Overpressures relief sizing

Explosion calculations, 499-504 Estimating destruction, 501 Overpressure, 502 Pressure piling, 501, 504 Relief sizing, 505 Scaled distance, 502, 503 Schock from velocity, 503 TNT equivalent, 499-504 Explosion characteristics of dusts, 515 Explosion suppression, 518 Explosion venting, gases/vapors, 504 Bleves, 504 Explosions, 482 Blast pressure. 496 Combustion, 482 Confined, 482 Damage, 498-501 Deflagration, 482 Detonation, 483... 

Within this Workbook, the maximum pressure required to fully open the pressure relief device will be referred to as the "relief pressure". (Caution some papers on relief sizing refer to "set pressure" but mean "relief pressure"). For a bursting disc, the relief pressure will be the maximum specified bursting pressure and for a safety valve, it will be the set pressure plus 10% overpressure (or whatever percentage overpressure the valve has been certified at). 

A reactor has a volume of 2 m3. The worst case runaway reaction has been identified and the data from a suitable adiabatic, low thermal inertia test, with a thermal inertia ( ) of 1.05, is given in Figure 6.4. Under these conditions, the reactor would contain 793 kg of reactants. The reacting system is a vapour pressure system. It is desired to relieve the runaway via a safety valve, if possible, with a set pressure of 0.91 barg (relief pressure of 1.0 barg = 2.0 bara). Evaluate the required relief size for an overpressure of 30% of the absolute relief pressure, which gives a maximum pressure of 2.6 bara = 1.6 barg. 

Definitions of terms used in connection with overpressure relief systems. (Reproduced courtesy of the American Petroleum Institute from API Recommended Practice 520, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part I - Sizing and Selection,... 

Transient sources. The response of normal overpressiu relief devices is usually too slow to provide effective protection against these sources. In some cases (e.g., a pocket of cold water entering a hot tower), the relief requirement depends on the pocket size and is therefore extremely difficult to estimate (10). Normal overpressure relief is usually not designed to deal with these sources (10, 45). 

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. 

Few chemical reactors are operated without any pressure relief system, but it is not uncommon for the relief system to be sized only to deal with overpressures from service fluids or fire engulfment. In such situations, where the relief system does not protect against the consequences of a runaway reaction, the safety of the reactor system must be assessed thoroughly to demonstrate compliance with legal requirements and recommended standards. The company operating the process should be able to justify its decision not to provide adequate overpressure relief. ... 

Determine the size and location of relief devices required to protect an exchanger from overpressure during a tube rupture. 

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 17. Variable or constant back pressure sizing factor, Kw for 25% overpressure on balanced bellows safety relief valves (liquids only). The curve represents conqiromise of the valves reconunended by a number of relief valve manufacturers. This curve may be used wiien the make of dw valve is not known. When the make is known, the manufacturer should be consulted for the correction factor.

Do not allow nitrogen or air supplies to overpressure tanks or vessels. Tanks and vessels could be designed to withstand the air and nitrogen header pressure. Another solution is to install a pressure relief valve downstream of a pressure reducing station sized to relieve the entire flow on failure of the station. 

The most important safety devices in a production facility are the pressure relief valves, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressures higher than their design pressures. Relief valves must be designed to open rapidly and fully, and be adequately sized to handle the total flow of gas and liquids that could potentially cause an overpressure situation. They relieve the pressure by routing this stream to a safe location where it can be vented to atmosphere or burned. 

A vessel can only be overpressured if the upstream vessel has a higher pressure than the vessel in question. A compressor scrubber with a MAWP of 285 that gets flow from a 285 MAWP separator does not need to have a relief valve sized for blocked discharge. The upstream relief valve will keep the upstream separator pressure from going higher than 285, so there is no way it can oveipressure the downstream scrubber. The scrubber PSV only needs to be sized for fire. 

Rupture disks are used for the same purpose as safety valves and, in addition, serve to relieve internal explosions in many applications. If the pressure rise can be anticipated, then the volume change corresponding to this change can be calculated by simple gas laws, and the capacity of the disk at the relieving pressure is knowm. The system must be examined and the possible causes of overpressure and their respective relief capacities identified before a reliable size can be determined. See Figure 7-14. 

Figure 9-1 Required vent area as a function of overpressure for two-phase flow. The vent area is decreased appreciably as the overpressure increases. Data from J. C. Leung, Simplified Vent Sizing Equations for Emergency Relief Requirements in Reactors and Storage Vessels, AICHE Journal (1986), 32(10).

Figure 9-3 Overpressure correction Kp for spring-operated reliefs in liquid service. Source API RP 520, Recommended Practice for the Sizing, Selection, and Installation of Pressure-Relieving Systems in Refineries,...

NFPA 30 and API Standard 2000 provide guidance for design of overpressure protection involving storage tanks that operate at or near atmospheric pressure. In particular, NFPA 30 focuses on flammability issues, while API 2000 addresses both pressure and vacuum requirements. The ASME code (Sections I and VIII) and API RP 520 are the primary references for pressure relief device sizing requirements. 

It is recommended that the chosen sizing method be evaluated for a range of overpressures. This will give an indication of the likely consequence of undersizing, in terms of how far the pressure may be expected to rise above the maximum permitted for the vessel. For moderate overpressures (typically 10-30% of the absolute relief pressure), the consequence of undersizing may sometimes be small. 

A proper sizing criteria Is to determine the liquid volume that could be dumped before the ESD system shuts in the relief valve source of overpressure. Thus, a dynamic liquid holdup determines the size of the relief drum. It will generally be smaller with the liquid holdup sizing criteria than a comparable horizontal API sized vessel. 

Install, inspect, and maintain properly sized pressure-relief systems to prevent overpressurization of equipment. 

Another of Dowell and Hendershots good deeds involves overpressure protection design. This good deed is the overgenerously sized rupture disks and relief valves on new... 

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