A diaphragm valve with an assembly or cartridge that facilitates replacement of the diaphragm, the valve seat or both. The assembly may be a valve assembly having a diaphragm, a seat carrier and a valve seat that are combined into a discrete subassembly or cartridge for a diaphragm valve. The assembly as a discrete cartridge can be easily removed and replaced in order to change out the diaphragm or the valve seat or both. In one embodiment, the diaphragm has a perimeter that is welded to one side of the seat carrier. Flow capacity is favored by providing a gap between a seat carrier and a flow port of the valve body where the gap in part defines a flow area that is the same or greater than the flow area of the flow port. Methods for installing and replacing a valve assembly into a valve body are also disclosed.