They're normally held in place by a sort of "spectacle" clip which attaches with 2 screws. There's also a matching gasket. However I have seen them swaged into place with 3 equally spaced punch hits. Not too clever and care will be needed if this is the case. Use 3 new positions for the punch. Photograph or make a drawing of the orientation and position of the valves before you remove them. It's easy to put them back wrong. Annoying if you have the clip, VERY annoying if using a punch.