desired) and regardless of the system load, if the load does not  exceed the designed capacity of the reducer.  [Figure 12-46] In certain fluid power systems, the supply of fluid to a  subsystem must be from more than one source to meet  system requirements. In some systems, an emergency system  is provided as a source of pressure in the event of normal  system failure. The emergency system usually actuates only  essential components. The main purpose of the shuttle valve  is to isolate the normal system from an alternate or emergency  system. It is small and simple; yet, it is a very important  component.  [Figure 12-47]  The housing contains three  ports—normal system inlet, alternate or emergency system  inlet, and outlet. A shuttle valve used to operate more than  one actuating unit may contain additional unit outlet ports.  Enclosed in the housing is a sliding part called the shuttle. Its  purpose is to seal off one of the inlet ports. There is a shuttle  seat at each inlet port. When a shuttle valve is in the normal  operation position, fluid has a free flow from the normal  system inlet port, through the valve, and out through the outlet  port to the actuating unit. The shuttle is seated against the  alternate system inlet port and held there by normal system  pressure and by the shuttle valve spring. The shuttle remains  in this position until the alternate system is activated. This  action directs fluid under pressure from the alternate system  to the shuttle valve and forces the shuttle from the alternate  system inlet port to the normal system inlet port. Fluid from  the alternate system then has a free flow to the outlet port but  is prevented from entering the normal system by the shuttle,  which seals off the normal system port.  Shutoff valves are used to shut off the flow of fluid to a  particular system or component. In general, these types of  valves are electrically powered. Shutoff valves are also used  to create a priority in a hydraulic system and are controlled  by pressure switches.  [Figure 12-48] In shuttle valves that are designed with a spring, the shuttle  is normally held against the alternate system inlet port by  the spring. The shuttle may be one of four types:  Shuttle Valves Shutoff Valves 1.  Sliding plunge 2.  Spring-loaded piston 3.  Spring-loaded ball 4.  Spring-loaded poppet Figure 12-47.  A spring-loaded piston-type shuttle valve in normal  configuration (A) and with alternate/emergency supply (B).  Figure 12-48.  Shutoff valves. Figure 12-46.  Operating mechanism of a pressure reducing valve.  12-30 Outlet Outlet Normal system inlet Alternate system inlet A B 100 pound spring Relief valve 750 psi Reduced pressure  200 psi square inch shouder To reduced pressure operated sub-system System pressure 1,500 psi Bleed orifice 1 square  inch piston 200 psi