Safety Instrumented System merupakan suatu sistem yang berfungsi untuk mempertahankan Basic Process Control System (BPCS) tetap berada didalam kondisi yang aman dengan melakukan aksi-aksi penyelamatan secara otomatis. Dalam proyek akhir ini telah direalisasikan suatu penerapan Safety Instrumented System berbasis PLC pada sistem cooling tower PLTP yang bertujuan untuk mendeteksi kondisi bahaya dari plant dan mengembalikannya ke dalam kondisi yang aman. Terdapat beberapa indikasi bahaya yang dapat terdeteksi oleh sistem ini, diantaranya pada saat kondisi air di tangki Hot Basin, Cold Basin, dan Kondensor berada di posisi level High-High atau Low-Low, dan pada saat terdapat tekanan berlebih di pipa saluran antara tangki Cold Basin dan Kondensor. Pada sistem ini digunakan PLC sebagai logic solver dengan metode perancangan diagram keadaan dalam proses pembuatan ladder diagram-nya. Hasil pengujian menunjukkan bahwa sistem mampu mendeteksi kondisi bahaya dengan baik. Selain itu sistem juga mampu mengembalikan kondisi plant ke dalam kondisi yang aman dengan membuka SIS Emergency Valve bahkan dengan melakukan Emergency Shutdown (ESD) jika kondisi paling bahaya terdeteksi. Kata kunci: Safety Instrumented System, Emergency Shutdown, Valve, Cooling Tower.

Safety Instrumented System is a system that serves to maintain the Basic Process Control System (BPCS) remains in a safe condition by performing rescue actions automatically. In this final project has been realized an application of Safety Instrumented Systems based on PLC on geothermal power plant’s cooling tower system that aims to detect hazardous conditions of the plant and put it back in a safe condition. There are some indications of the dangers condition that can be detected by this system, i.e when the condition of water level in the Hot Basin Tank, Cold Basin Tank, and the Condenser Tank is in the High-High or Low-Low level position, and when there is excess pressure in the pipeline between the Cold Basin Tank and Condenser. This system is using the PLC as a logic solver with the state diagram method in the process of making its ladder diagrams The results show that the system is able to detecting hazardous conditions well. Beside that, the system is also able to restore the plant to a safe condition by opening the SIS Emergency Valve even by the Emergency Shutdown (ESD) if the most hazardous condition is detected. Keywords: Safety Instrumented System, Emergency Shutdown, Valve, Cooling Tower.

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