unheading - снятие днища (head) резервуара, где происходило коксование:
http://osdir.com/patents/Distillation-apparatus/Insulated-tr...
Removal of coke from a full coker vessel, also known as decoking, typically is a time consuming and potentially dangerous process that generally involves cooling the multi-ton coke mass with water, drilling and cutting the coke mass from the drum with a specialized drilling system and dumping the hot, disaggregated mass along with steam and hot water into a chute through a hole in the coke vessel bottom. Opening the hole in the coker vessel bottom (or the top hole for drill insertion) for coke removal in older systems involves removal of a head device, which is designed to tightly seal the coker vessel during the coking phase of the cycle. The process of removing and replacing the removable top head and bottom units of the vessel cover is called heading and unheading or deheading.

А что же делает клапан? Читаем дальше:
Recently, however, significant improvements in the process of opening and closing pressure vessels, such as coker vessels have been achieved; for example, the “unheading” valve described in PCT Patent WO 02/07371. This new valve easily and automatically opens and closes a coker drum and is repetitively operable through numerous coking/decoking cycles, thus eliminating the cyclic heading and deheading process as described above.

Т.е. это клапан открывания днища для выгрузки кокса; unheading - историческое название, снятия днища не происходит на самом деле, когда используют этот клапан