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Slurry Pump: What is it, and how does it work
Slurry pumps are often subjected to severe 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 on the stuffing box.
The favorred methodology for packing a slurry pump is the "flush" seal shown in Figure thirteen-5a. Here the lantern ring is positioned in entrance of the packing rings and a copious supply 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 really helpful for severe abrasive services.
Another method for sealing is shown on Figure thirteen-5b. Here the lantern ring is positioned between packing rings. This configuration is called a "weep" seal. Again, clean liquid ought to 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. Nevertheless, the barrier so created is not 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 requirements for a "flush" type packing arrangement for conventional throat restriction gadgets where no attempt has been made to curtail using flushing water and the place the pressure differential is 15 psi is displayed in Figure thirteen-6. Such a restriction will have an annular radial clearance within the order of .007 instances the sleeve diameter. The length of the throat bush will be about the same as the width of one turn of packing.
It is impossible to predict the exact amount of flushing water required when the packing is "weep" type, since this relies on shaft deflection and gland maintenance. Nonetheless, under regular operating conditions, weepage can be in the order of 5% of the values stated in Figure thirteen-6 for "flush" packing arrangement.
In most cases, seals and flush necessities are provided in ignorance of the real pressure prevailing at the stuffing box, which leads to extreme use of gland water and elevated maintenance.
Built into some slurry pump designs are methods to reduce pumped pressure on the stuffing box by hydrodynamic means. (For example, see Figures thirteen-7 and thirteen-eight 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 totally acknowledged by users. By proper application of impeller pump out vanes and expellers, the pressure at the box might be reduced to almost 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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