Abstract: A valve for introducing additive substances into a main process stream passing therethrough includes a valve body and a valve seat member carried in the body and having a bore therethrough communicable with the main process stream lumen. The valve seat member has a port communicable with the bore to permit flow of an additive into the bore. A first valve stem in the bore is moved to open and close the port for selectively permitting flow of additive through said port into the bore. A second valve stem moves relative to the valve seat member to open and close the bore for selectively injecting additive into the lumen. Additionally, the valve permits disassembly for cleaning without interrupting flow of the main process stream and provides for heating and cooling of the additive and the main process stream by the circulation of suitable medium through the valve body in a passageway disposed about the main process stream.