Plunger Housing Check-valve assembly Port B Port B Port A Port A Secondary system Primary system From pump Vent Spring Spring Poppet Poppet Piston Piston 12-27 Figure 12-42.  A hydraulic quick-disconnect valve. Figure 12-41.  Priority valve. Figure 12-40.  Mechanically operated sequence valve. A priority valve gives priority to the critical hydraulic  subsystems over noncritical systems when system pressure  is low. For instance, if the pressure of the priority valve is set  for 2,200 psi, all systems receive pressure when the pressure  is above 2,200 psi. If the pressure drops below 2,200 psi,  the priority valve closes, and no fluid pressure flows to the  noncritical systems.  [Figure 12-41]  Some hydraulic designs  use pressure switches and electrical shutoff valves to assure  that the critical systems have priority over noncritical systems  when system pressure is low.  Quick disconnect valves are installed in hydraulic lines to  prevent loss of fluid when units are removed. Such valves  are installed in the pressure and suction lines of the system  immediately upstream and downstream of the power pump. In  addition to pump removal, a power pump can be disconnected  from the system and a hydraulic test stand connected in  its place. These valve units consist of two interconnecting  sections coupled together by a nut when installed in the  system. Each valve section has a piston and poppet assembly.  These are spring loaded to the closed position when the unit  is disconnected.  [Figure 12-42] A hydraulic fuse is a safety device. Fuses may be installed  at strategic locations throughout a hydraulic system. They  detect a sudden increase in flow, such as a burst downstream,  and shut off the fluid flow. By closing, a fuse preserves  hydraulic fluid for the rest of the system. Hydraulic fuses  are fitted to the brake system, leading edge flap and slat  extend and retract lines, nose landing gear up and down  lines, and the thrust reverser pressure and return lines. One  type of fuse, referred to as the automatic resetting type, is  designed to allow a certain volume of fluid per minute to pass  The safe and efficient operation of fluid power systems,  system components, and related equipment requires a means  of controlling pressure. There are many types of automatic  pressure control valves. Some of them are an escape for  through it. If the volume passing through the fuse becomes  excessive, the fuse closes and shuts off the flow. When the  pressure is removed from the pressure supply side of the fuse,  it automatically resets itself to the open position. Fuses are  usually cylindrical in shape, with an inlet and outlet port at  opposite ends.  [Figure 12-43] Pressure Control Valves Hydraulic Fuses  Quick Disconnect Valves  Priority Valves 