A steam jacket is a device that is put over a valve
for cooling or heating purposes. An alternate method of heating is heat tracing
For example, when asphalt is made it is poured through a valve at the end of the line. This valve cannot be allowed to cool as the asphalt will harden and it will be impossible to operate the valve. To prevent this, a steam jacket is placed around the valve and a heat conducting cement is used to fill any holes between the valve and the jacket. Steam is run through the jacket which has an inlet or an outlet. Some designs will have 2 inlets and 2 outlets.
Also, instead of steam, coolant like glycol can be run hrough the steam jacket for keeping a valve cool.