Pressure retaining valve

A pressure retaining valve, comprising:

• a valve housing having a main control piston and a non-return piston being releasably engaged in direct contact with one another, both of said main control piston and said non-return piston being individually and longitudinally displaceable in said valve housing between
• a pressure holding switching position closing a fluid-conducting connection between supply, tank and actuator connection ports in said valve housing by said pistons where said supply connection port supplies fluid media, said tank connection port returns fluid media and extends through said housing and said actuator connection port leads to a hydraulic actuator connected to said valve housing,
• a load-lifting switching position in which said pistons move relative to one another in said valve housing into a release position connecting said supply connection port in said valve housing with said actuator connection port in fluid communication and blocking said tank connection port from fluid communication with said supply connection port and said actuator connection port, and
• a load-lowering switching position spacing said pistons from one another by a predefinable distance connecting said tank connection port in said valve housing with said actuator connection port in fluid communication and blocking said supply connection port from fluid communication with said tank connection port and said actuator connection port;
• and
• a monitoring device coupled to said valve housing determining the switching positions individually of said pistons ensuring reliable functioning of the pressure retaining valve.