事故案例一则（english with comments of process safety expert)

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The process and the incident
[size=78%]§In a refrigeration unit an organic liquid loop was cooled down by evaporation of
liquid ammonia. During a shut down for cleaning a catastrophic rupture of the evaporator occurred .
Causal chain
[size=78%]§During the shutdown, an isolation valve was closed between the heat exchanger (= evaporator) and a pressure-relief device designed to protect the heat exchanger from possible over-pressure. (This might have been done to dismantle the safety valve for testing).

[size=78%]§Steam was used to clean out process piping; the steam also flowed through the heat exchanger tubes.
The steam heated the liquid ammonia remaining in the exchanger shell, which caused the pressure to build.
With the path to the pressure-relief device blocked, the heat exchanger ruptured catastrophically.
Lessons learned
[size=78%]§Avoid installation of a block valve on the path to a safety valve; if it is necessary to disconnect a safety valve, while the system may be pressurized install twin safety valves (with alternating shut-off valves) ensuring that the path to one safety valve is always open
[size=78%]§If a block valve is on the path to a safety valve, it must be locked open. The block valve may only be closed following a strict procedure ensuring that the system to be protected cannot experience a pressure higher than design pressure.



Remarks:
[size=89%]§According Bayer standard, liquid twin safety valves (with alternating shut-off valve) are installed on equipment used as buffer for liquid ammonia and in which an energy input occurs (as the evaporators mentioned here above) to allow maintenance and inspection of the safety valve without having to empty the equipment. Other equipment is equipped with single safety valves with manual locked open (car-sealed) valves on both sides. This kind of equipment is put out of service before maintenance or inspection work on safety valve.
[size=89%]§Nevertheless the safety valves on the evaporators are not designed for an extreme heat
input as described here above!. If on the process side cleaning is necessary, it is recommended to empty the evaporator (on the ammonia side) before starting cleaning operation. If this is not done, following measures are necessary:
[size=89%]–the evaporator must be on the process side safely disconnected from the liquid loop
[size=89%]–the evaporator must be on the ammonia side open to the ammonia gas line to the ammonia compressors
[size=89%]–the pressure on the ammonia side must be monitored during the cleaning operation.
[size=89%]§If such operations have to be performed, it is necessary to involve the personal responsible for operating the central refrigeration unit

谢谢楼主分享