Sequence of Operations |
Sequence of Operations |
AGC-CVC HPU Off - Energy Stored in Accumulators |
Even when the hydraulic power unit is turned off, stored energy presents a serious hazard. The stored energy is present from two sources:
1) Energy (pressurized hydraulic fluid) stored in accumulators: The AGC-CVC hydraulic system includes many accumulators, none of which are automatically vented of pressure when the electric motors for the hydraulic pumps are turned off. Instead, each accumulator is connected to an Accumulator Safely Valve. This valve assembly includes a vent valve which must be manually opened in order to vent the pressure from the accumulator to which it is connected. Each accumulator must be individually vented of pressure!
2) Energy (pressurized hydraulic fluid) which exists as a result of induced pressure. Most induced pressure is caused by two sources:
a) The weight of the piston and rod assembly in a hydraulic cylinder acts on the hydraulic fluid trapped between the assembly and a directional control valve, a closed pilot operated check valve, or some other valve which traps hydraulic fluid in the cylinder. This trapped hydraulic fluid is under pressure. In order to vent the pressure, the cylinder must be extended or retracted (one or the other, depending on the circuit configuration) so that the hydraulic fluid is no longer supporting the rod and piston assembly.
b) If a cylinder is supporting a load (the mass of a machine structure), then induced pressure will be present.
3) Before performing maintenance, lower the load or safely block the machine member in place so the cylinder is no longer supporting the load. Ensure induced pressure is not present, caused by the mass of the rod and piston assembly (see above). NEVER “crack a fitting” to relieve trapped pressure.
4) This brief summary is in no way intended to be a complete procedure list describing how to ensure stored energy has been safely vented in the various circuits of the hydraulic system.
5) Refer to the attached schematics which show where stored energy is likely to be present.
6) When the hydraulic system is turned off, Solenoid Vented Relief Valve 261.01 remains energized to its high pressure setting. As a result, pressurized hydraulic fluid continues to be stored in the accumulators in the entire AGC-CVC hydraulic system. Check valves (152.01 through 157.07) located immediately downstream from each of the pressure line filters (051.01 through 051.07) also prevent the AGC Accumulator Stand from venting. Therefore, the rod ends of the AGC Operator and Drive Side cylinders on all six mill stands will be pressurized unless Accumulators 236.06 through 236.12 are manually vented.
7) As is step 6 above, the Rod Side Bending Accumulator Valve Stand is not vented either. As a result pressure ports P2 and P3 in this accumulator stand will remain pressurized. P2 is connected to the DIN Cartridge Valve which is connected to the cap end of the Backup Roll Balancing Cylinder on each of the six mill stands. If the DIN valve is piloted open, pressurized hydraulic fluid will flow from Accumulator 244 into the cap end of the balancing cylinder. On mill stand F1, the DIN cartridge valve insert is 358.01 and the directional control which controls the opening and closing of the insert is 358.01.P3 is connected to an accumulator and the rod ends of the Operator and Drive Side Bending/Balancing cylinders on all six mill stands. The rod ends of these twenty four cylinders will be pressurized until the accumulators on the accumulator stand and in each of the circuits is manually vented.
8) Unless the AGC Operator and Drive Side cylinders are fully retracted, the cap ends of the cylinders will be pressurized by the induced load of the cylinder piston and rod assemblies as well as by the machine members each cylinder supports. On mill stand F1 Manual Shutoff Valve 079.01, Pilot Operated Check Valves 154.01 and 154.02 and Solenoid Vented Relief Valve 260.01 will prevent the hydraulic fluid from flowing out of the cap end of the cylinder, if the solenoid valve on the relief valve is actuated to the parallel flow pattern. Note that when Directional Control Valve 307.02 is de-energized, the pilot operated check valve will close.
9) Unless the Operator and Drive Side Bending/Balancing cylinders are fully retracted, the cap ends of the cylinders will be pressurized by the induced load of the cylinder piston and rod assemblies as well as by the machine members each cylinder supports. On mill stand F1 Pilot Operated Check Valves 153.11 and 167.31 and Relief Valve 291.01 will prevent the hydraulic fluid from flowing out of the cap end of the cylinder. Note that when Directional Control Valves 303.23 and 305.01 are de-energized, the pilot operated check valves will close.