Overpressure Relief Devices

Relief systems are expensive and introduce considerable environmental problems. Sometimes it is possibly to dispense with relief valves and all that comes after them by using stronger vessels, strong enough to withstand the highest pressures that can be reached. For example, if the vessel can withstand the pump delivery pressure, then a relief valve for overpressurization by the pump may not be needed. However, there may still be a need for a small relief device to guard against overpressurization in the event of a fire. It may be possible to avoid the need for a relief valve on a distillation column... 

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 Relief Devices The most common method of overpressure protection is through the use of safety rehef valves and/or rupture disks which discharge into a containment vessel, a disposal system, or directly to the atmosphere (Fig. 26-13). Table 26-8 summarizes some of the device characteristics and the advantages. 

Blockage of relief device by solids deposition (polymerization, solidification). Possible loss of overpressure protection. 

Determine the size and location of relief devices required to protect an exchanger from overpressure during a tube rupture. 

Selection of Pressure Relief Device - From the range of available pressure rehef valves and other devices, selection is made of the appropriate type for each item of equipment subject to overpressure. Instrumentation, check valves, and similar devices are generally not acceptable as means of overpressure protection. 

APg = allowable venting overpressure (psi), that is, the maximum venting pressure minus the relief device set pressure Pp = maximum venting pressure (psig)... 

Overpressure A pressure increase over the set pressure of the relief device, usually expressed as a percentage of gauge set pressure. 

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. 

When pressure relief devices are intended primarily for protection against overpressure due to external fire or heat, have no permanent supply connection, and are used for storage at ambient temperature of non-refrigerated liquefied compressed gases, they are excluded from requirements of Par. UG-125c (1) and C (2), with specific provisions. See ASME code [1] for detailed references and conditions. 

Critical pressure will normally be found betw een 53% and 60% of the upstream pressure, P, at time of relief from overpressure, including accumulation pressure in psia. That is, P represents the actual pressure at which the relief device is blowing or relie ing, w hich is normally above the set pressure by the amount of the accumulation pressure, (see Figure 7-7A). 

P2 = back pressure or downstream at outlet of safety relief device, psig, or psia, depending on usage p = rupture pressure for disk, psig or psia p° = overpressure (explosion), lb force/sq in. p = pressure, psi abs... 

A tank containing hydrogen sulfide gas (molecular weight 34) has been overpressured and the relief device has been opened. In this case the relief device has a 3-cm diameter, and the flow through the relief is equivalent to the flow obtained through a 3-cm-diameter hole in the tank. In this case the flow of gas has been calculated to be 1.76 kg/s. 

The problem is solved by assuming no overpressure during the relief. The relief vent area calculated is larger than the actual area required for a real relief device with overpressure. 

A typical water heater contains 40 gal of water and has a heat input of42,000 Btu /hr. If the heater is equipped with a 150-psia spring relief device, compute the area required for relief. Hint Two-phase flow is expected. Assume no overpressure. 

A pressure relief system must be designed to protect the beverage bottle from overpressure. The relief device will be installed in the C02 line where it enters the beverage container. 

A relief device must be installed on a vessel to protect against an operational upset. The relief must discharge 53,500 lb/hr of hydrocarbon vapor. The relief temperature is 167°F, and the set pressure is 75 psig. Assume an overpressure of 10% and a backpressure of 0 psig. The hydrocarbon vapor has a molecular weight of 65, a compressibility of 0.84, and a heat capacity ratio of 1.09. Determine the diameter of the relief. 

NFPA 30 and API Standard 2000 provide guidance for design of overpressure protection involving storage tanks that operate at or near atmospheric pressure. In particular, NFPA 30 focuses on flammability issues, while API 2000 addresses both pressure and vacuum requirements. The ASME code (Sections I and VIII) and API RP 520 are the primary references for pressure relief device sizing requirements. 

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