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Slurry pumps are often subjected to extreme shock loading and shaft whip because of the presence of solids and system upsets. For these reasons soft compression packing is still favored as a way of sealing at the stuffing box.

The choosered methodology for packing a slurry pump is the “flush” seal shown in Figure 13-5a. Here the lantern ring is positioned in entrance of the packing rings and a copious provide of clean liquid is injected at a pressure higher than the prevailing slurry pressure within the stuffing box. The clean liquid acts as a barrier and prevents the ingress of abrasive particles that cause packing and sleeve wear. The disadvantage of this system is that giant amounts of flushing water are required and the pumped product will be diluted. This system is beneficial for severe abrasive services.

An alternate methodology for sealing is shown on Figure thirteen-5b. Here the lantern ring is positioned between packing rings. This configuration is called a “weep” seal. Once more, clean liquid should be injected at a pressure higher than the prevailing slurry pressure near the stuffing box. Product dilution is significantly reduced compared to the “flush” seal design. However, the barrier so created is just not very effective, causing abrasive particles to penetrate and cause wear. If the service is only mildly abrasive, then grease can be utilized in lieu of liquid.

An approximation of flushing necessities for a “flush” type packing arrangement for standard throat restriction devices where no try has been made to curtail the use of flushing water and the place the pressure differential is 15 psi is displayed in Figure 13-6. Such a restriction will have an annular radial clearance in the order of .007 occasions the sleeve diameter. The length of the throat bush will be about the same as the width of one turn of packing.

It is not possible to predict the precise amount of flushing water required when the packing is “weep” type, since this depends on shaft deflection and gland maintenance. Nonetheless, under regular working conditions, weepage could be within the order of 5% of the values acknowledged in Figure 13-6 for “flush” packing arrangement.

In most cases, seals and flush requirements are provided in ignorance of the real pressure prevailing at the stuffing box, which results in extreme use of gland water and increased maintenance.

Constructed into some slurry pump designs are strategies to reduce pumped pressure on the stuffing box by hydrodynamic means. (For example, see Figures 13-7 and thirteen-8 for diagrams of pump out vanes on impellers and expellers.) The side-suction-pump configuration is subjected only to suction pressure and has an advantage over finish-suction pumps, one not fully acknowledged by users. By proper application of impeller pump out vanes and expellers, the pressure at the box could be reduced to virtually zero. This is called a dry box arrangement. In these cases, weep-type seal is satisfactory, with either water or grease being injected into the cavity formed by the lantern ring.

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