Pilot Operated Check Valves

Transcription

1 Pilot Operated Check Valves The check valves in Fig.1 operate like a standard check valve but are able to have reverse flow when required. They are called Pilot to Open Check Valves since they are normally closed but can be opened for reverse flow from an external pilot supply. The first cutaway, Plain Pilot to Open check valve in Fig. 1, is a standard design using a Pilot Piston with a stem to unseat the check valve poppet for reverse flow. Material with permission from Bud Trinkel 1 of 12

2 Pilot to Open Check Valves Plain Pilot to Open Pilot to Open With Decompression Pilot to Open With External Drain Out Free Flow In Controlled Flow Pilot Piston Pilot Poppet In Free Flow Out Controlled Flow Poppet Seat Dia Decompression Poppet Pilot Piston Dia. External Drain Symbol Symbol Symbol Fig. 1 2 of 12

3 Pilot Operated Check Valves The Pilot Piston diameter has an area of 3-4 times that of the Poppet Seat Dia. giving it increased force to open the Poppet against back pressure. Some pilot operated check valves have area ratios up to to allow a very low pilot pressure to open against high back pressure. (See Fig.2 for a typical circuit using pilot operated check valves.) The second valve, Pilot to Open With Decompression, has a small inner Decompression Poppet to allow a low pilot pressure to open a small flow to reduce back pressure. The release of high back pressure allows the Pilot Piston to easily open the main Poppet for full flow to tank. This does not work when the high back pressure is from load induced pressure or other continuous forces. 3 of 12

4 Pilot Operated Check Valves The third valve, Pilot Operated With External Drain, isolates the stem side of the Pilot Piston from the In Free Flow port back pressure that would resist pilot pressure trying to open the Poppet. Notice in the other two cutaways any pressure in the In Free Flow port pushes against the Pilot Piston stem side and resist pilot pressure trying to open the Poppet. pressure could be from a downstream flow control or counter balance valve in some circuits. The External Drain port can also be used to make the Pilot Piston return when using the valve as a pilot operated 2-Way function.. 4 of 12

5 Pilot Operated Check Valves The circuit in Fig.2 shows a typical and part damage. Almost immediately pressure to the pilot operated check valves pilot port builds again and the running away-stop scenario repeats until the cylinder meets resistance or something fails. The best valve for controlling running away loads is the counter balance valve. 5 of 12

6 Typical Pilot Operated Check Valve Circuit Line Pilot Pilot Operated Check Valve Float Center Directional Valve Fig. 2 6 of 12

7 Pilot Operated Check Valves Another problem with using a pilot operated check valve to hold back a running away load is shown in Fig. 3. A pilot operated check valve may not open when signaled to let a cylinder with an oversize rod and heavy load advance. When the directional valve shifts to extend the cylinder load induced pressure is holding the pilot operated check valve poppet closed. It may take 3 to 400 PSI to force the poppet open even with its 3 or 4 to one area difference. 7 of 12

8 Cylinder With Running Away Load Unable To Start Extending Pressure Increasing 2:1 Oversize Rod Load Symbol Fig. 3 8 of 12

9 Pilot Operated Check Valves Pressure builds at the pilot port but at the same time it builds at the cylinder cap end and with a 2:1 rod differential cylinder it adds PSI to the load induced pressure. The added down force causes pilot pressure to increase which causes more down force which causes more pilot pressure until the circuit reaches maximum pressure and the relief valve bypasses or the pump compensator kicks in to stop flow. The cylinder cannot start extending and even if it could the action would be the erratic as in Fig of 12

10 Cylinder With Running Away Load Stopping Suddenly After Losing Pilot Pressure Pressure Spiking Symbol Load Fig of 12

11 Pilot to Close Check Valves There is also a pilot to close check valve that is seldom used in circuits. It is usually not necessary to have a valve that always stops flow in one direction to also be capable of stopping it the opposite way. Notice in this cutaway the Spring Loaded Poppet does not have communicating holes through it to the spring chamber Flow from Inlet to Outlet is free to pass until a pilot signal is fed to the Pilot port. Since the Pilot port side of the Main Flow Poppet has more area than the Inlet side the valve can be closed against free flow. 11 of 12

12 Pilot to Close Check Valve Pilot Outlet Spring Loaded Poppet Inlet 12 of 12