Page 35 - Valve Selection Handbook

22 Valve Selection Handbook

When such packing is compressed in the stuffing box, axial shrinkage
of the packing causes friction between itself and the side walls that pre-
vents the transmission of the full gland force to the bottom of the pack-
ing. This fall in axial packing pressure is quite rapid, and its theoretical
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value can be calculated. -
The theoretical pressure distribution, however, applies to static condi-
tions only. When the stem is being moved, a pressure distribution takes
place so that an analysis of the actual pressure distribution is difficult.
The pressure distribution is also influenced by the mode of packing
installation. If the packing consists of a square cord, bending of the pack-
ing around the stem causes the packing to initially assume the shape of a
trapezoid. When compressing the packing, the pressure on the inner
periphery will be higher than on the outer periphery. Preformed packing
rings overcome this effect on the pressure distribution.
When the fluid pressure applied to the bottom of the packing begins to
exceed the lateral packing pressure, a gap develops between the packing
and the lateral faces, allowing the fluid to enter this space. In the case of
low-pressure applications, the gland may finally have to be retightened to
maintain a fluid seal.
When the fluid pressure is high enough, the sealing action takes place
just below the gland, where the fluid pressure attempts to extrude the
packing through the gland clearances. At this stage, the sealing action has
become automatic.
Readings of the fluid pressure gradient of leakage flow along the stuff-
ing box of rotating shafts, as shown in Figure 2-3, confirm this function
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of the stuffing box seal. * The pressure gradient at low fluid pressures
is more or less uniform, which indicates little influence by the fluid pres-
sure on the sealing action. On the other hand, the readings at high fluid
pressure show that 90% of the pressure drop occurs across the packing
ring just below the gland. This indicates a dominant influence of the fluid
pressure on the sealing action.
In the case of high fluid pressures, therefore, the packing ring just below
the gland is the most important one, and must be selected for resistance to
extrusion and wear and be carefully installed. Also, extra long stuffing
boxes for high-pressure applications do not serve the intended purpose.
If the packing is incompressible in bulk, as in the case of soft rubber,
the axial packing pressure introduced by tightening of the gland will pro-
duce a uniform lateral packing pressure over the entire length of the
packing. Fluid pressure applied to the bottom of the packing increases
the lateral packing pressure by the amount of fluid pressure, so the seal-