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.  
   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).

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.  
   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.

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