3 key types of seals for centrifugal slurry pump

Gland packing 

This is the traditional type of seal for centrifugal slurry pumps. Soft, rope-like packing, known as the packing gland, is packed around the shaft-sleeve. The packing is held stationary against the pump housing, while the shaft rotates inside it. Although this may initially appear to be the most economical solution, the disadvatages include:

• The packing needs to be physically in contact with the shaft (or shaft-sleeve) to reduce leakage, this means slightly more power required to drive the shaft or shaft-sleeve.
• The packing is worn down over time by friction, which increases leakage until it is adjusted, and eventually replaced.
• To work effectively, some of the liquid being pumped needs to leak through the seal for cooling and lubrication. This is a major problem is this liquid has corrosive or other unpleasant qualities.
• Constant rubbing means the shaft or shaft-sleeve will wear down over time.

Gland packing is the seal type which requires the most maintenace time, so the cost of maintenance and down time is an important consideration. Also, removing the liquid that has leaded can be costly.

A modification of the gland packing seal is the flushed gland, here clean water is flushed through the bland packing into the pump, which eliminates any leakage and keeps the shaft cool. However, the amount of water required makes this option costlier, plus the liquid being pumped gets diluted which is not always desirable.

Expeller seals 

An improvement on gland packing, this is where a rotary expeller (often combined with pump-out vanes on the back of the impeller) reduces the hydraulic pressure at the shaft by pushing the liquid away from it. This can reduce leakage to nearly zero when the pump is operating at optimal conditions. However, when the pump is stationery or running sub-optimally the liquid can still leak. Usually some form of gland packing is used in conjunction with the expeller seal used to minimise such leakage.

The main advantage of expeller seals is that the leakage is reduced without the significant expense of purchasing a mechanical seal. Also, as the gland packing is not pushed so tightly to the shaft, the wearing on the shaft (or shaft-sleeve) is reduced. 

The disadvantages of expeller seals are that leakage still occurs, a critical factor when pumping corrosive or unpleasant liquids, and that a significant amount of power (typically 2% of total power) is needed to run the expeller.

Expeller seals are fitted as standard in slurry pumps for the quarrying industry due to lower maintenance requirements, and because some leakage is acceptable. For other industries this leakage cannot be tolerated. For example, in the recycling industry, when pumping digestate from anaerobic digestion plants, such leakage is unpleasant and can lead to corrosion of pump parts, increasingly mechanical seals are being fitted instead. 

Mechanical seals 

These are the most effective, and most expensive sealing arrangements for slurry pumps.

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.

The advantages of using mechanical seals are:

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

The main disadvantage of mechanical seals is their much higher purchase price. However, this is offset by lower running costs and the elimination of any costs associated with leakage. Slurry pumps can be supplied fitted with mechanical seals, or they can easily be retro-fitted.

These are the most effective, and most expensive sealing arrangements for slurry pumps.

For more information, you can always contact one of our specialists.

Pump Engineer: Ms. Fiona Li

Email:[email protected]

Tel: +86-13673231378