Assessment of Safety Integrity of LDPE Tubular Reactor With Emergency Pressure Relief Requirement Consideration

The production of low density polyethylene (LDPE) with tubular reactor process always involves ultra high pressure (up to 310 MPa (3059 atm)) and medium temperature (normally less than 310°C (590 F)). As polymerization is highly sensitive to temperature and pressure fluctuation, emergency relief valves are required to prevent the runaway of a reaction (C₂H₄ decomposition, a strongly exothermal reaction). The pressure increase during decomposition develops rapidly (normally in several second). The pressure relief speed plays an important role in the safety integrity of reactor. The purpose of this paper is to demonstrate the evaluation of safety integrity of tubular reactor by taking process deviation and pressure relief requirement into consideration, and to determine the safety requirement specification of the emergency relief. A state of the art analysis is proposed in this paper. The time-dependent pressure fluctuation and relief capacity, as well as various decomposition scenarios during emergency pressure relief are studied. The analysis is then used to determine the required redundancy of the emergency relief valves. The probability of failure on demand (PFD) of multi-valve relief operation is calculated with the classic reliability model. The pressure fluctuation due to emergency pressure relief is calculated with a proven-in-use relief model. Necessary assumptions are made about the relief operation. It is concluded that 2 out of 5 (2005) emergency relief valves are sufficient for the emergency pressure relief in order to prevent the overpressurization of the tubular reactor under a given condition that the emergency relief valve fully opens in less than 3 seconds.