Relief capacity

System relief capacity must be multiplied by a factor of 0.90. 

Failure of Cooling Water assume all cooling mediums fail, determine relief capacity for the total v apors entering the vessel, including recycle streams. See [3] and [10]. 

In calculating the relief capacity to take care of external fire the following equation is used ... 

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. 

At least the following conditions or some combination must be considered in establishing the normal pressure-vacuum relief capacity requirements [26] ... 

Two types of pressure relief devices are available, safety valves and relief valves. Although these terms are often used interchangeably, there is a difference between the two. Safety valves are used with gases. The disk overhangs the seat to offer additional thrust area after the initial opening. This fully opens the valve immediately, giving maximum relief capacity. These are often called pop-off safety valves. 

Failure to consider appropriate challenges when determining the required relief capacity (e.g., maximum rates of liquid withdrawal or cooling of vessel contents). Credible contingencies (e.g., thunderstorm cooling a vessel during steam-out) should be considered. 

API (Venting Atmospheric and Low-Pressure Storage Tanks, Standard 2000, Washington, 1998) provides guidance for vacuum protection of low-pressure storage tanks. Where vacuum relief devices are provided, they should communicate directly with the vapor space in the vessel and should be installed so that they cannot be sealed off by the liquid contents in the vessel. Valves should be avoided in the inlets or outlets of vacuum relief devices unless the valves are reliably car-sealed or locked open, or excess relief capacity is provided (e.g., via multiple-way valves). 

Safety Relief Valves (SRVs) Conventional safety relief valves are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting of the valve is reduced by the amount of superimposed backpressure expected. Higher built-up backpressures can result in a complete loss of continuous valve relief 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. 

For both conventional and balanced SRVs, the inlet pressure loss, including the mounting nozzle entrance loss, rupture disk flow resistance, and inlet pipe friction, is recommended to stay below 3 percent of the differential set pressure, or else valve instability may occur, resulting in degraded relief capacity. 

The two-phase mass relief capacity per unit area, G, ignoring friction, can be estimated using Tangren et al. s method (see 9.4.3). This requires the void fraction in the reactor, a0, which is approximately 0.6 for this case. Tangren s method is ... 

Calculate two-phase relief capacity per unit area, G, using appropriate method (see Chapter 9)... 

The evaluation of the two-phase mass relief capacity per unit area, G, is discussed in Chapter 9. The additional parameters which are required for tempered hybrid systems are PJP, the ratio of the vapour pressure to the absolute pressure, and , the closed vessel temperature rise as the pressure rises from the relief pressure to the maximum pressure. 

A growing number of computer codes are available for relief system sizing or for the evaluation of the flow capacity (and hence the mass relief capacity per unit area, G) for a given relief system. The types of code available are discussed below under the following headings ... 

For simple relief systems and reacting systems for which it is applicable, the Omega method (see Annex 8) can be used to find the mass flow capacity of the relief system. However, if the Omega method is inapplicable, or if the relief system has sections of different diameter, then the use of a suitable computer code to obtain the mass relief capacity may be required. 

The available pressure range from the relief pressure to the maximum accumulated pressure is divided into pressure steps (or the corresponding temperature range is divided into temperature steps). Average physical properties, heat release rate per unit mass (q) and relief capacity per unit area (G) need to be evaluated for each... 

PROCEDURE FOR CALCULATION OF RELIEF CAPACITY FOR A LINE WITH TWO SECTIONS OF DIFFERENT DIAMETER... 

An essential point, when considering emergency rehef, is that as the relief system suddenly opens, reaction mass may be entrained by the gas or vapor, leading to two-phase flow that decreases the relief capacity of the system. Thus the... 

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. 

Check that the de-rated relief capacity is equal to or greater than the required capacity. If this is not the case, have a close look at either the inlet piping configuration or at increasing the size of the valve. 

Typical solutions include performing rigorous calculations of the relief capacity available via outflow from the low pressure side, re-rating the low pressure side to meet the two-thirds rule or installing a relief device. 

This is clearly a double jeopardy failure two unrelated events occurring at exactly the same time. One has nothing to do with the other. Therefore, you need to calculate the relief capacity for one scenario at a time. For the loss of power to a pump scenario, the relief load would be based on the amount of vapour generated at the normal rate of steam. For the steam control valve failure scenario, the relief capacity would be based on the amount of vapour generated by the heat provided by a wide-open steam valve even accounting for the amount of vapour condensed in this failure, the condenser would still be in operation. So the SRV should be sized for the worst condition. 

Is the relief capacity adequate for process upsets, valve or tube failure, fire,... 

In the case of a fire, there is a pressure buildup and the required relief capacity can be determined provided that the heat absorbed can be estimated. API 520 gives recommendations for heat of absorption during vapor relief [12]. Not all the causes will happen simultaneously, but the pressure relief or safety relief should be sized for conditions that require the greatest relief. 

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