CHAPTER 14: Sequence Valves and Reduction Valves (2023)

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Pressure regulators (except relief and relief valves)

There are some parts of fluid power circuits that require pressure control. (Chapter 9clogged relief valves and relief valves that control pressure in pump circuits). Other types of pressure controls include sequence valves, balancing valves and reducing valves. Although the inner workings (and the symbols) are similar, these three pressure regulators perform completely different functions. Sequence valves and counterbalance valves are normally closed, like dump valves and dump valves, but generally allow two-way flow, thus requiring a bypass check valve in their body. Sequence valves always have an external drain that connects directly to the tank. Equalizing valves are internally drained, except when used in some regeneration circuits.

Reducing valves are normally open and respond to outlet pressure to prevent outlet flow from rising above set pressure. It may also have a bypass check valve. Reducing valves always have an external drain that connects directly to the tank. Any back pressure in this drain line will increase the valve spring adjustment.

Relief valves, discharge valves, sequence valves, equalizing valves, and reducing valves are the most difficult to identify on a schematic drawing because their symbols are so similar. Take great care when diagnosing a problem to ensure these valves are correctly identified and their function is understood.

sequential valves

There are times when two or more actuators working in a parallel circuit need to move in sequence. This is only possible with separate directional control valves and limit switches or limit valves. This function ensures that the first actuator has reached a certain position before starting the next operation. If there is no safety concern or potential for product damage if the first actuator does not cycle before the second one begins, a sequence valve can be an easy way to control the actions of the actuators.

The symbols and cuts inFigure 14-1They are for hydraulic and pneumatic sequence valves. The main difference between these valves is that most hydraulic sequence valves are single use and must be used in series with a directional control valve, whereas many air sequence valves are spring-adjustable pilot operated directional control valves. return. In either case, a preset pressure must be reached before the valves allow fluid to pass or change flow paths. Many manufacturers offer a direct acting, internally piloted hydraulic sequence valve such as the design shown in Fig.Figure 14-1. This valve can be converted to external pilot control on site if required.

Figure 14-1. Hydraulic and pneumatic sequence valves

Several manufacturers also offer pilot-operated sequence valves. Pilot operated sequence valves remain closed at 50 psi or less than the set pressure. Direct acting sequence valves can partially open at pressures 100 to 150 psi below the set pressure, allowing premature actuator travel.

A balanced spool, supported by an adjustable force spring, blocks fluid at the inlet of the hydraulic sequence valve. When the inlet pressure reaches the spring setting, the pressure in the internal control line pushes the spool up to allow sufficient flow to the outlet to prevent pressure buildup. The inlet pressure will never drop below the set pressure when there is flow to the outlet. When the outlet pressure exceeds the set pressure, the valve opens fully and the pressure in both ports equalizes. Note that the drain port connected to the tank can be pressureless or constant, as any pressure in that line will contribute to spring adjustment. (Remember that a sequence valve must always have an external drain.)

A bypass check valve allows reverse flow when the valve is used in a two-way flow line. In some applications, a sequence valve may be piloted externally by another process. Most valves can be rebuilt in the field. (The designer should always change the part number to reflect the conversion.)

Pneumatic sequence valves are usually 5-way valves with adjustable springs to adjust their switching pressure. They are used to start a second operation after the previous one is completed. Some older machines have a solenoid valve to start the cycle and multiple sequence valves to extend and retract all other actuators. Some cares:
• A sequence valve ignites a pressure buildup and can prematurely initiate a second operation if an actuator sticks or stalls for any reason. If personal injury or product damage could result from incomplete displacement, do not use sequence valves. Instead, use limit switches or limit valves and directional control valves for each workflow.
• If flow controls are required, they must be meter-in. Route the downstream line signal from the flow controller to the sequence valve, as the pressure at this point is as great as is required to move the actuator and its load.

the circuit inFigure 14-2is typical of air-powered machines.Zyl. 1extends to clamp a part when an electrical input signal moves the pilot operated solenoid valve. AsZyl. 1expands, the pressure beyond the gauge port at the end of the cap becomes as high as necessary to move the cylinder and its charge. When the sequence valve is set to 70 psi,Zyl. 2must not move untilZyl. 1the piece has been extended and is securely fastened. If for some reason the clamp does not give a full stroke, theZyl. 2Extending early will not damage the part or be unsafe. If the cuff is 70 psi or greater, the sequence valve will switch to extendZyl. 2.Both cylinders can be easily retracted at the same time.

Figure 14-2. Typical circuit of a pneumatic sequence valve

A great feature of a sequential shift is that it doesn't matter how far the first cylinder must move before the next operation takes place. Thick or thin parts are clamped with the same force before starting the next operation, as the pressure must reach the same level to activate the next sequence.

Zyl. 2Features flow regulators to slow movement and maintain pressureZyl. 1during the stamping process. By disconnecting the pilot solenoid valve, both cylinders can return to home position at the same time.

The hydraulic sequential circuit inFigure 14-3is typical of a machine that needs to arm and hold the print while a second operation is performed.follow valve 1set to 550 psi; cuff pressureZyl. 1must be at least 550 psi before drillingZyl. 2can spread on hitZyl. 2is extended, the pressure in the circuit never drops below 550 psi. If the punching operation requires more than 550 psi, pressure will build throughout the circuit until the relief valve is adjusted.

Figure 14-3. Typical Hydraulic Sequence Valve Circuit 2 Sequence Valve(set at 450 psi) maintainsZyl. 1to receive a withdrawal signal forZyl. 2He's back and the pressure is on.

A pilot operated check valve maintains clamping force as the punch barrel retracts. The signal to open the pilot operated check valve comes from the line betweenfollow valve 2YZyl. 1,so there is no signZyl. 2fully retract. (This circuit is unsafe if pressure buildup is from a source other than clamp contact or end of stroke, causing the punch cylinder to operate prematurely.)

Figure 14-4. kickdown sequence valve

Sequence valves often generate a lot of heat because the first actuator to move requires more pressure than subsequent actuators. This means that there is often a large pressure drop across a sequence valve, resulting in wasted energy. In some circuits, a kickdown sequence valve can reduce energy loss. The section view and the symbol inFigure 14-4Show the inner workings of a kickdown sequence valve to explain how it controls opening pressure and then releases it.

Inlet fluid flows through the control orifice and up the adjustable cone where it is blocked. The resulting pressure tries to open the seat, while equal pressure and a slight spring acting on the opposite side keeps it closed. If the pressure builds up enough to loosen the adjustable plug and there is more flow through the plug than the control orifice, the pressure imbalance will cause the plug to rise. When the seat moves enough to allow trapped fluid to flow through the bypass port, the seat pressure drops because the bypass port is larger than the control port. At this point, the only force acting to keep the plug closed is the spring force and back pressure on the outlet port. If delivery stops, the cone closes again through pressure compensation and spring force on the cone.

Figure 14-5. Hydraulic circuit with sequential kickdown valves

the circuit inFigure 14-5is the same as 14-3, except it includes kickdown sequence valves instead of standard sequence valves.Zyl. 2will not advance in this circuit until there is pressureZyl. 1reached 750 psi. The difference is that when a kick-down sequence valve is opened at its pressure setting, it allows fluid at 50 psi plus whatever is needed to overcome downstream resistance. This means that the entire pump circuit for all actuators is above 50 psi.Zyl. 2Resistance. Pilot operated check valve inZyl. 1Cap end connector keeps it at near full power while under pressureZyl. 2expands with little force. Energy waste is very low, so heat generation is minimal. (More sequence valve circuits can be found in the eBookFluid energy circuits explainedby the author of this guide, which will be published in the coming months).

balancing valves

The fourth and final normally closed pressure control valve in hydraulic circuits is the compensation valve. Cylinders with external forces such as B. Weight of a sinker, machine parts or tools: working against them will overflow when timed unless their oil flow is restricted. A current loop with a meter output is one way to control overload loads, but it has a major drawback. The speed of a flow controller is fixed except when manually adjusted or when an infinitely variable proportional type is used. Since the flow is constant, the actuator will continue at the same speed even if the work flow increases or decreases. Therefore, the control is minimal and a great waste of energy can occur. (Figure 13-8shows a flow control loop for uncontrolled loads).

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A counterbalance valve prevents an actuator from slipping regardless of flow changes because it responds to pressure signals, not flow. A counterweight valve is almost the same as a sequence valve, except that it normally does not have an external drain port. The clippings and symbols inFigure 14-6Show the physical structure of three different overcenter valves and how they are represented on a schematic drawing.

The two cutouts and icons on the left are coil designs with inner and outer pilots. The valve on the right is a poppet valve with an internal and external pilot. Each valve type has advantages in different circuit arrangements which will be discussed later. A counterweight valve usually has a bypass check valve for reverse flow as it is most commonly used to control actuators with trailed or towed loads.

Figure 14-6. Three types of counterbalance valves

An internal pilot operated counterbalance valve switches to allow excess fluid to flow to the outlet when the inlet pressure increases to the pressure set by the pressure set. The inlet pressure will never drop below the set pressure when there is flow at the outlet. The flow from the inlet to the outlet is sufficient so that the back pressure in the actuator never drops below the set pressure. This means that the actuator will only move as fast as it is powered and will stop when the input flow stops.

Pressure adjustment on an internally piloted overcenter valve is normally accomplished by first screwing the pressure adjustment all the way in. To ensure that the valve is able to generate high enough pressure, turn on the pump and increase the load a little. Next, center the directional valve, which connects the fitting on the piston rod end of the cylinder to the tank, to see if it holds. If the load will hold, lift the load in increments, checking every few inches to see if the load will hold. With the load suspended, begin to slowly reduce the pressure set on the balancing valve until the load slides forward. If load starts to drop slowly, increase pressure until movement stops, then increase pressure setting another 1/4 to 1/2 turn. This tuning method generally uses less energy, always stopping and holding the load.

The main disadvantage of an internal pilot operated counterbalance valve is that the back pressure is constant and retained even when the actuator requires maximum force. Another disadvantage is that an internally piloted balance valve must be reset each time the load changes to maintain optimal performance. The main benefit of the valve is that it creates smooth cylinder movement as you proceed to work.

An external pilot operated check valve switches to allow excess material to flow when the pressure in the opposite port of the cylinder reaches the pressure set by the pressure set. The inlet pressure will never drop below the induced load pressure plus the pressure set in the pressure setting when outlet flow is present. The flow from the inlet to the outlet is sufficient for the actuator to move as fast as it is powered and stop when the flow to the actuator stops.

The pressure adjustment on the external pilot operated brake valve can be made on a dynamometer by adjusting the pressure setting between 100 and 200 psi. If the machine pressure needs to be adjusted, set the pressure setting above 200 psi and increase the load a little to ensure it stops and holds. If you can handle it, continue to lift the load enough to give yourself some time for the next step. Now turn off the load and observe the pump pressure. Pump pressure when lowering the load must not exceed 200 psi. Continue this action until the pump pressure is between 100 and 200 psi as the head drops. This tuning method generally wastes less power as charging always stops and holds.

The main disadvantage of an externally piloted counterbalance valve is that it can cause failure or even stop cylinder movement when moving to work. The main benefit is that back pressure is only present when the actuator advances towards the workpiece. On the normally open contact, pressure at the actuator inlet increases, forcing the brake valve fully open, removing all back pressure. Another advantage is that it is not necessary to readjust an external pilot operated down brake valve when the load changes.

The internally and externally piloted overcenter valves will alternate when the pressure in the internal pilot section reaches the pressure set in the pressure setting, allowing excess flow to the outlet. The inlet pressure will never drop below the set pressure when there is flow at the outlet. The flow from the inlet to the outlet is sufficient so that the back pressure in the actuator never drops below the set pressure. This means that the actuator will only move as fast as it is powered and will stop when the input flow stops.

Pressure adjustment on an inner and outer pilot operated overcenter valve is normally accomplished by first screwing the pressure adjustment all the way in. To ensure that the valve is able to generate high enough pressure, turn on the pump and increase the load a little. Next, center the directional valve whose rod end of the cylinder connects to the tank to see if it holds. If the load will hold, lift it gradually; check every few inches to see if the load is holding. With the load suspended, begin to slowly reduce the set pressure until the load slides forward. If load starts to drop slowly, increase pressure until movement stops, then increase pressure setting another 1/4 to 1/2 turn. This tuning method generally wastes less power as charging always stops and holds.

An inner and outer pilot operated counterbalance valve smoothly reduces loads and opens fully as pressure builds at the actuator inlet upon contact with the workpiece. The valve must be readjusted when the load changes, but that's a small price to pay for good control.

Figure 14-7It shows a vertically oriented cylinder with the rod pointing down and a load trying to extend it. To prevent cylinder overflow, the brake valve must be able to withstand the weight pressure related to the load. Pressure due to load can be calculated and brake valve preset 100-150 psi higher on a dynamometer, but pressure adjustment is usually done on the machine (as mentioned above).

Figure 14-7. Internally piloted brake valve switching

Note that the directional control valve has portsAYBconnected to the tank in medium state. A build-up of additional pressure in the control line is excluded when the circuit is at rest. if the doorsAoBblocked, pressure can build up and open the relief valve, allowing the cylinder to be bypassed.

excite magnetic coilA1directs flow from the pump to the end of the head. If the pressure increases there, the pressure at the head of the gasket also increases. When the rod end pressure reaches 100 to 150 psi above the induced loading pressure, the cylinder begins to extend as fast as the pump fills the cap end. When the flow increases, the cylinder speed increases and when the flow decreases, the cylinder speed decreases.

As noted in the explanation of the counterweight valve, there is back pressure on the rod end of the cylinder throughout the entire extension stroke. As a result, the working cylinder contact force is reduced by the balance pressure multiplied by the cylinder rod end area. The total weight of a press and tools plus the amount of additional pressure on the equalizing valve cannot be used for the job. Energy is expended to lift the weight but is not recovered during the work cycle. excite the magnetic coilB1directs fluid around the counterbalance valve, through the bypass check valve and to the rod end of the cylinder for retraction.

Figure 14-8. Switching an externally controlled down brake valve

the circuit inFigure 14-8shows the same cylinder with an external pilot operated counterbalance valve. An externally piloted valve can be set to approximately 100 to 200 psi regardless of the charge pressure induced in the cylinder. This is particularly useful in applications with constantly changing loads. It's also the best use of energy, as the brake valve fully opens when the cylinder encounters resistance, allowing the weight to do some of the work. Since the back pressure at the end of the piston rod is zero, more power is available.

excite magnetic coilA1sends fluid to the end of the cylinder head to extend it. When pressure builds up at the end of the head, it pressurizes the outer pilot and opens the relief valve. The valve will only open enough to drain fluid when the bonnet end is under pilot pressure. If pilot pressure is too low, the relief valve can open too quickly, causing cylinder leakage and a drop in pilot pressure. At that point, the relief valve closes abruptly and the cylinder stops. Almost immediately, pressure builds up again at the end of the cylinder head, the relief valve opens again, and the same scenario repeats until the cylinder meets resistance. An input control on the external control line can help, but it is very difficult to adjust. excite the magnetic coilB1directs fluid around the counterbalance valve, through the bypass check valve and to the rod end of the cylinder for retraction.

Internal and external pilot operated counterbalance valve inFigure 14-9combines the best properties of both valves. The inner pilot provides a smooth, low-force feed, while the outer pilot fully opens the valve to remove back pressure from the rod end of the cylinder as it contacts the workpiece. (Like the internally piloted valve, this version must be reset with each load change to keep efficiency and energy losses low.)

Figure 14-9. Circuit with internally and externally piloted down brake valve

The symbols in these example circuits show a direct acting pressure control valve. Several vendors offer a pilot operated version which is more stable and has a smaller pressure differential between cracking and full flow operation.

The circuits shown here work just as well with hydraulic motors, except that a counterbalance valve will not stop and hold an uncontrolled load on an engine that is not idling. All hydraulic motors have internal leakage, which increases as the motor wears out. The relief valve may not have a bypass, but regardless of its design, fluid will flow through engine parts.

There are no equalizing valves for the air circuits. Air circuits rely on flow controls to prevent an actuator from blowing. Typically, an air circuit uses a 2-position valve that holds the retract side pressure at rest so it stays in place at the end of the stroke. When it is necessary to stop a charge mid-stroke, the usual way to try is a 3-position valve with cylinder ports blocked mid-stroke. A pilot operated check valve is also available for air operation, providing some control to stop and hold an air cylinder in mid-stroke.

Reglas de Luftleitung

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Most facility air systems generate pressures between 90 and 125 psi, while most air circuits are designed to operate between 75 and 85 psi. Other systems can operate at pressures as low as 15 to 20 psi. To meet these ranges, a method is needed to reduce system pressure without wasting energy. A relief valve that releases air from the system to atmosphere and tries to bring down the entire system is not a good solution. The airline regulator shown inFigure 14-10reduces outlet pressure by shutting off flow if outlet pressure attempts to rise above regulator setting. There is very little energy loss as the air simply expands from its higher pressure to meet the lower pressure demand. In other words, an air compressor running at 120 psi only needs to run about a third of the frequency when controlling or stepping down to 40 psi.

Figure 14-10. Air line regulators (or reducing valves)

This points to the main reason why an airline regulator needs to be set high enough to get the job done. Not only does it cost more to operate a machine without a governor, but the machine repeatedly tries to run with varying pressure and therefore varying forces and speeds.

Sectional views and symbols inFigure 14-10show two common direct acting airline regulators. They are typically available in sizes up to 2" with pipe threads. (Larger sizes are built but are usually pilot-actuated using a small direct-acting regulator.) Air flows freely from inlet to outlet until outlet reaches the pressure setting. The adjustable spring keeps the check valve off its seat, stretching the diaphragm when there is free flow. As pressure continues to build at the outlet, it travels through the pilot channel towards the bottom of the diaphragm . When pressure builds, the diaphragm pushes the adjustable spring back, allowing the shut-off valve to close. The light spring pushes the shut-off valve closed. The outlet pressure is now stable at its reduced setting, as long as the pressure inlet is equal to or greater than the outlet pressure Any drop in outlet pressure reduces the pressure below the diaphragm and the adjustable spring pushes the shut-off valve open again to allow inlet of more air.

If there is a possibility that the vacuum line is over-pressurized for any reason, use the relief regulator shown at right.Figure 14-10. This valve closes a hole through the center of the diaphragm with the isolation valve stem. Once the set pressure is reached, the stop valve cannot be opened. Any additional pressure buildup below the diaphragm lifts its center section off the isolation valve stem and allows air to leak through the vent to atmosphere. This feature should not be used as a relief valve feature if pressure builds up during each cycle; it is only for occasional overpressure situations.

Any pneumatically operated machine should have a regulator set to the lowest pressure that will produce good product. Costly overpressure must be eliminated in any case. Use an air regulator whenever the job can be performed at a pressure lower than the factory air supply.

Another application for overhead line regulators, which can save compressor energy, is to reduce the pressure on the return stroke of actuators, which may use a small amount of force to retract. Many cylinders require a lot of force to extend and do work, but the retraction portion of the cycle requires very little force. An airline regulator positioned as shown inFigure 14-11You can save air on most circuits for a portion of each cycle in many cylinder events.

Figure 14-11. Pneumatic switching with air saving regulator

A 5-way spool valve, connected to a dual pressure port as shown, can provide normal cycle time while conserving compressed air in the system. Back pressure is set in the regulator, which provides the lowest possible pressure to the rod end of the cylinder that maintains cycle integrity. A drop of just 20 psi below working pressure can compensate the regulator in no time. The cylinder is extended by moving the 5-way valve. (There will be a slight bounce as the low pressure air at the end of the rod is compressed to contain the higher pressure at the end of the cap.) To control the cycle time, adjust the cylinder speed with the flow control on the end of the rod. . When the 5-way valve resets the cylinder again, the end cap flow control must be adjusted to a faster rate as the return force is limited.

Pressure reducers and reducing/relief valves

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In hydraulic circuits with several actuators, it can happen that the system pressure is too high for some actuators, while others need maximum force. A suggested solution is the circuit inFigure 14-12.cylinder 1requires 2000 psi to maintain power whilecylinder 2Product can be damaged if pressure exceeds 800 psi. to addrelief valve 2(set at 800 psi) takes care of thatcylinder 2Positive pressure, but limits the entire circuit to 800 psi.The pressure in a circuit with more than one relief valve will never be greater than the lowest valve setting.The correct way to have two or more pressures in a single circuit is to install reducing valves. (Figure 14-14Diagrams of a circuit that uses a reducing valve to obtain two pressures).

Figure 14-12. Two-pressure hydraulic circuit with two relief valves

The cut and the symbol inFigure 14-13shows a pilot-operated reducing valve that allows flow from inlet to outlet at reduced pressure until the pressure reaches the setting of the direct-acting relief valve in the pilot section. Unlike the other four pressure regulators (relief, relief, sequence, and balancing valves), a reducer valve is normally open and blocks flow at the set pressure.

Figure 14-13. pilot operated reducing valve

The normally closed direct acting relief valve in the pilot section intercepts fluid from the reduced pressure outlet port through the pilot port on top of the spool when the pressure is below its set point. The spool remains in the normally open position because pressure at both ends hydraulically balances it while the light spring holds it down. Eventually, as the pressure at the reduced pressure outlet port continues to build, the direct acting relief valve in the pilot section begins to open. Then some liquid flows through the drain hole into the tank. When the flow through the direct acting relief valve is greater than the control orifice can handle, the pressure at the top of the spool will drop and the pressure at the bottom of the spool will close the spool. The spool is never fully closed because as long as the outlet pressure is less than the inlet pressure, there will be flow through the drain port. The flow at the drain port is approximately 60 to 90 inches.³/Minimum. This flow is wasted energy and can cause a system to overheat if more restrictor valves are installed than necessary. When the pressure drops below the setting of the direct acting relief valve in the pilot section, the valve closes, forcing the spool into the open position.

A restrictor valve is normally open, so apparently backflow shouldn't be a problem. However, when the valve is operating, it is nearly closed and can be held closed by reverse flow when the actuator begins to return.Whenever a restrictor valve needs to bypass backflow, choose a valve with a built-in bypass check valve to eliminate the possibility of blocked backflow.

It is also very important to have a free drain connection with very low (if any) back pressure. Back pressure at the discharge port helps adjust the direct acting relief valve and can cause erratic results if the discharge pressure fluctuates. (Our upcoming eBook,Explanation of technical fluid circuits,explains how this drain port can be used to advantage in a dual pressure circuit. This book will be released in the coming months.)

Figure 14-14. Two-pressure hydraulic circuit with relief valve

The modified circuit in Fig. 14-14 allows two pressures without decreasing system pressure (as inFigure 14-13). A pressure reducing valve instead ofrelief valve 2allows adjustment of pressure, e.g.cylinder 2without affecting the pressurecylinder 1This reducer valve never backflows, so a bypass check valve is not required.

When in service, a reducer valve is nearly closed, allowing very little backflow unless you have a bypass check valve. Even so, the return flow must have a higher pressure than the inlet. If such high pressure cannot be tolerated, use the reducer/relief valve shown inFigure 14-15.

Figure 14-15. Pilot operated reduce/discharge valves

Reducing/Relief valves work exactly like reducing valves until an external force starts to increase the pressure at the reduced pressure outlet above the pressure set by the pilot section. When the outlet pressure is 4-6% above the set pressure, the spool moves upward until the outlet connects to the tank. Any liquid with a pressure greater than the set pressure will flow back into the tank, so the outlet pressure will not increase further. The flow from the tank comes only from the reduced pressure outlet, not from the pump through the inlet. When the outlet gauge pressure drops, the reducer/relief valve continues to perform its reducing function.

Notice that the left section view has an internal flow to the pilot section. This avoids connecting a separate drain line for the control current. However, if the back pressure in the tank line is high or may fluctuate due to other feedback functions, the pilot section setting will increase and the pressure at the reduced pressure outlet may increase above allowable levels. If back pressure in the tank line is high or pressure fluctuations cannot be tolerated, use an external drain valve. If backflow is required, specify a model with a built-in bypass check valve for more convenient piping.

Figure 14-16. Modular sequencing, balancing and reduce/dump valves

Figure 14-16shows most modular valve configurations for sequential, balancer, and reducer valves. Modular valves simplify plumbing and eliminate many connections that can create back pressure or add potential leak points.