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Fusible Plug Activation

Fusible Plug Activation

(OP)
Hi all,

A question came up in a recent relief device review. We have installed fusible plugs (melting point 450°F) on the liquid-filled jackets of some equipment to protect them in the event of an external fire. The question that came up was whether the alloy is designed to melt from the external fire (temperatures of 1660°F+), or if the plug is designed to melt from the temperature of the fluid in the jacket. A third-party has said that the fusible plug would not activate during an external fire since the fluid will never reach 450°F during a fire (temperature reaches around 300°F corresponding to saturation temperature). However we believe the plug should melt just from the very high external temperature (the plug is located at a low point in the jacket).

Does anyone have experience with this, or a reference that provides more detail on this? I haven't yet been able to receive a response from the manufacturer of the plugs.

RE: Fusible Plug Activation

My experience is that the plug would melt due to heat from the fire. I have always placed the plugs to be impacted by possible flames.

RE: Fusible Plug Activation

Fusible plugs are not intended be used as primary protection for systems vapourizing liquids, so your use of the word "saturation" has me concerned- systems containing vapourizing liquids should be protected by pressure relief. A fusible plug might be used to protect a piece of equipment which would rapidly boil dry in a fire case, but in that case it would be used in concert with a pressure relief valve.

In vapourizing liquid service, even if the plug is directly impinged on by flames, if there's liquid water present inside and it can boil, the plug metal temperature may not rise high enough to fail until all the liquid is gone. Unless pressure relief is also provided, failure of the equipment may occur before that point.

They are primarily intended to protect gas-filled equipment, and for equipment filled with liquids which boil at very high temperatures (with decomposition) such as heat transfer oils. They protect against the loss of material strength due to fire, which a pressure relief valve does not. A non-resealing device such as a rupture disk may also be used in those services.

RE: Fusible Plug Activation

(OP)
Thank you moltenmetal, that was exactly the answer I was looking for.

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