Sizing fire case

Where PRDs normally sized (except fire case where it is 21%)... 

For example, API 521 for fire case calculations basically allows you to ignore heights above a certain height when considering how much vessel surface to include in a fire zone calculation (see Section 2.3.2). Some companies go up to 8 m while others go up to 30 m for fire sizing and others simply have no height limit, considering that for a fire in a tank farm, for instance, it has been demonsttated that flames can reach over 100 m. 

A toluene surge drum has capacity 500 gal and is normally operated 60% full at 100°E under 300psig of hydrogen in the head space using a level-controlled outflow The normal flow rate into the vessel is 30,000 Ib/h. Determine the vessel dimensions if the vessel is vertically mounted. Evaluate the relief loads for the blocked outflow and external fire cases and hence determine the relief valve size. 

New process equipment installed within 7.6 meters from the grade/ground level will require pressure relief devices due to fire scenario. For revamp cases, it may be advantageous to place vessels on platforms at >7.6 meters if fire case relief load is most credible and bottlenecks the existing flare system. Fire circle or zone is defined as the maximum affected area during any equipment fire in the facility. API 521 (2014) defines its area as 230 to 460 m. Addition of process equipment inside an existing fire circle may increase the fire circle size. Hence, care shall be taken to review the fire circle size with each equipment addition. It will impact the peak relief load during the fire scenario. 

Thermal expansion and fire cases are not required to be checked, if the existing equipment is re-used, with the same service and also the same level control setting. Overpressure relief requirements due to each utility failure, fire cases and any other combination scenarios need to be estimated. API 521 (2014) has a comprehensive list of effects for utilities failure. All the PRV manifolds shall be checked to estimate back pressures at the PRVs. PRD overpressure calculations for equipment shall be documented as shown in Table 3.4. Vacuum relief (if the vessel/s is/are not designed to withstand full vacuum) shall also be documented. All the flare scenarios and flare network shall be properly documented. An example of PRV sizing calculations for the system shown in Figure 3.5 is presented in Table 3.4. 

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. 

T, = Inlet temperahire at relieving conditions, K. Note For the special case where a PR valve is being sized for fire, T, may be estimated by raising the midboiling point of the liquid to the inlet pressure P, using vapor pressure charts. 

Note that economics will in some cases favor the provision of additional insulation (beyond that required for heat conservation), in order to reduce the required capacity of the pressure relieving system, when it is sized by fire considerations. 

Bath-type heat exchangers can be either direct or indirect. In a direct bath exchanger, the heating medium exchanges heat directly with the fluid to be heated. The heat source for bath heaters can be a coil of a hot heat medium or steam, waste heat exhaust from an engine or turbine, or heat from electric immersion heaters. An example of a bath heater is an emulsion heater-treater of the type discussed in Volume 1. In this case, a fire tube immersed in the oil transfers heat directly to the oil bath. The calculation of heat duties and sizing of fire tubes for this type of heat exchanger can be calculated fom Chapter 2. 

Determine size and specifications for all safety relief valves and/or rupture disks for process safety relief (including run-a-way reactions) and relief in case of external fire. 

Where a.c. supplies exist, transformer-rectifiers are the most economical source of d.c. for cathodic protection systems. In the case of pipelines, standard transformer-rectifiers, either oil or air cooled, can be employed. They range in size from 5A, 5V for small systems to 100 A, 48 V for major pipeline schemes. A typical output for a well-coated cross-country pipeline in the UK would be 5 A, 48 V. In the case of sea-water jetties where the voltage required is usually low because of the lower sea-water resistivity, a typical rectifier size for a major installation would be 500 A, 18 V. For offshore pipelines and loading platforms where a fire hazard exists, it is usual to employ certified flameproof or intrinsically safe rectifiers to overcome any possibility of fire hazard should faults develop in the unit. 

LEs are very intuitive and therefore easy to apply. For the simplest case of directed fire, for example, they embody the idea that one side s attrition rate is proportional to the opposing side s size. However, LEs are applicable only under a... 

Three different types of furnaces are generally in use for calcination. The shaft furnace is considered to be the most suited for calcining coarse limestone. Furnaces of the rotary kiln type are used for handling materials of mixed particle sizes and lumps which disintegrate during the process. Calcination can be carried out in a fluidized bed-reactor for materials of small and uniform particle size. These furnaces are usually fired with gas, oil or coke in some cases electric heating is resorted to. 

In some cases, detailed dispersion modeling tools may not be available or their use is not warranted. To calculate the size of the flammable portion in the vapor cloud, other less precise, though sufficiently conservative, methods are available. Reference 5 cites a number of company, insurance, and governmental practices for estimating quantities of materials that could become involved in an explosion or fire. Some conservative approaches for determining the quantities of materials released include ... 

Virtually all the fires resulted in a CO concentration in the room upper level which was sufficiently high to cause serious concern. However, in all single plenum cases, the size of the lower level (cold layer) in the room and its CO concentrations were such that escape was virtually always possible. 

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