Cast ball valves are sturdy and durable and may be used in specifically demanding conditions that entail high temperatures and pressures. The spring provides the Original load (as a result of low elasticity of Teflon), although the fluid stress provides the load to drive the lips around the sealing surfaces. https://manueljylwi.wikibriefing.com/3349142/helping_the_others_realize_the_advantages_of_ball_valves