Shaft seals keep fluids from leaking out of a spinning or reciprocating shaft. This is critical for all pumps, and for centrifugal pumps, numerous sealing solutions will be available, including packing, lip seals, and all sorts of mechanical seals – single, double, and tandem, including cartridge seals. Rotary positive displacement pumps, such as gear pumps and vane pumps, are available with packing, lip seals, and mechanical seals. Reciprocating pumps have unique sealing challenges and typically rely on lip seals or packing. Magnetic drive pumps, diaphragm pumps, and peristaltic pumps, for example, do not require shaft seals. These 'seal-less' pumps contain stationary seals to prevent liquid leakage.
Types of pump shaft seals
Packing
Packing (also known as shaft packing or gland packing) is made of a soft substance that is frequently braided or moulded into rings. To form a seal, this is forced into a chamber around the driving shaft known as the stuffing box. Compression is normally applied axially to the packing, however a hydraulic medium can also apply compression radially.
Lip seals
Lip seals, also known as radial shaft seals, are simple circular elastomeric parts held against the driving shaft by a rigid outer casing. The seal is formed by the frictional contact between the 'lip' and the shaft, which is frequently supplemented by a spring. Lip seals are commonly found on pumps, hydraulic motors, and actuators in the hydraulic sector. They frequently serve as a backup seal for other sealing systems, such as mechanical seals.
Mechanical seals
Mechanical seals are made up of one or more pairs of optically flat, highly polished faces, one of which is stationary in the housing and the other rotating and attached to the driving shaft. The faces need to be lubricated, either by the pumped liquid or by a barrier fluid. In practise, the seal faces come into touch only while the pump is at rest. During use, the lubricant creates a thin, hydrodynamic layer between the opposing seal faces, decreasing wear and assisting with heat dissipation.
|
Seal |
Method |
Advantages |
Disadvantages
|
|
Packing
|
Braided or formed material compressed in stuffing box |
• Economical • Excellent for sealing viscous fluids • Seldom catastrophically fails |
• Poor seal for thin liquids, especially at higher pressures • Requires lubrication - packing must be allowed to leak slightly • Can damage the pump shaft |
|
Lip seal
|
A circular elastomeric seal element in a rigid outer housing |
• Economical • Wide variety including cartridge triple lip seals for higher viscosity liquids |
• Typically limited to lower pressures • Poor for thin, non-lubricating liquids or for abrasives • Can catastrophically fail |
|
Mechanical seal
|
Highly polished faces (one stationary and one rotating) running against each other |
• Wide variety of designs and materials for nearly every application • Easily replaced or repaired |
• Higher-end double cartridge seals and gas barrier seals are expensive • Requires external support equipment • Can catastrophically fail |
