MECHANICAL SEALS FOR WATER AND WASTEWATER PUMPS

Because mechanical shaft seal failures are the number one cause of pump downtime, the writer decided to dedicate this column to mechanical seal basics.

Years ago, most pump shafts were sealed using rings of soft packing, compressed by a packing gland, but this type of shaft seal required a fair amount of leakage just to lubricate the packing and keep it cool. Then came the development of the “mechanical seal,” which accomplishes the job of restraining product leakage around the pump shaft with two very flat surfaces (one stationary and one rotating).

Main elements of a Mechanical seals

Here is mentioned below, there are two seal face :

• A set of (very flat) machined and lapped primary sealing faces: The very close (near) contact between these two flat mating surfaces, which are perpendicular to the shaft, minimizes leakage. Dissimilar materials are usually used for the faces, one hard and one softer, in order to prevent adhesion of the two faces. One of the faces is usually a non-galling material such as carbon-graphite. The other surface is usually a relatively hard material like silicon-carbide, or ceramic. However, when handling abrasive, two hard surfaces are normally used:
• One face is held stationary in a housing
• The other face is fixed to, and rotates with the shaft.
• A set of secondary static seals, typically O-rings, wedges and/or V-rings.
• One static seal, seals stationary component(s) to the housing
• The other seal, seals the rotating component(s) to the shaft (it normally moves axially on the shaft or shaft sleeve)
• A spring member to maintain face contact, such as a single spring, multiple springs or metal bellows.
• Other mechanical seal hardware, which includes shaft sleeves, gland rings, collars, compression rings, and/or pins.

Mechanical seals require clean water, or other compatible liquid, for the lubrication of the seal faces. The faces in a typical mechanical seal are lubricated with a boundary layer of gas or liquid between the faces. Lubrication can be provided from the pumped liquid itself or from an external source, depending on system requirements.