Using solenoid relief valves as 2-way valves
High-flow (above 50 gpm) 2-way valves are not readily available for hydraulic circuits. To get around this problem, use a solenoid relief valve. Several circuits shown here are in use in many hydraulic applications.
For flows above 150 to 200 gpm, use slip-in cartridge valves (as explained in Chapter 4). Slip-in cartridge valves use simple directional controls to operate large poppets that can handle flows in excess of 600 gpm.
Figures 18-15 through 18-16 show the schematic diagram for a high-flow, on-off, bleed-off flow control circuit. Set the solenoid relief valve higher than system pressure so it never passes fluid unless vented. The normally closed bleed-off circuit shown in Figure 18-15 passes fluid when the solenoid is energized. Figure 18-16 shows a normally open solenoid-operated relief valve. With this valve, energizing the solenoid stops flow.
The circuits in Figures 18-17 and 18-18 show solenoid-operated relief valves bypassing large flow controls in a 2-speed circuit. Because there is backpressure downstream from the solenoid relief valve in this type circuit, use a valve with an external drain. (Backpressure at the outlet of a relief valve causes it to shut when internally drained.) Externally draining the relief valve eliminates backpressure at the vent port so it stays open when bypassing.
Solenoid reliefs used as shut-off valves do not cause as much shock as spool-type valves because relief valves cushion as they close.
In Figure 18-19, a normally open solenoid-operated relief valve protects a large hydraulic motor from overpressure, and also starts and stops it for a single-rotation application. Energizing the solenoid on the relief valve blocks its vent, causing it to close. Closing is smooth because pressure builds to relief setting quickly, providing some fluid a path to tank while the motor comes up to speed. When the motor reaches full speed, the relief valve closes completely. The motor then continues at full speed at whatever pressure it takes to keep it rotating.
Deenergizing the solenoid on the solenoid-operated relief valve connects pump flow to tank and the hydraulic motor coasts to a stop. A brake valve (Chapter 12) would stop the motor quickly and smoothly if required.
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.
Figures 18-21 and 18-22 show a symbol for a remote relief valve setup. Figure 18-21 shows the simplified symbol; Figure 18-22 shows the complete symbol. All pilot-operated relief valves have a vent port. The vent port tees into the pilot line that connects system pressure to the direct-acting relief valve’s pilot section. The vent port tees in after the control orifice. With the vent blocked, the relief valve functions normally. With the vent open to atmosphere, the relief valve opens at the pressure of the internal main poppet or piston spring. This is usually between 20 and 70 psi. Figure 18-21 shows a small, direct-acting relief valve piped to the vent port of a pilot-operated relief valve. The small direct-acting relief functions the same as the pilot-valve section on the main relief. An operator can use the remote relief to adjust main system pressure from any convenient location within 10 to15 feet of the system relief valve.
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.
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.