Delavar, A. ., M. . Mirza Ebrahim Tehrani, and S. S. . Alizadeh. “Layers of Protection Analysis to Achieve Safety Integrity Level (SIL), Case Study: Hydrogen Unit of Refinery”. Scientific Journal of Pure and Applied Sciences, vol. 5, no. 11, Nov. 2016, pp. 515-23, http://sjournals.com/index.php/sjpas/article/view/168.