The sealing of mixers and agitators 3-7
Mixer applications are the same as process pump applications, except for a couple of things:
- The mixer seal is subject to much more radial and axial shaft movement.
- Dual seals are usually required (especially in top entering mixers) because the seal is frequently located above the fluid causing a single seal to run dry.
- Many mixers run under vacuum conditions requiring the use of balanced seals.
- Bottom entering designs usually have a lot of solids in the seal area.
Please refer to the following drawing for the bearing arrangement of a typical mixer. You’ll note that the motor turns the shaft through some gearing that’s well supported by bearings. The problem starts when the mixer shaft passes through the stuffing box area.
As the impeller turns in the liquid it can cause excessive shaft whip and radial movement. As shown in figure “A” the packing helps to stabilize the shaft and is acting as another support bearing. Packing never was designed to act as a bearing so the result is major shaft wear and excessive leakage problems. The shaft movement is aggravated by several factors:
- If the liquid level in the tank is too low it can cause “vortexing.” To prevent this vortexing you should measure the diameter of the mixer blade and then insure that the fluid level is at least one and one half times this distance above the turning blade.
- In many instances we are mixing a combination of liquid and a solid. Since it’s customary to add the solids first they can cause “skidding” of the blade as it rides on these solids.
- The length of the shaft is too long for the diameter. In pumping we call this the L3/D4 ratio and recommend that the number never go over 60 (2 in the metric scale). In most mixer applications the number runs into the hundreds.
If you attempt to convert mixer packing to a mechanical seal you usually run into trouble with the excessive shaft motion. Some consumers have attempted to solve the motion problem by installing another anti- friction bearing closer to the mechanical seal. Unless the equipment is “line bored” (and it never is) it’s not wise to put three anti- friction bearings on any rotating shaft.
Picture “B” is one method of solving the problem. The mixer shaft is placed inside a “Quill Shaft” that’s well supported by bearings and this quill shaft is then coupled to the mixer shaft. This design will allow the manufacture to place the second shaft bearing closer to the stuffing box area where it should have been in the first place. Unfortunately this new design adds several thousand dollars to the mixer cost. The manufacture typically packages a double seal and bearing into a spool piece that can be inserted into the two piece shaft.
This original equipment seal is a poor choice because most mixer manufacturers choose an inferior unbalanced “back to back” double seal that’ll damage the expensive stub shaft and clog easily if there are solids in the mixing liquid. The following illustration describes this ” back to back” double seal:
The best, and lowest cost solution is to install a motion seal in the stuffing box and solve the problem once and for all. I do not have an illustration for a motion seal, but you can check with your seal supplier for a catalog that’ll explain the seal in great detail.
Motion seals are different than pump seals:
- Because of the misalignment potential, they are almost always a stationary design. (The springs do not rotate with the shaft)
- The inner seal should be balanced in both directions because many of these applications can fluctuate between pressure and vacuum. This feature will also prevent the inner seal from blowing open and allowing the barrier fluid to contaminate the product if you loose barrier fluid pressure between the seals.
- The rotating hard faces must be wider than normal to accommodate the excessive radial motion. A design movement of plus or minus 0.125 inches (3 mm) would be typical.
- The internal clearances must be greater than pump seals to prevent contact during excessive radial movement.
- The stationary faces must be spring loaded in some manner to prevent loading one seal and unloading the other during periods of axial movement. The easiest way to do this is to spring load the faces together.
- Cartridge designs are necessary for correct and easy installation and to allow you to change seals without emptying the mixer in “Side Entering” and “Bottom Entering” applications. Be sure the seal has some sort of self-aligning feature to compensate for misalignment caused by the set screws in the cartridge.
Single split seals, mounted on a split sleeve, have become very popular for mixer type applications (see the following illustration). The shaft movement can be partially stabilized by installing a split sleeve bearing in the stuffing box. A wide variety of materials are available for this split bearing. Check with your local distributor.
The split seal is also a good choice for those mixers that have a moveable shaft that seals a Teflon ring against the tank wall and allows you to change the seal without emptying the mixer. This is a major improvement over the original equipment design because you do not have to remove the shaft spool piece.
It should be obvious that when the spool piece is removed, there’s usually no force to hold the Teflon® ring against the mixer wall.
Bottom entering shafts create a different set of problems. Look at the following illustration:
Dirt and solids fall into the stuffing box and can’t be centrifuged out. Flushing liquid tends to channel through the solids and does a poor job of removing them from the stuffing box.
There are a couple of possible solutions.
- Cut off the stuffing box and move the seal into the vessel so that centrifugal force will throw the solids away from the flexible seal components
- Use a higher specific gravity fluid in the stuffing box to prevent gravity from pulling the solids into the seal area. A high density grease could be a good solution.
- If you can’t tolerate grease you should be able to find some type of a compatible fluid that’ll work.
- On February 13, 2018