5.3 Stuffing box arrangement

Stuffing box arrangement

Pump stuffing box arrangement can either be with or without barrier fluid. In pump stuffing box arrangement without barrier fluid, the pumped liquid forms a fluid film and must therefore be clean in order to avoid unacceptable wear. This design, see figure 5.3a, can cope only with moderate temperatures where the dissipation of heat takes place solely by means of the flow of leakage. There is a risk of air being sucked in at low inlet pressures as well as in the case of negative pressures.

Figure 5.3a Stuffing box without barrier fluid

Stuffing box with barrier fluid

For stuffing box with the pumped medium as barrier fluid, see figure 5.3b, the pumped medium must also be clean. The barrier fluid pipe is taken from a point where there is a higher pressure than in the box, thus eliminating the risk of air being sucked through the stuffing box.

Figure 5.3b Stuffing box for barrier fluid with normal location of lantern ring.

In case of contaminated, hot and dangerous fluids, a stuffing box with separate barrier fluid is required, see figure 5.3c. The fluid supplied must be selected taking into account the pumped medium and should maintain a pressure which exceeds the pressure at the seal by 0.1 – 0.15 MPa. The quantity of barrier fluid which leaks into the pumped fluid depends, among other factors, on the rigidity and roundness of the shaft and is normally only a fraction of a litre per minute. In the case of extremely abrasive media, the lantern ring can be located at the bottom of the box, see figure 5.3c, in order to prevent abrasive particles from entering into the box. In this case the quantity of sealing liquid which leaks into the pumped medium is considerable.

Figure 5.3c Box for barrier fluid with the lantern ring located at the bottom of the box.

Stuffing box with cooling

With fluid temperatures in excess of 80-120°C, depending upon the packing material, cooling of the box should be undertaken. External cooling of the packing rings is relatively ineffective because of the poor thermal conductivity of the packing material. The cooling chamber should therefore extend in front of the packing rings in the case of higher temperatures, figure 5.3d. For fluid temperatures in excess of 130-140°C cooling of the gland should also be provided. Such an arrangement decreases the vapour leakage and prevents heat being conducted through the shaft to the front bearing.

Figure 5.3d Stuffing box with cooling sleeve and cooled gland.