12-28 Figure 12-43.  Hydraulic fuse. Figure 12-44.  Pressure relief valve. pressure that exceeds a set pressure; some only reduce the  pressure to a lower pressure system or subsystem; and some  keep the pressure in a system within a required range. Hydraulic pressure must be regulated in order to use it to  perform the desired tasks. A pressure relief valve is used  to limit the amount of pressure being exerted on a confined  liquid. This is necessary to prevent failure of components  or rupture of hydraulic lines under excessive pressures. The  pressure relief valve is, in effect, a system safety valve. The design of pressure relief valves incorporates adjustable  spring-loaded valves. They are installed in such a manner as  to discharge fluid from the pressure line into a reservoir return  line when the pressure exceeds the predetermined maximum  for which the valve is adjusted. Various makes and designs  of pressure relief valves are in use, but, in general, they all  employ a spring-loaded valving device operated by hydraulic  pressure and spring tension.  [Figure 12-44]  Pressure relief  valves are adjusted by increasing or decreasing the tension  on the spring to determine the pressure required to open the  valve. They may be classified by type of construction or uses  in the system. The most common types of valve are: Pressure relief valves cannot be used as pressure regulators  in large hydraulic systems that depend on engine-driven  pumps for the primary source of pressure because the pump  is constantly under load and the energy expended in holding  the pressure relief valve off its seat is changed into heat. This  heat is transferred to the fluid and, in turn, to the packing  rings, causing them to deteriorate rapidly. Pressure relief  valves, however, may be used as pressure regulators in small,  low-pressure systems or when the pump is electrically driven  and is used intermittently.  Pressure relief valves may be used as: 1.  System relief valve—the most common use of the  pressure relief valve is as a safety device against  the possible failure of a pump compensator or other  pressure regulating device. All hydraulic systems  that have hydraulic pumps incorporate pressure relief  valves as safety devices. 1.  Ball type—in pressure relief valves with a ball-type  valving device, the ball rests on a contoured seat.  Pressure acting on the bottom of the ball pushes it off  its seat, allowing the fluid to bypass. 2.  Sleeve type—in pressure relief valves with a sleeve- type valving device, the ball remains stationary and  a sleeve-type seat is moved up by the fluid pressure.  This allows the fluid to bypass between the ball and  the sliding sleeve-type seat. 3.  Poppet type—in pressure relief valves with a poppet- type valving device, a cone-shaped poppet may have  any of several design configurations; however, it  is basically a cone and seat machined at matched  angles to prevent leakage. As the pressure rises to its  predetermined setting, the poppet is lifted off its seat,  as in the ball-type device. This allows the fluid to pass  through the opening created and out the return port.  Relief Valves  Adjusting screw cap Pressure  adjusting screw Compression spring Return port Ball Pressure port