SEALING SYSTEMS FOR CENTRIFUGAL 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 dissuade this fluid from leaking out of the pump in any volume.
The challenge is to contain this pressurised 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 three key types of seals are gland packing, expeller seals and mechanical seals.
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 (or 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 disadvantages include:
- The packing needs to be physically in contact with the shaft (or shaft-sleeve) to reduce leakage, this means slightly more power is 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 if 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 maintenance time, so the cost of maintenance and downtime is an important consideration. Also, removing the liquid that has leaked can be costly.
A modification of the gland packing seal is the flushed gland – here clean water is flushed through the gland 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.
Key: 11. Gland Assembly 12. Gland Clamp Bolt 13. Gland Bolt 14. Shaft Sleeve 15. Shaft Sleeve O-ring 16. Gland Packing 17. Lantern Restrictor 18. Shaft Spacer 19. Impeller O-ring 47. Stuffing Box
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.
Key: 11. Gland Assembly 12. Gland Clamp Bolt 13. Gland Bolt 14. Shaft Sleeve 15. Shaft Sleeve O-ring 16. Gland Packing 19. Impeller O-ring 38. Expeller Ring 39. Lantern Ring 40. Neck Ring 41. Expeller
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 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
- 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.