Positive isolation can be obtained with the correct selection of valves.  Use a DBB (double block and bleed) valve and you can have the isolation that is required.  Think about installing DBB trunnion ball valve or DBB plug valve.  Both are often used in tank farms for positive isolation and can be operated by hand.  Note: These valves can only be considered as positive isolation when the cavity is monitored or opened to atmosphere.  In the tank farms that I have seen, these DBB valves are only periodically tested by opening the cavity port to verify isolation of product. Check ASTM F 1020 regarding Line-Blind Valves.  The following definitions may help. 
A blank or what I called a blind is a solid one-piece circular unit inserted into a pipeline to prevent flow.
A line-blind valve—an assembly consisting of a spectacle
plate, bolting, and body, the purpose of which is to provide a convenient means to align a piping system to an open or positively closed configuration. The assembly is designed to provide a simplified method of changing over the flow control spectacle plate without the necessity of plate removal from the valve body.
A spectacle plate or spectacle blind is a figure-eight shaped unit with one end open for flow and the other solid to prevent flow.
The term "positive" means absolutely no flow such as might occur with slight valve leakage.

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