In this post we will describe basic information about sealing pumps and motorpumps that use mechanical seals to seal the shaft.
(Vilmar Luis Rossi – Engineering)
Anyone who uses or works with pumps and motor pumps has certainly heard about liquid leaks. The solution to this problem in systems that use mechanical seals can be, above all, challenging.
What is mechanical seal sealing?
First of all, we must understand that the mechanical seal is a set of sealing components present in the motor pump. Also called a sealing box, its function is to prevent the passage of liquid, air or foreign bodies in the region where the shaft passes through the housing. Likewise, it is precisely this that isolates the internal and external media of the pump.
The most common forms of sealing in pumps and motor pumps are static sealing and dynamic sealing. In summary, here we will talk about the basic concepts of radial dynamic sealing, which is also called mechanical seal sealing.
Above all, the mechanical seal is a set of components that have the purpose of preventing the passage of liquid along the interface of two rotating surfaces. Typically, one surface remains still (stationary), while the other rotates along with the axis (rotary).
A great advantage of adopting mechanical seals is that the lubrication of their sliding tracks is carried out by the pumped liquid itself. Furthermore, this type of seal does not present visible leaks that could harm or contaminate the place where the motor pump is installed.
The geometry of mechanical seals
The geometry of mechanical seals may vary according to the models sold by different manufacturers. However, the fundamental parts will usually be similar in geometry and function.
In figure 1 it is possible to visualize the parts that make up the mechanical seal.
Figure 1 – Mechanical seal components
- Stationary face: This part is fixed to the housing.
- Rotating face: This part is fixed on the driver shaft.
- Mola: Its function is to maintain contact between the stationary and rotating faces.
- Primary Seal: it happens “naturally” due to the geometry of the two faces, stationary and rotating, being in perfect contact, preventing the passage of the liquid, through the formation of a lubricating film (which is nothing more than a barrier that the liquid itself forms );
- Secondary Seal: it is made using elastomers present in the components of the stationary and/or rotating faces.
Still, you may be wondering: “Does every mechanical seal have this same geometry and these same components?” We can say that there are other geometries and different parts, as you can see in figure 2:
Figure 2 – Examples of mechanical seals
These illustrated mechanical seals are the most common and manufactured for simple applications, for example, for pumping clean water. We can say that the geometries of mechanical seals are the most diverse. Either way, it may even be a different geometry to suit a specific application or simply due to the manufacturing process of each mechanical seal manufacturer.
Materials
The rotating and stationary faces as well as the sealing rubber are produced from different materials. In this sense, to find out a little more about this, we suggest consulting the booklet that is available for download on the FAMAC website (Click here). Read this material to understand that each component of the mechanical seal can be produced from different materials. Understanding this, you will choose a mechanical seal for each piece of equipment whose manufacturing materials are compatible with the liquid being pumped and, of course, with the application.
Causes of liquid leaks in systems with mechanical seals
The main causes of liquid leaks in systems with mechanical seals are, in short:
- Wear: Over time, mechanical seal components can wear out due to friction and continuous exposure to pumped liquids. This can lead to small gaps that allow leakage.
- Improper Installation: If the mechanical seal is not installed correctly, misalignments or distortions will occur, creating spaces through which liquids can escape.
- Contamination: solid particles or corrosive substances in the pumped liquid can damage the mechanical seal components, compromising their effectiveness and resulting in leaks.
- Excessive pressure and temperature: Operating the pump in conditions of high pressure or temperature outside the mechanical seal specifications may cause failures and leaks.
Consequences
- Loss of efficiency: constant leakage reduces the pump's efficiency, requiring more energy to do the same work.
- Additional wear: The leak can cause corrosion and wear on other parts of the pump and system, increasing maintenance and repair costs.
- Contamination risk: In sensitive industries, such as chemicals or pharmaceuticals, leaks can lead to product contamination, resulting in significant financial losses and safety risks.
special cases
For temperatures above 60ºC, some pump models are already fitted with special seals to meet this condition. In fact, when the temperature limitation for the model is not specified, it is recommended that the water temperature is between 15 and 25°C, in order to guarantee hydraulic performance and reduce problems with pump cavitation. Furthermore, for normal mechanical seals, it is not recommended to exceed a maximum temperature of 60ºC, in order to guarantee their useful life.
For specific applications, such as submersible pumps, there are models that have a stuffing box with single or double mechanical seals, with specific materials, for application in abrasive liquids with the presence of solids. In this equipment, the mechanical seal is normally mounted in an oil chamber, with the purpose of keeping the seal lubricated and cooled. This allows the installation of a sensor that detects water ingress, which makes it possible to replace the mechanical seal before water ingress affects the useful life of the entire equipment.
Figure 3 – Cutaway image of a submersible pump from FAMAC’s FBS-NG line
For the mechanical seal to maintain the expected functions and time of use, the dimensions and assembly tolerances such as perpendicularity, concentricity, parallelism, runout and roughness must be strictly observed. Therefore, it is essential to observe the pump manufacturer's specifications before replacing the mechanical seal, ensuring that its operation and lifespan remain in accordance with the original design.
But this will be the subject of another post here on the blog, presenting details about the main precautions during maintenance and how to identify whether the failure was due to time of use, improper application or a defect in the mechanical seal.
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