"阀杆密封" can be translated into English as "stem seal" (referring to the sealing mechanism for the valve stem) or "valve stem packing" (more specifically referring to the packing material used to seal the valve stem).
"Stem seal" is a general term for the sealing arrangement around the valve stem.
"Valve stem packing" emphasizes the packing material (such as gaskets, O-rings, or other sealing elements) used to prevent leakage along the valve stem.
Choose the term that best fits the context of your usage.