General knowledge about mechanical seals

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

    1. Selection method

    According to different working conditions and media properties, mechanical seals include high temperature and low temperature resistant mechanical seals, high pressure resistant and corrosion resistant mechanical seals, particle resistant mechanical seals and mechanical seals suitable for light hydrocarbon media that are easy to vaporize. Use mechanical seals with different structural types and materials.

    The main parameters for the selection are: pressure of the sealed cavity (MPa), fluid temperature (℃), working speed (m/s), fluid characteristics and effective space for installation and sealing.

The basic principles of selection are:

1. According to the pressure of the sealed cavity, determine whether the sealing structure adopts balanced or unbalanced, single end face or double end face, etc.

2. According to the working speed, determine the use of rotary or static, fluid dynamic pressure 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, and cooling.

4. According to the effective space for installing the seal, determine the use of multiple springs or single springs or wave springs, built-in or external.

2. Requirements for the installation and use of mechanical seals

    1. Mechanical seal requirements for machine accuracy (take mechanical seal for pump as an example)

(1) The maximum radial runout tolerance of the shaft (or sleeve) where the mechanical seal is installed does not exceed 0.04～0.06mm.

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

(3) The maximum runout tolerance of the positioning end facing the shaft (or sleeve) surface of the sealing cavity and the sealing end cover shall not exceed 0.04～0.06mm.

    2. Confirmation of seals

(1) Confirm whether the installed seal is consistent with the required model.

(2) Check carefully with the assembly drawing before installation, and check whether the number of parts is complete.

(3) The mechanical seal is driven by a parallel spring. The spring can be divided into left and right rotation, which must be selected according to the rotation direction of the 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's instruction manual or sample.

(2) Before installation, the shaft (sleeve) and gland should be free of burrs, and the bearing should be in good condition; the seals, shaft, seal cavity, and gland should be cleaned. In order to reduce the friction resistance, the part where the mechanical seal is installed on the shaft should be coated with a thin layer of oil for lubrication. Taking into account the compatibility of the rubber O-ring, if oil is not suitable, soapy water can be used. Floating static ring has no anti-rotation pin structure, so it is not suitable to be oiled and should be dry installed in the gland.

(3) Install the static ring and gland on the shaft first, taking care not to collide with the shaft, and then install the moving ring assembly. The set screws of the spring seat or transmission seat should be evenly tightened in several times.

    Before fixing the gland, push the compensation ring by hand to compress it axially. After loosening, the compensation ring can automatically spring back without jamming, and then evenly lock the gland bolts.

    4. Use

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

(2) Before operation, turn the car by hand, and pay attention to whether the torque is too large, there is no rubbing or abnormal sound.

(3) Pay attention to the direction of rotation, whether the coupling is centered, whether the lubricating oil at the bearing part is added properly, and whether the piping is correct.

(4) Before operation, first open the medium and cooling water valve, check whether the gas in the sealed cavity is completely discharged to 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.