a hybrid wellhead system is assembled using a plurality of threaded unions , such as spanner nuts or hammer unions , for securing respective tubular heads and a flanged connection for securing a flow control stack to a top of a tubing head spool . the tubing head spool is secured by a threaded union to an intermediate head spool . the intermediate head spool is secured by another threaded union to a wellhead . each tubular head secures and suspends a tubular string in the well bore . the hybrid wellhead system is capable of withstanding higher fluid pressures than a conventional independent screwed wellhead , while providing a more economical alternative to a flanged , or ranged , wellhead system because it is less expensive to construct and faster to assemble .