5.1 Pump packing

Pump packing

A pump packing usually consists of 2 to 6 rings which are compressed by the axial tightening of a gland against a bottom ring, see figure 5.1a. A radial reaction pressure is exerted on the shaft as a result of this compressive force. Due to the friction from, above all, the surrounding housing, the radial reaction pressure is greatest at the gland and decreases towards the bottom ring.

Figure 5.1a Pump packing with pressure distribution without regard to liquid pressure.

The fluid under pressure will penetrate between the rings and the shaft to form small fluid recesses. There are thus two different pressures to be considered, i.e. the fluid pressure in the recesses themselves, and the radial abutment pressure between the recesses. In the case of a soft packing charged with fluid pressure, the radial pressure varies as shown in figure 5.1b. The fluid pressure falls through the soft packing atmospheric pressure at the gland.

A direct consequence of the distribution of the radial pressure along the length of the soft packing is that wear on the shaft is greatest at the gland (at A in figure 5.1a). If wear occurs at B then it is due to the action of abrasive particles in the liquid.

Figure 5.1b Pressure distribution in a pump gland packing.

The extent of leakage depends on the radial pressure which in its turn is regulated by means of the axial compressive force. Leakage must always be present in order to remove frictional heat and to prevent the fluid from evaporating. The leakage rate in the case of functioning pump packings which are properly adjusted is 1-10 cm³/min, i.e. from a few drops per minute up to about 1 drop per second.

Pump packings with a lantern ring have a somewhat different pressure distribution — see figure 5.1c. The barrier fluid should be admitted in the middle of the box. In the case of five rings the lantern ring should be placed with two rings inboard and three outboard in order to avoid excessive axial movement of the lantern ring during adjustment, thereby blocking the supply of barrier fluid.

Figure 5.1c Pressure drop curve for pump gland packing fitted with lantern ring.