What are mechanical seals ?
Power machines that have a rotating shaft, such as pumps and compressors, are generally known as "rotating machines." Mechanical seals are a type of packing installed on the power transmitting shaft of a rotating machine. They are used in various applications ranging from automobiles, ships, rockets and industrial plant equipment, to residential devices.
Mechanical seals are intended to prevent the fluid (water or oil) used by a machine from leaking to the external environment (the atmosphere or a body of water). This role of mechanical seals contributes to the prevention of environmental contamination, energy saving through improved machine operating efficiency, and machine safety.
Shown below is a sectional view of a rotating machine that requires the installation of a mechanical seal. This machine has a large vessel and a rotating shaft at the center of the vessel (e.g., a mixer). The illustration shows the consequences of cases with and without a mechanical seal.
Cases with and without a mechanical seal
Without a seal
- The liquid leaks.
With gland packing (stuffing)
- The axis wears.
- It needs some leaks
(lubrication) to prevent wear.
With a mechanical seal
- The axis does not wear.
- There are hardly any leaks.
This control on liquid leakage is called "sealing" in the mechanical seal industry.
- Without a seal
- If no mechanical seal or gland packing is used, the liquid leaks through the clearance between the shaft and the machine body.
- With a gland packing
- If the aim is solely to prevent leakage from the machine, it is effective to use a seal material known as gland packing on the shaft. However, a gland packing tightly wound around the shaft hinders the motion of the shaft, resulting in shaft wear and therefore requiring a lubricant during use.
- With a mechanical seal
- Separate rings are installed on the shaft and on the machine housing to allow minimal leakage of the liquid used by the machine without affecting the rotating force of the shaft.
To ensure this, each part is fabricated according to a precise design. Mechanical seals prevent leakage even with hazardous substances that are difficult to mechanically handle or under harsh conditions of high pressure and high rotating speed.
How are mechanical seals used?
The following are example uses of a mechanical seal in a pump.
Mechanical seal in pump structure
- 1.Internal structure of pump
- A pump is a machine that works to move fluids, as in lifting a liquid to its inside. For example, its impeller rotates to move water.
- 2.Mechanical seal installation location in pump
- A mechanical seal is installed on the impeller rotating shaft. This prevents the liquid from leaking through the clearance between the pump body and the shaft.
- 3.Mechanical seal installation location on shaft
- Mechanical seals are mostly comprised of two rings: a rotary ring on the shaft and a stationary ring on the pump housing.
- 4.Mechanical seal structure
(stationary and rotary rings)
- The rotary ring rotates with the shaft. The stationary and rotary rings rub against each other ensuring a clearance in the order of micrometers maintained between them. Where they rub against each other, they are referred to as "face materials."
- 4.Mechanical seal structure
The essential point is to control leakage and friction.
The face materials where the stationary ring and the rotary ring rub against each other are the most important portions as a barrier to the fluid. If the clearance is too small, the friction increases, hindering the shaft motion or resulting in seal breakage. Conversely, if the clearance is too large, the liquid will leak. Consequently, it is necessary to control the clearance in the order of micrometers to prevent leakage, but at the same time ensuring lubrication by the fluid, thereby reducing the sliding torque and avoiding hindrance to the machines' rotation.
Technologies behind mechanical seals
The mechanical seal technology is a sum of mechanical engineering and physical property technology due to the above-mentioned functions and applications. More specifically, the core of the mechanical seal technology is the tribology (friction, wear and lubrication) technology used to control the surfaces where the stationary and rotary rings rub (slide) against each other.
Mechanical seals with improved functionality will not only prevent the liquid or gas handled by a machine from leaking to the outside, but also improve machine operating efficiency, thereby helping achieve energy saving and prevent environmental contamination. Moreover, in some cases, rotating machines handle media that, in the case of leakage, can lead to a dangerous accident. Therefore, mechanical seals are required to be highly reliable through manufacturing backed by solid engineering expertise.
These functions and roles will make mechanical seals increasingly important functional parts in the future. Their further technical innovation is anticipated. To positively respond to these expectations, Eagle Industry is working on technical research and development every day.