Meaning
Mechanical load imposed on the actuation system by the internal sealing components. Stem packing drag is a necessary byproduct of the compression required to prevent fugitive emissions of the process fluid. It represents a significant portion of the total torque or thrust needed to operate a valve, particularly in high pressure services.
Torque Allocation
Sizing calculations for actuators must include a specific value for the resistive force of the seals. If the stem packing drag is underestimated, the valve may move slowly or fail to reach its full travel position. Designers often use empirical data from packing manufacturers to establish the load based on the gland bolt torque.
Seal Configuration
Graphite and ptfe rings are the most common materials used to manage the trade off between sealing and friction. Adding more rings to the stuffing box increases the stem packing drag but provides more barriers against leakage. Live loading systems use springs to maintain a constant pressure on the packing, which stabilizes the friction over thousands of cycles.
Emission Compliance
Environmental regulations drive the requirement for tighter seals to eliminate the release of volatile organic compounds. Increasing the gland pressure to meet these standards raises the stem packing drag and may necessitate a larger actuator. Facility operators balance the cost of larger hardware against the potential fines for exceeding leakage limits.
