The drive shaft of a centrifugal pump is used to drive the impeller. It is then immersed in the pump's fluid. To prevent any fluid from leaking out, a shaft seal is necessary. It is difficult to contain the pressurized, often abrasive liquid by using a seal that can withstand friction from the rotating shaft in the stationary pump housing.
These are the main types of seals: gland packing, and expeller seal pump
This is the traditional type of seal for centrifugal slurry pumps. The gland packing is a soft, rope-like packing that is wrapped around the shaft-sleeve (or safe). While the shaft rotates within the housing, the packing is fixed against it. This may seem like the best solution at first glance, but it has its drawbacks.
To reduce leakage, the packing must be in direct contact with the shaft (or shaft sleeves). This requires slightly more power to drive the shaft. Over time, the friction causes the packing to wear down, increasing leakage until it is fixed and then replaced. Pumping liquids must leak from the seal to ensure that they cool and lubricate properly. This can be a problem if the liquid is corrosive, or has other undesirable qualities.
This is an improvement to gland packing. A rotary expeller reduces hydraulic pressure at the shaft by pushing liquid away. When the pump is in its best condition, this can reduce leakage to almost zero. The liquid may still leak if the pump is stationary or not operating at its best.
To reduce such leakage, gland packing can be used with the expeller seal. Expeller seals have the advantage that leakage can be reduced without having to spend a lot on a mechanical seal. The gland packing is also not too tight to the shaft so the wear on the shaft (or shaft sleeves) is less.