Value-Added Upgrades to Extend Pump Life

For many years, traditional pump repair was simply a matter of changing the existing parts.

This started to change when some enlightened souls in the industry started to identify the various pump problems, and how each one could be solved. It was identified that for nearly 100 different pump problems, there were only 34 solutions required, and they could be classified in six groupings:  

1. Personnel Training
2. Better Sealing Devices
3. Component Modification
4. Upgraded Materials
5. System Change
6. New Pump

This greatly simplified the problems experienced when a pump is received at a service center. Using the traditional "Repair Approach", the pump would be returned to the "as new" condition with original materials, parts and clearances. Such an approach will be perfectly adequate if the cause for repair is the result of normal wear and tear, and the customer is satisfied with the length of time the pump has run since the last overhaul.

However, if the cause for repair is the result of premature failure, then bringing the pump back to the "as new" condition will invite the same failure for the same reason in the future. Under such conditions, the more enlightened companies usually prefer to receive a value-added upgrade of the pump to extend its serviceable life. This approach is particularly effective if the pump has been subjected to premature failure.

With the "Value-Added" approach, we can consider the six general solutions and focus on the upgrades that are considered universally beneficial for most pumps, and particularly for the end suction, single stage units that make up more than 80 percent of the pump population in most areas. The four most effective upgrades include:

1. Shaft Sealing
2. Large Bore Seal Chamber
3. Stronger Shafts
4. Effective Lubrication Protection

The extent to which these upgrades are accepted will depend solely on the level of the customer's desire for reliable equipment in a particular plant. This is a topic that needs detailed discussion with the customer before and during the pump repair process to ensure an appropriate balance between the desire for reliability and the investment that may be necessary.

Shaft Sealing

For more than 100 years, the leakage of liquids along the shaft from the pump casing was minimized by means of an arrangement of materials, collectively referred to as packing. Despite holding the dubious distinction of being the oldest part of the design of a modern process pump, packed stuffing boxes are still widely used owing to a low initial cost, and because their operation is familiar to plant personnel.

Although the materials from which packing is manufactured have changed considerably since it was first introduced during the 19th century, it still provides the same advantages and drawbacks (see Table 1).

Advantages	Disadvantages
It is relatively inexpensive to purchase. It is rarely the cause of catastrophic pump failure. It can be adjusted or replaced without pump disassembly. Most maintenance personnel are accustomed to its use.	It lowers pump efficiency. The packing requires regular adjustment. Adjustment requires the touch of an experienced millwright.  Packing and sleeve require regular replacement. It is required to permit constant leakage. It often requires considerable volumes of flush water.

Table 1.

Although constant leakage is required to ensure lubrication between the packing and sleeve, that is now only acceptable if the pumps are handling clean water. In view of society's increasing awareness of environmental concerns, the leakage required by packing is becoming unacceptable with the more aggressive liquids now common in our industrial processes. Consequently, the first level of sealing upgrade would be to a mechanical seal.

Mechanical Seal

All seals operate by having two flat faces running against each other. The rotating face is secured to the pump shaft while the stationary face is held in the gland. As one face is moving while the other is held stationary, this type of seal is referred to as a dynamic seal.

In a basic seal, four possible leak paths must be secured:

1. Between the two seal faces.
2. Between the rotating face and the shaft.
3. Between the stationary face and the gland.
4. Between the gland and the stuffing box.

The last two seal paths are usually static seals as there is no relative motion between the two parts. They are frequently referred to jointly as the tertiary seal, and may consist of a flat gasket or an O-ring in materials compatible with the pumpage.

In the older seal designs, the secondary seal under the rotating face will move marginally back and forth on the shaft, thus causing fretting corrosion and premature failure. However, in the newer seal designs, the secondary seal will be static, thus avoiding fretting corrosion problems on the shaft.

In normal pump operation, the rotating and stationary faces are held closed by the pressure of the liquid in the stuffing box acting as the closing force. During start-up and shutdown, the stuffing box pressure is augmented (and often replaced) by the spring force.

While some liquids are fairly simple to work with, others can be difficult. It is essential that all the individuals involved (including the seal supplier) are made aware of all factors that will influence the seal selection. These factors should include the following: 

• Pressure
• Temperature
• Corrosiveness
• Abrasiveness
• Viscosity
• Tendency to crystallize
• Toxicity
• Rotational speed
• Operational frequency
• Previous field experience

At this point it is worthwhile to acknowledge the benefit of onsite experience. If there is a history of a particular seal operating well in the same service under similar conditions, then this should take precedence over any other consideration.

It is also in the best interests of the end user to become sufficiently well informed about mechanical seals and their ongoing development to be able to identify the particular seal needed in any application.

For example, a component seal is one where each part of the seal must be assembled on the pump individually. This requires considerable skill and significant time investment on behalf of the maintenance department and the manufacturer's critical installation procedures must be followed with the utmost accuracy.

To simplify these safeguards and eliminate the need to make critical positioning measurements during installation, the use of a Cartridge Seal design is preferred.

The cartridge seal is a completely self-contained assembly, which includes all the components of the seal, the gland and the sleeve in one unit.

As it does not require any critical installation measurements, it simplifies the seal installation procedures while simultaneously protecting the faces and elastomers from accidental damage. It also effectively reduces the time spent on maintenance by simplifying seal installation and change-out procedures.

Cartridge arrangements are available in almost every type of seal on the market, and can therefore eliminate the risk factors and the extra maintenance hours inherent in the use of conventional component seals.

Large Bore Seal Chamber

Traditionally, the radial clearance between the shaft and stuffing box on the average process pump was sized to accommodate the 3/8-in square section packing. This resulted in a 1-5/8-in diameter shaft running in a 2-3/8-in bore stuffing box.

When a mechanical seal was introduced into this area, the minimal annular space left available was considered inadequate for reliable operation of the seal and contributed to a high incidence of seal failure. Consequently, the radial clearance between the shaft and the bore of the stuffing box was increased to, at least, 7/8-in. This has proved to be extremely beneficial in ensuring seal reliability.

By enlarging the bore of the stuffing box, the problem identified as seal rub was eliminated. This is a condition where shaft deflection causes the mechanical seal to contact the bore of the stuffing box, resulting in premature seal failure.

The larger volume of the seal chamber also increased the quantity of liquid around the mechanical seal. This permits greater heat dissipation and allows the seal to operate in a cooler environment.

A number of different seal chamber designs are currently in use. The large cylindrical bore chamber shown in Figure 4 is the same design as the stuffing box except that the bore diameter is larger only in the area occupied by the seal, and the traditional close clearance is maintained at the bottom of the chamber. This design permits temperature or pressure control of the seal chamber. However, it should always be used with a flush arrangement to minimize the possibility of creating air pockets in the chamber. 

A through bore chamber eliminates the close clearance at the bottom of the chamber and opens the seal to the full effect of the conditions at the back of the impeller.

Other seal chamber designs incorporate various devices and modifications that change the flow pattern to encourage active circulation of the liquid and eliminate abrasive particles from the seal chambers. This provides a cleaner environment for the seal faces.

Stronger Shafts

In a horizontal, end suction centrifugal pump, frequent and regular seal failure with different seals indicates an undersized shaft being subjected to excessive deflection.

The same thing is true of a packed pump that cannot maintain a minimal amount of leakage for any length of time and seems to leak excessively regardless of the amount of time and expertise spent on minimizing that leakage. This problem is frequently blamed on the last individual that repacked that pump, or even on the type of packing that is used, resulting in many different packing styles being tried. In this case, the underlying source of the difficulty is also an undersized shaft being subjected to excessive deflection.

Under ideal operating conditions where the pump will be running at the Best Efficiency Point (BEP), the radial forces exerted on the shaft through the various hydraulic loads on the impeller will be minimal and not affect the shaft. However, when a pump is not operating close to the BEP, the radial forces exerted on the shaft through the various hydraulic loads on the impeller will be excessive, and have a tendency to deflect the shaft.

In most end-suction process pumps, the amount of deflection that will take place will depend on the effective diameter of the shaft. If the effective diameter is large enough, the deflection will be minimal. However, if the effective diameter is too small, then the deflection will be excessive and cause premature seal and packing failure as described above.

If the shaft sleeve is shrunk onto the shaft, the effective diameter will be the sleeve diameter. However, if the sleeve is secured by a hook design or is keyed to the shaft, the effective diameter becomes the diameter of the shaft under the sleeve. This results in a much weaker shaft that is almost twice as susceptible to deflection in the event of a hydraulic upset condition such as may occur close to the shut-off point on the pump performance curve.

The good news is that this can be quickly corrected by eliminating a shaft sleeve from the pump and using a solid shaft with the original sleeve diameter in that area, thus reestablishing the shaft strength.

Bearing Lubrication Protection

To ensure that bearings provide long-lasting, trouble-free service, it must be recognized that they are only a part of the total bearing arrangement. Other important aspects include the support and the protection of these bearings. Such support includes a strong shaft and housing to minimize the effect of any externally induced stresses or vibration. It also requires accurate machining of the housing and the shaft as well as the correct fits for the bearings.

Protection of the bearings is supplied by the lubricant, which is required to separate the rolling elements and the raceway contact surfaces, to minimize the effect of friction and prevent corrosion. The selection of the lubricant is a consideration of its viscosity, and depends on the operating temperature, the bearing size and its rotational speed. The bearing manufacturer can identify the minimum viscosity required for all these conditions, and the chosen lubricant should provide a higher viscosity than the minimum identified. This reduces the friction losses in the bearing and extends the operating life.

Protection of the lubricant itself is also vital. With any solids contamination of the lubricant, one study implies that 40 percent of bearing life is lost under what is referred to as "Normal" operating conditions. This seems to indicate that considerable improvement is possible on the cleanliness of our bearing environments.

Where water contamination is a problem, it is generally accepted that a mere 0.002 percent water content in wet oil results in a loss of 48 percent of bearing life, and 3 percent of water content results in a loss of 78 percent of bearing life.

All of this underscores the necessity for protecting the quality of the lubricant used to ensure bearing reliability in centrifugal pumps.

Lubricant Protection

A variety of methods are used to keep the contaminants out of the lubricant in pumping equipment. The most commonly used device to keep contaminants out of the bearing housing is the lip seal, despite the fact that it has been proven to be inappropriate for its intended function in a centrifugal process pump. It should be noted for clarification purposes that the function of the seal in the bearing housing is to keep contaminants out of the housing, and not to keep the lubricant in the housing.

The lip seal is designed for the equivalent of an L10 life of 1,000 operating hours. In a pump operating on continuous duty, that translates into a period of less than six weeks. This life span is also dependent on the lip seal being well lubricated, yet most process pumps have little or no lubricant in the immediate area of the lip seals, thus contributing to their early failure. In addition, the lip seal is a contact seal that will inevitably groove the shaft.

Bearing Isolator

A less damaging option is a non-contacting labyrinth seal or bearing isolator, and these are available in a variety of configurations. All of them have a rotor secured to the shaft by O-rings that drive the rotor without inflicting any fretting damage. The stator collects the liquid centrifuged away from the rotor, and drains it away from the lubricant to the outside of the housing.

Most of these seals are designed to operate on horizontal shafts with a drain hole that is required to be installed at the lowest (or six o'clock) position.

Earlier designs exposed the inside of the bearing housing to external ambient conditions when the pump was not running. While this was acceptable in many instances, in areas of high humidity it permitted the ingress of sufficient moisture to perpetuate the problem. To combat this, some bearing isolator designs are now available that will seal off the housing when the pump is stationary. Under these conditions it would also be advisable to seal off the traditional breather cap.

Conclusion

These recommendations do not take into account any special problem conditions that the pump may be experiencing as a result of its relationship with the system. These must be evaluated and considered separately. However, it will be found that, on many of these occasions, one or more of the recommended upgrades are likely to form a part of the overall solution.

Reference

Mackay, Ross. The Practical Pumping Handbook. ISBN 1-85617-410-7

Pumps & Systems, March 2009