a. Check the requirement if the equipment and lines are:按(1) Cleared of all chemical residue by flushing or steaming, and
(2) isolated by physical disconnection, a blank, or a double-block- and-bleed. (A single or double valve is insufficient protection.) Follow procedures in Section 2, Lock-out Procedures, and Section 3, First Break Procedures.
(3) Combustible dust and solids have been removed from horizontal surfaces to < 1/16” build up.
主要是翻译这个词“double-block- and-bleed”. 急，谢谢！
參考答案:具有两个密封副的阀门