Slurry pumps are often subjected to extreme shock loading and shaft whip due to the presence of solids and system upsets. For these reasons soft compression packing is still favored as a means of sealing on the stuffing box.
The wantred technique for packing a slurry pump is the “flush” seal shown in Figure thirteen-5a. Right 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 large amounts of flushing water are required and the pumped product will be diluted. This system is really useful for extreme abrasive services.
Another methodology for sealing is shown on Figure thirteen-5b. Right here the lantern ring is positioned between packing rings. This configuration is called a “weep” seal. Once more, clean liquid needs 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 shouldn’t be very efficient, 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 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 times the sleeve diameter. The length of the throat bush will be about the identical 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 relies on shaft deflection and gland maintenance. Nonetheless, under normal operating conditions, weepage could be within 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 on the stuffing box, which leads to excessive use of gland water and increased maintenance.
Constructed into some slurry pump designs are methods to reduce pumped pressure at the stuffing box by hydrodynamic means. (For instance, see Figures 13-7 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 fully recognized by users. By proper application of impeller pump out vanes and expellers, the pressure at the box can 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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