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Basic Fusible Plug

Basic Knowledge Fusible Plug
Basic Instrumentation - Basic Fusible Plug


Actually the construction of fusible plug as a tubing fitting so sometimes some people call it fusible fitting,
the main part of fusible plug contain a eutectic alloy plug. The eutectic alloy plug has a specific melting point. For example you can see the rating of each melting point as in the table. The temperature rating is signed by color.

Stamp Color Melting Point in °F (deg °C)
160 Black  160 °F (71 °C)
212 Orange  212 °F (100 °C)
255 Green  255 °F (124 °C)
281 Red  281 °F (138 °C)

* reff: fusible plug swagelok

Sensor Fusible Plug:



Basic Principle Fusible Plug 1
Basic Instrumentation - Basic Fusible Plug


The sensor cann't work alone in application we must apply a pressue to the sensor via tubing liner (usually stainless steel). When the sensor is exposed to the temperatures (fire) " at or above" the melting point, the eutectic alloy plug will melting and the pressure inside the fitting or tubing will be released to the atmosphere.



basic fusib 



Picture above ilustrate when fusible plug stand alone before installed in the application.

Basic Principle Fusible Plug 2
Basic Instrumentation - Basic Fusible Plug


After that the sensor will be exposed to the high temperature, for example fire
and then reach " at or above" the melting point of the sensor. What happen with the sensor? the euctectic alloy plug will melting as shown picture below.



basic fus4

In this condition fusible plug sensor still in off service, or stand alone. This is the basic principle how fusible plug working, so later will be continued working principle fusible plug in installation/application.


Basic Principle Fusible 3
Basic Instrumentation - Basic Fusible Plug


Now we will apply pressure to the fusible plug sensor by installed tubing liner. The ilustration   less over like this picture. You can compare the different with the posting before " basic working principle fusible plug 2 ". Now we have pressure inside the sensor.

basic fus2



So in here pressure keep standby inside the tubing liner and the sensor. Now we apply fire at around the fusible sensor or by increasing temperature on the fusible sensor. When temperature around the sensor reach the melting point of fusible, euctectic alloy will melting and pressure inside the sensor will release to the atmosphere. Decreasing the pressure will we use to drive other equipment or instrumentation depend on the application. Usually we use in the shutdown system of process.



 basic fus3


Decreasing that pressure can be detected by pressure transmitter (as PSLL), or in pneumatic system detected by T-way pneumatic valve. Both have function to trigger signal shutdown.


Basic Application Fusible Plug
Basic Instrumentation - Basic Fusible Plug


Beside " Fusible Plug System " we also frequently mention " Fusible Loop System ".
Basically is the same. When we talk fusible loop, means fusible plug is installed in a real application, not only stand alone. Here for example fusible loop used to initiate signal shutdown/trip and the system use the Pressure Transmitter to monitor the loop pressure. Commonly we mention the pressure transmitter here as PSLL (Pressure Switch Low Low), base on main function. Then signal will be send to PLC. below is illustration of the installation, assume initial condition of pressure in the loop is 0 Barg, because uncharged before.





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