It depends on which valves are you speaking; for example the first valves that you have take care in their selection are the control valve to the heater because those are going to work with high delta P and solids also their bypass ; since those can generated high vibration when they are used, some units design that bypass with the same control valve or they install robust valves, since in some units they have generated leak and fires The other valves are the called switch valves and valves SP-6 (entry of the coke drums), as well as the valves associated to the top of coke drums Sp-1/2/3 and those that go towards to the blowdown system. Exist several companies that offer very good designs for example Velan company (they offer ball valve (switch and SP-6/1/2 and ring valve (SP-3)) . In the past these valve were gate or wedge pulg valve You can find more information in the old discussion forum,and also from the sponsoring of this forum
There are several companies that manufacture similar models of valves such as valve KEE; homines valve; METSO company
Currently most of the designs are specified with ball valves, in many cases is due to its operation is simpler than the wedge plug valve or switch, in both valves is essential for the proper operation keep a good injection of the steam purge
in these directions you find information that can help you http://www.osha.gov/dts/shib/shib082903c.html
