IPE-EP-5-6-4

IPE Engineering Practice IPE-EP-5-6-4

Document number: IPE-EP-5-6-4 · Section: 5 — Piping

CoInafsletcatilo nA Prouinbta E nEgningeeirninege Prriancgtic Pesractices Piping for Pressure Relief Systems 4.3 Selection of pipe and piping components for pressure relief piping systems shall be in accordance with EP 5–2–1. 4.4 Fabrication, erection and testing of pressure relief piping systems shall be in accordance with EP 5–5–1, EP 5–5–2, and EP 5–5–3. 4.5 Pressure relief systems for unfired steam generators shall be in accordance with the provisions of ASME Section VIII, Division I, unless prohibited by state law or other regulatory agencies. 4.6 Pressure relief valve requirements for fired steam boilers furnished to ASME Code Section I shall be per this code. 4.7 Pressure relief valves shall meet all requirements of EP 5–3–14. 4.8 Locations for pressure relief devices shall be selected to satisfy process and instrument design requirements and to facilitate access for maintenance. 4.9 Pressure relief valves shall be mounted vertically. 4.10 Pressure relief devices shall be installed in such a manner that the inlet drains back to the equipment being protected. In addition, the outlet shall be drained toward the header. Wherever possible, relief devices should be placed directly on the equipment or pipeline they are protecting. 4.11 Typical pressure relief valve installations are generically shown in API RP 520 Part II. Specific details of each installation shall be individually designed and detailed on the piping drawings. 4.12 Bonnets of bellows–type pressure relief valves shall be vented. 4.12.1 Bonnet vents of bellows–type pressure relief valves may discharge directly to the atmosphere when such discharge will not impinge on pipework or equipment, or personnel walkways or platforms. Where necessary, a short nipple and elbow shall be installed to direct flow away from these areas. The elbow should be equipped with a fine wire mesh to prevent insects and other obstructions from plugging up the bonnet. 4.12.2 (*)Bonnet vents of bellows–type pressure relief valves shall be piped to a closed non– pressurized system separate from the relief valve discharge, when specified by Owner’s Engineer. 4.12.3 If bonnet vents are piped to such a closed system, the piping shall be as follows: 1) Sized and arranged to prevent liquid traps or any appreciable back pressure. 2) Equipped with a car sealed open (CSO) isolation valve near the bonnet and a normally closed bleeder between the isolation valve and the bonnet to check for leakage of the bellows. 4.13 The size of all pipe, valves, and fittings between a pressure relief valve (PRV) and the equipment it protects shall be at least equal to the inlet flange size of the PRV. Where two or more PRVs are necessary to provide the required relieving capacity and the discharge from the vessel to the PRVs is through a common nozzle, this nozzle and any other common piping to the PRVs shall have an equivalent flow area at least equal to the combined area of the PRV inlets determined at the inlet flanges. 4.14 Pressure relief piping, including vent piping, and supports shall be designed to: 1) Resist reactive forces from discharge. 2) Prevent excessive vibration.

CoInafsletcatilo nA Prouinbta E nEgningeeirninege Prriancgtic Pesractices Piping for Pressure Relief Systems 6.1.3 Gate valves shall be installed with stems oriented horizontally. If this is not feasible, the stems shall be oriented downward per EP 5–1–2. 6.1.4 Double block and bleed requirements for isolation valves shall be in accordance with EP 5–3–1, Table 4. 6.2 (*)For new installations, when isolation valves are installed directly upstream of a pressure relief device, other than thermal relief valves, an additional relief device or a full size bypass vent line is required so that 100% design relieving capacity is available while any relief device is out of service. The Owner’s Engineer can override this requirement on a case by case basis. However, in any case, strict management procedures must be in place that ensure continuous pressure monitoring and enable immediate system shutdown or venting while a PSV is out of service for testing or repair. Such procedures must also prohibit the inadvertent actuation of isolation valves in pressure relief system piping. 6.3 For existing units, it is recognized that there are installations where isolation valves are located directly underneath pressure relief devices, without 100% spare capacity or a full bypass vent line available. It is not the intent of this Practice to require retrofitting these installations. Appendix M, of the ASME Code allows isolation valves under pressure relief valves provided strict management procedures are followed while the pressure relief device is out of service. Extreme caution should be used when following the guidelines of Appendix M. 6.4 Thermal relief valve installations typically do not require 100% design relieving capacity or a full size bypass vent line when isolation valves are used under the thermal relief valves for the following reasons: 6.4.1 Thermal relief valves are typically not removed and sent to a shop for inspection and testing. It is usually cheaper to replace them. The protected equipment is not unprotected for long periods of time during replacement of the thermal relief valve. 6.4.2 Thermal relief valves are typically designed to prevent overpressure due to a blocked–in failure scenario. The likelihood of a blocked–in situation occurring during the brief time that a thermal relief valve is being replaced is considered to be extremely low. 6.5 When isolation valves are required directly upstream of a pressure relief device (pressure relief valves, thermal relief valves, rupture discs, etc.) discharging to a closed system, the isolation valves shall also be fitted on the discharge side of the pressure relief device. See API RP 520 Part II for specific installation requirements. 6.6 Isolation valves directly upstream of a pressure relief device shall meet the requirements of paragraph 6.1 and the following: 1) Isolation valves shall be full port. 2) (*)When 100% spare capacity is provided, a mechanical interlock or interlocking procedure shall be provided, unless otherwise approved by the Owner’s Engineer. 3) (*)Three–way isolation valves are permitted provided the installation meets the inlet pressure drop requirements and the additional requirements specified in API RP 520 Part II. A mechanical device other than a key system shall be used when interlocks are required. Such installations require approval by the Owner’s Engineer.

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