API 521 7 th Edition Ballot Item 7.8 Work Item 4 Gas Breakthrough NOTE: This is a reballot of previously approved API 521 7 th Edition Ballot Item 6.3 which was modified based on comments. Comments should only address changes being balloted, truly address technical issues if they are technical comments, and propose alternative language. Background Work Item 4: # Source Section Comment Proposed Change Volunteer 4 6 th Edition Ballot 2 comme nt 67; ballot 3 comme nt 5 4.4.8.3 API 521 6 th Edition Ballot 2: Add new criteria on liquid displacement considerations following gas blowby. Ballot 3: Liquid displacement can occur due to number of scenarios that involve HP/LP interfaces (Reverse flow, tube rupture, etc) not just vapor breakthrough. I would like to suggest that this be a possibly be part of a separate section that looks at this subject on from a higher level and then provide detail for individual scenarios. Liquid displacement definition will be included. Solutions (mitigation plans) will be included, such as increasing the volume of downstream vessel, installing a relief device upstream. Cascading break through will not be included in the first draft API 521 6 th Edition Ballot 2: If the vapour space is insufficient to prevent the downstream vessel from filling prior to gas breakthrough, pressure relief valves sized only for the gas breakthrough flow may not prevent overpressure of the vessel. Overpressure may be prevented by one of the following options sizing the pressure relief valves to discharge liquid at the volumetric equivalent of the incoming vapour flow increasing the size of the downstream vessel to provide additional vapour space to accommodate the maximum anticipated liquid inventory providing a HIPS on the high pressure vessel which isolates the flow to the low pressure vessel on low liquid level. Evaluate cascading break through. P. Frey; M. Brewer; M. Porter; E. Vatland Johansen; T. Bevilacqua; K. Campbell; L. Mcdaniel; W. Ciolek; J. Burgess; F. Self / A. Aldeeb
Proposed Modifications to API 521 6 th Ed: 1. Add the following sentence at the end of Paragraph 3 in Section 4.4.8.3. Annex G provides additional considerations to evaluate the impact of the vapor breakthrough or gas blowby on low-pressure system and overpressure relief requirements. 2. Add Annex G (Informative) Liquid Displacement : Annex G (Informative) Vapor Breakthrough into Liquid- containing SystemsDisplacement Failure of high-pressure vessel liquid bottoms level control and bypass valves discharging into a low-pressure system may result in significant increase in the lowpressure system liquid level. Depending on the high-pressure and low-pressure systems volumes and liquid inventories, the low-pressure downstream system may liquid overfill. Of special concern, in certain cases this scenario may be followed by loss of liquid level in the high-pressure system that can result in Vapor vapor breakthrough through the inlet level control and bypass valves to the low-pressure systems (the scenario described in Section 4.4.8.3) is generally preceded by the high-pressure system vapor phase displacing the liquid phase in the low-pressure system until eventually vapor breakthrough occurs at the inlet control valve. As the vapor passes through the inlet level control valve, the vapor will expand and push (displace) the liquid in the downstream system until a relief path is established. This transient scenario is commonly described as liquid displacementthis is commonly described as liquid displacement. During the scenario, the liquid level in the low-pressure vessel can rise creating the potential for liquid or two-phase relief. This can result in increased lowpressure system relief requirements relative to a vapor breakthrough with only vapor relief. Due to the difficulties of establishing the required relieving capacity and ensuring that this is continuously available, it is conservatively assumed no credit is taken for
vapor outflow and the low-pressure system reaches relieving pressure. The consequences of liquid displacement are very sensitive to the size of the low-pressure system and liquid inventories in the high and low-pressure systems prior to the start of the scenario. Hence, a review should be undertaken to identify the worst case operating conditions for the liquid displacement assessment including consideration of non-routine situationsincluding process start-up and shutdown. In this annexsection, general considerations are described to identify potential liquid displacement relief requirements associated with the vapor breakthrough scenarios. Other publications provide more detailed guidanceinformation on liquid displacement analysis [1] [2]. It should also be noted that other overpressure scenarios may result in similar liquid displacement effect such as control valve failure, heat exchanger tube rupture, inadvertent opening of manual valves, reverse flow, instrument air failure, or power failure. However, these scenarios are not addressed in this section. The phase quality and flowrate of the relieved stream depends on following factors: high and low pressure systems operating temperature and pressure; high and low pressure systems fluid composition; high and low pressure systems liquid levels; i.e., liquid inventories; prior to failure of the inlet valve(s) liquid-vapor onset/disengagement regime: foamy, bubbly, and churn-turbulent fluids; vapor superficial velocity in low-pressuredownstream vessel; low-pressure vessel orientation (vertical or horizontal); low-pressure vessel and inlet nozzle elevation; low-pressure vessel inlet flow distributor geometry and orientation (if present); location of the pressure relief device; and assumption for continuation of high-pressure and low-pressure systems liquid outflows at normal minimum rates Depending on the high and low pressure systems liquid inventories, there are two potential relief scenarios that may be relevant: A. Upstream system liquid inventory is less than the downstream system vapor volume at relief conditions. The user should determine the liquid level in the low-pressure vessel assuming all of the liquid from the high-pressure vessel is transferred. A vapor-liquid disengagement analysis should then be performed to determine the phase of the
relief stream. The DIERS Project Manual [3] and the CCPS Guidelines for Pressure Relief and Effluent Handling Systems [4] provide guidance to determine if two phase relief will occur due to liquid swell (e.g., the increase in liquid level due to liquid and vapor mixing) or liquid entrainment caused by the velocity of the vapor across the surface of the liquid. Complete vapor-liquid disengagement with no liquid entrainment will result in vapor relief. The Required required vapor relief rate should be determined using the guidance provided in Section 4.4.8.3. In case it is assumed that liquid will continue to flow from the high-pressure system, then the impact of flashing liquid on relief requirements should be evaluated. However, if the vapor-liquid disengagement analysis revealed reveals a two-phase flow, then the guidance provided in the DIERS Project Manual [3] and the CCPS Guidelines for Pressure Relief and Effluent Handling Systems [4] can be used to estimate the two-phase relief stream quality. This guidance considers the fluid regime, i.e., foamy, bubbly, or churn-turbulent. The required relief rate should be equal to the volumetric flow of fluid entering the system through flow limiting element (wide-open control valve, bypass valve, restriction orifice) at relief conditions taking into consideration the mixing with swelled liquid phasebased on the volumetric vapor flow rate through wide-open control and/or bypass valves mixed with the swelled liquid phase. If the low-pressure vessel s inlet nozzle becomes submerged, then when the vapor breakthrough occurs the vapor will be sparged into the low-pressure vessel causing the liquid level to rise further. If it is determined User should evaluate if a that twophase relief will not occur due to inadequate vapor-liquid disengagement, then two phase flow or due to high flow vapor re-entrainingentrainment of liquid should be considered[2]. B. Upstream system liquid inventory is more than the downstream system vapor volume at relief conditionsthe vapor volume in the downstream system is insufficient to prevent it from filling with liquid prior to vapor breakthrough. In this case, the low pressure vessel overfills before the vapor breakthrough occurs. The initial relief will be a steady-state liquid relief (or steady-state two-phase relief). However, it is expected that the liquid level in the high-pressure vessel will eventually be lost which would result in vapor breakthrough. Since the low-pressure system is full, The the required relief rate should be calculated based on the displaced liquid at a rate equal to the volumetric flow of fluid entering the system through the flow limiting element (wide-open control valve, bypass valve, restriction orifice) at relief
conditions volumetric rate equivalent to the actual volumetric vapor flow rate through the flow limiting element (wide-open control valve, bypass valve, restriction orifice). Liquid displacement should be calculated using liquid and vapor densities at the relieving conditions. The user may apply a credit for continuous high-pressure system liquid out flow to reduce the vapor breakthrough rate across the control valve and hence reduce the liquid displacement relief rate. In these cases, the vapor phase density in the lowpressure system should account for liquid flashing at relief conditions. It should be noted that the relief rate calculated for liquid displacement almost invariablyoften results in inadequate substantial relief requirements capacity due to substantially large liquid relief rate. The following options may be considered to mitigate the overpressure scenario or reduce the relief requirements of liquid displacement: Designing an inherently safer system by increasing the low-pressure system MAWP to eliminate the applicability of the overpressure scenario. However, the impact on the equipment downstream of the low-pressure system should be evaluated. Increasinge the size and vessel capacity of the low- pressure system to allow vapor-liquid disengagement to occur and to prevent overfillingincreasing the size of the low pressure system. Modifying the liquid levels to provide additional vapor space to accommodate the maximum anticipated liquid inventory. Providing HIPS on the high pressure system which isolates the flow to the low pressure system on low liquid level in the high pressure system and/or high level in the low pressure system. Providinge HIPS on the low-pressure system to progressively open a dump valve which would allow liquid to leave the low-pressure system and prevent low-pressure system. Performing detailed dynamic simulation of inlet control valve failure and potential liquid displacement per guidance in Section 4.3.3. Sizing the pressure relief devices for liquid displacement. Restricting inlet flow through the flow limiting element. Taking credit for reduction in relieving flow due to flowing resistance in full segment piping between high-pressure and low-pressure systems. Considering downstream overfill protection per guidance in Section 4.4.7.
Determininge if credit can be taken for the downstream system out flow paths. If the selected option to mitigate the overpressure scenario or reduce the relief requirements of liquid displacement resulted in liquid release to the disposal system, then the impact on PRD size, slug flow in disposal system piping, and knockout drum capacity should be evaluated. References: [1] N. Faulk and A. Aldeeb, Understanding Gas Blowby Scenario Calculations, Proceedings of the 11th Global Congress on Process Safety, American Institute of Chemical Engineers, Austin, Texas, 2015 [2] G. Melhem, M. Brewer, and M. Porter, The Anatomy of Liquid Displacement and Vapor Breakthrough, Proceedings of the 12th Global Congress on Process Safety, American Institute of Chemical Engineers, Houston, Texas, 2016the Spring 2014 DIERS Users Group Meeting, Houston, Texas [3] H. G. Fisher, et al., Emergency Relief System Design Using DIERS Technology, ISBN 0-8169-0568-1, American Institute of Chemical Engineers, New York, 1992 [4] CCPS, Guidelines for Pressure Relief and Effluent Handling Systems, 2017, ISBN 978-0-470-76773-3