Overpressure relief

This chapter reviews the common practices of relieving the column to avoid overpressure, outlines several considerations and factors that affect overpressure, highlights the pitfalls of undersizing and oversizing relief devices, and provides guidelines for avoiding pitfalls. 

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Install flame arresters on atmospheric vents to prevent fire on the outside of the tank from propagating back into the vapor space inside the tank. Provide fire resistant insulation for critical vessels, piping, outlet valves on tanks, valve actuators, instruments lines, and key electrical facilities. Provide remote controlled, automatic, and fire-actuated valves to stop loss of tank contents during an emergency provide fire protection to these valves. Valves should be close-coupled to the tank, and must be resistant to corrosion or other deleterious effects of spilled fluids. Vessels should be provided with overpressure relief protection. 

Install weak sections in piping and duct work to provide overpressure relief... 

As long as pressure, level, and temperature control devices are operating correctly, the safety system is not needed. If the control system malfunctions, then pressure, level, and temperature safety switches sense the problem so the inflow can be shut off. If the control system fails and the safety switches don t work, then relief valves are needed to protect against overpressure. Relief valves are essential because safety switches do fail or can be bypassed for operational reasons. Also, even when safety switches operate correctly, shutdown valves take time to operate, and there may be pressure stored in upstream vessels that can overpressure downstream equipment while the system is shutting down. Relief valves are an essential element in the facility safety system. 

To avoid this situation, some regulators are designed with a built-in overpressure relief mechanism. Overpressure relief circuits usually are composed of a spring-opposed diaphragm and valve assembly that vents the downstream piping when the control pressure rises above the set-point pressure. 

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. 

Chemical Process Retrofitting and Revamping Table 3.4 Summary of overpressure relief calculations. 

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. ... 

The overpressure relief containment system (see detail in Figure 12.5) uses a stainless steel vent stack (4 inch diameter pipe) to the roof of the building. It is terminated by an oversize hydrophobic filter. Calculations indicate that, in the event of a burst, the volume of liquid that will be discharged can be accommodated in an extension (ca. 10 feet long) of the vent stack below the discharge point. This accommodation is necessary to... 

The nuclear pressure relief system consists of safety/relief valves (SRVs) located on the MSLs between the RPV and the inboard MSIV. There are four SRVs per MSL. The SRVs provide three main protection functions overpressure safety, overpressure relief, and depressurization operation, which is discussed below separately. 

The suppression pool contains borated water to provide a diverse backup to the gravity-driven control rods. Core cooling and decay heat removal is assured, with water returned to the reactor vessel and steam produced by decay heat vented to the suppression pool. The containment overpressure relief periodically opens to vent steam from the suppression pool. There is a three-day supply of water available to accept decay heat. No operator action is required during this time. For longer periods the suppression pool is manually refilled. Emergency diesel generators and core cooling pumps are not required. 

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