These are sometimes referred to as piercing valves. They consist of a small clamp which is in two sections fitted to the pipework.
Totally sealed systems will involve the use of this type of valve in order to gain access to the system. A special charging/gauge adaptor is screwed on to the piercing valve assembly (see Figure 8).
The valve assemblies are available for a wide range of pipe sizes and the adaptor is a standard fitting to the assemblies.