EATONAerospace Group TF100-84D May 2013 3 Figure A reflects a lanyard operated pilot valve shown in the open position. The operation of the hydrant valve, whether the pilot is lanyard or air operated, is identical. Figure B reflects the same hydrant valve with the air pilot in a closed position. The only differences between the two units are in the operating mechanism that supplies the power to open and close the pilot valve. In the air operated pilot, the closing lanyard and opening latching mechanisms are replaced with an air operated piston. Servicing Valve Closed/ Pilot Valve Open or Closed The closing of the servicing valve has the same affect as closing the pilot valve. That is, the flow passage from the piston chamber to the downstream side of the piston is blocked. The piston chamber pressure begins to equalize to the inlet pressure (P1) through the check valve. The piston area is greater than the effective seal area, hence the unbalance of forces caused by the equal pressure, plus the spring, will cause the valve to stay closed. Figure A Figure B Valve Operation Pilot Valve Open/ Servicing Valve Open The open pilot valve allows the continuous passageway from the main piston chamber and from the closing control orifice. The piston chamber is vented through an opening control orifice and the open servicing valve to a point in the lower valve half. The pressure (P2) at this point is less than the inlet pressure (P1). The piston chamber pressure is also maintained at P2 causing an unbalance of forces on the piston. The inlet pressure force is greater than the combined piston pressure force plus the spring force hence the valve will open to allow flow. This is assuming that the outlet adapter poppet in the upper valve half has been opened by a coupler. The pilot poppet is maintained in the open position by one of two methods: yy Lanyard operated pilot: The pilot is opened by the pull of the “T” handle located on the top of the pilot valve. When it is pulled upward, the spring loaded latch attached to the lanyard pivots to lock the pilot into the open position. yy Air operated pilot: Air pressure applied to the pilot piston will maintain the pilot in the open position until the pressure has been depleted (by release of deadman) Pilot Valve Closed/ Servicing Valve Open Pulling the lanyard, or depleting the air supplied to their respective pilots, will allow the spring loaded pilot poppet to close. This action blocks off the venting of the piston chamber to the lower pressure area downstream. The piston chamber begins to equalize to the inlet pressure (P1) through the check valve. The piston area is greater than the effective seal area, hence the unbalance of forces caused by the equal pressure plus the spring will cause the piston to begin to close. As the piston moves toward the closed position, the piston chamber volume increases and must be filled through the two series orifices. The primary orifice is considerably larger than the secondary (slot). During the initial and majority of the travel of the piston, the primary orifice is fully exposed to the inlet pressure, hence the rate of closure is controlled by this orifice. When the piston moves far enough closed to cover the primary orifice, the secondary (smaller) orifice begins to control the closure rate. Hence the valve begins to close relatively rapidly and then slows down as it nears its closed position. The relative size and locations of these two orifices allows the valve to close to provide a minimum of overshoot and yet limit the surge pressure shock, on closing, and still maintain a closure rate in accordance with applicable international specifications. On defueling option J, the pilot valve is manually held closed by the thumb screw to allow defueling flow.
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