Drinking water is an essential resource for life and with the climate change we are facing in these years it is becoming an increasingly precious asset. This year in particular, the lack of rain and the high temperature are getting challenging the water supply of our country. So, it become crucial minimize the amount of water waisted both in the domestic and industrial environment, but it is even more important to stop the leakage in our distribution system.
It has been established that more than 30% of the water released into the Italian distribution networks is dispersed due to old and inefficient pipes. Modern and maintained pipelines would allow to face the lack of rain and snow, thus avoiding serious damages to the economy.
It will be important for the design of new systems or the modernization of the existing one consider the design guidelines for sealing systems. Exist indeed specific criteria to define the sealing performance of each joint over time depending on the type of seal and the assembly condition.
Let’s start to distinguish the different categories and see the main recommendation for each one.
We can identify 3 main categories based on the working conditions:
Axial compression
Diametral compression
Deformation
Let’s analyse them in detail:
Axial compression: it happens when the seal is compressed between a groove and a cover. The seal has the aim to ensure the sealing between these two surfaces thanks to the deformation of the material. For this application seals with rectangular cross section are often used but also o-ring, when the sealing profile is circular.
In this case there are 4 main condition to consider:
Seal has to be compressed from 10% to 35%. The lower limit represents the minimum compression in order to ensure the sealing function, while the upper limit is necessary in order to avoid high loads that could damage the rubber over time. These limits must be respected also considering the combination of minimum and maximum tolerances of each component (seal, groove and cover).
Since the seal have to be compressed, the initial height of its section will be higher than the groove. We need to verify if the volume of the undeformed gasket is lower than the volume available inside the groove, otherwise the seal will not have enough space for the deformation and the assembly will become impossible.
The contact length between gasket and sealing surfaces have to be at least twice the maximum dimension of the porosity present in the surfaces. This in order to ensure that production defect will not influence the sealing function.
The slenderness ratio, the ratio between height and width of the seal, must not be higher than 2,6 otherwise the seal risk to be instable during compression, it may bend and the compression may results lower then the expected and insufficient for the sealing.
Diametral compression: it happens usually with oring that seal around a shaft. The groove of these seals can be placed directly on the shaft or inside the female component.
Also in this case, as in the first one, the gasket undergoes a compression but this time in the diametral direction. The same condition reported above have to be considered in addition to the following:
In case the groove is placed on the shaft the seal should not undergo a stretching higher than 100% during the assembly and have also to be lower then 8% in the groove.
In case the groove in places in the female component, the external diameter of the seal should not be bigger then 5% of the groove diameter in order to make a correct assembly.
Deformation: this represent the most variable situation where due to the geometry of the groove or the assembly conditions is not possible to compress the seal axially or diametral. In this case the cross section of the gasket can have different shape, usually designed specifically for each application.
To verify the proper functionality of the seal in this condition, other than practical assembly and sealing tests, we can use mechanical simulation software. Through this software, the whole assembly can be represented with its boundary conditions, and we can verify how the seal is deformed and how it interact with the sealing surfaces.
But, how to valuate if the final configuration is sufficient or not to ensure the sealing and the stability of the gasket in the groove?
First of all we have to verify the reliability of the simulation. It is necessary to have real data concerning the condition of work like temperature, friction, mechanical properties of the material etc. The better are the starting information the higher will be the reliability of the results obtained
Contact with the surfaces: gasket can not seal if not in contact with both the sealing surfaces. It may seems trivial but sometimes in the image showed on the screen the rubber seems to be in contact with the groove even if there is a small gap between them. So, it is fundamental to evaluate the results not only through the images but also through the numerical valour calculated by the software.
Contact pressure: in order to ensure the sealing, the contact pressure between gasket and sealing surfaces must be at least higher than the working pressure of the fluid transported. If not, the fluid would have enough force to separate the gasket from the sealing surface and cause a leakage.
And you, have you ever used these design guidelines on your seals?
Get in touch with us in order to design together your next seal!
