Mechanical Shaft Seal Leakage MitigationBy PumpWorks Engineering Team / March 16, 2021
If a pump operator is fortunate to have equipment with long service life and high reliability, very often the temptation is to overlook these units and focus on other units that are not performing so well. Do not be deceived! A high reliability focused organization will look ahead to potential problems.
One area of potential problems that can become serious is the issue of leakage between the ID of mechanical seal sleeves and shafts.
There are two broad categories of API pumps to consider:
- The simpler overhung category – OH2 and OH3/4
- The more complicated and difficult to address – BB1, BB2, BB3, and VS6
The Impact of Mechanical Shaft Seal Leakage
In many situations the root cause of the leakage through this annular space is caused by fretting of the O-ring seal (or in the case of hot service, Grafoil packing) on the shaft. It is true that leakage can occur because of damage during installation—however, I am referring to leakage that begins after many months or probably years of service.
One day, hazardous material is seen dripping out the back of the seal near the drive collar. Maintenance is performed and the leak continues. Polishing the fretted area can provide some short term relief, but will not solve the problem.
Short Term Solution
For OH2 and OH3 (and OH4) a shaft replacement is very likely practical. These pumps are often spared and the maintenance cost is not extreme. The long term solutions addressed below are nevertheless good choices for these pumps.
For BB1, BB2, BB3, and VS6 pumps for which shaft repair and/or replacement is difficult to schedule and implement, a modified seal can be an effective solution. This of course requires significantly more planning.
For these applications, I have implemented a relocation of the seal sleeve O-ring. Working carefully with your mechanical seal supplier, a simple relocation of this O-ring can provide a very effective solution. When you are aware of the problem, assess the location (inspect the shaft yourself – don’t rely on others) of the damaged area on the shaft and move the O-ring accordingly. If the seal repair would have required a new sleeve anyway, the incremental cost of this short term solution can be quite low. A very important word of caution here – managing this subtle change with your seal inventory is all important here.
Long Term Solution
- During pump overhauls/rebuilds, specify a shaft upgrade to a high quality martensitic material such as ASTM A479 Type 410.
- Coat the shaft underneath the seal sleeve. Tungsten Carbide (or a Nickel–tungsten carbide) is a good choice for almost all applications. It is very likely that your mechanical seal is at least partially tungsten carbide. Be careful with Nickel substrate coating in amine service. Consult your surface coating contractor for their recommendation.
- Request from your mechanical seal supplier a modified drawing showing the position of the seal sleeve O-ring as referenced to the face of the seal chamber and the overall length of the sleeve. An important word of caution – do not coat the shaft area surrounding the drive collar set screws. Leave ¼”-½” of clear length from the drive collar set screws.
- Coating the entire length of the seal sleeve (minus the drive collar set screw) may seem excessive. If the O-ring is at the front of the seal, the area behind the O-ring is open to atmospheric condition. This area will often become packed with atmospheric contaminants, thus making the seal very difficult to remove. Difficult removals often result in additional seal damage and can result in shaft damage which can lead to infancy seal failure. My experience is that coating the entire length of the shaft will essentially eliminate this issue.
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