Fusible loop fire protection system Fusible loop is a very common kind of fire protection method in offshore oilfield processing .Usually the aim of fusible loop protection is processing equipment that may catch fire or explode such as pipeline pump or high/low pressure vessel .It was widely used in offshore processing since this method got the advantage of low cost in designing and maintenance ,also because this method is very durable and very easy to install to any shape or volume equipment .In the following pages will introduce the basic theory and principle of fusible loop protection system. The basic element in a fusible loop protection system is a constant Instrument air supply and a throttle which continuously supply small amount of air to tubing to keep tubing air pressure in case of small leaking in the following tubing net, other two important element in the following tubing net are the tubing itself and the fusible head . tubing’s function is lead air to everywhere we want to protect and the most important parts are the fusible head itself ,the fusible head normally looks like a tubing cap but in the end of the cap was blocked with a kind of special designed alloy ,which will melt quickly and vent fusible loop when the temperature around fusible head reach the designed setpoint . Of course ,the dropping of fusible loop tubing air pressure will catch by a pressure switch which will send fire signal to control system .Also, the dropping of pressure will release pneumatic relay(normal open ) which was used to control Deluge valve (normal close)and cause fire water or other kind of extinguisher released from Deluge pipeline. Sometimes we didn’t got deluge system, At that case the fusible loop just used to give a electrical fire signal out . Some import point in fusible loop is listed below : 1) The air supply’s pressure must be constant .usually we use a air regulator to supply air for fusible loop ,the pressure range in fusible loop is choose by the fusible head. Most fusible head’s safe working pressure is below 60 PSIg .we must take care not using above this limit, otherwise the fusible head may be blow off by the tubing pressure and trip a alarm . 2) The throttle we used to keep the fusible loop pressure normally paralleled with a check valve to prevent some incident supply pressure dropping and to isolate different loop .the throttle should adjust and fasten since accident close of that throttle will cause tubing pressure slowly dropdown and that will be difficult to troubleshooting .Also, usually we paralleled a pneumatic charge button to raise the speed of restarting since the throttle ‘s supply speed is very low in case of long tubing or big protection area . 3) The temperature setpoint of fusible head was choosen by the protection area status, if the protection equipment is area around the axis of pump ,usually we choose 250゜F style. For normal vessel protection ,we choose 160゜F style instead .there are a serial style of fusible head we can choose .the temperature setpoint is curved in the surface of fusible head and must be attention to choose the same type while changing or replacing. 4) Checking the fusible loop leakage normally use fresh water which were pumped to fusible loop after stoping the fusible loop. Avoid using seawater since if sea water will dry and left salt accumulated in fusible loop ,it will cause tubing blocking and will be hard to get rid of them . 5) Normally the pressure switch which detect the dropping of tubing pressure is just set at the 2/3 of normal pressure .The reason for this is avoiding some incident little change of tubing pressure that sometimes may occur. the number of fusible loop and the length of tubing is also limited and can’t extend as big as we wish .the dropping of pressure when one fusible head melt must great than 2/3 of normal pressure to left enough space for pressure switch acting . Just like we point above, fusible loop protection system is ve