Integral hydraulic blocking and relief valve

An integral blocking and relief valve for blocking the flow of hydraulic fluid unless the pressure of said fluid exceeds a predetermined value, comprising:

• a first input port adapted to be connected to the hydraulic fluid source to be blocked and relieved;
• a first piston P.sub.A slidably constrained in a first chamber, said first chamber being connected to said first input port and said piston P.sub.A having an area A.sub.A at a first end, said area A.sub.A being exposed to hydraulic pressure in said first chamber;
• a second piston P.sub.B having a first portion being slidably constrained in a pressure relief chamber and a second portion integral with the first end of said first portion and being slidably constrained in, and exposing an area A.sub.SB to a relief bias chamber, said second piston P.sub.B further comprising a plunger portion extending from the second end of said first portion;
• a second input port connected to said pressure relief chamber and adapted to be connected to a system controlled hydraulic flow line;
• a pressure bias port adapted to be connected to a source of system hydraulic pressure;
• a bias chamber connected to said pressure bias port and joining said first chamber with said relief bias chamber such that the second end of said first piston P.sub.A can abut the free end of said second portion;
• a relief port connected to said relief bias chamber and adapted to be connected to a system return pressure;
• and
• a poppet valve being slidably constrained in a poppet chamber, said poppet chamber joining with said pressure relief chamber and said first input port such that hydraulic fluid at said first input port is controllably passed to said pressure relief chamber dependent on the position of said poppet, said poppet exposing an area A.sub.p to said pressure relief chamber and being engageable by said plunger portion of the second piston to drive the poppet to a position to allow flow from said input port to said pressure relief chamber, said relief bias chamber being directly connected to the system return pressure in all positions of the poppet valve
• the first piston exposed area A.sub.A, the poppet exposed area A.sub.p and the second piston second portion exposed A.sub.SB being defined by:
• A.sub.SB .perspectiveto.A.sub.A -A.sub.p
• whereby the poppet is driven to allow flow from the first input port to the relief chamber at a predetermined pressure of fluid in said first input port independent of the level of system pressure.