an Engineering & Service Bulletin
Technical Topics
J.H. WRIGHT & ASSOCIATES
TOP ENTRY MIXERS WITH MECHANICAL SEALS
With today's increasing pressure on pollution, totally sealing up the entry point of a mixer into the top of a tank is becoming a necessity in many cases. Since the use of a mechanical seal here poses special problems, it is worth an in-depth discussion.
A typical mixer configuration looks like this:
Since split mechanical seals are not only expensive, but something less than a screaming success, it is obvious that the location of the gearbox poses a real problem when you need to replace the seal.
Several solutions have been designed; one is to put a removable coupling in the shaft between the seal and the gearbox. However, this solution doesn't provide room to get the seal off (10 or 12 inches), so you need to drop the lower shaft or raise the gearbox shaft, either one not so easily done.
Also any time you start increasing the distance from the gearbox bearings to the seal - shaft deflection starts to go up. Since most top entry mixers have no lower shaft steady bearingdeflection goes up fast!
The best solution up until recently is to mount the seals on a spool piece with a bearing built in:
This system also has the advantage of being fully testable in the shop before installation. Price however, is the problem since this seal can easily double the cost of your entire mixer. Should you need some exotic seal parts hold on to your hat! One other problem is that this results in a three bearing system; two in the gearbox, one in the seal. Alignment of two bearings is easy but to do three in the field can be tough.
Two other problems for seals in top entry mixers are cooling and lubrication. You cannot depend on product lubrication and cooling as you can in a pump. Therefore, you often need separate cooling and/or lubrication systems that usually require double seals.
Since leakage into your tank may be disastrous, elaborate alarm systems are also often required.
Now just to make things really a problem, let's put the entire sealing system way out of sight on the top of a large tank!
It's no wonder some serious top entry mixer seal problems exist.
Lest I sound too much like the evening news "Doom & Gloom", recently a new gearbox design has appeared that can be a solution. Consider this design:
You start by unbolting the shaft coupling. The gearbox and motor then swing out of the way on the pivot point. Remove the lower shaft coupling and you're looking down at the mechanical seal and can pull it right off the shaft.
The neat thing about this design is that it can be retrofitted to almost any mixer in the field. You just machine a taper on the mixer shaft at the proper point to receive a removable coupling. If you have a top entry seal problem, punch our "Bingo Card" and we will engineer and price up a conversion for you.
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