In the past, there were two ways to seal a pump: the mechanical seal method or the compression packing set. Either choice was subject to potential leakage and/or hidden costs. For compression packing users, pump leakage has become so widely accepted that many companies have focused on better ways to divert the leakage rather than eliminate the leaks. For mechanical seal users, it is not uncommon for a company to have a spare pump with a mechanical seal to replace an operating pump should the mechanical seal fail.
The ideal pump seal does not contaminate the process, handles system upsets, runs in less than perfect conditions and is easy to install. These criteria eliminate many existing pump seals, but process-exclusive pump seals remain one option. These seals are neither mechanical seals nor braided compression packing. Before discussing them in more detail, we will review some of the issues involved with mechanical seals and compression packing.
Mechanical Seals
Mechanical seals are the most common method of sealing pumps today. The performance of these seals is generally acceptable; however, some issues with mechanical seals can be costly during the seal's life.
Under nearly ideal process conditions, mechanical seals usually perform well; however, run-out and vibration, abrasives, viscous media, cavitation or air entrapment can cause a mechanical seal to fail or require costly variations. For shaft run-out in excess of the manufacturer's recommendation, the shaft must be re-aligned or bearing replacement must be performed. It is common practice to install expansion joints to absorb vibration.
For abrasives, a double mechanical seal is required, but if abrasives find their way between the mechanical seal faces, they could cause seal failure. For certain applications involving sticky, viscous media, the face seal technology is unsuitable. For cavitation that occurs inside the pump, the solution is to re-route the fluid so that air does not enter the process, which may not be possible. For air entrapment, the flush water must be rerouted so that air bubbles are eliminated and the stuffing box must be purged.
When dealing with a flushed mechanical seal, the flush must be clean and reliable. A "dry" situation can quickly cause seal failure, and a flush that is contaminated with particles can destroy the seal faces.
Installing a mechanical seal can be expensive and time-consuming. If the mechanical seal is not a split, the pump must be disassembled to install it, which can take many hours. If the pump is to be removed from the system for assembly, overhead cranes and forklifts may be required. Some mechanical seals may require an "above average" level of knowledge and training to properly install them. This may require onsite technical assistance from the seal manufacturer.
Mechanical seal failure is not progressive. When it occurs, it is immediate and there is no possibility of repairing or reducing the leak in service (see Figure 1).
Figure 1. Typical failures modes of mechanical seals under less than ideal operating conditions
Compression Packing
Compression packing is the oldest type of pump seal available. It was the most popular method of sealing pumps prior to the advent of mechanical seals. This simple method is reliable, and performance is generally acceptable in the applications for which it is typically selected. However, there are also hidden costs involved with compression packing.
Water usage can be one of the largest hidden costs. Some packing sets must be flushed to cool the set and lubricate the packing/shaft interface. Flushing is also used to exclude abrasive particles from the stuffing box area. A flushed packing set may consume water at a rate of 0.50 to 7 gallons per minute. For example, a single pump that runs at 3 gpm will use 1,576,800 gallons of water per year. In addition to the water usage, there are costs associated with the treatment prior to returning it to its source. Water dilution of the process fluid incurs hidden costs as it must often be evaporated or removed from the process. This can mean expensive equipment costs and decreased productivity.
Power consumption is a direct cost associated with the use of compression packing. The friction required to obtain a seal between the packing set and the shaft can create drag on the shaft or sleeve and will require more amperage to operate the pump. This increased amperage means approximately twice the electrical usage of a balanced mechanical seal.
Compression packing creates a seal through compression and resulting radial expansion. Too much radial expansion can cause significant shaft wear, particularly in the area nearest the gland follower where higher radial loads are generated (see Figure 2). The shaft must be replaced or a new sleeve must be installed if the wear is significant.
Figure 2. Radial expansion of conventional compression packing can cause significant shaft wear, necessitating replacement or installation of a new sleeve
Process fluid loss can pose another hidden cost for compression packing. In the case of a pump that has no flushing fluids, the process fluid is used to lubricate the packing/shaft interface. The loss of this process fluid is usually not recoverable.
The easy-to-install nature of these products makes them desirable in plants with inexperienced or reduced maintenance groups. Pumps can be repacked with compression packing in the field and do not have to be broken down. Emergency pump repair can be much quicker with compression packing because the product can be easily cut from a spool and installed in a pump.
Process Exclusive Technology
A relatively new pump seal technology is process exclusive technology. This technology uses a specially configured cage to direct a pressure-controlled flush to hydraulically load the seal and prevent process fluid from entering the stuffing box area, while dramatically reducing the flow rate of the flush. Similar in installation to a compression packing set, these seals can handle the same adverse pump conditions, yet run with the lower amperage draw and water usage of a balanced mechanical seal (see Figure 3). Much like compression packing, process exclusive seals can be adjusted while equipment is running to control leakage; however, the rate of flush water consumption is typically 60 times less than a comparable compression packing.
Figure 3. Process exclusive sealing technology directs a pressure-controlled flush to hydraulically load the seal, prevent process fluid from entering the stuffing box and dramatically reduce flush water consumption
These seals perform well in abrasive applications with a flush. In addition, they operate longer and more reliably and can cost considerably less.
Due to the hydraulic loading of the set and a patent-pending barrier ring, abrasive particles cannot enter the stuffing box and compressive load from the gland follower is lower, so shaft and sleeve wear and power consumption are reduced.
Figure 4. Note the power consumption of process exclusive sealing technology (blue), compared with a baseline pump running without any type of seal (green), an unbalanced mechanical seal (aqua) and a balanced mechanical seal (magenta)
Conclusion
Notwithstanding their limitations, mechanical seals and compression packing remain viable options for sealing pumps. As noted, however, some mechanical seals do not perform particularly well under adverse conditions, and compression packing can increase water usage and power consumption. Process exclusive technology overcomes these limitations to offer pump manufacturers and users an effective and economical sealing solution.
Pumps & Systems, October 2009