Assembly is a very important step to achieve tightness and function of stuffing box stem seals.
During assembly, a good seating of the packing has to be reached, first. This is realized by plastic deformation of the packing rings. Existing gaps (between stem and packing / packing and housing / between the rings) have to be filled and internal leakage paths have to be closed. This process is accelerated with increased load.
After this pre-deformation a given assembly stress value (tightening torque) has to be applied. Usually, the packing is compressed axially using a gland and bolts; the main tightening surfaces are between stem and packing / packing and housing. Therefore, regarding tightness a sufficient radial force (better: radial stress) has to be maintained during every state of operation.
On the other side radial force / stress induces friction between stem and packing. Regarding function of a valve limited friction is essential.
As a consequence, tightening of stuffing box packings during assembly is a kind of an optimization task: Regarding tightness the assembly stress value has to be high; regarding friction is has to be minimized.
Reliable assembly of stuffing box packings is challenging; thorough preparation is needed as well as skilled personnel.
The tightening procedure has to meet the demands. If the demands are high the amtec TEMES stb.ctrl procedure is an option to consider.