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Mechanical Seal Selection Method And Use Requirements
- Oct 17, 2018 -

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

First, the selection method

Mechanical seals according to different working conditions and medium properties, high temperature resistant, low temperature resistant mechanical seals, high pressure resistant, corrosion resistant mechanical seals, mechanical seals resistant to particulate media, and mechanical seals for light hydrocarbon media that are easy to vaporize, etc. The use of mechanical seals for different structural types and materials. The main parameters of the selection are: sealed chamber pressure (MPa), fluid temperature (°C), working speed (m/s), fluid characteristics and effective space for sealing. The basic principles of selection are:

1. According to the pressure of the sealed cavity, it is determined that the sealing structure adopts balanced or unbalanced type, single end or double end.

2. According to the working speed, it is determined to adopt rotary or static type, fluid dynamic pressure 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 cycle protection system such as lubrication, flushing, heat preservation and cooling.

4. According to the effective space for installing the seal, it is determined to use a multi-spring or single spring or wave spring, built-in or exterior.

Second, the mechanical seal installation and use requirements

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

(1) The radial runout tolerance of the shaft (or bushing) on which the mechanical seal is installed shall not exceed 0.04 to 0.06 mm.

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

(3) The locating tolerance of the positioning end face of the sealing cavity combined with the sealing end cap to the shaft (or bushing) surface shall not exceed 0.04~0.06mm.

2. Confirmation of seals

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

(2) Before installation, carefully compare with the assembly drawing, and the number of parts is complete.

(3) The mechanical seal adopting the parallel spring transmission, the spring has the left and right rotation points, and 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 methods are almost the same. The installation steps and precautions are as follows:

(1) Determination of installation size

When installing, the installation dimensions of the mechanical seal should be guaranteed according to the product's instruction manual or sample.

(2) Before loading, the shaft (sleeve sleeve) and gland should be free of burrs and the bearing condition is good; the seal, shaft, seal chamber and gland should be cleaned. In order to reduce the frictional resistance, the mechanical seal on the shaft should be thinly coated with a layer of oil for lubrication. Considering the compatibility of the rubber O-ring, if it is not suitable for oil, it can be coated with soapy water. The floating static ring does not have an anti-rotation pin structure. It should not be oiled. The gland should be installed dry.

(3) First install the static ring and the gland on the shaft, taking care not to touch the shaft, and then load the moving ring assembly. The set screws of the spring seat or the drive base should be tightened evenly several times.

Before the gland is fixed, the compensating ring is manually pressed for axial compression. After the loosening, the compensating ring can automatically spring back without jamming, and then the gland bolt is evenly locked.

4. Use

(1) When the temperature of the conveying medium is too high, too low, or contains impurity particles, flammable, explosive, and toxic, it is necessary to take corresponding measures such as blocking, rinsing, cooling, and filtering.

(2) Use the hand-cranked car before the operation, pay attention to whether the torque is too large, whether there is any rubbing or abnormal sound.

(3) Pay attention to the direction of rotation, whether the coupling is centered, whether the lubrication of the bearing part is appropriate, and whether the piping is correct.

(4) Before the operation, first open the medium and cooling water valves, check whether the gas in the sealed chamber is completely discharged, prevent the static pressure from causing leakage, 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.

Third, the main technical structure and solutions

The mechanical seal of these pumps consists of two sealing elements (static ring and moving ring) perpendicular to the shaft, the surface of which is smooth and straight, and fits to each other. The flatness of the sealing end face is not more than 0.0009 mm, the surface roughness, the metal material Ra = 0.02 μm, and the non-metallic material Ra = 0.04 μm. The moving ring and the stationary ring are relatively rotated, and the friction pair material is graphite, alumina ceramic, silicon carbide, silicon nitride, tungsten carbide hard alloy, and the like. During operation, the pressure of the spring and the sealing medium (working solution) exerts a proper pressing force on the contact surface of the rotating moving ring and the stationary ring, so that the end faces are closely fitted and a very thin layer is formed between the end faces. The liquid film achieves the purpose of sealing. The liquid film has an outer layer hydrodynamic pressure and an inner layer static pressure (or molecular affinity), which acts as a lubrication and balance pressure.

Fourth, the following technical points should be noted when repairing.

(l) When the static ring or moving ring seal is difficult to install in place, apply lubricant here.

(2) When the spring force is insufficient, a gap is easily generated between the moving ring and the static ring sealing surface, and the fixing screw can be moved to increase the spring force or replace the spring. A small number of leaks are caused by normal wear or have reached the end of their service life, and most of them are caused by large changes in working conditions or improper operation and maintenance.

(1) evacuation, cavitation or rolling for a long time, resulting in seal damage;

(2) The actual output of the pump is too small, a large number of medium pumps circulate, heat accumulates, causing gasification of the medium, resulting in seal failure;

(3) The return flow is too large, causing the sediment at the bottom of the suction side container (tower, kettle, tank, pool) to rise and damage the seal;

(4) For a longer period of time, there is no manual turning when restarting, and the friction pair tears the sealing surface due to adhesion;

(5) Increased corrosive, polymerizable, and gelatinous substances in the medium;

(6) The ambient temperature changes drastically;

(7) Frequent changes or adjustments in working conditions;

(8) Sudden power failure or downtime. The centrifugal pump suddenly leaks during normal operation. If it cannot be found in time, it will often cause a big accident or loss. It must be taken seriously and effective measures taken.