Hydraulic Pilot Operated Relief Valve
A pilot-operated relief valve has the same function as a direct-acting relief valve; however, it has a different pressure vs. flow curve. The performance curves for the two types of relief valve are given in Fig. 3.7. The pilot-operated valve opens completely over a narrow pressure range. This allows the circuit to operate over a wider pressure range without loss of fluid over the relief valve.
A functional diagram of a pilot-operated relief valve is shown in Fig. 3.8. The main spool has a small hole (orifice) drilled in the skirt. Because of this hole, pressure is the same on the top and bottom of the skirt. As long as there is negligible flow through the orifice, there is no pressure drop across the orifice.
The pilot section of the valve is the top section. A dart is held in place by the pilot spring. When the hydraulic force on this dart becomes greater than the pilot spring force, the dart is unseated, and fluid flows from the cavity above the skirt, through an internal drain to the valve outlet. Flow through the orifice replaces the fluid lost from the cavity above the skirt. The spool is still held in position by the main spool spring.
At this point, discussion is facilitated if the two springs are assigned values. Suppose the pilot spring is a 1425-psi spring, and the main spool spring is a 75-psi spring. When pressure at the valve inlet reaches 1425 psi, the dart is unseated. Pressure in the upper cavity cannot increase above 1425 psi. The hydraulic force on the top and bottom of the skirt is equal, and these two forces balance. The main spool is held in position only by the spring force produced by the 75 psi spring.
What happens when the pressure at the valve inlet reaches 1425 psi? The relief valve stays closed. If pressure continues to increase and reaches 1500 psi, the spool lifts, and fluid is bypassed to the reservoir. As pressure continues to increase above 1500 psi, the main spool opens further until it is completely open. Only a small pressure increase is needed to completely compress the 75 psi spring. (In Fig. 3.7, the pilot-operated valve is fully open at 1585 psi.) In other words, the valve goes from cracking to full open with a very small increase in pressure. When the load is changing quickly, and sharp pressure spikes are created, the quick opening feature of a pilot-operated relief valve is needed to protect the circuit.
The key advantage of a pilot-operated valve is that it allows the designer to use pressure to within 100 psi of the valve setting to meet the functional objective of the circuit. In comparison, the direct-acting valve cracks open at 1500 psi, and pressure must increase to 2000 psi before it is fully open.
A pilot-operated relief valve can be used with a remote pilot as shown in Fig. 3.9. The remote pilot functions like the pilot built into the top of the main relief valve. It allows the designer to set two pressure levels with one main relief valve.
Suppose the pilot on top of the main valve has a 1925 psi spring, and the remote pilot has a 925 psi spring. If the pressure at the remote pilot reaches 925 psi, the dart unseats, and the pressure in the cavity above the skirt is limited to 925 psi. The main valve cracks open at 925 + 75 = 1000 psi. If pressure at the remote pilot location stays below 925 psi, then circuit pressure can continue to build until it reaches 1925 psi, the setting of the pilot built into the housing of the main valve. The main valve now cracks open at 1925 + 75 = 2000 psi. In this illustration, no information is given on where the remote pilot is connected in the circuit. We only know that it is someplace other than the main pressure line from the pump.
Categories: Hydraulic Pressure Control | Tags: Hydraulic, Pilot Operated, Pressure Control, Relief Valve | Leave a comment