Relief Header Design

The pressure in the relief piping is usually equal to atmospheric pressure as long as no relief valve is relieving. There is a common miscon- 

The relief piping should be segregated into pressure relief and atmospheric relief systems. If there is a wide range of set pressures on relief valves, the pressure relief piping is sometimes divided into high pressure and low pre.ssure systems. 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

The percent absolute back-pressure for conventional and pilot-operated safety relief valves is  

K(j = valve coefficient of discharge = 0.92 for pilot-operated Pi = flowing pressure, psia MW = molecular weight of gas = 23.2 Z = compressibility factor = 0.9334 C - gas constant based on ratio of specific heats Cp/C  

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The designer should consider, when configuring the tie-ins to a relief header, any possible reactions between the effluent streams. On my alky unit, the reaction between strong H SO (which is noncorrosive) and acid-free H O will produce terribly corrosive weak H SO, which will destroy the welds in carbon steel piping in a matter of just a few days. 

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. 

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. 

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. 

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. 

It is wortli noting tluit design calculations for tlie sizing of relief systems (relief valves, headers, scmbbers and knock-out drums, etc.) are conservative in order to protect tlie integrity of vessels and relief systems. Tlie calculations used for risk assessments are tliose which most accurately describe the discliarge rate from tlie luizardous incident being modeled. 

It should be noted that vapor depressuring may not be practical when the vessel design pressure is less than 100 psig (690 kPa) because valves and piping can become unreasonably large and costly or when the vapor depressuring load governs the size of pressure relief and flare headers. Refer to API RP 521,... 

Variable superimposed backpressure Usually the result of one or more SRVs discharging into a common header. The backpressures may be different at each moment and at each relief cycle Bellows or pilot design is always required since no predetermined set pressure is possible when the oudet pressure is acting on the trim of the valve therefore direcdy influencing the set pressure, and the set point will vary with backpressure (Figure 3.9). 

The design of a flare system includes the sizing of safety and relief valves, inlet and discharge piping, and flare header. All these should be adequately sized to prevent overpressuring of equipment in case of operational failure, such as fire, inlet or outlet blockage, reflux failure, power failure or instrument failure. 

However, in many applications, the reduction in vent header and protective relief equipment, together with the lower probability and frequency of discharge, can justify this technique. The author has experienced one case with a high-pressure, close-boiling system where the reduction in vent header and protective equipment cost was by itself sufficient to pay for the required increase in column design pressure. 

The design of relief valve systems includes the criteria of local stresses at the header-to-relief valve inlet piping junction and the stresses in the relief valve inlet piping and header. 

A. Operation Near Atmospheric Pressure. The simplest approach to hydrogen header control at very low pressure uses large water-sealed vents designed to maintain a fairly precise back-pressiure with little or no fluctuation. Figure 11.35 shows such a vent. Section 9.1.10.1 gives design details for this type of system as a pressure-relief device with little heed to gas flow distribution. For more precise control, it is important to distribute the gas as small bubbles over a wider area of the water seal (10-20 nun deep) in order to provide minimum pressure drop and fluctuation in flow. 

Each primary safety valve discharge line is designed to pass a maximum steam flow of 630,000 Ibm/hr from the safety valve to one of the main header lines in the Steam Relief System (Section 6.8.2.2.2). The safety valve discharge flow analysis includes the effects of steam flow and Rapid Depressurization Function flow which result in two-phase flow inthe Steam Relief System. The safety valve discharge lines collect into two headers which are routed from the pressurizer cubicle to the In-containment Refueling Water Storage Tank (IRWST). See Section 6.8 for a description of the IRWST. Each Steam Relief System discharge line will pass the maximum steam flow from two safety valves, 1,260,000 Ibm/hr (2 valves time 630,000 Ibm/hr). 

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