Stewart’s Electric Motor Works, Inc.
Split case pump seals are commonly the most difficult installation of any mechanical seal. The replacement of these seals usually is determined by the type originally found in the pump. The problem is that the existing seal type may not be the best for the fluid pumping application. The fluid to be pumped often is the last thing considered for a system designer specifying a pump for a general application. And if the fluid contains more than just water, problems can start to arise. Even closed recirculation systems have additives that can interfere with proper sealing.
For the purpose of terminology, when original equipment manufacturer (OEM) seals are mentioned we will be referring to “Single Spring Elastomeric Bellows” seals.
A pump that has been worked on previously might even contain an improper seal. The seal may fit into the pump, but is the wrong working length or there might have been an improper selection of face materials. It might also have improper elastomers for the pumping application.
Common premature failures
Premature seal failure is generally described as any seal that does not run for an expected (and reasonable) length of time. Premature failure is not to be confused with catastrophic seal failure. That’s where a seal fails all at once, usually due to dry run conditions or breakup of the seal for mechanical reasons. Keep in mind that these examples can happen in a matter of minutes, hours, or days and even months. A successful installation is measured by the time in service between seal replacement.
Some of the most common premature failures of split case pumps with OEM seals in them will be:
- Pumps that pass the hydrostatic test and then on startup will immediately leak.
- Upon startup the seal isn’t leaking but in a short length of time will develop a drip that continues to get worse the longer the pump runs.
- Installations that while the pump is running, the seals do not leak. They will leak when the pump shuts down; when the pump starts again the leak stops.
The factors you will want to consider to ensure the seals are installed correctly are: cleanliness, proper set length (especially on pumps with locking collars), component handling, and material selection. The two most common mistakes during installation are hyperextension of the bellows (the elastomeric flexible component of the seal head) or the seal head itself being mounted on the shaft diagonally. The inherent complex design of the pump and the necessary steps required to assemble add to the challenge of proper installation. Making sure that the head and seat mate as close to being perfectly parallel in whatever operational plane they function will lead to successful pump operation.
In most applications you can’t see the seal. So how do you determine proper installation? The answer to that: In many cases you really cannot. Only runtime will tell. Good installation practices have to be followed to best ensure that the seal will work properly. It’s difficult to determine if the seals are installed properly until the pump is in operational mode.
Using correct seal type
One of the problems with replicating existing OEM seals that are installed in the pump is to make sure that they are the correct type of seal. Then you must determine if the seals are correct for that application. Knowing what makes up the fluid components of the pumpage will help you decide what materials the seal may include. For example, corrosive fluids, even chlorine, can shorten the life of the elastomer drastically. Suspended solids will find the small gap between the precision lapped mating faces to start leakage. It is important to remember when making the seal selection for the pump and application not to replicate someone else’s mistake by failing to consider all the parameters.
Even if the pump is hydrostatic tested in your service center, passes and does not leak, there will be times when the pump is installed and operational in the field and you will still experience premature seal failure.
There will be occasions where an end user will separate the casings and only bring the rotating assembly in for recondition. Situations such as these are prime candidates for leakage after reassembly. Your technicians are trained to ensure the shaft sleeves run true, the bearings are mounted properly, the seal components are carefully installed, the impeller is balanced…only to have the end user drop the assembly on the way back to the building. The idea was to save time and money by not removing the entire pump. If you have worked on split case pumps, then you have experienced one if not all of these premature failure issues at one time or the other.
Most failure issues with OEM seals can be avoided by using cartridge seals. If the split case pump has enough clearance between the seal cage end of the stuffing box and the next obstruction, then a cartridge may be used. Most of the problems associated with OEM seals may be eliminated because of the ease of installation of a cartridge seal.
The next obstruction, referred to as the down shaft restriction (DSR), almost always will be the bearing journals. If the space is there to accommodate the gland and locking collar outboard length of the seal, then a cartridge seal may be the best for your application.
The reason a cartridge seal will outperform an OEM seal is because the set length of the seal and the proper alignment of the faces are built into the seal. The ease of installation and the elimination of possible seal failure and rework created by failure will offset the pricing difference between the OEM seal and a cartridge seal.
The proper face alignment and set length of any seal is paramount if you want longevity. A cartridge seal in a stationary design will derive the face alignment from the centerline of the shaft. Conversely, a rotary design seal will derive the face alignment from the face of the seal cage or the way it is put onto the shaft.
The cartridge seal preload centering clips keep the faces of the cartridge seal together and in proper orbital alignment during the assembly process. The preload centering clips will also eliminate possible contamination of the faces.
The construction of the cartridge seal will allow you to assemble the rotating assembly and install the completed assembly into the pump housing without having to deal with all of the issues associated with movement that can occur and will affect the performance of OEM type seals.
After the pump is assembled, bolt the cartridge to the pump, place the set screws and pull the preload centering clips; you will know the seal is properly installed.
Cartridge seals create a reliable liquid seal between the pump housing and the pump shaft. Pump speeds usually up to 3600 rpm create heat and wear from friction. The OEM seals are unbalanced and with increased face loads, will accelerate the wear.
A cartridge seal is a balanced seal. Most cartridge seals in production today are double hydraulically balanced. This is done internally in the cartridge seal and allows an offsetting force to lighten the face load of the primary sealing faces.
When the face load is offset, then the seal will run cooler and will have better lubrication. This will extend the life of any seal. Lubrication is forced in between the seal faces by the process pressure. The more lubrication that is provided, the cooler the seal will run and the better the seal will perform.
Any seal must have adequate fluid between the faces for proper lubrication and operation. Should there be a period void of lubrication, the seal will burn up immediately. Dry running will destroy all seals.
A single cartridge seal, like any other, will contain the same components. The seal will contain primary lapped faces, one rotating and one stationary. The cartridge seal also has static and dynamic secondary components. They are usually o-rings to accomplish the total sealing of the pump unit.
Simplicity of putting a cartridge seal onto the shaft, assembling the rotating unit and then the assembling of the pump make it easy. After the assembly of the pump, all that is needed for completion is to fasten the cartridge seal by bolting the seal gland to the pump housing, setting the set screws and pulling the centering clips. Mounting the cartridge seal is simple.
Remember that mechanical seals are a tried, tested and a proven technology. Mechanical seals are the best way to create a reliable liquid seal. Also remember that any seal must be kept cool and lubricated by the fluid it pumps.
Most seals don’t fail because they wear out. Running dry, vibration, cavitation, erosive fluids, contamination, and mechanical failures of shafts and bearings, all have detrimental effects on the life of the pump and its seals.
All of these issues must be taken into consideration if you have a problematic sealing application with continual or premature failure. Identifying the mode of failure will be elusive at times. A lot can be determined by looking at the seal faces and components when you have seal failure. Most of the time a mode of failure can be determined by just looking at the seal components. After correctly determining the mode of failure, you can select the proper seal for your application.
When you have a problematic application, your knowledge of selecting traditional mechanical seals and cartridge seals allows you to offer the customer corrective action and solutions to his mode of failure. The help provided by you from your analysis of the situation will demonstrate the value of your expertise and ensure return business.
More on pump repair
If you want to learn more about pump maintenance and repair, I recommend that you attend EASA’s “Fundamentals of Pump Repair” seminar.