Pressure-Control Valves and Flow-Control ValvesPressure-control valves are used in hydraulic circuits to maintain desired pressure levels in various parts of the circuits. A pressure-control valve maintains the desired pressure level by(1)diverting higher-pressure fluid to a lower-pressure area，thereby limiting the pressure in the higher-pressure area, or(2)restricting flow into another area. Valves that divert fluid can be safety, relief, counterbalance, sequence, and unloading types. Valves that restrict flow into another area can be of the reducing type.A pressure-control valve may also be defined as either a normally closed or normally open two-way valve. Relief, sequence, unloading and counterbalance valves are normally closed, two-way valves that are partially or fully opened while performing their design function, A reducing valve is a normally open valve that restricts and finally blocks fluid flow into a secondary area. With either type of operation, the valve can be said to create automatically an orifice to provide the desired pressure control. An orifice is not always created when the valve is piloted from an external source. One valve of this type is the unloadingit is not self-operating；it depends on a signal from an external source. Relief, reducing counterbalance, and sequence valves can be fully automatic in operation, with the operating signal taken from within the envelope. In this chapter we shall study the different types of pressure-control valves and learn how they are used in various hydraulic circuits.Types of Pressure-Control ValvesEight popular devices for pressure-control service are:Safety valve. Usually a poppet-type two-way valve intended to release a secondary area when the fluid pressure approaches the set opening pressure of the valve. This type of valve protects piping and equipment from excessive pressure.Relief valve. Valve which limits the maximum pressure that can be applied in that portion of the circuit to which it is connected.Counterbalance valve. Valve which maintains resistance against flow in one direction but permits free flow in the other direction.Sequence valve. Valve which directs flow to more than one portion of a fluid circuit, in sequence.Unloading valve. Valve which allows pressure to build up to an adjustable setting, then bypasses the flow as long as a remote source maintains the preset pressure on the pilot port.Pressure-reducing valve. Valve which maintains a reduced pressure at its outlet regardless of the higher inlet pressure.Hydraulic fuse. Device equipped with a frangible disk which establishes the maximum pressure in a hydraulic circuit by rupturing at a preset pressure valve.Pressure switch. Switch operated by fluid pressure and responsive to a rise or fall in fluid pressure.Compound Relief ValveIn the study of ISO hydraulic symbols it was stated that simplified symbols are widely used. Because of this, pressure-relief valves used in common hydraulic circuits are rarely shown complete with all auxiliary devices and connections. Instead, the simplified symbol shows only the basic relief valve, pressure input, tank connection, valve spring, and the offset arrow indicating that the valve is normally closed. A slash arrow as shown on the bias-spring of the pilot-relief valve in Fig.3-20a may be added to the bias-spring symbols of the simplified valve as shown in Fig.3-20b if the valve is adjustable, particularly if this information is significant to circuit operation. Figure 3-20a shows the complete symbols for a compound relief valve. All adjacent controls are shown, along with the main relief element. The envelope surrounding all the elements may have five connections. These are(1)pressure input, (2)tank connection, (3)remote-control-station connections, (4)test station, and (5)external drain for the pilot relief (not shown) that is provided only on special order. Fig.3-20 (a)Complete symbol for a compound relief valve (b)Simplified symbol for compound relief valve The input pressure and tank connection provide the major flow through the valve. Only enough fluid need flow to the test-station and remote-control connections for the respective functions. The test station is generally used for a gauge connection to check fluid pressure. This does not require a flow of fluid. The remote-control connection passes the quantity of fluid coming through the fixed internal orifice at the rate established by the spring in the main relief element. An external drain from the pilot-relief valve, if fitted, will not pass more fluid than passes through the fixed internal orifice.Figure 3-21 shows a cutaway view of compound relief valve. Note that the main spool is held by the spring in a position that blocks the passage from the pressure input port to the tank port, just as the symbol in Fig.3-20 shows.Input pressure is directed to the bottom of the spool below the spring cavity wit