Pressure relief requirements

Other Utihties - Failure of other uOhties, such as inert gas to seals and pinge systems, or compressed air (when used by the process) may in some cases determine pressure relief requirements. These cases are evaluated on a single contingency failure basis similar to the above. 

Grolmes, M. A., Pressure Relief Requirement for Organie Peroxides and Otlier Related Compounds, Int. Symp. on Runaway Reaetions, Pressure Relief Design and Effluent Elandling, AIChE, pp. 219-245, Mareh 11-13, 1998. 

Total pressure relief requirement = 6000 -t 371 = 6371 SCFH free air... 

Vapor Treatment. The vapors from the tank space can be sent to a treatment system (condenser, absorption, etc.) before venting. The system shown in Fig. 9.1 uses a vacuum-pressure relief valve which allows air in from the atmosphere when the liquid level falls (Fig. 9.1a) but forces the vapor through a treatment system when the tank is filled (Fig. 9.16). If inert gas blanketing is required, because of the flammable nature of the material, then a similar system can be adopted which draws inert gas rather than air when the liquid level falls. 

Once the process route has been chosen, it may be possible to synthesize flowsheets that do not require large inventories of materials in the process. The design of the reaction and separation system is particularly important in this respect, but heat transfer, storage, and pressure relief systems are also important. 

Pressure-relief-device requirements are defined in Subsec. A. Set point and maximum pressure during relief are defined according to the service, the cause of overpressure, and the number of relief devices. Safety, safety relief, relief valves, rupture disk, breaking pin, and rules on tolerances for the reheving point are given. 

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. 

Vent Sizing Package (VSP) The VSP is an extension of ARC technology. The VSP is a bench-scale apparatus for characterizing runaway chemical reactions. It makes possible the sizing of pressure relief systems with less engineering expertise than is required with the ARC or other methods. 

If vapour return is used, what pressure is required and is it below the tanker relief valve setting ... 

Pressure relief is not required when the heat exchanger, including upstream and downstream piping and equipment, is designed for two-thirds of the high pressure side design pressure (section 3.18.2). 

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. 

In some cases where the ASME Code woidd not require pressure relief protection, the 1.5 Times Design Pressure Rule is apphcable. This rule is stated as follows Equipment may be considered to be adequately protected against overpressure from certain low-probability situations if the pressure does not exceed 1.5 times design pressure. This criterion has been selected since it generally does not exceed yield stress, and most Ukety would not occur more frequently than a hydrostatic test. Thus, it will protect against the possibility of a catastrophic failure. This rule is applied in special situations which have a low probability of occurrence but which cannot be completely ruled out. 

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.)... 

Pumparound Flow Failure - The relief requirement is the vaporization rate caused by an amount of heat equal to that removed in the pumparound circuit. The latent heat of vaporization would correspond to the temperature and pressure at PR valve relieving conditions. "Pinchout" of steam heaters may be considered. 

Absorbent Flow Failure - For lean oil absorption generally, no relief requirement results from lean oil failure. However, in a unit where large quantities of inlet vapor may be removed in the absorber, loss of absorbent could cause a pressure rise to relief pressure, since the downstream system may not be adequate to handle the increased flow. In such cases, the effect of this additional vapor flow into downstream equipment must be analyzed. 

The required relieving rate for a pressure relief valve is determined from consideration of the contingencies which can cause overpressure. Basic... 

Table 4 provides an example of a typical Pressure Relief Valve specification sheet. The following notes indicate the basis for the times which are required in the Design Specification. 

This section describes the requirements for the design and installation of pressure relief valve inlet and outlet piping manifolds and valving, including safety valve and flare headers. 

Isolation Valves for Pressure Relief Systems - Block valves for maintenance isolation purposes are permissible in pressure relieving systems, provided that they are car sealed open and comply with the requirements of CSO valves. The particular locations where such CSO valves are p>ermitted are ... 

A distinction must be made regarding the length of service of the pressure reducing systems. Fatigue failure of any mechanical system depends on time, i.e., the number of cycles to failure. Therefore, the treatment required for a continuous service may not be justified for a short term service. A System in short term service is defined as one which operates a total of 12 hours or less during the life of the plant. Pressure relief valves typically meet this limit. Systems in short term service exceeding the screening criteria indicated above should be evaluated. 

Some method of pressure relief is required on all pressure vessels and for other proeess equipment where inereasing pressure might rupture the vessel. Mueh of the piping used in modern ehemieal operations also requires overpressure proteetion. Safety relief valves or rupture dises are employed for pressure relief. In many eases, either a rupture dise or a safety relief valve ean be used. Safety relief valves are usually used for proeess proteetion and rupture dises are used for vessel proteetion. The safety relief valve or rupture dise must be designed to operate at a known pressure and prevent the pressure within the system from inereasing. Therefore, it is important to eon-sider the flowrate the valve ean handle. 

An important application of a rupture disc device is at the inlet of a pressure relief valve. The sizing of the pressure relief valve or rupture disc device combination requires that the pressure relief valve first be sized to meet the required relieving capacity. The normal size of the rupture disc device installed at the inlet of the pressure relief valve must be equal to or greater than the nominal size of the inlet connection of the valve to permit sufficient flow capacity and valve performance. The failure modes of rupture discs are [40] ... 

Superimposed back pressure The static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources. 

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