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Rotary seal is a term of very frequent use in the production and maintenance of companies that work with some type of fluid. For instance, chemical, petrochemical, oil and gas companies, among many others. If you are in a company that operates a liquid or a gas, you will hear about it, the mechanical seal.
Many of the fluids operated by these companies have risks. W can classify it into occupational risks, such as:
We will cover these details in future articles.
As nothing works for free, equipment is needed to move or transform these fluids. Enter the so-called rotating equipment. These, in turn, have gaps between the stationary and rotating parts, in such a way that a residual leak is expected. This leak will depend mainly on the type of seal chosen. The most common are gaskets and rotary seal.
But after all, what is a rotary seal?
By definition, rotary seal is a type of machine element, used only in the sealing of rotating equipment. It operates some type of, where centrifugal pumps are the most frequent. But it can also be present in:
We already know what it is, but how does it work?
In order to understand how it works, we must first understand how it is constituted.
In a very simple way, any mechanical seal will have one or more pairs of sacrifice pieces that are called sealing faces. These are, in most seals, very smooth as they are polished and polished. One face is stationary while the other rotates in solidarity with the axis.
The stationary face is called the seat, mating ring, depending on the manufacturer, of primary ring or slip ring.
As the faces are moving parts and their assembly is not done by interference, a gap will exist at the assembly site. There we have two more leak zones, behind the faces. To prevent this, another sealing element is used that is called secondary seals. These can be O-rings, joints or teflon wedges, “V-ring” , among others.
An important component of the seal is the elastic element. Many believe that the function of the spring or elastic element is to promote the closing of the rotary seal during the operation of the equipment. But in practice, the main responsible for this is the hydraulic pressure of the equipment.
The spring is essential for closing the seal while the equipment is out of operation. That is, when there is no operating hydraulic pressure.
To complete the construction of the rotary seal, the torque transmission elements are missing. Who guarantees that the rotating part will rotate jointly with the axis? This is where the pins and the case appear, which also has an assembly function.
The set of parts that has the compensation or axial movement of the mechanical seal is called the seal head. In conventional stamps, it is usually the rotating part. Hence the name rotating head, but it is not a rule. There are many seals, mainly the cartridges, which the seats are rotating.
Now that we know the components of a rotary seal, we can understand how it works.
When the equipment is stopped, the faces of the mechanical seal are fully in contact, due to the spring force.
From the moment the equipment starts operating, a hydraulic force starts to act. By Pascal’s Law, the force acts in all directions. And because of that part of the fluid operated penetrates between the faces, separating them. This liquid that is between the faces is called a liquid film or interfacial film.
For didactic and illustrative purposes, we changed the color. And we are exaggerating the distance. This distance is very small and imperceptible to the naked eye.
I usually say that these faces operate at the same time, together and separately. It may seem strange, but it is anyway. During the operation of the equipment, they are generally separated by a distance of 2 to 10 microns.
This distance is not enough to avoid contact between the peaks of the surfaces of the faces. But it is also not close enough to affirm that they are solidary. Due to this relative contact between faces, we have the first generation of heat in the rotary seal.
The liquid film has direct contact with the environment and this is where the residual leak comes from. But when generated, this liquid evaporates.
That is why we do not see the leak of the rotary seal with the naked eye. And when the seal is damaged people, in general, say: “The mechanical seal is leaking”.
There are two basic types of seals:
Static seals are used where there is no relative movement in the joint to be sealed. Joints and “O” rings are typical static seals.
Dynamic seals are used when one surface moves in relation to the other.
A common application of rotary seal is to seal the rotating shaft of a centrifugal pump. For a better understanding, it is advisable to quickly review the operating principle of a pump.
Any pump converts the energy of a drive, such as an electric motor or turbine, into energy in the form of speed and pressure to the fluid being pumped.
The liquid enters the suction flange and flows into the rotor eye. It is driven by the rotor fins being accelerated in the direction of rotor rotation. As the liquid leaves the rotor, the speed of the liquid approaches the speed of the fin ends.
The housing is in the form of a volute or spiral. The liquid moves at high speed from a narrow clearance area to a wider clearance area. The energy is converted into pressure and the fluid approaches the discharge nozzle, being directed to it.
The discharge pressure will force some of the fluid behind the rotor, where it will attempt to escape around the rotating shaft. Pump manufacturers use several techniques to reduce the pressure of the fluid that tries to escape, such as:
1) Drilling balance holes in the rotor. So that most of the pressure escapes to the suction side of the rotor
2) the addition of small blades on the rear face of the rotor.
However, as there is no way to completely eliminate this pressure. It is necessary to use sealing devices to limit the leakage of the product into the atmosphere. These devices are typically compression gaskets or rotary seal.
The choice of a sealing method needs to be carefully analyzed for each application. In relation to installation, maintenance, energy consumption, product loss and effluent treatment costs.
Simple Mechanical Seals
The mechanical seal was developed to eliminate the disadvantages of compression gaskets. Leakage can be reduced to a level that meets current standards. And the maintenance cost will be much less. On the other hand, the training of maintenance personnel will need to be of a better level.
The initial cost may be higher, and the mechanical seal may have to be specifically designed for the application. There are two main groups of seals for rotary shafts.
One group is characterized by the theoretical absence of friction between the rotating and stationary elements of the rotary seal. Mechanical bushing and labyrinth seals belong to this group. The second group is characterized by direct friction between the two surfaces. It depends on the rotation of the axis. Within this group, there are two main subgroups.
If you are looking for the best rotary seals, we can assist you in this regard. Dshseals is the home of premium class high quality seals. It is serving industries globally. Hence, you can choose your reliable sealing partner here.