i have to purchase a valve to isolate a 30'' outline located at the bottom of a storage tank of Bunker150000Brales.I will connect a future line to this valve in few months. However, the tank will already be in operation.Should I installed a standard gate valve, a parallel slide gate valve or a knife gate valve ?I think a gate valve could do the jobs but will be more expensive. I don't know ''parallel slide gate valve" but I have seen that type of valve installed on similar application.Knife gate valve : my feeling is that it's leaks.Maybe a gate valve but they do leak. Parallel slide: expensive and the packing will rpobably leak. Knife-gates are for slurries like paper stock, but not for tight isolation. I would look first at a butterfly valve. You may need a fire-safe valve. THese are available with a zero-leak resilient primary seat and a metal secondary seat for post-fire shutoff. Given the size of the line being 30" and I am assumeing an ANSI 150# or 300# tank rating or less I would suggest you look at a Triple Offset Butterfly Valve. These are typically firesafe to API 607 4th edition and are zero leakage. I would first contact Adams Valve in Houston TX or if you are in Europe they are located in Germany.