1、先導(dǎo)式安全閥的工作原理

 先導(dǎo)式安全閥由主閥和導(dǎo)閥組成。導(dǎo)閥隨設(shè)備介質(zhì)壓力的變化而動作，主閥則由導(dǎo)閥的驅(qū)動或控制而動作。下圖是先導(dǎo)式安全閥的動作原理圖。            

   當(dāng)被保護(hù)系統(tǒng)處于正常運(yùn)行狀況時，導(dǎo)閥閥瓣處于關(guān)閉狀態(tài)。系統(tǒng)壓力從主閥進(jìn)口通過導(dǎo)管和導(dǎo)閥傳入主閥閥瓣（活塞）上方氣室。由于活塞面積大于閥瓣密封面面積，系統(tǒng)壓力對閥瓣產(chǎn)生一個向下的合力，使主閥處于關(guān)閉、密封狀態(tài)。

    當(dāng)系統(tǒng)壓力升高達(dá)到整定壓力時，導(dǎo)閥開啟，同時滑閥向上移動封閉導(dǎo)閥的進(jìn)氣通道。主閥閥瓣上方氣室的介質(zhì)經(jīng)由打開的導(dǎo)閥排出，使主閥閥瓣上方壓力降低。主閥閥瓣在進(jìn)口壓力的推動下打開而使系統(tǒng)卸壓。

當(dāng)系統(tǒng)壓力降低到一定值時，導(dǎo)閥回座并帶動頂桿頂開滑閥。系統(tǒng)壓力再次通過導(dǎo)閥傳入主閥閥瓣上方氣室，并推動主閥閥瓣關(guān)閉。

2、先導(dǎo)式安全閥的分類

 按閥門動作類型分為突開式和調(diào)節(jié)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.

 導(dǎo)閥可分為流動型和非流動型（過流型和非過流型）

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.