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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 preferred methodology for packing a slurry pump is the “flush” seal shown in Figure thirteen-5a. Here the lantern ring is positioned in front 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 large quantities of flushing water are required and the pumped product will be diluted. This system is beneficial for severe abrasive services.

Another method for sealing is shown on Figure 13-5b. Right here the lantern ring is positioned between packing rings. This configuration is called a “weep” seal. Again, clean liquid should be injected at a pressure higher than the prevailing slurry pressure close to the stuffing box. Product dilution is significantly reduced compared to the “flush” seal design. However, the barrier so created isn’t very effective, causing abrasive particles to penetrate and cause wear. If the service is only mildly abrasive, then grease can be used 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 using 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 times the sleeve diameter. The size of the throat bush will be about the identical because the width of one turn of packing.

It is unattainable to predict the precise quantity of flushing water required when the packing is “weep” type, since this relies on shaft deflection and gland maintenance. Nevertheless, under normal working conditions, weepage could be within the order of 5% of the values said in Figure thirteen-6 for “flush” packing arrangement.

In most cases, seals and flush necessities are provided in ignorance of the real pressure prevailing on the stuffing box, which ends in extreme use of gland water and elevated maintenance.

Built into some slurry pump designs are strategies to reduce pumped pressure at the stuffing box by hydrodynamic means. (For instance, 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 end-suction pumps, one not totally acknowledged by users. By proper application of impeller pump out vanes and expellers, the pressure at the box might be reduced to nearly 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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