关键词: 等熵闪蒸法, 安全阀, 计算, 泄放量, 泄放面积, 工况分析, 水力学计算

Abstract:

This paper introduces the general design process of pressure relief valve in petrochemical units. An example of sizing the pressure relief valve which locates on the recycle gas discharge line at the top of the cold high pressure separator of diesel hydrotreating unit overseas is also introduced. When considering external fire case of the cold high pressure separator, isentropic flash method is utilized for the simulation of relieving fluid under high pressure and high temperature. With this method, the calculated relief area is more accurate and used for determination of the valve orifice size. Conduct hydraulic calculation on the suction and discharge pipelines of the pressure relief valve based on isometric diagrams to determine the size of pipelines and guide the valve type selection. For the suction pipelines, the total pressure loss should not exceed 3% of the pressure relief valve set pressure which prevents valve failure from chattering. For the discharge pipelines, the ratio of calculated built-up backpressure to pressure relief valve set pressure is used for valve type selection. This guarantees the pressure relief valve will pop open normally under set pressure. If the ratio is less than 10% for normal cases and less than 21% for external fire cases, conventional type valve can be used. If the ratio is more than 10% but less than 50%, balanced type valve should be used. And if the ratio is more than 50%, pilot-operated type valve should be used.

Key words: isentropic flash method, pressure relief valve, calculation, relieving capacity, orifice area, case analysis, hydraulics calculation