The mechanical seal of a centrifugal pump and its application

01 Definition of mechanical seal and its application in centrifugal pumps

A mechanical seal, also known as a face seal, is a shaft sealing device for rotating machinery. It consists of at least one pair of end faces perpendicular to the axis of rotation, which, under the combined action of fluid pressure, the elastic force of the compensation mechanism, and the cooperation of auxiliary seals, remain in contact and slide relative to each other, thus preventing fluid leakage.

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As a rotary machine, the centrifugal pump has a transmission shaft that penetrates both the inside and outside of the equipment, resulting in a circumferential gap between the shaft and the equipment. The medium inside the equipment leaks out through this gap. If the pressure inside the equipment is lower than atmospheric pressure, air leaks into the equipment. Therefore, a shaft seal device must be installed to prevent leakage.

The development of 02 seals and the advantages of mechanical seals

Currently, significant advancements have been made in pump seals, which are primarily categorized into two types: contact seals and non-contact seals. Among these, mechanical seals (also known as face seals) are the commonly used shaft seals in centrifugal pumps. Prior to the maturity of mechanical seals, rotating body seals employed various sealing methods such as packing, labyrinth seals, and rubber oil seals. Compared to these types of seals, mechanical seals offer the following advantages:

1. Excellent sealing performance, with leakage controlled at 2-3ml/h, and even down to 0.01ml/h;
2. Reliable operation and long service life;
3. No frequent adjustment is required, as the mechanical seal has a compensation mechanism, unlike the packing seal which requires frequent adjustment and tightening;
4. The power loss is small, only 20%-30% of that of the packing seal;
5. The shaft surface is resistant to wear, eliminating the need to replace the shaft sleeve;
6. Strong vibration resistance, good cushioning, and wide range of applications. Image 2.png

03 Simple classification of mechanical seals

1. Installation location: internal and external

“Built-in type” refers to the seal being installed inside the pump chamber, characterized by high sealing pressure. “External type” refers to the seal being installed outside the pump chamber, with low sealing pressure but suitable for corrosive media.

2. Rotating type and stationary type

Rotating type refers to the compensation ring rotating with the shaft, while stationary type refers to the compensation ring not rotating with the shaft and remaining stationary, which is suitable for high-speed rotating applications.

3. Balanced and unbalanced types

The distinction between balanced and unbalanced seals is determined by whether the slip diameter db is increased. Unbalanced seals are generally used for fluid pressures below 1 MPa, while balanced seals are typically designed for pressures up to and including 5 MPa.

The mechanical seal of our CDL series products is an internal/rotating/balanced type mechanical seal.