School of Chemical and Biomedical Engineering Division of Chemical and Biomolecular Engineering Plant Safety LOPA Analysis Name: Ong Ding Shan Matric number: U1920546E Current Existing System Figure 1 shows the selected operation that was used for analysis. Figure 1 Solvent intermediate storage tank A layer of protection analysis was done on a solvent intermediate storage tank targeting the potential overpressure of the vessel. In essence, this study evaluates the functionality of current existing independent protection layers that reduces the likelihood of vessel rupture and proposes new implementations to minimize the overall likelihood at greatest risk to be less than the target hazard of 3 × 10−5 per year. In the event of vessel rupture, the initial failure will spread and the sudden release of flammable gas will result in potential fire balls and explosions, causing damages to lives and properties. Some assumptions were made within the scope of this study. - Steam pressure control valve is periodically tested. PSV is meant to proect the over pressurization of pipeline for trapped liquid expansion scenario. Check valve maintained in accordance with manufacturer’s recommendations. - A vessel failure will result in a tear because the vessel is made from ductile carbon steel. (The maximum pressure the vessel can withstand is 3 barg.) Table 1 shows the LOPA analysis done on the selected operation. Scenario Overpressure of a Solvent Intermediate Storage Tank leading to PV Energy Explosion/Deflagration of Flammable Liquid Targeted Frequency 3.0E-05 Calculated Frequency 4 5 2 6 8 9 10 BPCS Relief Valve Check Valve Discharge Isolation Block Valve Unmitigated Likelihood 0.1 1 0.1 1 1.0E-03 1 1 0.1 1 1.0E-02 1 2 3 Initiating Event Description Initiating Cause Cause Likelihood Tank design Fatality BPCS Vent Valve Short circuit causing electrical signal from level switch failing to reach transfer pump 0.1 1 1 Clogging of Vent Line 0.1 1 1 Backflow of highpressure solvent steam mixture causing substantial overpressure ratio Build up of pressure within the storage tank due to lack of venting 7 Protective Layers S/No. 1 1.1E-02 Table 1 LOPA analysis of unmitigated operation The level switch protects the transfer pump by translating the level of the solvent in the tank to the operation of the transfer pump via an electrical signal. A short circuit will cause the electrical signal to be jammed and the transfer pump to cease operation. - The initiating frequency of short circuit: 0.1/year. Maximum pressure that the vessel can withstand without rupture is no greater than 3 barg, 50’C. The vessel contains no innate ability to rectify increasing pressure hence it was attributed a factor of 1. Probability of a person present was assumed to be at 100%, hence fatality was given a factor of 1. Vent line relieves the pressure within the tank. The probability of it being clogged was assumed a conservative value of 0.1/year. In the second scenario, the vent pipe was assumed to have failed, hence it has no credit. Relief valve allows high pressured steam solvent mixture to return to the vessel. It does not relief pressure within the tank hence it was given a factor of 1. Check valve prevents the backflow of steam solvent mixture back to the pump, thus preventing a build up of pressure in the tank. It has a probability of failure of 0.1/year. Discharge isolation block valve is kept open always, and thus does not mitigate the overpressure scenario, hence it was given a factor of 1. References for the probability of failure in these scenarios were retrieved from lecture slides and notes. Modified System Figure 2 shows the selected operation that was modified. Figure 2: Modified Solvent intermediate storage tank Table 2 shows the LOPA analysis done on the modified operation. Scenario Targeted Frequency S/No. 1 2 Overpressure of a Solvent Intermediate Storage Tank leading to PV Energy Explosion/Deflagration of Flammable Liquid 3.0E-05 Calculated Frequency 4 5 Cause Likelihood Tank design Fatality Short circuit causing electrical signal from level switch failing to reach transfer pump 0.1 1 1 0.1 1 Clogging of Vent Line 0.1 1 1 1 1 1 2 3 Initiating Event Description Initiating Cause Backflow of highpressure solvent steam mixture causing substantial overpressure ratio Build up of pressure within the storage tank due to lack of venting 1.1E-05 6 7 8 Protective Layers BPCS BPCS Check Vent Relief Valve Valve Valve 9 10 11 Additional Layers 12 Discharge Isolation Block Valve Alarm Rupture Disc Mitigated Likelihood 0.1 1 0.1 0.01 1.0E-06 0.1 1 0.1 0.01 1.0E-05 The design of the intermediate storage tank can be improved by adding the following implementations. 1) Independent pressure tapping on the storage tank with an alarm, pressure alarm high PAH (PFD = 0.1) This device tracks the pressure within the storage tank and is set at 2 x MAWP of the vessel (2.0 barg). At 2.0 barg, the critical alarm will be set off. Operators will be activated and respond to the situation on ground to take effective action. For instance, shutting down the plant. The probability of failure of this implementation is linked to the operator failure that is tagged to be at 10−1 per opportunity, found from a reference material of LOPA analysis [1]. Periodic maintenance should be scheduled to ensure operating conditions of the alarm. These checks should be recorded and logged for auditing purposes. 2) Installation of rupture disc with an emergency tank (PFD 0.01) A rupture disc is a one-time use pressure safety disc that fails at a set differential pressure. For this operation, the pressure will be set at 2.5 x MAWP of the vessel (2.5 barg). At 2.5 barg, the membrane ruptures and causes solvent within the storage vessel to flow into the emergency tank. This provides instant response to critical pressure levels of the tank. The probability of failure of this implementation was tagged to be at 0.01, found from a reference material online. Periodic checks should be done to ensure that the rupture disc is not damaged, and it should be replaced as accordance to manufacturer’s recommendations. Conclusion The LOPA analysis highlighted that the initial design did not satisfy the target hazard of 3 × 10−5 per year. Hence, two solutions were crafted to minimize the target hazard down to 1.1 × 10−5 per year. In the initial analysis, two potential initiating events were considered that assumes the failure of different independent protection layers and computing if the vessel will rupture. Every equipment was analyzed to understand if their function and deviation could impact the pressure of the storage tank. The probabilities of both scenarios were added that highlights a vessel rupture chance of 1.1 × 10−2 per year. The installation of the PAH and the rupture disc would relieve the pressure within the storage vessel at critical pressure. These initiatives are independent of the existing measures and serves to mitigate the likelihood of vessel rupture. They are also accessible to the operator, hence, auditable. Hence, the hazard risk was mitigated to 1.1 × 10−5 per year, meeting the targeted hazard tolerance of 3 × 10−5 per year. References [1] Raman, B. (2020). Layer of protection analysis [pdf]. Singapore: Nanyang Technological University. [2] American Institute of Chemical Engineers, Guidelines for Initiating Events and Independent Protection Layers in Layer of Protection Analysis, New Jersey: Wiley, 2015.
