A blow-out proof stem is a valve stem assembled from inside the body with an integral shoulder larger than the stem bore, so internal pressure can never eject the stem through the packing gland — even if the gland nut or packing is removed while the valve is pressurised. It is a mandatory safety feature in ball valve standards.
The hazard is real and has killed maintenance workers: on a primitive valve whose stem screws in from outside, line pressure acts on the stem cross-section like a piston. Back the gland off under pressure — or let the packing extrude — and the stem launches out of the valve like a projectile, followed by the process fluid.
Blow-out proof construction reverses the assembly direction. The stem is inserted upward from inside the body cavity and carries an integral (not pinned or clipped) shoulder that bears against a matching internal step in the bonnet or body neck. Pressure now pushes the shoulder harder against its retention step; the only way the stem leaves the valve is by disassembling the depressurised body. ISO 17292, API 608 and API 6D all mandate the feature, and specifications commonly add that the shoulder must be one-piece with the stem — a circlip that can corrode or shear does not qualify.
Verification is visual at the design review stage: ask for the valve sectional drawing and check the stem cannot be withdrawn outward. Every VTXFLOW ball valve uses a one-piece internally-loaded blow-out proof stem in combination with anti-static continuity.