What is Double Mechanical Seals

Author:Site Editor     Publish Time: 2020-12-17      Origin:Site

Compared with the single end face mechanical seal, the double mechanical seal has a better sealing performance. It is suitable for the sealing of strong corrosion, high temperature, suspended particles and fiber media, gas media, flammable, explosive, volatile, and low viscosity media, and high vacuum conditions. The double end face mechanical seal is not just the repetition of the single mechanical seal. In some key occasions in the chemical industry, it can effectively prevent the leakage of dangerous, toxic, flammable, and explosive substances, and has a high-performance insurance function. When sealing high-pressure media, it can reasonably distribute the pressure difference between the two ends of each seal and increase the working pressure range of the seal. For some media with poor self-lubrication, easy to condense, and vaporize, the efficiency and service life of the seal can be greatly improved by injecting the sealing liquid(barrier fluid) into the sealing cavity to improve the sealing conditions.

 

Double end face mechanical seal working principle 


The principle of the dual mechanical seal is basically the same as that of the single mechanical seal. Both rely on the end surface (friction pair) perpendicular to the axis of rotation to maintain a fit and relative sliding under the action of fluid pressure and the elastic force of the compensation mechanism and the auxiliary seal and constitute a device to prevent fluid leakage. The double acting mechanical seal structure is shown in Figure 1.


When working, under the action of the resultant force caused by the fluid pressure (medium pressure) and the elastic force of the elastic element, an appropriate specific pressure (pressing force) is generated on the end face of the seal ring, so that the two contact end faces (moving ring, static ring) are closely attached, and a very thin liquid film is maintained in the very small gap between the end faces achieving the purpose of sealing. The double cartridge mechanical seal has two end-face seals. If the primary seal fails, the secondary seal can still be sealed to prevent leakage. Generally, double-end face seals require an external sealing liquid system, and the sealing liquid is introduced into the sealed cavity for plugging, lubrication and cooling, and it is mostly used for circulating cooling. The sealing fluid(double mechanical seal barrier fluid) can not only wash the friction pair to improve the working environment of the mechanical seal but also serve as an important detection method for the failure of the primary sealing surface.


 Figure 1 Schematic diagram of double mechanical seal

1

1. Static ring anti-rotation bolt

2. Static ring on the medium side

3. Moving ring on the medium side

4/7. Moving ring spring

5. Built-in pump effect ring

6. Bushing

8. Mechanical seal chamber jacket

9. Atmospheric side moving ring

10. O-ring

11. Atmospheric side static ring

12. Shaft sleeve positioning sleeve


 

Structure of dual mechanical seal


Compared with the single end face mechanical seal, the double acting mechanical seal has one more friction pair. The two friction pairs can be composed of two moving rings and two static rings respectively, or two moving rings can share a static ring or two static rings can share a moving ring.

 

Classification of double mechanical seals


There are many classification methods for double-end mechanical seals.


1. According to the pressure of the sealing liquid system, it can be divided into pressure double seal and pressureless double seal.

 

The pressure of the pressure double seal sealing liquid system is generally 0.01--0.03 MPa higher than the pressure before the primary seal. When the primary seal leaks, the sealing liquid will enter the pump, ensuring that the medium in the pump will not leak to the outside. As the sealing liquid enters the pump, the pressure and liquid level of the sealing liquid system are reduced, and leakage monitoring and alarming of the sealing can be realized.

 

The pressure in the non-pressure double seal sealing system is generally normal pressure or micro pressure, which is lower than the pressure in the seal chamber before the first level seal. When the primary seal leaks, the medium in the pump will enter the sealing liquid system, causing the pressure and liquid level of the sealing liquid system to rise. Leak monitoring and alarms can be realized through a pressure switch or a liquid level switch. The use of a sealing fluid system to monitor the failure of a double cartridge mechanical seal is also one of the most important forms that distinguish it from a single-end mechanical seal.

  

2. According to the arrangement of friction pairs, there are four types: radial, face-to-face, back-to-back and tandem, as shown in Figure 2.

 

1) Radial double mechanical seal

The radial dual mechanical seal consists of a pair of mutually matched moving rings and static rings. The main technical structure is characterized in that ring grooves are correspondingly opened on the mating end faces of the moving ring and the static ring to form two radial matching friction pairs. The static ring is provided with a drain hole to form a radial double end face mechanical seal. Its overall structure is simple and compact, with small structural space and convenient installation and use. It is especially suitable for occasions with limited radial installation dimensions. 


2) Face to face double seal

The face-to-face double acting mechanical seal has two forms: static ring face-to-face and moving ring face-to-face. The static ring face to face is an integral moving ring, and the moving ring face to face is an integral static ring. However, due to problems such as processing accuracy and replacement cost after mechanical seal failure, face-to-face double cartridge seals are rarely used in mechanical seals for fluid pumps, and are generally used where the size of the seal cavity is small. Light hydrocarbons, viscous, polymer fluids, and harmful gases use dry gas seals, non-contact design can use face-to-face mechanical seals. Face-to-face mechanical seals require that the sealing liquid pressure be greater than the medium pressure, which is a double seal under pressure, otherwise the force on the dynamic and static rings of the inner mechanical seal is unreasonable. The face-to-face structure is mostly used in a container structure, which can be used as a single end face or double end face with sealing liquid. 


3) Back-to-back double mechanical seal

The back-to-back double cartridge mechanical seal can also be made into a container, which is convenient for disassembly and assembly, and easy for maintenance, so it is used more. The double-end mechanical seal of the sulfur device is back-to-back. The back-to-back double seal has two moving rings and two static rings respectively, and the primary seal is an outflow type. Because of its wide application conditions and wide application, the dual mechanical seal in the narrow sense refers specifically to the back-to-back double-end mechanical seal. Back-to-back double seals are pressured double seals. When the primary seal fails, the pressure and liquid level of the sealing liquid system will decrease. The power of the sealing liquid circulation is provided by the built-in pump effect ring. 


4) Series double seal

The series mechanical seal is composed of two or more single-end mechanical seals arranged in the same direction. Two sets of single end face mechanical seals form a series double mechanical seal. This sealing method reduces the pressure of the sealed cavity step by step, and the medium does not leak. Due to its compact structure, the tandem double end seal can be applied to medium-load occasions with small shaft diameters, small axial dimensions, and large speeds. The inner end is a balanced mechanical seal, and the outer end is an unbalanced mechanical seal with a pump effect ring. The tandem double acting mechanical seal can be made into a pressure double seal or a pressureless double seal. The pressureless double seal is the most common. The sealing liquid(double mechanical seal barrier fluid) provides cooling and lubrication of the sealing surface and is circulated by the built-in pump effect ring.


Figure 2 Double mechanical seal structure diagram

2

 

Failure analysis


Among the failures of rotating equipment, the shutdown due to mechanical seal leakage accounts for about 50% of the total maintenance equipment. The failure mechanism of the double-end mechanical seal is basically the same as the failure mechanism of the single mechanical seal, mainly the failure of the sealing surface. During the operation of the pump, the temperature of the sealing surface will increase due to the heat generated by friction and stirring. In addition to the influence of the medium, gasification of the sealing surface medium, deformation of the sealing ring, thermal wear, and even thermal cracking will occur. Some media will aggravate solidification, polymerization, crystallization, coking and corrosion when the temperature changes. Various abnormal working conditions will aggravate or directly lead to seal failure in the short term.  Relevant agencies have conducted investigations on pump machinery for several years. The reasons for seal failure and their proportions are shown in the table 3.


Table 3 Analysis of causes of mechanical seal failure

 

Causes of seal failure

Proportion (%)

 Operational problems (interrupted flushing fluid, etc.)

40

 Mechanical problems (installation error, improper clearance, improper adjustment)

24

 High level error of fluid circuit (insufficient negative pressure, insufficient flushing fluid, improper pump characteristics)

19

 Improper selection of sealing elements (wrong material or basic structure)

9

 Other reasons

8


It can be seen from the above table that the main cause of dual mechanical seal failure is operation problems. When analyzing the failure, it is necessary to conduct an in-depth study of the entire process, including the leakage state before disassembly, the amount of leakage, the inspection during the disassembly, and the detection of sealing parts. Find the leakage point and the true cause of the failure, and then take corresponding measures.

 

Daily maintenance

 

It can be seen from Table 3 that the main reason for the seal failure is the operation problem, so do daily maintenance work. Providing a reasonable working condition for the double-end mechanical seal is a prerequisite for normal and good seal operation. Do daily maintenance and ensure the following points.

1) Check whether the auxiliary device is normal before starting. Ensure that the liquid level and pressure of the sealing fluid system are within the operating range, and all parts are well sealed. Press the steering wheel to fill the machine seal with liquid through the double-end machine seal pump effect ring.

2) Do not evacuate the pump to avoid dry friction on the sealing end surface.

3) Frequently check whether the auxiliary device is working properly:  

a. Pay attention to the position of the sealing liquid level. If the oil level is lowered, it should be replenished frequently, and the pressure should be supplemented to maintain the pressure.

b. Drain the leaked liquid regularly.

c. Frequently check the cooling water pipes, flushing pipes, etc., to prevent blockages, and prevent the sudden stop of water from causing poor cooling and flushing effects, resulting in seal failure.

4) Prevent the influence of vibration and heat generation on the seal.

a. Pay attention to abnormal sound, vibration may cause the anti-rotation pin to loosen or fall out.  

b. Pay attention to abnormal heating, such as whether the bearing heating is normal, and make reasonable adjustments.

 

From the perspective of the sealing mechanism, whether it is a double mechanical seal or a single mechanical seal which is difficult to control and has a large leakage, is transformed into an end seal (friction pair), static seal and pseudo static seal that is easier to control. The two-stage end face seal gives the double acting mechanical seal safety, reliability and no leakage to the outside. According to the arrangement of friction pairs and the sealing fluid system, the dual mechanical seal has a variety of structural forms to meet the requirements of various working conditions. There are many reasons for the failure of the double-end mechanical seal, and they are also very complicated, which can be solved by taking corresponding measures. The leakage of the double mechanical seal is directly related to the process operation, installation and maintenance. In daily production, operation should be standardized, management should be strengthened, and a summary analysis of failures should be done to ensure and improve sealing efficiency and life.


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