Stuffing boxes

Because stuffing boxes meet only minimal sealing requirements, their use is limited to straightforward problems involving nonhazardous substances outside of potentially explosive atmospheres. Though unavoidable, leakage can be tolerated in the absence of potential hazards.

The actual sealing element is the packing cord, which is placed under axial compression to apply sealing pressure between the packing and abutting parts. Friction between the packing and the shaft or housing causes the tension to decline from the gland to the base packing ring. The seal's tightness and the friction behavior depend on the packing construction, shaft workmanship, lubrication and wear conditions, and the operating parameters of pressure, temperature, and relative velocity.

Simple agitator stuffing boxes, like the one shown in Fig. 4, are operated in the mixed friction range. The packing material used must therefore have a low coefficient of friction. Lubrication-enhancing reimpregnation and lubrication during operation can prolong the service life of this stuffing box substantially.

To prevent excessive wear as a result of shaft deflections and keep the stuffing box from changing the critical speed of the agitator shaft inadmissibly, the seals must be machined very precisely and aligned perfectly concentrically with the shaft. The packing material has to remain elastic. Cup springs are used to apply pressure to the gland follower for these reasons. Because packing wear quickly reduces the effect of this axial spring loading, the stuffing box is designed to permit retightening while the machine is running.

Table 1 lists the application limits for this type of stuffing box.

If the temperatures prevailing in the agitated vessel are relatively high (between 130 and 400° C), water-cooled thermal barriers are installed to keep the packing materials from overheating.

Even at low loading levels, stuffing boxes have to be lubricated and cooled to perform reliably. Even then they are so prone to wear that they often require servicing (with resultant downtime) for renewing packings and sometimes reworking the shaft contact surface.

Depending on a stuffing box's condition and its particular duty conditions, the temperatures reached at the seal sometimes exceed the ignition temperatures of explosive gas mixtures. Wherever potentially explosive atmospheres are present, therefore, mechanical seals or magnetic couplings have to be used instead of stuffing boxes to avoid sources of ignition.