Mechanical seals are a very effective tool.
Any centrifugal pump must fit a rotating shaft while mechanically retaining the liquid or gas at the wet end of the pump.
Mechanical seals mainly consist of a “rotating sealing surface” with a rotating mechanism that rotates at the same speed as the pump shaft, and a “fixed sealing surface” that is connected to the rotating sealing surface and using a gland. or in some pump models, a pressurized assembly (using a set of springs or a metal accordion) that seals the rotating surface to prevent contamination and leakage when the pump is not operating and when the pump shaft is rotating. Holds firmly against the fixed surface.
Static sealing gaskets, “O-rings” or elastomeric cylinders are strategically placed to complete the sealing assembly.
Figure 1: Mechanical floods
Conditions for choosing the right mechanical seal
In order to choose a mechanical seal correctly, you should first pay attention to its rotating part and then evaluate the type of fixed surfaces.
The pressure factors of the flood environment, the temperature and speed of the rotating axis of the machine (RPM), the type of pumped fluid and the size of the diameter of the rotating axis were taken into consideration.
Because when a valve fails, a significant amount of money is spent on replacing it, and in most cases, the process equipment is shut down until a new mechanical valve is installed.
Therefore, choosing the right mechanical seal according to its operating environment can increase productivity in the production process.
Advantages of mechanical seals
Among the advantages of the mechanical seal is preventing the leakage of expensive, corrosive and dangerous materials, toxic liquids and gases, pollutants and carcinogens, radioactive liquids, explosives, etc.
Basics of fluid pump sealing
A mechanical seal is a simple method of protecting the fluid in a conduit or pipe (usually pumps, mixers, etc.) in which a rotating shaft passes through a fixed housing or sometimes the housing rotates around the shaft.
When sealing a centrifugal pump, the challenge is to allow a rotating shaft to enter the wetted part of the pump without allowing too much pressurized fluid to escape. (figure below)
To meet this challenge, a seal or seal is needed between the shaft and the pump housing that can maintain the pressure of the process being pumped and resist the friction created by the rotation of the shaft.
Before examining how mechanical floods work, it is important to understand other methods of flood formation.
One of these methods that is still widely used is “gland packing”.
In fact, gland packing is a rope-like and woven material that is wrapped around the shaft and physically fills the gap between the shaft and the pump housing.
This material is still commonly used in many applications.
Figure 2: Gland packing
However, users increasingly want mechanical seals for the following reasons:
The friction of the rotation of the shaft over time causes the wear of the packing, thus increasing the leakage until the packing is adjusted or replaced.
In order to reduce leakage, it is necessary to compress the packing against the shaft, and this means that the pump needs more thrust to turn the shaft, and this means wasted energy.
Since the packing is in contact with the shaft, as a result of wear, a groove is created in it, which can be expensive to repair and replace.
Mechanical floodgates are designed to overcome these problems:
Figure 3: Showing the pump housing and rotating shaft
A basic mechanical seal consists of three levels of sealing:
The fixed part of the seal that is mounted on the pump housing with a static seal. This may be secured with an o-ring or gasket sealed between the fixed part and the pump housing.
The rotating part of the seal that is usually fixed to the shaft with an O-ring. Also, this sealing part is considered as a static part because it rotates with the shaft.
The mechanical seal itself is the interface between the static and rotating parts of the seal.
Figure 4: The part marked in red. Left: fixed part and right: rotating part
While the two sealing levels in the flood design are simple static floods, the flood between the rotating and stationary sections needs a little more attention.
This initial flood is the basis of the design of all floods and is necessary for its effectiveness.
Figure 5: Initial flood. Yellow: the rotating part of the flood. Blue: fixed part of the flood.
The primary seal is mainly a vertical bearing that consists of two very flat surfaces: one fixed and the other rotating, which run against each other.
The seal faces are compressed together using a combination of hydraulic force from the sealing fluid and spring force from the seal design.
In this way, a seal is created to prevent the leakage process between the rotating shaft and the fixed areas of the pump.
Mechanical flood surfaces are flattened to a high degree. If the sealing surfaces rotate against each other without any lubrication, they will wear and break quickly due to surface friction and heat.
For this reason, a type of lubrication between the rotating surface and the fixed seal is required, which is known as a fluid film.
In most mechanical seals, they are lubricated by maintaining a thin film of fluid between the seal surfaces.
This film can be supplied either through the process fluid being pumped or from an external source.
The need for a liquid film between surfaces,
It presents a design challenge: adequate lubrication of the flow between the flood surfaces without leaking an unacceptable amount of process fluid or introducing contaminants between the surfaces that could cause damage to the flood itself.
This can be achieved by maintaining a precise gap between the faces, which should be large enough to allow a small amount of clean lubricating fluid to flow through, but to prevent contaminants from entering through the gap between the flood faces. be small enough.
Figure 6: Display of fluid film and distance between flood levels
The distance between the surfaces in a typical flood is 1 micron, which is 75 times narrower than a human hair.
Because this gap is so small, particles that would damage flood surfaces cannot enter, and the amount of liquid that leaks through this space is so small that it appears as vapor.
Mechanical flood 2 movie 1
This microgap is maintained by using springs and hydraulic force to compress the flood surfaces together while the fluid pressure between the surfaces (the fluid film) acts to separate them.
In fact, without the presence of a fluid film, the two sealing surfaces will be in full contact, which is known as dry running and leads to rapid failure of the seal.
Mechanical flood 9
Pressing the surfaces together without process pressure (and force from the springs) separates these flood surfaces too far, allowing fluid to leak out.
Mechanical seal engineering focuses on increasing the life of the sealing surfaces by ensuring the high quality of the lubricating fluid and by selecting the appropriate material for the sealing surfaces for the pump process.
In short, why do we use mechanical seals?
Floods leak a film of fluid on surfaces as vapor without visible leakage.
Modern “cartridge seal” designs do not damage the pump shaft or bushing.
Daily maintenance is reduced because the seals have internal springs that self-adjust them.
Seals have light surfaces that consume less power than gland packings.
Bearing contamination is reduced in normal operation because the lubricant is not affected by flood leakage and is washed out.
If the product is present in the pump, the plant equipment will suffer less from corrosion.
With this technology, the vacuum can also be sealed, a problem for packing that attracts air to the pump.
Less product is wasted and costs are saved, even water is expensive and there is less need to clean the surface.