SUBJECT: Sealing products that are sensitive to agitation. 6-12
This category of sealing is the one that is the least understood by most of the people that are involved in the process industry. It is easy to understand how temperature can change an "easy to seal liquid" into a difficult to seal crystallized product, a solid, or a gas, but it is hard to see how agitation alone can have much of an affect, because pumped liquids are continually being agitated.
Whether or not you are going to have a problem often depends upon how long the fluid is going to be agitated, and how fast the agitation takes place. We all know that cream becomes butter with agitation and if you beat it fast enough, and long enough, an egg white (a fluid) will become a solid.
The fluid we find in a pump stuffing box seldom gets the proper circulation. The stuffing box lantern ring connection is commonly used for this purpose and if you will look at the area closely you'll see that the fluid is trapped in the seal face area where it is exposed to long periods of high speed agitation.
If the fluid is not affected by agitation or mixing, we say it is a Newtonian fluid (you remember, the apple fell on his head and he discovered gravity). These fluids are not considered a sealing problem for us unless they are sensitive to temperature or pressure changes, or contain lots of solids. The fluids we are concerned about are the non Newtonian fluids, and the problem ones fall into three neat categories:
Dilatants. The more you agitate them, the more viscous they become and in many cases they can solidify. Any time a fluid becomes viscous it can interfere with the ability of the mechanical seal to follow shaft "run out" or vibration. This hysteresis or delay will allow solids to penetrate between the lapped faces or allow fugitive emissions to escape to the atmosphere.
Thixothrophic fluids are the opposite of dilatants. Their viscosity decreases with agitation.
Plastic materials release their viscosity suddenly and present the very same problems as thixotrophic fluids.
When dealing with any of these problems be sure to keep the agitation in the stuffing box to a minimum. In some isolated cases the seal hydraulic balance diameter could be lowered and/or the spring face load reduced to lower the amount of shear. If you are running at higher than conventional motor speeds this can be a real problem.
The use of two seals with a pressurized lubricant as a barrier fluid can keep a lubricant between the faces and diminish the color contamination problem. If color contamination is a real problem, the use of two hard faces is recommended.
As is the case with just about any fluid sealing problem the use of a reliable, clean, compatible, liquid flush is the universal solution. It is often the only solution if you find that none of the above suggestions are practical in your application.
For information about my CD with over 600 Seal & Pump Subjects explained, click here
Link to Mc Nally home page