the main pipe material shall be made of stainless steel (or material having a higher exemption pressure) upstream (if there is no check-valve) and downstream the isolation valve up to one dn passed the by-pass branch connections, 在截止阀的上下游，旁通支管路接头一个dn长度范围内，主管材质应采用不锈钢（或许用压力更大的其他材质）。 fyi
