API 520 PT II – 2003 pdf download.Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries Part II—Installation.
1 Scope This recommended practice covers methods of installation for pressure-relief devices for equipment that has a maximum allowable working pressure (MAWP) of 15 psig (1.03 bar g or 103 kPA) or greater. Pressure-relief valves or rupture disks may be used independently or in combination with each other to provide the required protection against excessive pressure accumulation. As used in this recommended practice, the term pressure-relief valve includes safety relief valves used in either compressible or incompressible ßuid service, and relief valves used in incompressible ßuid service. This recom- mended practice covers gas, vapor, steam, two-phase and incompressible ßuid service; it does not cover special appli- cations that require unusual installation considerations.
4.1.1 Flow and Stress Considerations Inlet piping to the pressure-relief devices should provide for proper system performance. This requires design consid- eration of the ßow-induced pressure drop in the inlet piping. Excessive pressure losses in the piping system between the protected vessel and a pressure-relief device will adversely affect the system-relieving capacity and can cause valve instability. In addition, the effect of stresses derived from both pressure-relief device operation and externally applied loads must be considered. For more complete piping design guide- lines, see ASME B31.3. 4.1.2 Vibration Considerations Most vibrations that occur in inlet piping systems are ran- dom and complex. These vibrations may cause leakage at the seat of a pressure-relief valve, premature opening, or prema- ture fatigue failure of certain valve parts, inlet and outlet pip- ing, or both. Vibration in inlet piping to a rupture disk may adversely affect the burst pressure and life of the rupture disk. Detrimental effects of vibrations on the pressure-relief device can be reduced by minimizing the cause of vibrations, by additional piping support, by use of either pilot-operated relief valves or soft-seated pressure-relief valves, or by pro- viding greater pressure differentials between the operating pressure and the set pressure. 4.2 PRESSURE-DROP LIMITATIONS AND PIPING CONFIGURATIONS For pressure-drop limitations and piping conÞgurations, see Figures 1, 2, 4, and 5. 4.2.1 Pressure Loss at the Pressure-Relief Valve Inlet Excessive pressure loss at the inlet of a pressure-relief valve can cause rapid opening and closing of the valve, or chattering. Chattering will result in lowered capacity and damage to the seating surfaces. The pressure loss that affects valve performance is caused by non-recoverable entrance losses (turbulent dissipation) and by friction within the inlet piping to the pressure-relief valve.
4.2.3 Remote Sensing for Pilot-Operated Pressure-Relief Valves Remote sensing for pilot-operated pressure-relief valves can be utilized when there is excessive inlet pipe pressure loss or when the main valve must be located at a pressure source different from the pilot sensing point because of service limi- tations of the main valve (see Figure 6). 126.96.36.199 Inlet Pipe Loss Remote sensing permits the pilot to sense the system pres- sure upstream of the piping loss. Remote sensing may elimi- nate uncontrolled valve cycling or chattering for a pop action pilot-operated pressure-relief valve and will permit a modu- lating pilot-operated pressure-relief valve to achieve full lift at the required overpressure. Although remote sensing may eliminate valve chatter or permit a modulating pilot-operated pressure-relief valve to achieve full lift at the required overpressure, any pressure drop in the inlet pipe will reduce the relieving capacity.
188.8.131.52 Installation Guidelines Remote sensing lines should measure static pressure where the velocity is low. Otherwise, the pilot will sense an artiÞ- cially low pressure due to the effect of velocity. Ensure that the pilot sensing point is within the system pro- tected by the main valve. For ßowing pilots, remote sensing lines shall be sized to limit the pressure loss to 3 percent of the set pressure based on the maximum ßow rate of the pilot at 110 percent of set pressure. Consult the manufacturer for size recommendations for the remote sensing line. For non-ßowing pilots, remote sensing lines with a ßow area of 0.070 in. 2 (45 mm 2 ) should be sufÞcient since no sys- tem medium ßows through this type of pilot when the main valve is open and relieving. Consult the manufacturer for remote sensing line size recommendations. Consider using pipe for remote sensing lines to ensure mechanical integrity. If a block valve is installed in the remote sensing line, the guidelines in Section 6 should be followed. A closed block valve in a remote sensing line renders the pressure-relief valve inoperative and may allow the valve to open.