Usually apply for large-tonnage pressure machines. When the hydro-cylinder in its position, it could save a large flow LFP (Low Fuel Pump). It saves power and cost by using hydrovalve. Oil pump press to big cylinder. Big cylinder runs out and retract with small cylinder’s driving. Hydrovalve works under high pressure. With opening the control side of hydrovalve, oil in cylinder return back to top petrol tank to build high pressure in cylinder. After compressing in cylinder, hydraulic oil would be supplemented from top petrol tank to big cylinder. Essentially, the hydrovalve is a large drift diameter fluid control one-way valve. Hydrovalve opens automatically to make vacuum environment in cylinder. It is mainly used in large-tonnage pressing machines.