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Home > News > Content
Talking About Double Mechanical Seal
Aug 19, 2018

1 Introduction

Compared with single-end mechanical seal, double-end mechanical seal has good sealing performance, suitable for strong corrosion, high temperature, with suspended particles and fiber medium, gas medium, flammable and explosive, volatile low viscosity medium, high vacuum working condition seal. The mechanical seal of the pump for alkylbenzene equipment in our workshop is a single-end mechanical seal. The new sulfur plant uses a double-face mechanical seal because of the high sealing requirements for media such as MDEA. The double-end mechanical seal is not only a repetition of the single-end mechanical seal, but also can prevent the leakage of dangerous, toxic, flammable and explosive substances in some key occasions of the chemical industry, and has a high-performance insurance effect. When sealing the high-pressure medium, the pressure difference between the two ends of each seal can be reasonably distributed to improve the working pressure range of the seal. For some self-lubricating, easy-coagulation, vaporization medium, the sealing condition is improved by injecting sealing liquid into the sealing cavity, which greatly improves the efficiency and service life of the sealing.

2, the principle of sealing

The principle of double-end mechanical seal is basically the same as that of single-end mechanical seal. It is formed by the end face (friction pair) perpendicular to the axis of rotation, which is protected by the action of fluid pressure and the elastic force of the compensating mechanism and the auxiliary sliding. A device for fluid leakage. The double-end mechanical seal structure is shown in Figure 2-1.

 During operation, under the combined force caused by the fluid pressure (medium pressure) and the elastic force of the elastic member, an appropriate specific pressure (compression force) is generated on the end face of the seal ring, so that the two contact end faces (dynamic ring, static) The end faces of the ring are closely attached to each other, and a very thin liquid film is maintained in a very small gap between the end faces to achieve the purpose of sealing. The double-end mechanical seal has two end face seals. If the first-stage seal fails, the secondary seal can still be sealed to prevent leakage. Generally, the double-face seal requires an external sealing liquid system, and a sealing liquid is introduced into the sealing cavity for sealing, lubrication and cooling, and is mostly used for circulating cooling. The sealing fluid not only can rinse the friction pair to improve the working environment of the mechanical seal, but also can be used as an important detection means for the failure of the primary sealing surface.

3, structure

Compared to single-end mechanical seals, the double-end mechanical seal has one more friction pair. The two friction pairs may be composed of two moving rings and two static rings, or two moving rings sharing one static ring or two static rings sharing one moving ring. There are several classification methods for double mechanical seals.

3.1 According to the sealing fluid system pressure can be divided into pressure double seal and pressureless double seal.

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

The pressure in the pressureless double seal system is generally normal pressure or micro pressure, which is lower than the seal chamber pressure before the first stage seal. When the first-stage seal leaks, the medium in the pump enters the seal liquid system, causing the pressure and liquid level of the seal liquid system to rise, and leakage monitoring and alarm can be realized through the pressure switch or the liquid level switch. The failure of the double-end mechanical seal to monitor the failure of the double-faced mechanical seal is one of the most important forms that distinguish it from the single-end mechanical seal.

3.2 According to the arrangement of friction pairs, it is divided into radial, face-to-face, back-to-back and series, as shown in Figure 3-1.

The radial double-end mechanical seal comprises a pair of moving dynamic rings and static rings. The main technical structure is characterized in that a ring groove is formed on the moving ring and the static ring engaging end surface to form two radial friction pairs. A drain hole is opened in the stationary ring to form a radial double-face mechanical seal. The overall structure is simple and compact, the structure space is small, and the installation and use are convenient, and is particularly suitable for occasions where the radial installation size is limited.

The face-to-face double-face mechanical seal has two forms: a stationary ring face-to-face and a moving ring face-to-face. The static ring face to face is an integral moving ring, and the moving ring faces the surface and is a whole static ring. However, due to the processing accuracy, replacement cost after mechanical seal failure, etc. Face-to-face double seals are used less in mechanical seals for fluid pumps, and are generally used in applications where the size of the seal chamber is small. Back-to-back or tandem mechanical seals have been fully adapted to cover the application of face-to-face mechanical seals. Light hydrocarbons, viscous, polymeric fluids, and hazardous gases are sealed with a dry gas and non-contact design with face-to-face mechanical seals. The face-to-face mechanical seal requires that the sealing fluid pressure be greater than the medium pressure, which belongs to the pressure double seal, otherwise the inner mechanical seal dynamic static ring is unreasonably stressed. The face-to-face structure is mostly used for the assembled structure, and can be used as a single end face or a sealing liquid to form a double end face.

The back-to-back double-face mechanical seal can also be made into a package, which is convenient to disassemble and assemble, and easy to repair, so it is used more. The double mechanical seal of the sulfur plant is back to back. The back-to-back double seal has two moving rings and two static rings, and the first-stage sealing is an outflow type. Because of its wide application conditions and wide application, the double-end mechanical seal in the narrow sense refers to the back-to-back double-face mechanical seal. The back-to-back double seal belongs to the pressure double seal, and the pressure and liquid level of the seal liquid system will decrease when the first seal fails. The power of the seal fluid circulation is provided by the built-in pump effect ring.

The series mechanical seal is composed of two or more sets of single-end mechanical seals arranged in the same direction to form a series mechanical seal, and two sets of single-end mechanical seals constitute a series double-end mechanical seal. This sealing method reduces the pressure of the sealed chamber step by step, and the medium does not leak. Due to its compact structure, the series double-face seal can be applied to medium load cases where the shaft diameter is not large, the axial dimension is small, and the rotational speed is large. The inner end is a balanced mechanical seal, and the outer end is an unbalanced mechanical seal with a pumping ring. The series double-face mechanical seal can be made into a pressure double seal or a pressureless double seal, and the most common pressureless double seal. The sealing liquid provides cooling lubrication of the sealing surface and is circulated by the built-in pumping effect ring.

 

4, failure analysis

In the case of rotating equipment failure, 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 that of the single-end mechanical seal, mainly the failure of the sealing surface. During the operation of the pump, the heat generated by friction and agitation will increase the temperature of the sealing surface, and the influence of the medium will cause vaporization of the sealing surface medium, deformation of the sealing ring, thermal wear, and even thermal cracking. Some media can exacerbate curing, polymerization, crystallization, coking, and corrosion as temperature changes. Unusual conditions can exacerbate or directly lead to seal failure in the short term.

The relevant agencies have conducted investigations on pump machinery for several years. The reasons for the failure of the seal and its proportion are shown in Table 4-1.

Table 4-1 Analysis of Causes of Mechanical Seal Failure

Seal failure reason

proportion(%)

Operational problems (flushing fluid interruption, etc.)

40

 

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

twenty four

 

 

High fluid circuit error (underpressure, insufficient flushing fluid, improper pump characteristics)

19

 

Improper selection of sealing elements (material or basic structural error)

9

 

 

As can be seen from the above table, the main cause of seal failure is operational problems. When analyzing faults, it is necessary to conduct in-depth research on the whole process, including the state of leakage before disassembly, the amount of leakage, the inspection during disassembly, and the inspection of sealing components, to find the leak point and the real cause of the fault, and then take corresponding measures.

5, routine maintenance

It can be seen from Table 1 that the main cause of the failure of the seal is the operation problem. Therefore, it is a prerequisite for the normal operation and good sealing operation to do a proper maintenance work. The following points should be guaranteed for daily maintenance.

1) Check whether the auxiliary device is normal before starting, and ensure that the liquid level and pressure of the sealing liquid system are within the operating range, and the seal is good everywhere. Press the steering wheel to pass the double-end machine to seal the pumping effect ring to fill the machine seal with liquid.

2) Do not evacuate the pump to avoid dry friction on the seal face.

3) Always check if the auxiliary device is working properly:

a, pay attention to the position of the liquid level of the sealing liquid, the oil level should be replenished frequently, and the pressure should be reduced to maintain the pressure.

b. Discharge the leaking liquid regularly.

c. Always check the cooling water pipes and flushing pipes to prevent blockage and prevent sudden water stoppage, which will result in poor cooling and flushing, resulting in seal failure.

4) Prevent the vibration and heat of the machine itself from affecting the seal.

a. Pay attention to the abnormal sound, and the vibration can cause the anti-rotation pin to loosen and escape.

b, pay attention to abnormal fever, such as: whether the bearing fever is normal, and make reasonable adjustments.

6 Conclusion

From the perspective of the sealing mechanism, both the double-face mechanical seal and the single-end mechanical seal convert the shaft seal which is difficult to control and has a large leakage into a relatively easy-to-control end seal (friction pair), static seal and pseudo static seal. The two-stage end face seal gives the double-end mechanical seal safe and reliable, and has no leakage characteristics. According to the arrangement of the friction pair and the sealing liquid system, the double-face mechanical seal has various forms in the structure to meet the requirements of various working conditions.

There are many reasons for the failure of double mechanical seals, and they are complicated and can be solved by taking corresponding measures. Double mechanical seal leakage is directly related to process operation and installation and maintenance. It is necessary to standardize operations, strengthen management, and summarize and analyze faults in daily production, which is necessary to ensure and improve sealing efficiency and life.

Jiaxing Burgmann Mechanical Seal Co., Ltd

Contact: Doris Sun

TEL: +86-573-84133388

Fax: +86-573-84133388

E-mail: doris@mechanicalseal.com.cn