Double segregation is necessary
Shutdown and isolation of pressurized heat, liquid, and gas systems
For the isolation of chemical, thermal, liquid, and gas systems where pressure and fluidity are present, the valve shall be closed, cut off, locked, and emptied (these systems shall also cut off power to the motor or pump involved), but at least double isolation shall be taken in the cases listed below.
Access to restricted space requiring access permission; The presence or past presence of flammable materials;
Logistics with PH less than 2 or greater than 12 exist or have existed;
Pressure greater than 100PSI (5,171.5 mmHg) exists or has existed;
A temperature greater than or having been 140°F (60 ° c)
Presence or past presence of oxidants, cryogenic or toxic substances;
The presence or past presence of harmful bacteria or viruses (e.g., cooling towers);
Double isolation measures refer to the combination of two types of mechanical isolation, such as "double partition emptying", single valve and blind plate isolation, single valve and removal/displacement pipeline isolation and disconnection operations, which must follow the relevant provisions of the disconnection pipeline procedure.
If only one form of mechanical isolation is allowed on site under the above process conditions, sufficient personal protective equipment must be used as a second line of defense.