Conventional gland packing Vs. mechanical seals

Thinking of making the switch from conventional packing to mechanical seals?
Before we make the decision, let’s read on for a basic understanding of mechanical seals, and how they differ from packing.

WHY DO PUMPS NEED TO BE SEALED? 
Let's go back to some pumping basics and review how a centrifugal pump operates. Fluid enters a centrifugal pump through the suction nozzle at the center of a rotating impeller. As the impeller rotates, its vanes fill with fluid, then force it out to the pump casing where it exits the pump through the discharge nozzle.
Unfortunately, the discharge pressure will force fluid back behind the impeller, where it attempts to exit by way of the rotating drive shaft. Attempts have been made by pump manufacturers to minimize this leakage through design, but the leakage won't be stopped entirely.

Some form of sealing method must be put in place to minimize the leakage.

USING PACKING AS A SEAL Packing materials were originally the sealing method of choice, using soft, flexible materials. It's a very popular method of sealing pumps because of its low upfront cost, easy installation, and readily available materials.

USING MECHANICAL SEALS The biggest benefit of using mechanical seals is the drastic reduction in leakage. That reduction in leakage also has an effect on a number of other things as well. Things like a safer work environment without a wet floor, fewer bearing failures caused by leaking fluid, no excessive wear on the shaft sleeve damage from packing.
However, Mechanical seals are complicated pieces of equipment, with a wide variety of materials and configurations to choose from, be sure that what you invest in is exactly what you need for your application.

Thumb of rules: If the medium use is water, gland packing will be the choice because of it convenience, but when come to flammable or oil base medium the mechanical shall be the top selection.