Comparison Data of Packing Ring Stress

The lateral pressure coefficient test was conducted to evaluate the stress distribution characteristics of different packing structures under uniform loading conditions. Using Japanese Fuji pressure-sensitive paper, both axial and lateral stresses were measured on each packing ring under a ballast load of 35 MPa. The lateral pressure coefficient (n) was then calculated as the ratio of lateral stress (σr) to axial stress (σφ), expressed as n = σr/σφ. For each of the five packing rings, this ratio was determined separately (n₁–n₅), providing insight into the uniformity of stress transmission within the packing set.

Stress Analysis of Packing Rings

The test examined both axial and radial stress conditions of the packing rings. Axial stress represents the compressive force exerted along the stem axis, while lateral stress reflects the force distributed radially against the gland walls. These measurements help identify whether the applied axial load is effectively converted into uniform radial sealing pressure.

Test Results

Three types of packing were tested—braided packing, anti-emission packing, and flexible graphite packing—each composed of five rings. The following summarizes the average lateral pressure coefficients observed:

• Braided Packing: Average coefficient 0.84. Axial and lateral stresses decreased gradually from the 1st to 5th ring. Over-compression appeared in the upper layers (yellow marks on pressure paper) and under-stress in the lower layers (green marks), indicating uneven stress transmission.
• Anti-Emission Packing: Average coefficient 0.91. Stress distribution was more uniform, with moderate over-compression in the first two rings and under-stress at the fifth. The impregnated structure provided improved load transfer and sealing balance compared to standard braided packing.
• Flexible Graphite Packing: Average coefficient 0.75. A clear downward stress gradient was observed from the first to last ring. Over-compression occurred at the top ring, while under-stress dominated from the fourth ring downward, reflecting poor mechanical load transfer in pure graphite materials.

Summary of Findings

The axial and lateral stress distributions revealed that deeper packing arrangements do not enhance sealing effectiveness. On the contrary, excessive packing depth increases the likelihood of stress relaxation and torque loss during thermal cycling—especially under repeated startup and shutdown conditions in power plant pipelines. The loosening of jack bolts observed in these tests serves as direct evidence of such stress failure.

To ensure consistent low-leakage performance in small-bore forged steel valves—including full compliance with ISO 15848-1 testing at 400°C, three thermal cycles, and 310 switching operations—it is essential to reduce stuffing box depth. This optimization allows the graphite packing to operate within its ideal stress range, ensuring efficient mechanical conduction and stable sealing force.

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