Mechanical seals for slurry pumps.

In a centrifugal slurry pump, the drive shaft is inserted into the pump to drive the impeller, which is immersed in the fluid being pumped. A shaft seal is required to stop this fluid from leaking out of the pump in any volume.

The challenge is to contain this pressurized and often abrasive liquid using a seal that will also withstand the friction generated by the rapidly-rotating shaft running in the stationary pump housing.

The traditional type of seal is gland packing – soft packing held in place by a rope-like sleeve known as a packing gland. Although this may be the more economical solution, the disadvantages include:

• The packing is worn down over time by friction, which increases leakage until it is repaired or replaced. 
• The packing needs to be physically in contact with the shaft to reduce leakage, this means more power is required to drive the shaft; also the shaft will wear down over time.
• To work effectively, some of the liquid being pumped will leak through the seal. This is a major problem if this liquid has corrosive or other unpleasant qualities.
• The packing also needs flushing with a large volume of clean water to keep it cool.

Using a mechanical seal can overcome these problems.

A mechanical seal consists of two parts, one section is fixed to the shaft, the other fixed to the stationary pump housing. The interface of these two components is the mechanical seal. One of the sections will be fixed in place, while the other will be pushed up to it using a spring-loaded fixing. This allows a tiny amount of liquid, called the fluid film, to flow into the minute space between these two surfaces, creating a seal, but also providing the needed lubrication and cooling.

This liquid between the two faces of the seal can either come from the fluid being pumped or from an external source. There will be some leakage into the dry area of the pump, but as the gap is so small (as narrow as 1 micron) this will be virtually undetectable.

Cost savings come from the reduction in wear to pump parts, a reduction in down-time because repairs and parts are needed less often, a reduction in the liquid needed for cooling, and a prevention of any discernible leakage of the pumped liquid.

Common brands of centrifugal slurry pumps are Warman and Slurrypro. The pumps can be supplied with mechanical seals if requested, or they can easily be retro-fitted.

Bill Noakes- Atlantic Pumps