Pressure-relieving devices

The autoclave is not the only component of an LDPE plant which may be exposed to a decomposition. Local hot spots in a secondary compressor may initiate a decomposition reaction consequendy it is necessary to protect these units from serious overpressure by pressure relieving devices and to release the products of the decomposition reactions safely. The problem of the aerial decomposition referred to eadier has been largely overcome by rapidly quenching the decomposition products as they enter the vent stack. 

Pressure Relief Valves Definitions for pressure relief valves, relief valves, pilot-operated pressure relief valves and safety v alves, are found in the ASVIE Boiler and Pressure Assel Code, Section Division 1, Rules for Construction of Pressure Assels, Paragraphs UC-125 and UC-126, The pressure-relief valve is an automatic pressure relieving device designed to open when normal conditions are exceeded and to close again when normal conditions are restored. Within this class there are relief valves, pilot operated pressure relief valves, and safety valves. 

API RP 520. Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries. Part I, Sizing and Selection, 5th ed., July 1990, and Part II, InstaUation, 3d ed., November 1988. American Petroleum Institute, Washington, D.C. 

National Board of Boiler and Pressure Vessel Inspectors. 1992. Pressure Relieving Device Cei tification (Red Book), National Board of Boiler and Pressure Vessel Inspectors, Columbus, Ohio. 

API Recommended Practice 520 , Sizing, Selection, and Installation of Pressure Relieving Devices in Refineries, 1993. 

Relief Valve - A relief valve is an automatic pressure-relieving device actuated by the static pressure upstream of the valve, and whieh opens in proportion to the increase in pressure over the opening pressure. It is used primarily for liquid service. 

Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid.) Safety relief valves are classified as "Conventional" or "Balanced", depending upon the effect of back pressure on their performance. 

Pressure levels at the flare header depend largely on the type and nature of pressure relieving devices that have been specified to protect upstream equipment, as well as the pressure levels of all equipment connected directly to the flare stack. 

Testing Procedures for Pressure Relieving Devices Discharging Against Variable Back Pressure... 

Types of Positive Pressure Relieving Devices (see manufacturers catalogs for design details)... 

Safety-Relief Valve this is an automatic pressure-relieving device actuated by the static pressure upstream of the valve and characterized by an adjustment to allow reclosure, either a pop or a non-pop action, and a nozzle type entrance and it reseats as pressure drops. It is used on steam, gas, vapor and liquid (with adjustments), and is probably the most general tyqDe of valve in petrochemical and chemical plants (Figures 7-3, 7-3A, and 7-4). Rated capacity is reached at 3% or 10% overpressure, depending upon code and/or process conditions. It is suitable for use either as a safety or a relief valve [1,10]. It opens in proportion to increase in internal pressure. 

Process Safety and Pressure-Relieving Devices Pressure-Relieving Devices... 

Back Pressure pressure existing at the outlet or discharge connection of the pressure-relieving device, result-... 

Burst Pressure the inlet static pressure at which a ruf>-ture disk pressure-relieving device functions or opens to release internal pressure. 

When a pressure vessel is exposed to external heat or fire, supplemental pressure relieving devices are required for this excessive pressure. These devices must have capacity to limit the overpressure to not more than 21% above the maximum allowable working pressure of the vessel. (See Figures 7-7A and 7-7B.) A single relieving device may be used to handle the capacities of paragraph UG-125 of the code, provided it meets the requirements of both conditions described. 

Pressure relieving devices in process plants for process and utility steam systems must conform to the requirements of ASME [1] Par. UG-131b. This is not necessarily satisfactory to meet the ASME Power Boiler Code for applications on power generating equipment. 

Note that the intent of the ASME Code is to ensure that under those circumstances where a pressure relieving device can be isolated by a block valve from its pressure, or its discharge, that a responsible indhadual lock and unlock the block valve to the safe open position and that this individual remain at the block valve the entire time that the block valve is closed. 

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