I have managed to get a little exposure to pumps here over the past couple of years. This probably isnt the best description by I will try.
Pumps may have a packing gland. The packing we use is a segment of rope that is impregnated with graphite. It is sliced at an angle so that it forms a ring. Five or six of these rings are slid around the shaft and into the pump. A backing ring (not sure if that is the proper name) is bolted up on the shaft behind the packing to compress it and form it to the shaft and any cavities. Packing can leak, and we do not use it to seal pumps that are used for solvents. Over tightening the packing can cause the pump to over heat (I have seen a pump shaft red hot before).
I do not have much experience with mechanical seals. I am of the understanding there are several different types. We have a vacuum pump that had a mechanical seal fail. I wanted to change it myself but when asking the pump rep, he recommented against it unless I had alot of experience with them, as that looking at them funny will cause one to fail. I have been recommened that mechanical seals are better to use with solvents. Our dryer has a Johnson joint which is a type of mechanical seal I believe. It is basically a carbon ring and a spring presses it against a surface for the seal.
In addition to the two types of pumps mentioned above there are also mag drive pumps. I thought that might be what you ment by casing pumps. Mag drive pumps have no seals. Instead the motor turns a shaft that is connected to a magnet in the shape of a drum. In our case it looks like a coffee can. The impeller is connected to a smaller drum that fits inside of this drum. However the pump casing itself seperates these two pieces. This way there are no seals to leak. The shaft turns the first magnet that turns the second magnet connected to the impeller. I have been told that magnets can "spin loose"? if run dry though I have not seen this yet.
The direct answer your question of what is the difference between a pump with a mechanical seal and one with packing, the answer is - nothing, a pump is a pump and a sealing system is a sealing system, both mechanical seals and packing are used to seal the shaft where it enters the casing, deciding which one to use depends many factors. I suggest you do some research or sealing systems so that you better understand the differences.
