The level of liquid in your vessel can effect how you safely run your agitator. Here are some important rules to follow when running an agitator with the fluid at/below blade level as well as when the vessel is being emptied and filled.
When fluid is at or below blade level:
The agitator should never be run at full speed with the liquid level at the agitator blade level. When the agitator is run with the fluid in the vessel at approximately the same level as the blades, fluid flow can cause the shaft to “skate”, resulting in excessive stresses and runout in the seal area and upper end of the agitator. This can cause premature seal failure and even glass damage on the agitator itself. The simple fix is to make sure there is the right volume of product in your vessel before turning on the agitator. If this becomes a regular problem, then it’s time to look into getting a different sized reactor to meet your process needs.
When the vessel is being emptied:
An agitator should never be run at full speed while a vessel is empty or during slow draining of the vessel, especially if the liquid will be at the blade level for any appreciable time. However, when extra care is taken, an agitator can run at full speed during the emptying of the vessel. The caveats are: 1) it must be done quickly, 2) the liquid level should not be held at the blade level for any appreciable length of time (see above), 3) the viscosity is not increasing, and 4) the agitator should be slowed to 35% of the maximum speed once the liquid level drops to 8-10” above the blade level.
There is sometimes a misconception with the operating of mixing systems during filling. It is generally okay to start and run the agitator during filling, provided that the filling is done quickly and the liquid level is not held at blade level. When liquid is added the agitator should run slowly and the speed should be increased as the level increases. “Down-pumping” is required, in which the impeller is rotated so that the blades push the fluid in a downwards manner.