Mechanical seals have classified several types. In this article, we will see the basic classification of mechanical seal that is the “Mechanical Seal – Balanced and Unbalanced Type”.
What is the mechanical seal balance?
Mechanical Seal balance is used to refer the load act across the seal faces. If the load at the seal faces is too high, the liquid film between the seal rings could be squeezed out or vaporized. An unstable condition from thermoelastic instability could result in a high wear rate of the sealing surfaces.
Seal balancing can avoid these conditions and lead to an increase in the life of the mechanical seal and reduce power consumption.
The pressure in any stuffing box acts equally in all directions and forces the primary ring against the mating ring. The force (F) acts only on the diameter (Do) across the seal face, it acts as a closing force on the seal faces.
To relieve the force at the seal faces, the diameter of the shoulder on a sleeve or the seal hardware is decreased. Thereby the seal face pressure can be lowered. This is called seal balancing.
A seal without a shoulder in the design is an unbalanced seal. A balanced seal is designed to operate with a shoulder. Only metal bellows seal is a balanced seal that does not require a shoulder.
Seal balance Ratio:
The seal manufacturer shall design the seal faces and seal balance ratio to minimize seal-face-generated heat consistent with optimum seal life.
As per API 682, the seal balance ratio is calculated as follows,
For seals pressurized at the outside diameter, the seal balance ratio, B, is defined by the simplified equation:
B= (Do2 – Db2) / (Do2 – Di2)
where
Do is the seal face outside diameter;
Di is the seal face inside diameter;
Db is the balance diameter of the seal.
For seals pressurized at the inner diameter, the seal balance ratio is defined by the equation:
B= (Db2 – Di2) / (Do2 – Di2)
where
Do is the seal face outside diameter;
Di is the seal face inside diameter;
Db is the balance diameter of the seal.
For a balanced seal, the balance ratio is equal to 1.
Unbalanced Seals
Virtually all mechanical seals are available in either unbalanced ( Ref. Figure) or balanced versions. The term “unbalanced” is used when the stuffing box pressure times the area exposed to the pumped fluid (closing force), acting to close the seal faces, is greater than the average pressure between the seal faces (pressure gradient)times the area of contact between the faces. In other words, unbalanced mechanical seal exhibit net hydraulic closing forces which are generated by the actual pressures to be sealed.
For example, if there were a stuffing box pressure of 50 psig (3.4barg), the spring load would have to be added. Hence, the “face load” or closing force on the faces would be even higher than 50 psig times the face area. This, of course, limits the pressure sealing capacity of an unbalanced seal.
Unbalanced seals are often more stable than balanced seals when subjected to vibration, misalignment and cavitation. The disadvantage is their relatively low-pressure limit. If the closing force exerted on the seal faces exceeds the pressure limit, the lubricating film between the faces is squeezed out and the highly loaded dry running seal fails.
Balanced Seals
The balanced seal has the same opening (face) area as the unbalanced seal, but the closing area has been reduced about the face area. Because force equals pressure times area, reducing the closing area reduces the closing force. Consequently, less heat is generated and the seal generally has a longer life.
For example, if the stuffing box pressure were 300 psig (20.6 barg), then the net closing force would be substantially reduced to perhaps 90 psig (6.2 barg).
To simplify the explanation, balancing mechanical seal involves a small design change which reduces the hydraulic forces acting to close the seal faces. Balanced seals have higher pressure limits, lower seal face loading, and generate less heat. They are better able to handle liquids with low lubricity and high vapour pressures. This would include light hydrocarbons. Because seal designs vary from manufacturer to manufacturer and from application to application, it is not possible to standardize on either configurations or materials that cover all conceivable services. Available basic designs have variations that were often developed to meet specific applications. Each seal design has its own strengths and weaknesses.
Difference Between Balanced and Unbalanced mechanical seal:
| S.no | Description | UnBalanced Seal | Balanced Seal |
| 1. | Heat Generation | More heat generates across the seal face. | Less head generate across the seal face when compare with unbalanced seal |
| 2. | Seal Balance ratio | Greater Than 1 | Less than or Equal to 1 |
| 3. | Mechanical seal life | Seal life is less. | Seal life is more. |
| 4. | Stuffing box pressure | Installed in a pump having low stuffing box pressure. | Installed in all range of pressure. |
| 5. | Vibration and Misalignment | Most stable for vibration and misalignment | Less stable for vibration and misalignment. |
| 6. | Liquid handling | This type of seal is used for liquids very good lubricity. | This type of seal is used for liquids with poor lubricity & high vapour pressures such as light hydrocarbons. |
| 7 | Seal Face loading | High seal face loading | Low seal face loading |
Conclusion:
Nowadays most of the seal manufactures are used only balanced mechanical seal. In some special mechanical seals (ie., engineered seals) are designed with unbalanced mechanical seal.
Balanced mechanical seals are more preferred than unbalanced mechanical seals. Seal balance can range from 0.65 to 1.35, depending on operating conditions.
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