What Is a Mechanical Seal?
Mechanical seals reduce leakage of liquids from rotating shafts. Specifically, mechanical seals are used on the rotating shafts of machinery with rotating mechanisms, such as pumps and compressors, to prevent leakage of liquids such as water and oil from the rotating shaft.
Uses of Mechanical Seals
Mechanical seals use liquids and are widely used in industrial machinery, such as automobiles and industrial plants with rotating mechanisms, as well as in residential facilities.
Since each liquid has different characteristics, it is important to select the materials and mechanisms used in mechanical seals properly, according to the liquid. With proper selection, the mechanical seal can prevent leakage of hazardous liquids and help prevent environmental problems, while contributing to energy savings and improved equipment safety through efficient rotation.
Principle of Mechanical Seals
The basic structure of a mechanical seal consists of a rotating ring that rotates in the direction of the axis of rotation of the rotating part of the machine and a fixed ring that does not rotate. The ring-shaped “sealing” on the rotating ring is pressed against the “floating seat” on the fixed ring to slide, forming a gap between these sliding surfaces that prevents leakage of liquid.
Because of this structure and principle, some types of mechanical seal can prevent leakage of hazardous liquids under high rotation and high pressure.
Types of Mechanical Seals
There are various types of mechanical seals, and their characteristics vary depending on the sealing characteristics of the rotating ring, mounting position, and installation method. The most common types are unbalanced, balanced, rotating, stationary, inside, and outside types.
1. Unbalanced and Balanced Types
Unbalanced and balanced types are classified according to the sealing characteristics of the rotating ring. The factor that determines the pressure by the liquid is the pressure-sensitive area (A1) on the liquid side of the sealing of the rotating ring.
If the relationship between the pressure-receiving area (A1) and the sliding area (A2) is A1 > A2, the liquid pressure directly affects the sliding surface pressure. On the other hand, when A1 < A2, the pressure from the liquid is reduced.
The ratio of A1 to A2, A1/A2, is called the balance ratio (B.V.). The unbalanced type is affected strongly by the liquid pressure when B.V. > 1, while the balanced type is affected weakly by the pressure when B.V. ≤ 1.
2. Rotary Type and Stationary Type
The rotating type is a mechanism in which the sealing rotates in synchronization with the shaft, while the stationary type is a mechanism in which the sealing is fixed and does not rotate. The rotary type can be smaller than the stationary type, but the sealing is easily deformed when rotating at high speeds, which may lead to defects.
3. Inside Type and Outside Type
The inside type is a mechanism in which the leaking liquid travels from the outside to the inside, while the outside type is a mechanism in which the leaking liquid travels from the inside to the outside. The inside type is characterized by improved sealing because the liquid is affected by centrifugal force.
The outside type, on the other hand, has the advantage of being less susceptible to corrosion because it can be constructed so that the liquid has less contact with the Mechanical Seal.
Other Information on Mechanical Seals
1. Comparison of Mechanical Seal and Gland Packing
In addition to mechanical seals, gland packings are also useful in preventing liquid leakage from rotating parts of rotating machines. Therefore, we will explain the features, advantages and disadvantages of Mechanical Seal and Gland Packing.
Mechanical Seal
- Leakage volume: Minute
- Structure: Complex
- Cost: Initial (at time of installation) = large/ Running = small
- Life span: Relatively long
Gland Packing
- Leakage: Some leakage required for use
- Structure: Simple
- Cost: Initial = small/ Running = large (periodic replacement is necessary, consider the time required for retightening)
- Life span: Relatively short
Depending on the fluid used, mechanical seals and gland packings are used, but gland packings are generally used when there is no danger of leakage, such as water. On the other hand, mechanical seals are generally used when there is no danger of leakage from hazardous materials.
Gland packing is often used for equipment that uses powder in addition to fluid. However, consideration should be given when using mechanical seals for equipment that contains foreign matter in the fluid, such as wastewater, or viscous liquids, such as slurry.
If these fluids are mixed between sliding surfaces or adhered to the sliding surfaces, there is a high possibility that the sliding surfaces will be scratched and leakage will occur. In addition, if they get into and adhere to the springs that are used to press seals and mechanical seals together, the spring tracking ability may be impaired, leading to leakage in some cases.
2. Life of a Mechanical Seal
The life of a mechanical seal depends greatly on the specifications of the machine. The fluid used, machine operating hours, number of operations, and fluid temperature are the main factors that determine the service life. Basically, the approximate service life is tentatively determined based on the past installation experience and set as a cyclic replacement, but generally, the service life is 2 years.
For those that do not have a set replacement cycle, the timing for replacement depends on the importance of the equipment, but if the amount of visual leakage increases, it is time to replace the seal. Mechanical Seals provide a non-contact seal.
Therefore, they can be used maintenance-free as long as the springs that keep the sliding surfaces even and the packings that prevent fluid intrusion do not wear out. However, consumables must be replaced periodically to prevent leakage as they deteriorate over time.