帮忙翻译一句话,有关管道和阀门的

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.