【Principles of Chemical Safety Design】Safety Valve

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The design documents include a safety valve data sheet (safety valve specification), which covers the discharge capacity of the safety valve under the risk conditions considered in the design, as well as related physical property parameters. The designed discharge volume shall be no less than the maximum value under various operating conditions. The design requirement is that the pressure drop in the inlet pipeline of the safety valve should be less than 3% of the set pressure to avoid hammering; this means that the diameter of the inlet pipeline for the safety valve should not be smaller than that of the inlet flange of the safety valve, and the distance between the safety valve inlet and the protected equipment should not be too great. For different safety valves, the back pressure limit also varies. When the safety valve releases pressure, the back pressure of conventional safety valves is less than 10% of the set pressure; such safety valves are used when the back pressure stabilizes. The maximum back pressure for balanced bell-type safety valves can be 50% of the set pressure; these types of safety valves are used when the back pressure is unstable. When cut-off valves are installed on the inlet and outlet pipes of the safety valve, they should be sealed with lead or locked open, and the valve stems of the cut-off valves should be installed horizontally. Special consideration should be given to special circumstances. For example: a device in which the pressure drops significantly behind a pressure control valve will experience a pressure much higher than under normal operation once the control valve fails, and this will occur over a short period of time. To ensure safety and/or reduce the number of times the safety valve operates, the maximum operating pressure can be appropriately increased. Under normal circumstances, the fault-open condition of the control valve at the container inlet determines the size of the safety valve. This situation is more common when the pressure difference between the two containers is large. Will a control valve failure, with the bypass open by 50% while in the full-open position during an accident, cause overpressure in the downstream vessels? To address the low-probability risk of the bypass valve being in the 100% open position, the pressure relief capacity of the safety valve should be determined with both the control valve and the bypass fully open. Under normal circumstances, a minimum of 25% design pressure relief is required for the safety valve to remain in the open position. If it is less than 25%, the safety valve will experience a \"hammering phenomenon\". Designers should review the pressure relief capacity and the size of the safety valves to ensure that the pressure relief capacity of each valve is not less than 25% of the rated discharge capacity. When the discharge volume is below 25% of the safety valve’s rated capacity, consider arranging the two valves out of alignment to avoid hammering on the valve seat.