Abstract: The report will compare and discuss the existing control system implemented by a plant in Qatar and the newly designed control system for the plant based on reliability and probability of failure on demand (PFD) calculations. This report also describes the new control system in terms of designing, building and testing phases of the control system. This plant uses the blowback system to filter out product dust from the waste gas using a filter. The filter is cleaned using the air flow that comes from the air header which is controlled by a poppet valve (solenoid valve). The poppet valve purges air for fixed intervals of time. The failure of the poppet valve (stuck open) led to a continuous flow of air and the pressure in the air header was decreased from 7 bar to 3.5 bar. This triggered the emergency shutdown alarm and the plant was shut down. Due to the shutdown, the company loses time and money. The company fixed the problem by implementing one transmitter and one on/off valve. The purpose of this project is to design (based on reliability), build and test a control system which will monitor and control the pressure in the instrument air supply line and takes the process to a safe state by closing the valve in case the pressure decreases below the setpoint. The instrument drawings for the new control system were created. The reliability calculations were done for both the control systems (company’s solution and student’s solution). The new control system was more reliable and the probability of failure on demand rate was lower than the company’s solution. The control system was developed using programming languages (function block diagram and ladder logic diagram) with the help of RSLogix 5000 software and a HMI interface for the control system was created using Factory Talk View software. The developed program was capable of taking the process to a safe state (in case the pressure decreases below the setpoint) and the HMI interface displayed the pressure measurements and alarms triggered by the process. The recommendation with this report is to use a different type of voting system for the controller (2oo2 with diagnostics) which involves selecting the transmitters with Advanced HART diagnostics suite. The 2oo2 Diagnostics voting logic provides high safety and low false trips. The diagnostics feature continuously checks the health of transmitters and takes preventive measures by notifying the operator before abnormal process conditions can occur.
Keywords: Pressure, Safe, Process, Control System, Design, Air, PLC, HMI.
Title: Design a Safety Control System for the Instrument Air Supply in a Qatar Plant
Author: Haseeb Ahmed, Mouhsin Hassan, Daniel Mercier
International Journal of Mechanical and Industrial Technology
ISSN 2348-7593 (Online)
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