Bearing seals mechanical seal

The most common maintenance problem with centrifugal pumps is with the seals. Mechanical seal problems account for most of the pump repairs in a chemical plant, with bearing failures a distant second. The absence of an external motor (on canned pumps) and a seal is appeahng to those experienced with mechanical seal pumps. 

Mechanical losses - Bearings, lip seals, mechanical seals, packings, etc., all consume energy and reduce the pump s efficiency. Small pumps (less than 15 HP) are particularly susceptible. 

You can t begin to resolve problems in your centrifugal pumps, bearings and mechanical seals, until you learn the numbers 60°, and 240°, with respect to your pump cutwater. 

The mechanical design of the idler roUs is a function of the particular service under which the conveyor operates. Minimum industrial standards for roU dimensions, bearings, and appHcation criteria for different service conditions have been estabHshed (14). Idler life is deterrnined by a combination of factors such as bearings, seals, shell thickness, load density, and the operating environment. 

Potential problems associated with centrifuges include mechanical friction from bearings vibration leaking seals static electricity and overspeed. 

The bearings are W ater lubricated. The typical materials of construction are carbon, copper alloys, bakelite and ceramics. The mechanical seals, like a double oil seal protected w ith a cap called a Sand Guard, are robust and perfect in sealing the inotor to prevent the entry of pumped... 

Oil-filled motors Instead of water oil is now used with oil and mechanical seals at the bearings to prevent leakage of oil through them. 

Some solutions may not be practical, or economical, or timely and consistent with production. You could be forced to live with cavitation until the next plant shutdown to make the neces.sary corrections. In the meantime, the cavitation shock waves and vibrations will travel through the impeller, down the shaft to the mechanical seal faces, and onto the shaft bearings. We offer some. specific recommendations for surviving cavitation shock waves and vibrations in Chapters 13 and 14 on Mechanical Seals. 

This dial indicator is fixed to the volute mounting adapter collar of the pump and the needle is on the shaft (Figure 14-21). The shaft should be moved radially by hand (see the arrows) up and down. Note the movement in the indicator. This is a check of the radial tolerance in the bearing. Some people use the word run out . Radial dellection causes misalignment of the rotating and stationary faces of the mechanical seal. This shortens the seal life by eausing drive pins and springs to wear and rub in relative motion. 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

Very few industrial pumps come out of service and go into the maintenance shop because tlie volute casing or impeller split down the middle, or because the shaft fractured into four pieces, d he majority of pumps go into the shop because the bearings or the mechanical seal tailed. 

The leak size can be reduced by using double mechanical seals or a mechanical seal and a throttle bush, the space between the two being vented to a safe place. Major leaks may still occur, however, due to collapse of the bearing or seal. LFG pumps should therefore be fitted with emergency isolation valves (see Section 7.2.1), particularly if the temperature is low or the inventory that can leak out is high. 

Figure 6-46A. Exploded view of rotary lobe gas pump with mechanical seals and pressure lubrication for bearings. By permission, Roots Division Dresser Industries, Inc.

Because these machines require internal lubrication of the sliding-vane surface, no extreme effort is made to arrange special bearing and shaft seals, except that a gas seal of the conventional mechanical design is usually used to prevent gas from escaping to the outside or to prevent air inleakage in the case of vacuum service. For hazardous or... 

The efficiency of all mechanical seals is dependent upon the condition of the sealing area surfaces. The surfaces remain in contact with each other for the effective working life of the seal and are friction-bearing surfaces. 

Mechanical seals offer a more reliable seal than compressed packing seals. Because the spring in a mechanical seal exerts a constant pressure on the seal ring, it automatically adjusts for wear at the faces. Thus, the need for manual adjustment is eliminated. Additionally, because the bearing surface is between the rotating and stationary components of the seal, the shaft or shaft sleeve does not become worn. Although the seal will eventually wear out and need replacing, the shaft will not experience wear. 

---