帮忙翻译一句话,有关管道和阀门的

题目

帮忙翻译一句话,有关管道和阀门的
- The SGS provides active, safety-related overpressure protection of the steam generator and main steam piping upstream of the main steam isolation valve.
主要是 main steam piping upstream of the main steam isolation valve,这个短语是什么意思.upstream把整个句子搞乱了,想象不出他们之间的关系.

答案

主蒸汽隔离阀门上游的主蒸汽管道
upstream：上游