Mechanical Seals

Author:Site Editor     Publish Time: 2020-11-26      Origin:Site

Mechanical seals are one of the basic mechanical components with precision and complex structure. They are the 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.

 

Sealing knowledge


1. About leakage

To understand the seal, you must first understand the principle of leakage. There are three types of leaks.

1) Wear and leak. That is the leakage between the sealing surface gaps.

2) Leakage. That is the leakage of the sealed fluid through the capillary of the sealing material.

3) Diffusion. It refers to the material transfer of the sealing medium through the gap or the capillary of the material under the action of the concentration difference.


2. Sealing method

There are roughly several sealing methods.

1) Minimize the location of the seal

2) Blockage and isolation

3) Lead or inject

4) Increase leakage resistance

5) Add power components in the channel

6) Combination of multiple sealing methods


3. Common sealing form

Gasket seals, packing seals, mechanical seals, non-contact seals, and injection-type leak plugging under pressure are common sealing forms. Among them, the packing seal should be regarded as the most common. It also includes a soft packing seal, hard packing seal, and molded packing seal. Formed packing seals include our common O-rings, Y-rings, oil seals, and so on. Non-contact seals include gap seals, labyrinth seals, floating seals, dynamic seals, magnetic fluid seals, and fully enclosed seals.

 

mechanical seal 2


The basic concept of mechanical seal


Mechanical seal refers to a device that prevents fluid leakage by keeping at least one pair of end faces perpendicular to the axis of rotation under the action of fluid pressure, the elastic force (or magnetic force) of the compensation mechanism, and the auxiliary seal, which are kept close together and slide relatively. If the auxiliary seal of the compensation ring is a metal bellows, it is called a bellows mechanical seal.

 

Composition of mechanical seal


There are mainly the following four types of components.

1. Main seals: moving ring and static ring.

2. Auxiliary seal: seal ring.

3. Compression parts: spring, push ring.

4. Transmission parts: bomb base and key or fixing screw.

 

Selection of mechanical seals


According to different working conditions and media properties, mechanical seals include high temperature, low temperature, high pressure, and corrosion-resistant mechanical seals, particle-resistant medium mechanical seals, and mechanical seals suitable for light hydrocarbon media that are easy to vaporize. Mechanical seals with different structural types and materials should be selected according to different uses. Mechanical seals are widely used in various industries, such as water pump plants, printing and dyeing machinery plants, paper mills, power plants, chemical plants, pharmaceutical shipyards, and sewage treatment plants, and are widely used in hospitals, agriculture, equipment cooling, and leather industry, heavy machinery, etc. The main parameters of mechanical seal selection are: seal cavity pressure (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.

 

Installation of mechanical seals


1. Mechanical seal requirements for machine accuracy (take mechanical seal for the 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 should 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) For mechanical seals driven by parallel coil springs, the springs are divided into left and right rotations, 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 are as follows:

1) Determination of installation size

During installation, 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 all 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. Considering the compatibility of rubber O-rings, soapy water can be applied if the oil is not suitable. The floating static ring has no anti-rotation pin structure, so it is not suitable to be oiled and should be dry installed into 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 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 sealing, washing, cooling, and filtering must be taken.

2) Before turning the machine, pay attention to whether the torque is too large, there is no rubbing or abnormal sound.

3) Pay attention to the direction of rotation and whether the coupling is centered. Whether the lubricating oil for the bearing part is proper. Is the piping correct?

4) Open the medium and cooling water valve before the operation. Check whether the gas in the sealed cavity is completely discharged to prevent leakage caused by static pressure. Then start up and run.

5) Whether the work is normal and stable after the operation, whether there is abnormal rotation, abnormal sound, and overheating caused by shaft rotation.

 

5. Precautions

1) Precautions during disassembly

a. Be careful when disassembling the mechanical seal. Hand hammers and flat shovels are strictly prohibited to avoid damage to the sealing elements. A pair of wire hooks can be used to extend into the gap of the transmission seat in the direction of self-financing to pull out the sealing device. If the fouling cannot be removed, it should be cleaned before disassembly.

b. If mechanical seals are used at both ends of the pump, take care of each other during assembly and disassembly to prevent one from losing another.

c. For the mechanical seal that has been in operation, if the gland is loose and the seal moves, the dynamic and static ring parts must be replaced, and should not be re-tightened for continued use. Because after the movement, the original running track of the friction pair will change, and the sealing of the contact surface is easily damaged.

 

mechanical seal 1


2) Matters needing attention during installation

 a. Pay great attention to avoid installation deviation during installation.

(1) The tightening of the gland should be carried out after the coupling is aligned. The bolts should be evenly supported to prevent the end face of the gland from a deflection. Check each point with a feeler gauge, and the error should not exceed 0.05 mm.

(2) Check the matching clearance (ie concentricity) between the gland and the outer diameter of the shaft or sleeve. The circumference should be even. Use a feeler gauge to check that the tolerance of each point is not more than 0.01 mm.

b. The compression of the spring must be carried out in accordance with regulations, and no excessive or small phenomena are allowed, and the required error is 2.00 mm. If it is too large, it will increase the specific pressure of the end face and accelerate the end face wear. Too small will cause insufficient specific pressure and can not play a sealing role.

c. After the moving ring is installed, ensure that it can move flexibly on the shaft. After the moving ring is pressed against the spring, it should automatically spring back.

 

Normal operation and maintenance of mechanical seals


1. Preparations and precautions before starting

1) Comprehensively check the mechanical seal, whether the attachments and pipeline installation are complete, and whether they meet the technical requirements.

2) Before starting the mechanical seal, perform a static pressure test to check whether the mechanical seal is leaking. If there are too many leaks, find out the cause and try to eliminate them. If it is still invalid, it should be disassembled, checked, and reinstalled. Generally, the static pressure test pressure is 2~3 kg/cm².

3) Rotate the pump to the crank and check whether it is light and even. If the crank is difficult or does not move, check whether the assembly size is wrong and whether the installation is reasonable.

 

2. Installation and shutdown

1) Keep the sealed cavity filled with liquid before starting. When transporting solidified media, steam is used to heat the sealed cavity to melt the media. The car must be cranked before starting to prevent the soft ring from breaking due to a sudden start.

2) For mechanical seals that use the external oil sealing system of the pump, the oil sealing system should be activated first. The oil sealing system is finally stopped after stopping.

3) After the hot oil pump is stopped, the cooling water in the oil seal cavity and the end seal cannot be stopped immediately. The cooling water should be stopped when the oil temperature at the end seal drops below 80 degrees to avoid damage to the sealing parts.

 

3. Operation

1) If there is a slight leakage after the pump is started, it should be observed for a period of time. If the leakage does not decrease after 4 hours of continuous operation, the pump should be stopped for inspection.

2) The operating pressure of the pump should be stable, and the pressure fluctuation should not exceed 1 kg/cm².

3) During the operation of the pump, pumping down should be avoided to avoid dry friction on the sealing surface and damage to the seal.

4) The sealing condition should be checked frequently. In operation, when the leakage exceeds the standard, the heavy oil should not exceed 5 drops/min, and the light oil should not exceed 10/min. If there is no improvement trend within 2-3 days, the pump should be stopped and the sealing device should be checked.

 

The development of mechanical seals


The development of "sealing" in China has a long history. The ancients used cotton, linen, and other fibers to make seals for water-lifting machinery. Nowadays, there have been researches on sealing laws, sealing device design technology, and applied scientific principles. Discipline-"Sealing", scientific research institutions also have professional courses specializing in sealing. Sealing involves many professional fields, in addition to materials and machinery, there are also mechanics (including fluid mechanics, boundary layer theory, etc.), Tribology, automatic control, etc.


The role of mechanical seals makes it a basic component and accessory in the main industries of the national economy such as national defense, chemical industry, petroleum, coal, transportation, and machinery manufacturing. In recent years, with rapid economic growth, the mechanical seal manufacturing industry has maintained double-digit growth. According to the data of "Prediction of Production and Sales Demand and Transformation and Upgrading Analysis of China's Rubber and Plastic Seal Manufacturing Industry", China's mechanical seal manufacturing industry has achieved an industrial output value of nearly 10 billion, an increase of more than 20% year-on-year. From the perspective of the number of companies, sales revenue, and asset scale, China's mechanical seal manufacturing companies are basically concentrated in East China.

 


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