Fire relief

Equipment wetted surface will absorb the heat from fire. The amount of heat absorption from fire can be calculated by following equations, Eq. (la) assumes there is good drainage and Eq, (lb) assumes there is no good drainage. 

Q is heat absorption rate from fire, in btu/hr F is environment factor Aw Is wetted surface area, in fl2. 

For bare vessel, F-1. Most equipment insulation has aluminum jacket with stainless steel bands, but it ts not able to stand the fire. In order for the insulation to stand the fire, it should be able to stand the fire impingement, assuming at 1660 F up to two hours, and able to stand the high pressure water om the water hose. Insulation with stainless steel jacket and stainless steel bands will satisfy this requirement. Besides this, insulation material should be able to stand high temperature up to 1600 F. [2] Then, insulation credit can be taken and F can be calculated as follows  

Vapor relief fixim fire case is calculated by following equation  

W is vapor relief load, in Ihdir is latent heat, in btu/lb. Latent heat can be calculated using a process simulator. But if relief conditions are close to critical point, process simulator may have difficult to estimate the latent heat Most time, 50 btu/lb will be used as a conservative latent heat for this situation. 

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Two-phase fire relief equations are available for conservative design. Leung19 presented an equation for the maximum temperature based on an energy balance around the heated vessel. This assumes a constant heat input rate Q ... 

For fire reliefs with single-phase vapor flow the equations provided in sections 9-2 and 9-4 are used to determine the size of the relief. 

The Workbook does not deal with fire relief of vessels (except where external fire modifies the relief sizing for runaway exothermic reaction) or with the mechanical integrity of either the process vessels or relief system. Guidance on these is available elsewhere14. 

The external fire relief requirements were determined using equations presented in Appendix D of API RP 520, but which also depend on the adequacy of... 

C THIS PROGRAM PRINTS THE RESULTS OF FIRE RELIEF SIZING ONTO A... 

VL1 Fire relief It is important to note that PRD is used to relieve pressure during fire case. To put out fire, it wilt rely on firefighting equipment If fire prolongs, equipment may not able to stand the fire, and PRD is not going to save the equipment. 

For vapor relief from columns (other than fire relief), the rate of vapor relief and the fluid properties must be determined by simulation at relieving pressure. The relieving will be 110% or 116%, depending on the requirement. For a fire case, the relief rate depends upon the latent heat and wetted surface area. The latent heat shall be estimated at 121% relieving pressure and for the average liquid composition within the column. 

Relief systems are expensive and introduce considerable environmental problems. Sometimes it is possibly to dispense with relief valves and all that comes after them by using stronger vessels, strong enough to withstand the highest pressures that can be reached. For example, if the vessel can withstand the pump delivery pressure, then a relief valve for overpressurization by the pump may not be needed. However, there may still be a need for a small relief device to guard against overpressurization in the event of a fire. It may be possible to avoid the need for a relief valve on a distillation column... 

Care of Pressure Vessels Protection against excessive pressure is largely taken care of by code requirements for relief devices. Exposure to fire is also covered by the code. The code, however, does not provide for the possibility of local overheating and weakening of a vessel in a fire. Insulation reduces the required relieving capacity and also reduces the possibihty of local overheating. 

Pressure-Vacuum Relief Valves For apphcations involving atmospheric and low-pressure storage tanks, pressure-vacuum relief valves (PVRVs) are used to provide pressure relief. These units combine both a pressure and a vacuum relief valve into a single assembly that mounts on a nozzle on top of the tank and are usually sized to handle the normal in-breathing and out-breathing requirements. For emergency pressure rehef situations (e.g., fire), ERVs are used. API RP 520 and API STD 2000 can be used as references for sizing. 

A common cause of a BLE T] in plants of the hydrocarbon-chemical industry is exposure to fire. With an external fire below the liquid level in a vessel, the heat of vaporization provides a heat sink, as with a teakettle evolved vapors exit tnrough the relief valve. But if the flame impinges on the vessel above the liquid level, the metal will weaken and may cause the vessel to rupture suddenly, even with the relief valve open. The explosive energy for a BLE T] comes from superheat. This energy is at a maximum at the superheat hmit temperature. (SLT is the maximum temperature to which a hquid can be heated before homogeneous nucleation occurs with explosive vaporization of the hquid and accompanying overpressure.) The SLT... 

Install flame arresters on atmospheric vents to prevent fire on the outside of the tank from propagating back into the vapor space inside the tank. Provide fire resistant insulation for critical vessels, piping, outlet valves on tanks, valve actuators, instruments lines, and key electrical facilities. Provide remote controlled, automatic, and fire-actuated valves to stop loss of tank contents during an emergency provide fire protection to these valves. Valves should be close-coupled to the tank, and must be resistant to corrosion or other deleterious effects of spilled fluids. Vessels should be provided with overpressure relief protection. 

In the third e.vample, the line terminates at 53%. This means DO NOT run this pump at less than 53% of the BEP. 53% of 4500 gpm is 2385 gpm. Because this is a firewater pump and because firemen need to throttle the nozzles on their fire hoses, then we need to install a pressure relief valve on this system with a discharge bypass line so that the pump dumps the restricted water (less than 2400 gpm) back into the suction tank or lake. If not, this firewater pump is likely to suffer bearing failure during an emergency. 

Deflagration venting (see NFPA 68) is highly applicable to powder operations. This is because most powder operations are at atmospheric pressure, the rate of deflagration pressure rise is usually small enough for vent relief panels to be of a practical size, and the subsequent fire is limited because most powders cannot bum in bulk. 

Can effluent and relief systems cope with large or abnormal discharges, during start-up, shutdown, hot standby, commissioning and fire-fighting ... 

The basis for design overpressure described in this section is related to the ASME Boiler and Pressure Vessel Codes and ANSI B31.3, Code for Petroleum Refinery Piping. Compliance with these codes is a requirement, or is recognized as the equivalent of a requirement in many locations. Where more stringent codes apply, the local requirements must be met. Therefore, local codes must be checked to determine their requirements. For example, some countries do not permit the use of block valves underneath pressure relief valves, unless dual valves with interlocks are installed. Also, in some cases, 20% accumulation under fire exposure conditions is not permitted, and accumulation allowed may be lower than the ASME Codes. In the United States, the ASME Code is mandatory, since it is a requirement under the Occupational Safety and Health... 

The ASTM fired pressure vessels code requires pressure-relief devices to prevent pressures from rising more than 6% above the maximum allowable working pressure. 

A PR valve is not required for protection against fire on any vessel which normally contains little or no liquid, since failure of the shell from overheating would occur even if a PR valve where provided. Examples are fuel gas knockout drums and compressor suction knockout drums. (Note Some local codes require pressure relief valve protection for "dry drum" situations.)... 

Except for special sihiations, pressure relief devices are not provided for fire exposure of heat exchangers, air fins, or piping, nor are the exposed surfaces of such items included for calculating the fire exposure heat input. Special situations may be congestion and substandard spacing, or unusually large equipment with normal liquid holdup over about 4 m and/or which represents over 15% of the total wetted surface of the system to which it is directly connected for pressure relief. 

Protection from Fire Exposure and Pressure Relief Considerations - Pressure relief valves cannot protect a vessel that becomes locally overheated on an unwetted surface, although they do prevent the pressure from rising beyond the accumulation pressure of the valve. However, in such a case the vessel may be effectively protected against failure by either one of two methods for mitigating the effect of fire ... 

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

Figure 7. Pressure conditions for safety relief valve installed on a pressure vessel (vapor phase) supplemental valve used for fire exposure only.

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