The open, close or partially open place in the valve can be detected by taking a look at the place in the lever: Garza stresses that his presentation shouldn't be taken as holy writ, but as how his specific branch of ExxonMobil (i.e. Upstream) looks at the situation in its http://edgarxncpc.bloggosite.com/85258/what-does-double-block-and-bleed-valve-mean