2.  Thermal relief valve—the pressure relief valve is used  to relieve excessive pressures that may exist due to  thermal expansion of the fluid. They are used where a  check valve or selector valve prevents pressure from  being relieved through the main system relief valve.  Thermal relief valves are usually smaller than system  relief valves. As pressurized fluid in the line in which  it is installed builds to an excessive amount, the valve  poppet is forced off its seat. This allows excessive  pressurized fluid to flow through the relief valve to the  reservoir return line. When system pressure decreases  to a predetermined pressure, spring tension overcomes  system pressure and forces the valve poppet to the  closed position. pressure within a predetermined range. The other purpose  is to permit the pump to turn without resistance (termed  unloading the pump) at times when pressure in the system  is within normal operating range. The pressure regulator is  located in the system so that pump output can get into the  system pressure circuit only by passing through the regulator.  The combination of a constant-delivery-type pump and the  pressure regulator is virtually the equivalent of a compensator- controlled, variable-delivery-type pump.  [Figure 12-45] Pressure Reducers The term pressure regulator is applied to a device used in  hydraulic systems that are pressurized by constant-delivery- type pumps. One purpose of the pressure regulator is to  manage the output of the pump to maintain system operating  Pressure Regulators  Pressure reducing valves are used in hydraulic systems where  it is necessary to lower the normal system operating pressure  by a specified amount. Pressure reducing valves provide a  steady pressure into a system that operates at a lower pressure  than the supply system. A reducing valve can normally be set  for any desired downstream pressure within the design limits  of the valve. Once the valve is set, the reduced pressure is  maintained regardless of changes in supply pressure (as long  as the supply pressure is at least as high as the reduced pressure  Figure 12-45.  The location of a pressure regulator in a basic hydraulic system. The regulator unloads the constant delivery pump by  bypassing fluid to the return line when the predetermined system pressure is reached.  12-29 Return line Bypass line Filter Accumulator Check valves Pressure regulator Pressure gauge Relief valve Selector valve Emergency hand pump Pressure line Actuating unit Constant delivery pump Supply lines Supply Reservoir Air Fluid lines