HYDRAULIC PRESS :PRE- FILL VALVES

MODEL OF PRE-FILL VALVE

 Poppet-type pre-fill valves

Pre-fill valves operate similarly to pilot-operated check valves, but they are usually much larger. Some pre-fill valves can handle flows in excess of 6000 gpm at pressure drops of less than 4 to 8 psi. Their normal function is to fill and exhaust a large bore cylinder as it travels to and from contact with the work piece. Large, high-tonnage presses -- both vertical and horizontal -- use pre-fill valves to reduce pump size while maintaining cycle time.
The cutaway view and symbol in Figure 10-8 show the construction of a typical poppet-type pre-fill
valve. A large main-flow poppet seals the path between the tank and the cylinder ports. As the piston
advances, vacuum in the void behind it allows atmospheric pressure to push the main-flow poppet open so fluid from the tank can fill this void. On the retraction stroke, a signal to the pilot piston pushes the main-flow poppet open so fluid can return to tank. While a pilot-operated check valve’s pilot piston is larger than the poppet it opens, the main-flow poppet in a pre-fill is much larger in diameter than the pilot piston. Thus it is impossible to open the main-flow poppet against high back pressure. This keeps decompression shock from damaging pipes and components.Decompression shock occurs when large volumes of fluid at high pressure are released suddenly.
Because all hydraulic oil has some entrained air (bubbles so small they cannot be seen without
magnification), there is a 0.5 to 1% compressibility that must be dealt with when using large-bore
cylinders. On top of fluid compressibility, the cylinder tube may stretch diametrically and longitudinally.
In addition, the framework that is resisting the tonnage produced also can stretch. Summing all these
factors, a 50-in. bore cylinder with a 72-in. stroke can contain more than 25 gal of extra fluid at 3000
psi. If this trapped fluid suddenly has a large open path to atmosphere, its velocity at first release is
such that it can break fittings, blow hoses, straighten tubes or pipe bends with relative ease. Releasing this same trapped fluid in a controlled manner over a few seconds dissipates the excess energy and no damage is seen.
The plain pre-fill valve might be used on smaller cylinders or circuits that have other means for
decompressing. The pre-fill valve with decompression has a small poppet in the large poppet that is
easy to open at high pressure but will not allow the high flow that causes decompression shock. This
decompression poppet usually has a means to adjust how fast the cylinder decompresses.
Another pre-fill valve design is the sleeve type that must be externally shifted open and closed. Both
designs give the same results even though their operation is different.
See also
hydraulics
working-of-hydraulc-press 
hydraulic-directional-control-valve