Sequence of Operations |
Sequence of Operations |
Roll Change HPU Off - Energy Stored in Accumulators |
Even when the Roll Change pressure compensated pumps are not operating, there is stored hydraulic energy present in the roll change circuits for two reasons: 1) there are several accumulators installed in this system and there may be loaded actuators (hydraulic cylinders), both of which present a danger to personnel and the mill itself.
1) None of the accumulators in the system are automatically vented upon shutdown of the electric motors which power the pressure compensated pumps. The DIN Cartridge Check Valve Assemblies (366.01.371.01 through 366.07/371.07) located at each pump station prevent the accumulators from venting back through the pumps and there is not any sort of valve which will automatically vent any of the accumulators. As a result, these accumulators supply pressurized hydraulic fluid to the main pressure header line or into one specific section of a circuit. The circuits which include accumulators are: Pilot Pressure Supply, Hold Down Roller, Pinch Roll Exit Side, and Upper Pinch Roll Entry Side.
2) Actuators (hydraulic cylinders) which support a load by hydraulic fluid trapped between the one end of the cylinder and a closed pilot operated check valve or a blocked directional valve spool present a danger if someone disconnects a cylinder line or shifts a directional control valve that opens a flow path for the pressurized fluid to escape, or pilots open a pilot operated check valve, also opening a flow path for the pressurized fluid to escape. Cylinders which may be supporting a load include, but are not limited to, the following circuits: Descaling System Hood Swiveling, Liftable Roll, Hold Down Roller, Pinch Rolls (both Entry and Exit sides), Tilting Work Roll Changing, Lifting Rail, and Entry and Delivery Guides.
3) Before performing maintenance, ensure the accumulators are manually vented and that cylinders have been fully extended, or retracted, as appropriate, and that the cylinders are not supporting a load or that the loads are safely blocked in place.
4) Review the schematics for this event to locate the accumulators and sources of induced load pressures.