(55t) Guidelines for Storage and Handling of Reactive Chemicals

Emergency Relief valve sizing for external fire scenario is important for Low pressure storage tank. The fire hazard is an accumulation of flammable liquid that could be ignited and may create a pool fire. For calculations of heat fire flux, equations are available in API-2000 and API-520 & 521, NFPA-30. However, these standards do not give any idea for How long fire may burn and what time it will take to reach up to its bubble point temperature at relieving pressure or the onset temperature for reactive systems. For the calculations of Heat up time and Fire duration, empirical equations are available in journal of loss prevention in the process industries. This Paper provides importance of fire duration calculations and heat up time calculations for External fire scenario for storage tanks. This paper will try to elaborate on how to calculate heat up time and External fire duration with examples. This paper also includes how to determine effective pool area based on plant layout. Heat up time (HUT) is defined as the time between the fire start with the vessel at operating condition and the time where the vessel reach the bubble point temperature at reliving pressure or the no return or onset temperature of reactive systems. HUT is a function of mass inside the vessel, heat capacity of the fluid and effective heat flux to the vessel. HUT can vary from few minutes to hours or days too. HUT compared with fire duration time, if HUT exceeds fire duration time by sufficient margin, then it may be good to design the reliving system for not fire scenario. Vessel inventory has a significant impact on calculation of Heat up time. At lower inventories lower heat up times may result. As a best practice this inventory should be taken from the last 2 to 3 years tank level trend. Viscosity is also one of the Important factor to be considered while taking credit for long heat up times for highly viscous systems. For highly viscous system, temperature distribution from external fire source to fluid will not be uniform. For reactive case it may be possible that temperature at vessel wall may reach up to onset temperature. The reaction could continue if fire stops. In this case it may be better to size the vent for an external fire driven runaway reaction. Fire duration calculation is only used to determine the size of a relief device for runaway reaction in a vessel or for determining the possibility of the two phase flow venting of liquid filled vessels. For reactive scenario, if the fire duration is of sufficient duration, then a relief device must be sized for runaway reaction and if the fire duration is not sufficient then relief device should be sized based on non reactive external fire scenario without considering decomposition and runaway reaction. Same for two phase flow, if the fire duration is not enough to cause the relief device to open, then the vent is sized only for vapor flow instead of two phase flow. A fire duration can be calculated by determining the depth of liquid in a dike or drainage area and assuming that the burning rate caused by pool fire is 1 in/7 min. Liquid runoff due to drains is usually ignored. Providing a properly sized relief device does not necessarily assure that the vessel will not fail. Reclosing relief devices maintain pressure in the vessel. If the fire continue for long duration, metal of the vessel will become dry and ultimately it will fail near the set pressure of the relief device if fire spray is not provided around the vessel. If a fire has the potential to last long enough to cause vessel failure other protection measures include water sprays, use of rupture disk instead of pressure relief valves. All above guideline apply to vent sizing for the external fire case. As there may be additional overpressure scenario like Blocked outlet, Instrument failure or blowthrough scenario. If one of the alternate requires a larger vent size than the external fire case, then the larger size should be considered