Since seals are sometimes used in nasty, corrosive services, you might think that storage requirements would not be very demanding. But mechanical seals are precision devices having extremely smooth surfaces and often rely on close clearances and movement in order to function properly. Plus, the expected reliability, often measured in years, is very high. Therefore, in addition to be manufactured correctly, it is important that seals be stored properly.
Manufacturers are likely to have similar guidelines but each OEM should be contacted for specifics relating to their products. The following are representative guidelines to successful storage of mechanical seals.
Long Term Storage of Parts
In general, the guidelines for long term storage of seal parts and assemblies typically are:
- in the OEM packaging
- indoors in a controlled environment
- temperature between 15 to 25°C (59 to 77°F)
- humidity below 65%
- out of the sunlight
- not in solvents, oils
- away from ozone.
For cartridge seals that have been in storage, the cartridge assembly should be pressure tested on air before installation in the pump.
Secondary Sealing Elements
Elastomeric secondary sealing elements such as O-rings, V-rings, gaskets, etc. have a “shelf life” which is a consideration for long term storage. In general, the shelf life varies with the type of elastomer. SAE Standard ARP5316 as well as International Standard ISO 2230 lists the maximum recommended shelf life for O-ring and molded secondary seal materials. The shelf life is considered to have an initial storage life after which the component must be inspected followed by an extended storage life provided that the component passed the inspection.
The SAE standard considers shelf life to be 15 years or more when the secondary sealing elements have been properly stored.
ISO 2230 considers that typical nitrile and neoprene elastomers used in mechanical seals have an initial storage life of 7 years followed by a potential extended storage life of 3 years. Typical ethylene propylene, fluorocarbon and perfluorocarbon elastomers used in mechanical seals have an initial storage life of 10 years followed by a potential extended storage life of 5 years.
Although shelf life may appear to be relatively short, especially the ISO recommendation, it actually is not a major problem. After all, some end users report an average Mean Time Between Repairs (MTBR) of 4 to 8 years and the elastomers used therein obviously had already been in storage for some amount of time.
Over time, an elastomer constrained in a groove will take a compression set; that is, deform. Perfluorocarbon elastomers are especially prone to compression set. The resulting change in dimensions and decrease in sealing force can impact performance – especially for secondary sealing elements which must slide (i.e., the dynamic secondary sealing element of a pusher seal). Static sealing elements are at less risk.
Wear Faces
It is, or should be, well known that the lapped faces of seal rings and mating rings may “go out of flat” when stored for a long time. This seems to occur even when those parts have been properly stored. The basic causes for a change in flatness are believed to include:
- stored with active spring load for certain seal design/assemblies
- minute changes over time in shrink fits
- relaxation of internal stresses due to manufacturing process.
Whatever, the cause, it is good practice to check the flatness of seal faces that have been in long term storage and also to air test assemblies.
Short Term Storage of Seals in Pumps
New pumps or pumps recently removed from service may be adequately stored out-of-doors for a few months provided certain precautions are taken and good practices are followed. Such short storage requires attention to the needs of both pump and seal and their respective manufacturers should be contacted for recommendations. Typical considerations concerning the seal include
- drain the pump and seal chambers
- dry the pump and seal chambers using clean, dry compressed air
- cover and close all pump and seal openings, connections, etc.
- turn the shaft two revolutions by hand every month.
Long Term Storage of Pumps
Typical long term storage considerations for pumps are similar to short term except that the seals should be removed and stored indoors. As an added precautions, some end users prefer to apply an oil mist blanket to the pump while in storage.
Oil Mist
Oil mist lubrication is usually thought of as a means of lubricating rolling element bearings. The mist contains microscopic droplets of oil suspended in a large volume of clean, dry air. An oil mist system consists of an air supply, an oil mist generator and piping to distribute the mist to many sets of bearings. The oil mist is typically at a very low pressure.
Oil mist may also be used to protect new equipment that is being stored prior to installation. Some process plants may already be using oil mist lubrication and the new equipment storage area may be added onto the normal operating mist system.
API RP-686
The American Petroleum Institute (API) has a recommended practice, “Machinery Installation and Installation Design” (RP-686) which contains information about storage and commissioning of rotating equipment. RP-686 recommends oil mist storage before commissioning.
Conclusion
Mechanical seal components and assemblies can be successfully stored for relatively long periods of time provided that certain basic guidelines are followed. The primary considerations include proper packaging and a controlled environment followed by a simple air test before startup.
References
- Ehlert, Don, “Getting the Facts on Oil Mist Lubrication”, Turbomachinery Laboratory, Texas A&M University, 2011.
- Flowserve Inc., “Mechanical Seal and Seal Support System Storage”, 2013.
- Fluid Sealing Association, “What Procedures Should I use When Storing my Mechanicals Seals?”, Pumps and Systems Magazine, 2011.
- Lai, Tom & Kay, Peter. “Breakaway Frictions of Dynamic O-rings in Mechanical Seals”, Lubrication Engineering , 1993.