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Firefighting (FiFi) Tank Outlet Blockage

What happened?

A member reports a blockage of the outlet of a dive chamber fire fighting water tank, caused by the failure of a floating plug.

On this particular DSV the saturation diving chamber complex has six firefighting water tanks. Each tank has a water outlet at the bottom with a floating plug that blocks the outlet if all the water drains from the tank. See line drawing of the bottom of one of the FiFi tanks.

The floating plug assembly consists of a buoyant float and a nylon conical endcap plug with recessed O-ring, which sinks in water. The floating plug assembly is highlighted red in the line drawing. The float is connected to the endcap plug with a threaded rod through the centre, as shown on Figure 3. The floating plug is retained in the tank by a guide tube.

What were the causes?

The endcap plug unscrewed itself over time and eventually detached from the float. Because the endcap plug is heavier than water it sank to the bottom of the guide tube and blocked the tank outlet.

That would result in no water being supplied from the tank, and therefore it could have life-threatening repercussions in the event of an emergency. In normal circumstances it would only be detected during routine 6 monthly maintenance. (IMCA emphasis).

Actions taken? lessons learned?

The floating plug assemblies were removed from all 6 x FiFi tanks on the vessel and the endcap plugs were permanently bonded to the floats to prevent this failure from happening again.

Each DSV should review the water storage tanks in their system for any similar failure mechanisms. This failure was identified in a dive system FiFi tank, it could equally occur in other water tanks in any vessel.

Safety Event

Published: 23 October 2020
Download: IMCA SF 30/20

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