Pressure vessels supplemental requirements

Shell and Tube Heat Exchangers for General Piefineiy Seivices, API Standard 660, 4th ed., 1982, is published by the American Petroleum Institute to supplement both the TEMA Standards and the ASME Code. Many companies in the chemical and petroleum processing fields have their own standards to supplement these various requirements. The Jnterrelation.ships between Codes, Standards, and Customer Specifications for Proce.ss Heat Tran.sfer Equipment is a symposium volume which was edited by F. L. Rubin and pubhshed by ASME in December 1979. (See discussion of pressure-vessel codes in Sec. 6.)... 

The ASME code provides the basic requirements for over-pressure protection. Section I, Power Boilers, covers fired and unfired steam boilers. All other vessels including exchanger shells and similar pressure containing equipment fall under Section VIII, Pressure Vessels. API RP 520 and lesser API documents supplement the ASME code. These codes specify allowable accumulation, which is the difference between relieving pressure at which the valve reaches full rated flow and set pressure at which the valve starts to open. Accumulation is expressed as percentage of set pressure in Table 1. The articles by Rearick and Isqacs are used throughout this section. 

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. 

The foregoing supplemental requirements are not of an anticipatory nature. Each is based on a significant number of actual pressure vessel failures. 

The minimum required wall thickness for a component can be taken as the thickness in the new condition minus the original specified corrosion allowance. The minimum required wall thickness for pressure vessel components can be computed if the component geometry, design pressure (including liquid head) and temperature, specifications for the material of construction, allowable stress, and thicknesses required for supplemental loads are known. The values for thickness calculations must include future corrosion allowance—the amount of corrosion expected after several field inspections are performed. Refer to the API 579, Fimess-for-Service [2], for additional discussion. 

An adequate drainage system should be provided for all locations where a large amount of hydrocarbon liquids has the possibility of release and may accumulate within the terms of the risk analysis frequency levels. Normal practice is to ensure adequate drainage capability exists at all pumps, tanks, vessels, columns, etc., supplemented by area surface runoff or general area catch basins. Sewer systems are normally gravity flow for either sanitary requirements or oily surface water disposal. Where insufficient elevation is available for the main header, lift stations are installed with a forced pressure outlet header to a disposal or treatment system. 

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