Minor repairs had just been completed on the shell plate of the fresh water generator (evaporator) by a shore workshop while in port. As an original spare was not available, the damaged sight glass was substituted with a disc cut from a 5 mm thick acrylic sheet.
Upon restarting the plant after sailing, it was observed that the drum chamber was not developing sufficient vacuum. The ship’s engineers decided to carry out a pressure test of the casing to locate any leaks. Without considering the hazards, the crew introduced compressed air into the vessel and raised the internal pressure to about 3 bar. Suddenly, the acrylic sight glass shattered, injuring the electrical officer, who was applying soap solution to the shell’s exterior.
Lessons learnt1 It is very unsafe to subject vessels, tanks or containers to uncontrolled pneumatic pressure for testing purposes as there is great risk of permanent deformation or violent rupture;
2 A controlled hydrostatic test i.e. filling up the container with water to a permissible head (preferably under Class supervision) is the most appropriate and safe method for leak testing on board;
3 When a pneumatic test is considered the only practicable method, compressed air must be admitted through a suitable reducing arrangement and pressure must be closely monitored by a manometer/water column gauge, ensuring safe limits are never exceeded;
4 Fresh water generators should ideally be tested by creating an internal vacuum and applying a liquid dye externally on suspected areas.
Upon restarting the plant after sailing, it was observed that the drum chamber was not developing sufficient vacuum. The ship’s engineers decided to carry out a pressure test of the casing to locate any leaks. Without considering the hazards, the crew introduced compressed air into the vessel and raised the internal pressure to about 3 bar. Suddenly, the acrylic sight glass shattered, injuring the electrical officer, who was applying soap solution to the shell’s exterior.
Lessons learnt1 It is very unsafe to subject vessels, tanks or containers to uncontrolled pneumatic pressure for testing purposes as there is great risk of permanent deformation or violent rupture;
2 A controlled hydrostatic test i.e. filling up the container with water to a permissible head (preferably under Class supervision) is the most appropriate and safe method for leak testing on board;
3 When a pneumatic test is considered the only practicable method, compressed air must be admitted through a suitable reducing arrangement and pressure must be closely monitored by a manometer/water column gauge, ensuring safe limits are never exceeded;
4 Fresh water generators should ideally be tested by creating an internal vacuum and applying a liquid dye externally on suspected areas.
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