block flow diagram bfd process and instrument symbols process flow diagram pfd process and instrument diagram p amp id valves valves ball valve شیر توپی butterfly valve شیر پروانه اي globe valve شیر ساچمه اي gate valve شیر دروازه اي needle valve شیر سوزنی plug valve شیر مجرابند یا سماوري diaphragm valve check valve شیر یکطرفه pumps amp compressors pumps amp compressors centrifugal pumps پمپهاي گریز از مرکز positive displacement pumps پمپهاي جابجایی مثبت rotary دورانی reciprocating تناوبی pumps compressors positive displacement rotary positive displacement reciprocating centrifugal centrifugal heat exchangers furnace and boiler heat exchangers shell and tube heat exchanger مبدل هاي حرارتی لوله پوسته vessels vessels cooling tower reactor reactor tower line symbols instrument symbols control loop set point heat exchanger heat exchanger control system sensor transmitter also often called the primary and secondary elements controller the brain of the control system final control element often a control valve but not always other common final control elements are variable speed pumps conveyors and electric motors an electronic control loop controlling flow process flow diagram