The In-line Shutoff Valve is the perfect choice for situations where you need absolute dead tight shutoff. The valve has a raised seat so there is no pocket in the line. Other valves have a recess in which product can puddle and degrade. The raised seat makes it so the line is completely drainable on either side of the seat. Combine a line drain with the In-line Shutoff Valve and you have a perfect combination for polymer shut down. This design uses the plunger seal so it can be used in high temperature / high pressure processes including 2500# @ 1000F. Available in either full port or reduced port designs.
Another option for In-Line Shutoff is the inclusion of a small Line Blind to guarantee no leakage. Seen towards the bottom of the page was a recent design including an integral swing style line blind.