WHAT DO YOU KNOW ABOUT THE DESIGN AND TYPES OF MECHANICAL SEALS?

When it comes to gland packings, we often mention mechanical seals. The mechanical seal is a device widely used in industrial production equipment, and it is a sealing device that deals with dynamic seal of rotating shaft. The mechanical seal first applied for a patent in the United Kingdom around 1885. In 1956, a balanced seal patent and an intermediate ring seal appeared in the structure. With the special requirements put forward by the industrial development of atomic energy, aerospace and nuclear power, new structures such as fluid dynamic pressure seal and fluid static pressure seal, spiral mechanical seal combination and floating ring mechanical seal combination have emerged. The improvement of mechanical seal in structure and the innovation of principle make the performance of mechanical seal improve continuously.

1. Design of Mechanical Seals

1) Determination of the basic structure.

Since the working environment is high temperature, high pressure, and strong corrosion medium, the double end non-equilibrium structure is selected to introduce the isolation liquid for protection. Due to the large shaft diameter, the compensation mechanism uses a small spring structure. The thrust generated by the compensation mechanism acts on the static ring. The static ring can float in the axial direction. The moving ring is fixed on the shaft sleeve. The auxiliary seals of the dynamic and static rings all adopt the structure of a rubber O-ring and a PTFE retaining ring. This structure can prevent the deformation of the rubber seal ring under high pressure and increase the reliability of the auxiliary seal. In order to reduce the temperature of the sealing end surface, circulating water is used to cool. The cooling water is in and out independently.

2) Material determination

Sealed parts are subjected to chemical corrosion and electrochemical corrosion under corrosive media, especially at the sealing end. The corrosion products are destroyed due to friction. The periodic cycle of this phenomenon produces abrasion. Its abrasion speed is about 10-50 times the corrosion rate of the surface without friction. Therefore, materials that are resistant to corrosion and wear should be selected. Otherwise, the mechanical seal cannot work reliably. Therefore, the moving ring is made of silicon carbide ceramic (SiC). The static ring is made of immersed carbon graphite (M 106K).

3) Design of sealing end face

The sealing end face is composed of two parts, the moving ring and the static ring, and the sealing end face of the moving ring and the static ring is correspondingly made wide and narrow in order to work effectively. The soft material is used as a narrow ring and the hard material is used as a wide ring, so that the narrow ring is evenly worn without being embedded in the wide ring. The width of the end face should be as small as possible under the condition of sufficient material strength and rigidity. An excessively large width will reduce the lubrication and cooling effect of the end surface, increase the wear and leakage, increase power consumption.

4) Design of moving ring and static ring

The moving ring is fixed on the moving ring seat with a positioning ring. The four protrusions on the moving ring are embedded in the four grooves to drive the moving ring to rotate, and the moving ring seat and the sleeve are integral. The shaft sleeve is fixed to the shaft by a set screw for transmission and anti-rotation.

There are grooves on the back of the static ring to fix the two stud bolts through the cylindrical pin and the static ring seat, fixing the static ring from the side to prevent falling off. The moving ring seat is fixed on the upper and lower trays by two flat keys, so that it can only do axial movement. The snap ring is used to fix the static ring seat and the upper and lower trays, so that they cannot fall off the tray.

5) Design of auxiliary seal ring

The auxiliary seal of the dynamic and static rings adopts a 0-type rubber seal ring and a PTFE retaining ring. The design of the sealing part is on the back of the dynamic and static rings. This structure is an improvement of the traditional sealing method. The traditional auxiliary seal uses 0 type rubber sealing ring alone, its compression is not good. If the compression is small, the sealing performance is poor, and if the compression is large, it will make installation difficult, at the same time, friction resistance increases and the floatability is poor. These factors greatly affect the performance of mechanical seals, especially in the case of high-pressure media, the traditional seal ring is easy to deform and fail. The new auxiliary sealing method solves the above problems. In this case, the high compression ratio of the 0-type sealing ring is limited to the groove and is not easy to deform and fail. PTFE is an excellent sealing material.

2. Types of Mechanical Seals

According to structural types, the basic types are:

1) Balanced and unbalanced mechanical seals

2) Built-in and external mechanical seals

3) Internal flow and external flow mechanical seals

4) Static and rotary mechanical seals

5) Single spring seals and multiple spring mechanical seals

6) Single-end and double-end mechanical seals

7) Contact and non-contact mechanical seals

8) Single-stage and double-stage mechanical seals

9) Elastomer bellow seals