1、先导式安全阀的工作原理

 先导式安全阀由主阀和导阀组成。导阀随设备介质压力的变化而动作，主阀则由导阀的驱动或控制而动作。下图是先导式安全阀的动作原理图。            

   当被保护系统处于正常运行状况时，导阀阀瓣处于关闭状态。系统压力从主阀进口通过导管和导阀传入主阀阀瓣（活塞）上方气室。由于活塞面积大于阀瓣密封面面积，系统压力对阀瓣产生一个向下的合力，使主阀处于关闭、密封状态。

    当系统压力升高达到整定压力时，导阀开启，同时滑阀向上移动封闭导阀的进气通道。主阀阀瓣上方气室的介质经由打开的导阀排出，使主阀阀瓣上方压力降低。主阀阀瓣在进口压力的推动下打开而使系统卸压。

当系统压力降低到一定值时，导阀回座并带动顶杆顶开滑阀。系统压力再次通过导阀传入主阀阀瓣上方气室，并推动主阀阀瓣关闭。

2、先导式安全阀的分类

 按阀门动作类型分为突开式和调节式。

API RP520 2.2.2.6 The pilot that operates the main valve can be eithera pop-action or modulating-action pilot. The pop-action pilot,as shown in Figure 24, causes the main valve to lift fully at setpressure without overpressure. The modulating pilot, as shown in Figure 25, opens the main valve only enough to satisfythe required relieving capacity.

 导阀可分为流动型和非流动型（过流型和非过流型）

API RP520 2.2.2.7 The pilots may be either a flowing or nonflowingtype. The flowing type allows process fluid to continuouslyflow through the pilot when the main valve is open; the nonflowingtype does not. The nonflowing pilot-type is generally recommended for most services to reduce the possibility ofhydrate formation (icing) or solids in the lading fluid affecting the pilot’s performance.