The Difference between balanced unbalanced mechanical seal
Mechanical seals are perhaps one of the most used parts. When we look into the specifications, it can be classified into Balanced Type and Unbalanced type.
What is mechanical seal balance?
Mechanical seal balance means the load that acts across the seal faces. It is critically important that the load is optimum between the seal faces.
If it is exceptionally high, then the liquid film gets squeezed out. It vaporizes quickly and causes an unstable condition.
Due to the thermo-elastic instability, there is high wear and tear of the sealing surfaces.
Well-balanced seals avoid this situation and lead to reduce energy consumption and extend the life of the seal.
The pressure in a stuffing box applies equally in all directions. It presses the primary ring against the mating ring.
Since the force acts only on the diameter, it acts as a closing force on the seal face.
When the pressure is high, it is lowered by reducing the diameter of the shoulder on the sleeve. It can be reduced by decreasing the seal hardware as well.
Thus, the pressure can be brought to the optimum level again. The process is called seal balancing.
What is an unbalanced seal?
All mechanical seals are available in balanced and unbalanced versions.
Unbalanced seal generates multiple times higher hydraulic closing force generated by the actual pressure that seals the two faces.
Though they are more stable than the balanced ones, if the closing force on the seal faces crosses the pressure limits, then it wears out the lubricating film which squeezes out, and the dry seal fails due to heavy load.
All unbalanced seals are without a shoulder.
What is a balanced seal?
In the balanced seal, the opening area or face area is same as the unbalanced seal, but the closing area is maintained in proportion to the face area.
As we reduce the closing area, the closing force also gets reduced.
Thus, less heat gets generated, and we get extended life of the seal.
Key differences between Balanced and unbalanced mechanical seals
- More in unbalanced seals
- Less in balanced seals
- More in balanced seals
- Less in unbalanced seals
Vibration and stability
- More stable and aligned
- Less stable and misaligned
Liquid handling capacity
- Unbalanced seals can handle high capacities of high lubricity fluids
- Balanced seals can handle high capacities of poor lubricity fluids. They are best fit for light hydrocarbons
Seal face loading
- High in unbalanced
- Low in Balanced
- Heat generation across the face is more in unbalanced seals
- Heat generation across the surface is low in balanced seals
It requires a small design change to reduce the hydraulic forces acting across the seal faces, but the benefits obtained are marvelous.
Balanced seals bring several benefits over the unbalanced seals; making them more useful. Hence, they are preferred by the users. The ideal seal balance ration should be in the range 0.65 to 1.35 subject to the operating conditions.
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