求翻译：排污阀和放空阀应选用具有节流截止功能的阀门。DN300以下排污阀主要选用阀套式排污阀，手动放空管线上选用节流截止放空阀， 具有自动放空功能的管线上采用旋塞阀。是什么意思？

Drain valve and vent valve should be used with function of throttle stop valve. DN300 following main selection valve sleeve type sewage valve sewage valve, manually emptying use throttle stop venting valve on the pipeline, with automatic venting functions of plug valve on the pipeline.

正在翻译，请等待...

Drain valve and vent valve should be used with function of throttle stop valve. DN300 following main selection valve sleeve type sewage valve sewage valve, manually emptying use throttle stop venting valve on the pipeline, with automatic venting functions of plug valve on the pipeline.