Pressure relief valve design capacity

The set pressure tolerances of pressure relief valves are not to exceed +2 psi for pressures up to and including 70 psig and +3% for pressures above 70 psig. Indirect operation of safety valves, for example, by pilot valve, is not acceptable unless the primary unloading valve will automatically open at not over the set pressure and will operate fully in accordance with design relieving capacity conditions if some essential part of the pilot or auxiliary device should fail [1]. 

API 520 Definition A nominal or computed area used with an effective discharge coefficient to calculate the minimum required reheving capacity for a pressure relief valve per the preliminary sizing equations contained in API Standard 526. API Standard 526 provides effective discharge area for a range of sizes in terms of letter designations D through T. 

The DOT rules require that pressure cars have relief valves designed to limit pressure to 82.5 percent (with certain exceptions) of test pressure (110 percent of maximum operating pressure) when exposed to fire. Appendix A of AAR Specifications deals with the flow capacity of relief devices. The formulas apply to cars in the upright position with the device discharging vapor. They may not protect the car adequately when it is overturned and the device is discharging liquid. 

Cryogenic liquids must be stored, shipped, and handled in containers that are designed for the pressures and temperatures to which they may be subjected. Materials that are pliable under normal conditions can become brittle at low temperatures. Dewar flasks, which are used for relatively small amounts of material, should have a dust cap over the outlet to prevent atmospheric moisture from condensing and plugging the neck of the tube. Special cylinders insulated and vacuum-jacketed with pressure-relief valves and rupture devices to protect the cylinder from pressure buildup are available in capacities of 100 to 200 liters (L). 

Type CG-8, This is a combination device consisting of a rupture disk device followed by and in series with a pressure relief valve. The piping connecting the rupture disk and pressure relief valve is equipped with a sensing device to indicate pressure. The system is designed and constructed so the required relieving capacity is achieved and the operation of the relief valve is not impeded. 

The discharge capacity of safety and/or pressure relief valves in the RCS should be sufficient to limit pressure increases and to keep the pressure within the prescribed design limits during all operational transients and in the accident conditions considered in the design of the RCS, in accordance with the applicable pressure vessel code and standard. The number of valves should be sufficient to provide the necessary degree of redundancy. 

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

Back-pressure can affect either the set pressure or the capacity of a relief valve. The set pressure is the pressure at which the relief valve begins to open. Capacity is the maximum flow rate that the relief valve will relieve. The set pressure for a conventional relief valve increases directly with back-pressure. Conventional valves can be compensated for constant back-pressure by lowering the set pressure. For self-imposed back-pressure—back-pressure due to the valve itself relieving—-there is no way to compensate. In production facility design, the back-pressure is usually not constant. It is due to the relief valve or other relief valves relieving into the header. Conventional relief valves should be limited to 10% back-pressure due to the effect of back-pressure on the set point. 

The backpres.sure created by this drum is an additive to the pipe manifold pressure drops and the pressure loss through the separator. Therefore, it cannot be independently designed and not integrated into the backpressure system. The flow capacity of the relief valve(s) must... 

In sizing depressuring valves, it should be assumed that heater burners are shut-off, reboilers are shutdown, and normal flow in the vessel has ceased. Vapor depressuring valves should be designed such that the initial, instantaneous depressuring flow rates do not exceed the capacity of the closed pressure relief system and the flare. 

In the fine chemicals and pharmaceutical industries, reactors are often used for diverse processes. In such a case, it is difficult to define a scenario for the design of the pressure relief system. Nevertheless, this is required by law in many countries. Thus, a specific approach must be found to solve the problem. One possibility, that is applicable for tempered systems, consists of reversing the approach. Instead of dimensioning the safety valve or bursting disk, one can choose a practicable size and calculate its relief capacity for two-phase flow with commonly-used solvents. This relief capacity will impose a maximum heat release rate for the reaction at the temperature corresponding to the relief pressure. 

There are two main types of safety relief valves. The conventional type is shown in Figure 5-3. A balanced safety relief valve shown in Figure 5-4 is designed to limit the effect of back pressure on opening pressure, closing pressure, lift capacity, and relieving capacity. The balanced valves are mainly the piston type and the bellows type. 

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