For pressure-control cartridge valves, the poppet has an area ratio of 1:1. This means flow can only go from the A port to the B port. A 1:1 area ratio makes the valve stable when controlling flow at pressure. Notice that the poppet is straight sided and sits on a tapered seat.

The symbol and cutaway view in Figure 11-10 illustrate a relief valve, but the design would function as a sequence or counterbalance valve as well. A relief valve never needs to have reverse flow, while a sequence or counterbalance valve usually requires a reverse-flow check valve piped around it. Also, as a sequence valve, it would always need a separate drain line to tank. These valves and an unloading valve are shown and discussed next.

Figure 11-10 shows a direct-acting relief valve symbol in the cover with pilot lines and orifices connecting it to a 1:1 area-ratio poppet. The A port is always the inlet, and as a relief valve, the B port is always the outlet to tank. The operation of a slip-in cartridge valve as a relief valve is identical to the poppet-type relief valve discussed in Chapter 9. The main attraction of slip-in cartridge valves is that they come in larger sizes for high-flow applications. Most relief valves for everyday circuits handle less that 200 gpm. Cartridge relief valves can handle flows in excess of 1500 gpm.

Slip-in cartridge relief valves can be set up with all the features of the pilot-operated relief valves that were shown in Chapter 7. Solenoid venting and multiple pressure selection work in the same manner except for higher flow capabilities. Look at the symbols of the different types of arrangements for slip-in cartridge relief valves in Chapter 4.

Figure 11-11 shows an internally piloted, externally drained sequence valve. This is the same valve illustrated in Figure 11-10, except the drain fluid from the direct-acting relief valve section must be ported directly to tank. If fluid is drained to the B port, backpressure at that port would add to the spring set pressure. In some cases, the internal pilot is changed to an external pilot as the circuit dictates. As mentioned previously, a sequence valve circuit often needs reverse flow so a cartridge-type check valve would be piped around it to provide free flow in reverse.

The valve pictured in Figure 11-11 also can perform as a counterbalance valve. Counterbalance valves can be internally or externally piloted but they usually are internally drained. Chapter 14 discusses sequence and counterbalance valves in detail and shows why pilot and drain functions are not always the same. Without exception, counterbalance valve circuits must have reverse flow so they always need a bypass check valve.

To add an unloading valve function to a cartridge valve, a special cover is required. The symbol and cutaway view in Figure 11-12 show the setup for an unloading valve that could be used for a hi-lo pump circuit. Chapter 9 provides a complete explanation of unloading valves.

An unloading valve is similar to -- and functions just like -- a relief valve if the unloading port were not connected. Pressure would build enough to push the ball against the adjustable spring; then all excess pump flow would go to tank at set pressure. The addition of the unloading piston makes it possible to move the ball back far enough so that all pressure on top of the 1:1 poppet drops off. The valve then opens pump flow to tank at 30 to 50 psi. Pressure to the unloading port usually comes from a high-pressure pump. This keeps a high-volume pump unloaded during the work stroke.

The last of the pressure-control valves is the reducing valve. Unlike the other four pressure controls, a reducing valve is normally open instead of normally closed. The valve also is the one slip-in cartridge valve design that does not use a poppet to block fluid. Slip-in cartridge reducing valves are similar to in-line and subplate-mounted valves except that they have higher flow capabilities.

The symbol and cutaway view in Figure11-13 show a common construction for pilot-operated slip-in cartridge reducing valves. Notice the symbol for the cartridge is the standard ANSI-ISO drawing for any reducing valve. A cover containing a direct-acting relief valve sets the pressure. As fluid enters port A, it is free to go directly out port B . When outlet flow meets resistance, pressure starts to build and goes to the inlet of the direct-acting relief valve. The flow path is directly from ports A and B through the check valve. When pressure reaches the direct-acting relief valve setting, fluid starts to flow slowly through the valve. Flow increases as pressure continues to increase. At set pressure, flow through the direct-acting relief valve exceeds flow through the orifices. At that point, pressure on top of the flow-through spool drops. When this pressure drops enough, the spool raises and restricts outlet flow at set pressure. Flow never shuts off completely because flow from the reduced pressure is always going to tank through the direct-acting relief valve. When pressure at the outlet drops, the direct-acting relief valve closes and forces the passage through the spool to reopen.