PRESSURE RELIEF VALVES |
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Fire conditions
If the vessel is protected by two valves, one only to respond to an operational loss of control, it must be set at or below MAWP, and the second( supplementary valve), to respond to a fire condition external to the vessel, must be set at 10 % above MAWP and have the nozzle area also dimensioned for 10 % overpressure. This valve must prevent, even under fire conditions, the pressure inside the vessel from developing to a value greater than 21 % of MAWP. Note: Pressure vessel designers must establish the 10 %, 16 % or 21 % accumulation limits, assessing that the allowable stress reduction with the increase in temperature is still within the safety factors applied by the design codes( between 3 and 5 times). The accumulation values, for all the situations mentioned, mean that with all the pressure relief valves open and relieving, the pressure inside the protected equipment cannot exceed those percentages. Accumulation is, therefore, the primary parameter when sizing a pressure vessel or boiler, and is expressed as a percentage of the MAWP of that equipment. Note: Each section of the ASME code( I, IV, and VIII) provides its own rules regarding the percentage of pressure allowed to be raised above the MAWP of the equipment to be protected by the pressure relief valve during a process overpressure event. Note: A supplemental pressure relief valve for“ fire conditions” is a pressure relief device that provides a much higher flow capacity than the one that was sized and
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installed only to meet the flow capacity required by the process. It should always be applied when an additional hazard may be generated to a pressure vessel by its exposure to fire or another unexpected source of external heat. Therefore, the supplemental pressure relief device is only applied to devices already sized and installed for situations of operational loss of control, such as chemical reaction, blocked discharge, etc. The value of its opening pressure cannot exceed 110 % of the vessel’ s MAWP. The accumulation percentage is a value that depends only on the MAWP value of the vessel, the causes of the overpressure and the number of valves installed, and is independent of the opening pressure value of the safety and / or relief valve( s) that protect it.
Accumulation in boilers
In boilers built in accordance with ASME Section I, the maximum allowable accumulation is 6 % above MAWP, regardless of the number of pressure relief valves installed. Therefore, no more than 6 % above the highest pressure at which any pressure relief valve is set to open, i. e., when set below MAWP, or no more than 6 % above the boiler MAWP is allowed( PG 67.2). PG 67.3 of the ASME Section I code allows the second or last valve to be set to open up to 3 % above the boiler MAWP.
Overpressure
Overpressure is the same as accumulation only when the pressure relief valve is set to open at the same value as the MAWP
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of the pressure vessel or boiler, and there are no pressure losses in the inlet piping. The value of the overpressure depends on the construction code of the equipment to be protected and the value of the valve opening pressure. For example, for boilers that are built according to ASME Section I, this value is 3 % or 2 psi, whichever is greater, both in the steam drum and in the superheater. For pressure vessels built according to ASME Section VIII and process piping built according to ASME B31.3, the overpressure value must be 3 psi or 10 % above the valve opening pressure( whichever is greater), for gases, vapors and liquids. Overpressure is mainly necessary in valves with a spring under load and operating with compressible fluids, since the disc does not reach the maximum lift at the opening pressure, due to the increasing force exerted by the spring in the downward direction and opposite to the opening direction. Therefore, it is an increase in pressure above the opening pressure, necessary for the valve disc to reach its maximum lift, and, consequently, for the valve to reach its maximum flow capacity, i. e., the flow will be limited by the nozzle throat area. Note: This is the overpressure acting on the area of the external diameter of the nozzle ring( when this ring is correctly positioned), and the huddling chamber is what allows the disc to reach its maximum lift. The positioning of this ring can change both the disc’ s lifting stroke and the overpressure value achieved in relief conditions, in addition to changing the valve’ s closing pressure value. Thus, the overpressure exists to allow the pressure inside the nozzle and huddling chamber to overcome the increasing force exerted by the spring. The chattering that occurs due to oversizing is due to there being no overpressure inside the huddling chamber during the valve relief process. Overpressure is the difference between the accumulation existing inside the pressure vessel or boiler and the valve opening pressure. Therefore, the value of the overpressure depends on the flow capacity required by the process in relation to the maximum flow capacity of the valve. However, it is at the percentage of overpressure required by the building code that the nominal flow capacity of the valve is achieved. If the valve opening pressure is lower than the MAWP of the vessel, the overpressure may be greater than 10 %, provided that the maximum accumulation reached |
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