More products description :

secondary seals are used to seal around the production casing stub in the bottom of the bowl of the tubing head, the various types are described below:

double FS-seals:

This design consists of two elastomer-shaped seals located in grooves in the bottom of the tubing head. A seal is achieved by interference of the seal with the outside diameter of the casing and the bottom or OD of its retaining groove. A spring molded in each inside corner of the seal provides an anti-extrusion mechanism when sealing against pressure by bridging the gap between the tubing outside diameter and casing head seal bore. The shape of the seal allows it flexibility to maintain sealing interference with the large variations in tubing outside diameter. 

      RCMS Seals

The Rough Casing Metal Seal (RCMS) is designed to use up to 20,000 psi. This seal does not require special machining of the casing stub; is compatible with temperatures from -20°F to 350°F in corrosive environments; does not require special installation tools and is simple to install; is NACE compatible; is compatible with casing spool and tubing spool strength; is compatible with casing string strength properties.