Always use a relief valve with fixed-displacement hydraulic pumps. Pressure-compensated pump circuits also may use a relief valve for certain applications.
You will get efficient and thoughtful service from Huade Hydraulic.
Think of a relief valve in a hydraulic system as a fuse or circuit breaker in an electric circuit. An electric circuit never blows a fuse unless it overloads. When an electric circuit overloads, it is inoperable until reset. Usually the person responsible for resetting the fuse looks for the reason it blew and fixes the problem before restarting the machine. Many hydraulic circuits allow the relief valve to dump some or all pump flow to tank all or part of the time. The extra power to produce that unused flow is expensive. Also, heat generation from excess flow requires larger heat exchangers that are expensive to buy and operate.
Protecting the pump and the system from excess pressure is the only valid function for a relief valve. At no time should the relief valve be used to pass excess pressure fluid to tank. When excess pump flow goes to tank, it generates heat. The relief valve in a well-designed hydraulic circuit never relieves oil to tank — unless there is a circuit or control malfunction.
Figures 18-2 and -3 show the simple and complete symbols for a pilot-operated (or compound ) relief valve. This type relief valve has two sections. The pilot operator on top is a small, poppet-type direct-acting relief valve. The main flow section of the valve is a poppet- or piston-type, normally closed 2-way valve. Through internal porting, a small direct-acting relief poppet controls a large poppet or piston. A pilot-operated relief valve responds more slowly, but does not even partially open until system pressure reaches approximately 95% of set pressure. Pilot-operated relief valves are suitable for remote operation, they open to unload pumps at pressures below 50 psi, and they act as large 2-way valves in some circuits.
Examples of relief-valve circuits
Always locate the relief valve as close as possible to the outlet of a fixed-displacement pump. A pilot-operated relief works best because it does not pass any fluid until system pressure is very near the valve’s set pressure.
Figure 18-5 shows a pressure-compensated pump with a direct-acting relief valve to protect it against overpressure. Pressure spikes often occur in pressure-compensated pump circuits with high flow or fast cycling. When the pump must compensate rapidly or often from full flow to no flow, the resulting overpressure drastically shortens pump life.
In Figure 18-5, the pump would be at low pressure and full flow when cylinder CYL3 extends rapidly. When the cylinder stops, fluid requirement is zero, but pump flow is still 40 gpm. As pressure builds, the pump finally starts compensating at about or psi. It is still producing 40 gpm — with no place for the oil to go. Without a relief valve in the circuit, system pressure spikes during each cycle can reach four to ten times the compensator setting. Pressure spikes damage the pump and piping after a few hours of operation. The faster the cycle, the more quickly shock damage from pressure spikes causes problems.
A relief valve, installed in Figure 18-5, reduces pressure spikes to protect the system. When the pump shifts to no flow, excess flow goes to tank through the relief valve. When the pump reaches compensator pressure, the relief valve closes. (For another and better way to reduce pressure spikes and protect a pressure-compensated pump from rapid cycling, see Chapter 1, Figures 17-19.)
Set the relief valve in a pressure-compensated pump circuit at 150 to 200 psi higher than the pump compensator. With relief pressure below compensator setting, pump flow goes to tank and makes heat. With relief pressure set at compensator pressure, the relief valve starts dumping when the pump starts compensating. When the relief valve passes fluid, the pump sees a pressure drop, and starts flowing again. The resultant pressure drop allows the relief valve to close and the dump/flow cycle starts again. After a few hours of this erratic operation, the pump fails.
Adding a solenoid valve to the vent port of a pilot-operated relief valve makes an effective unloading valve. Figure 18-6 shows a fixed-displacement pump supplying three cylinders. There is no power to the solenoid on the relief valve with the cylinders idle, so pump flow goes to tank at low pressure. Energizing a solenoid on the relief valve and one cylinder’s directional valve causes an action. Energizing both solenoids at the same time sends pump flow to the cylinder until reaching maximum relief pressure. A solenoid relief valve always has a slight delay before blocking flow to tank after energizing the solenoid. The delay is in milliseconds so it usually is only noticeable on very fast cycles.
Figure 18-20 shows a normally open solenoid-operated relief valve that allows a large pressure-compensated pump to start at no load. A normally open solenoid-operated relief valve lets flow from the pressure-compensated pump go to tank until the electric drive motor is up to speed. A time delay or a flow meter with a flow switch energizes the solenoid on the relief valve to load the circuit. Deenergizing the normally open solenoid-operated relief valve unloads the pump any time to reduce power consumption, heat buildup, and noise.
Controlling a pilot-operated relief valve remotely
The system relief valve is normally located near the pump outlet on a typical hydraulic unit. The hydraulic unit could be at a distance from the operator, or cramped conditions could make the relief valve hard to get near. If an application’s relief pressure must change often, add a remote relief valve control to a pilot-operated relief valve for convenience.
To adjust a circuit with a remote relief valve, use the following procedure. First, set the main relief at minimum pressure and the remote relief at maximum pressure. Start the pump and check for obvious leaks and incorrect plumbing. Pressure is low during this part of the procedure. Next, slowly raise the main relief to maximum system pressure and lock it. Now, use the remote relief to set pressure to any setting less than the main relief. The operator can only adjust pressure to a level lower than the main relief setting. This is an important safety factor because it eliminates damage or injury from excess pressure caused by an inexperienced operator.
The company is the world’s best hydraulic relief valve supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
Most manufacturers recommend that the remote valve be located a maximum of 10 to 15 feet from the main relief. The greater the distance between the remote and main relief, the longer the response time of the main relief. An increase in response time allows higher pressure override, causing pressure spikes. Pressure spikes may cause premature pump, piping, or valve failure.
With a solenoid or manual valve to select more than one remote relief, it is easy to select multiple preset pressures.
Pilot-operated relief valves have a vent port. In Figures 18-21 and 18-22, the vent port is piped to a single, remote direct-acting relief for adjusting pressure remotely. Figures 18-23 through 18-25 show the vent port connected to directional valves and multiple remote reliefs. These circuits allow changes from maximum pressure to several preset or infinitely variable limits during a cycle.
Figure 18-31 shows a cutaway view of an unloading relief valve. It is similar to a standard relief valve but has an extra control piston in its head. There is an approximate 15% difference in the area of the control piston and the pilot-control poppet seat. As pressure builds, it pushes against both sides of the control piston and against the pilot-control poppet. Nothing moves until pressure starts to force the pilot-control poppet off its seat, as in Figure 18-32. Pressure drop in front of the control piston lets it move and force the pilot-control poppet completely off its seat, Figure 18-33. Forcing the pilot-control poppet off its seat unloads the pump at 20 to 70 psi. The pilot-control poppet stays open until system pressure drops approximately 15%, then closes to force pump flow into the circuit again. When pressure rises to maximum, the pilot-control poppet is pushed off its seat and unloads the pump. This action continues anytime the pump runs.
Pressure relief valves are used to prevent over pressurization in the hydraulic circuit. They are located close to the pumps and reservoir tank or close to the equipment to safeguard from over pressurization.
This is an illustration of a cartridge style "pilot" operated pressure relief valve. (There are also "poppet" operated relief valves, which have no pilot function.) Pilot operated relief valves, vs. poppet style relief valves, can be sized to handle more flow than a poppet style valve and have better response and flow characteristics. This type of valve has a spool that is controlled by a variable force spring. Utilization of spring tension is the most common means of controlling pressure from pumps, pressure relief valves, and pressure reducing valves. The spring’s force constant along with where the spring is set determine at what pressure the valve will dump to the port 2.
Turning the pilot spring screw clockwise increases the force exerted on the pilot check ball. This will increase the pressure at which the valve opens up. As pressure in port 1 increases, oil flows up the main spool orifice and lifts the pilot check ball. When the check ball opens, a small amount of oil is exhausted to port 2. This creates a pressure drop across the main spool orifice. This pressure drop lifts the main spool and opens the main path from port 1 to port 2. Pressure in port 1 will remain at the valve pressure setting. Dumping continues as long as there is flow past the pilot check ball.
In stage 1, the pressure is below the valve setting. There is small leakage around the main spool to port 2.
In stage 2, as pressure in line 1 increases above the valve setting, the pilot check valve opens and some oil flows to port 2. This flow creates a pressure drop across main spool orifice.
In stage 3, the pressure drop across the main spool orifice opens the main spool, dumping main port 1 to main port 2. When port 1 pressure decreases, the pilot ball check valve will reseat itself, eliminating flow through the pilot check. With no flow, the main spool spring will reseat the main spool and stop flow to port 2.
Pressure relief valves are adjusted to stay closed under normal operating conditions. They are not an efficient way to set system pressures as heat and noise are generated when the valve is open (recall that temperature rises when no useful work is being done). If a relief valve and downstream oil lines are hot, the valve is open and dumping oil. The relief valves are the first things to check if your oil is running hot. Verify system pressures are on target as well as the setting of the valve.
For more information on maintaining your hydraulic system, contact your Valmet representative.
Contact us to discuss your requirements of Superimposed Relief Valve ZDB10. Our experienced sales team can help you identify the options that best suit your needs.