A thermosyphon tank can also be used with mechanical seals. They use very little water, usually only a few liters per month. They are expensive and can cause damage quickly, which could lead to expensive repairs. Mechanical seals have their place.
There are two situations where mechanical seals can be used: sealing an extremely toxic acid, or when the pump inlet head is too high (e.g. series pumping).
But what about other situations that cover a large portion of pumps in a mill, is there another choice for the shaft seal of a slurry pump? Expeller seal pump that has a small impeller behind it. The small impeller (or expeller) spins when the pump runs and pushes or expels the slurry that is trying to escape through the opening.
There is absolutely no need to add gland water and there is no leakage from the back of your pump. When the pump is off, a few rows of grease-lubricated packing are used to seal it against leakage. Check out the attached video. Exempeller seals do have limitations. Yes. They are extremely affordable (only about 5-10% less than a pump) but they work well when used correctly.
Today, most centrifugal slurry pumps have shaft seals that use packing and gland water to seal around the shaft preventing the escape of slurry from the back-side of the casing. The gland water must be clean and injected into the pump casing at a pressure that is slightly above (10-15 psi) the pump's discharge pressure. Gland water dribbles out the back of the pump and packing must be tightened and replaced often quite frequently.