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Common sense and use of mechanical seal selection

by:Lepu Seal     2022-08-08

Mechanical seals are one of the basic mechanical components with precision and complex structure. They are the key components of various mechanical seals, pumps, reaction synthesis kettles, turbo compressors, submersible motors and other equipment. Its sealing performance and service life depend on many factors, such as model selection, machine accuracy, correct installation and use, etc.

First, the selection method
Mechanical seals include high-temperature and low-temperature-resistant mechanical seals, high-pressure and corrosion-resistant mechanical seals, particle-resistant mechanical seals, and mechanical seals adapted to easily vaporized light hydrocarbon media, depending on the working conditions and the properties of the medium. Mechanical seals with different structural types and materials should be selected according to different uses.

The main parameters of the selection are: sealing cavity pressure (MPA), fluid temperature (℃), working speed (M/S), characteristics of fluid mechanical seals, and effective space for installing seals.

The basic principles of selection are:
1. According to the pressure of the sealing cavity, determine whether the sealing structure is balanced or unbalanced, single-end or double-end, etc.

2. According to the working speed, it is determined to adopt rotary or static type, hydrodynamic type or non-contact type.

3. According to the temperature and fluid properties, determine the friction pair and auxiliary sealing materials, and correctly select the mechanical seal circulation protection system such as lubrication, flushing, heat preservation, cooling, etc.

4. According to the effective space of the installation seal, it is determined to use multiple springs or single springs or wave springs, built-in or external-mounted.

Second, the installation and use requirements of mechanical seals

1. The requirements of mechanical seals for machine accuracy (take the mechanical seals for pumps as an example)
(1) The maximum radial runout tolerance of the shaft (or shaft sleeve) where the mechanical seal is installed shall not exceed 0.04~0.06MM.

(2) The axial movement of the rotor does not exceed 0.3MM.

(3) The runout tolerance of the positioning end of the sealing cavity combined with the sealing end cover facing the surface of the shaft (or shaft sleeve) should not exceed 0.04~0.06MM at most.

2. Confirmation of seals
(1) Confirm whether the installed seal is the same as the required model.

(2) Before installation, check carefully with the general assembly drawing to see if the number of parts is complete.

(3) The mechanical seal is driven by a parallel coil spring, and its spring can be divided into left and right rotation, which must be selected according to the rotation direction of the rotating shaft.

3. Installation
The installation method varies with the type of mechanical seal and the type of machine, but the installation essentials are almost the same. The installation steps and precautions are as follows:

(1) Determination of installation size When installing, the installation size of the mechanical seal should be guaranteed according to the product instruction manual or sample.

(2) Before loading, the shaft (sleeve) and gland should be punctured, and the bearing should be in good condition; the seals, shaft, sealing cavity and gland should be cleaned. In order to reduce frictional resistance, the part where the mechanical seal is installed on the shaft should be coated with a thin layer of oil for lubrication. Considering the compatibility of rubber O-rings, if oil is not suitable, soapy water can be applied. The structure of floating static ring without anti-rotation pin should not be oiled, and should be installed in the gland dry.

(3) Install the static ring together with the gland on the shaft first, be careful not to touch the shaft, and then install the moving ring assembly. The set screw of the spring seat or transmission seat should be evenly tightened several times. Before the gland is not fixed, push the compensation ring by hand for axial compression. After loosening, the compensation ring can automatically spring back without jamming, and then the gland bolts are evenly locked.

4. Use
(1) When the temperature of the conveying medium is too high, too low, or contains impurity particles, flammable, explosive or toxic, corresponding measures such as blocking, flushing, cooling and filtering must be taken.

(2) Turn the car by hand before running, pay attention to whether the torque is too large, whether there is friction and abnormal sound.

(3) Pay attention to the direction of rotation, whether the coupling is centered, whether the addition of lubricating oil in the bearing parts is appropriate, and whether the piping is correct.

(4) Before operation, firstly open the medium and cooling water valve, check whether the gas in the sealing chamber is fully discharged, prevent leakage caused by static pressure, and then start the operation.

(5) Whether the work is normal and stable after driving, whether there is abnormal torque caused by shaft rotation, abnormal noise and overheating.
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