A carefully planned and carried out maintenance program extends centrifugal pump life, maintains high pump dependability and rated performance, and reduces overall operating costs. The three primary areas of pump care are general effects of erosion and specific problems of packing and bearings.
Erosion is wear caused from the impingement effect of the fluid. The wear from erosion is increased when abrasive solids are suspended in the fluid. Discontinuities in the flow passage, such as exposed gaskets, abrupt change of pipe size, and sharp corners, are particularly susceptible to erosion. They cause a change in the direction of flow that creates eddy currents and instantaneous velocity increases.
A centrifugal pump which has been carefully selected for its application will show less wear and that wear will be uniform thus affecting performance less. A pump that is the wrong size or the wrong design for its service can very likely fail prematurely.
As a pump wears, impeller clearances are increased, and the pump’s performance is reduced. Pumps that depend on close clearances for effective performance show the most rapid reduction in performance. For this reason only a pump designed for slurries should be used in that service. These pumps do not depend on such close impeller clearance, internal discontinuities are eliminated, fluid passages are large to minimize high velocities, and impeller diameters and shafts are larger so the pump can be run at reduced speeds.
Packing problems most usually are caused by difficulty in maintaining proper lubrication between the shaft and packing. The shaft and packing must be lubricated to prevent shaft scoring and wear as well as packing wear.
The most common method for lubricating packing is to allow leakage. The most common cause of packing difficulties comes from preventing this kind of lubrication by overtightening. Tight packing causes excessive heat that
wears the shaft and packing. As a result, the shaft is scored and packing must be replaced frequently. And it is virtually impossible to maintain reasonable packing life or to seal against a rough shaft.
Usually the line fluid is used as a lubricant. However, if it contains abrasives, it is not suitable and another lubricant must be introduced into the stuffing box at the lantern ring. Around a drilling rig the best such external lubricant is
water free from abrasives. But it’s pressure must be sufficient to force it into the stuffing box and to keep the abrasive line product from entering the stuffing box.
Packing life is also reduced at higher shaft speeds. So one more important point in pump selection is to pick the pump that will do the job required at the lowest speed.
When repacking the stuffing box, first make sure the box is clean and all old packing is removed. Place packing in the bottom of the box that, when compressed, the lantern ring will be in the proper location beneath the sealing tap.
The ring joints should be staggered. Draw up snug only by means of the gland. Pack the remainder of the box, draw up snug and back off the gland until the nuts are finger-tight. Packing expands with heat, and a box which is more than finger-tight when cold, will generally smoke when started up.
Tighten nuts half a turn at a time and wait to see if leakage has been controlled to desired rate. Do no run drop tight. Some drippage is required to cool packing.
Water to the packing lantern ring is recommended when the stuffing box pressure is below atmospheric pressure. Where water cannot be used, grease as often as required to maintain an air seal. Water flushing also prolongs packing life in abrasive service.
NOTE: Do not add extra packing rings when excessive leakage occurs
The third important factor in pump performance is proper care of the bearings. Several factors can affect their life and performance. Mechanical unbalance produces excessive loads, as does misalignment of the pump because of improper or poor piping foundation. Excessive cavitation also causes unusual vibration loads on the beatings, resulting in premature failure. Solids that ball up and plug the impeller cause a mechanical unbalance and corresponding vibration loads that are damaging. Overtightening the beatings cause the lubricant to break down while excessive lubrication causes beatings to overheat.
But the most important bearing problems come from contamination. Dirt and grit in the bearing race cause rapid failure. Moisture within a beating enclosure (usually entering from contaminated lubricant) causes rust and corrosion
with subsequent bearing failure. Cleanliness cannot be overemphasized. You should not need to regrease the pump unless the original grease becomes contaminated. Disassemble the pump and remove old grease. Hand pack the bearing and fill the bearing cap approximately 1/3 full with clean grease.
An increase in bearing temperatures (above 200°F) or noise indicates possible bearing failure. Complete bearing failure usually damages other pump parts. Try to prevent complete bearing failure by changing when the above conditions are detected.