Operated Pressure Control Valve Operation

Initially at start up the hydraulic system would be at low pressure, for a couple of seconds, and the relief valve shown above would be closed. The orifice ensures that system pressure is “felt” on both sides of the spool. Study the illustration to become familiar with the individual components.

Pilot Operated Pressure Control Valve Operation

In the illustration, the port subjected to 1000psi is the inlet port and is directed attached to the system. The port down below is connected to tank.

Pilot Operated Pressure Control Valve Operation

Since the dart is biased by a stiff spring, it will take more pressure to overcome it than the spring of the main spool which is light by comparison. The action of the dart opening starts the venting process by creating a pressure differential across the main spool.

Pilot Operated Pressure Control Valve Operation

Once the dart has begun to open, it will take an additional pressure build up of 25psi before the main spool opens fully. It should be noted that the main spool “throttles” in a relief valve. Main spool positioning is based on the demand for flow.

Pilot Operated Pressure Control Valve Operation

While venting, the dart is completely unseated and passing oil at high pressure. With the dart open, a pressure differential exists because the oil leaving the area above the spool is greater than the oil that can flow in through the tiny orifice. When pressure drops the dart closes and oil pressure builds on the spool, closing it.

Sequence Valve

Simply put, the purpose of a sequence valve is to cause a series of operations to occur in order. This normally not passing valve blocks oil to the “secondary” actuator until a preset limit of pressure is reached. When pressure is correct, the sequence valve opens and allows flow to reach the secondary actuator for movement. Unlike the relief valve, the sequence valve will open completely. Usually, a bypass check valve is included for reverse flow.

Counterbalance or Unloading Valve

This valve is remotely operated. Remember that the valve is watching the sense line for pressure, not its inlet, so we can control a tremendous load but allow it to move at relatively low pressure by use of the remote pilot.

Pressure Reducing Valve

The pressure reducing valve is the only normally passing pressure control used in hydraulics. Just like the pneumatic pressure regulator, the reducing valve controls pressure in hydraulic branch line circuits. Recall that we can control the clamping force of a cylinder with this valve.

Methods of Remote Control

Any time the oil on top of the main spool is vented, the main spool will open. All methods of remote control work on this principle.

Remotely Operated Pressure Control Valve

In the example above, the remote pilot valve serves to control the relief valve by venting the oil on top of its main spool. The remote pilot might be located some distance from the relief valve, maybe on a control panel. The other dart on the relief valve could be used for maximum pressure control of the valve.

Remotely Control of Relief Valve

Schematic Representation

There is no component enclosure symbol because these two valves are not physically together.

Alternative Remote Control

When examine the above illustration, please remember that any time the oil is released from above the main spool, this type of valve will actuate. You could even use two way hand valve to do the job.

Differential Unloading Relief Valve

This valve, unlike the others, gives us a “cut in” and “cut out” range for unloading a pump. This valve is usually used with an accumulator so that the pump may be unloaded during periods where the system is being powered by the stored volume of the accumulator.

Differential Unloading Relief ValveIn a Circuit

Notice the two sense lines. This valve is watching the pressure at the pump outlet as well as the accumulator. The check valve isolates the system from the pump during the unloaded periods.

Pilot Controlled Pressure Regulator

Some of the control methods used in hydraulics are also used in pneumatics. Here, pilot pressure, in this case air pressure, is used to push the piston down while pressure from the secondary port pushes up on the piston. Whenever the air on top is exhausted, the valve will close.

Review Part One1. What is override?

2. What is a “two stage” valve?

3. What is pilot pressure?

4. What is the sequence of operation within the pilot operated valve?

5. Which of the two stage valves opens first?

6. As above, which of the two flows the most?

7. What is the function of the control orifice?

8. What is remote operation?

9. Which of the two valves is “direct acting”?

10. What is the normal state of a pressure regulator.

11. What is the name given to the hydraulic pressure control valve that is normally passing?

12. What is the purpose of the pressure regulator?

13. From which port does the regulator sense pressure?

14. What does it mean if a regulator is venting air and it has not been adjusted?

15. Explain some of the differences between the relief valve and the pressure regulator.

Review Part Two16. What acts on the internal member of pressure control valves to help them achieve a normal condition?

17 Give at least two different functions for normally closed pressure control valves?

18. What determines the name given to a pressure control valve?

19. What is the normal state of a relief valve and to what is it’s outlet port connected?

20. What is the function of an un-loading valve?

21. Give the function of a sequence valve.

22. Give the name of a normally not passing pressure control valve used in hydraulics to counteract or to balance a weight attached to a load?

23. Describe what “internal” pilot pressure refers to in a counterbalance valve?

24. Describe the function of a brake valve.

25. Give the name of the only normally passing pressure control valve used in hydraulics.

26. Explain why a pressure control valve would need an external drain.

27. What would be the draining arrangement for a pressure control valve whose secondary port is connected to tank?

28. What is the function of a check valve in relation to a pressure control valve?

29. What is the normal state of the pneumatic pressure regulator?

30. What does the term “vented” refer to in regard the pressure regulator?