AS-C461 customized double cartridge mechnaical seal

AS-C461 double cartridge mechnaical seal is customized dual seal made as per client drawing.

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Case study on the transformation of high-risk medium pump mechanical seal and seal flushing scheme

The slurry circulation pump at the bottom of the catalytic fractionation tower is the key equipment of the catalytic cracking unit of the refinery. The pump is a single-stage single-suction cantilever centrifugal pump. The pump model is 200ZPY450; the flow rate is 376m3/h, the head is 150 m; the inlet pressure 0.2 MPa, outlet pressure 2.0 MPa, medium is oil slurry; temperature 340 ℃, bellows type mechanical seal is used. The medium transported by the oil slurry pump has high temperature and high density (840~1 010 kg/m3), and contains a relatively high concentration of solid particles such as catalysts (Al2O3 and SiO2). The maximum concentration can reach 25 g/L. These solid particles have strong adsorption properties. , adhere to the mechanical seal bellows, causing a large number of catalyst particles to be deposited in the corrugated troughs of the bellows, causing the bellows to lose elasticity, causing frequent leakage of the mechanical seal, and causing great hidden dangers to the long-term operation of the device.

1.Failure analysis of mechanical seal system
1.1.Introduction to mechanical seal system
The mechanical seal of the oil slurry pump in this factory adopts a single bellows type. The stationary seat of the mechanical seal is made of silicon carbide (SiC), the rotary face is made of graphite, the rotary face auxiliary sealing ring is made of flexible graphite, and the other materials are stainless steel.

The auxiliary system of the mechanical seal adopts an external flushing structure (PLAN32 plan). The seal flushing fluid is 30°C catalyzed self-produced diesel oil with a pressure of 0.6 MPa. The flushing fluid enters the seal cavity through the mechanical seal gland. The seal flushing fluid has a low temperature and can The friction heat is taken away in time to prevent coking of the friction pair and bellows of the mechanical seal.In addition, the mechanical seal also adopts a low-pressure steam cooling system. The cooling steam not only plays a cooling role, but also prevents liquid in the seal cavity from entering the elastic element cavity of the mechanical seal, so that the bellows mechanical seal can operate under cleaner conditions.

1.2.Analysis of causes of mechanical seal failure Through the dismantling and anatomy analysis of the failed mechanical seal, it was found that the main causes of mechanical seal failure and leakage are as follows:
1.2.1.Mechanical seal failure caused by process operation
During the daily start-up and shutdown process, when the inlet pressure of the oil slurry pump fluctuates greatly due to operational reasons, abnormal phenomena such as reverse evacuation, cavitation, and pressure suppression will occur in the oil slurry pump, causing large axial forces, causing stationary seat separated from the rotary face of the mechanical seal, causing leakage of the mechanical seal; In daily operation, the delivery medium of the oil slurry pump contains catalyst granular substances, which enter the friction pair end surface of the mechanical seal during operation and have a grinding effect, causing coking, carbon deposition and solid catalyst particles on the inside of the mechanical sealing bellows, causing the bellows to lose elasticity and causing severe damage to the sealing surface. Then mechanical seal failure due to wear and tear occurs.

1.2.2.Mechanical seal failure occurs during equipment installation and operation. The friction condition between the friction pair end faces of the mechanical seal is the main factor affecting the sealing life of the mechanical seal. During the installation and operation of the oil slurry pump, the mechanical seal has the following failures:
(1)Improper installation causes mechanical seal leakage
When installing the mechanical seal, the compression amount is too large, resulting in serious wear and scratches on the end face of the friction pair of the mechanical seal; the rotary face is too tight, and the spring cannot adjust the axial floating amount of the rotary face; the stationary seat sealing surface is too loose, and when the rotary face floats axially. At this time, the stationary seat is separated from the static ring seat.

(2) Mechanical seal leakage due to loss of lubricating oil film on the sealing end face Due to the existence of the end face seal load, dry friction occurs when the pump is started when the seal cavity is short of liquid, causing mechanical seal leakage; the medium pressure is lower than the saturated steam pressure, causing the liquid film on the seal end face to flash and lose lubrication, causing mechanical seal leakage.

（3）Mechanical seal leakage caused by mechanical seal flushing The auxiliary flushing system of the mechanical seal of the oil slurry pump should ensure the continuous supply of flushing fluid to the mechanical seal cooling system and prevent the end face flushing fluid from being interrupted. The flushing fluid is interrupted during the operation of the oil slurry pump, which will increase the temperature of the seal end face and cause the lubricating film to flash and reduce the seal end face lubrication, thereby aggravating wear and causing mechanical seal leakage.
2.Mechanical seal modification plan
2.1.Structure and working principle of tandem mechanical seal
The mechanical seal of the oil slurry pump is changed from a single end face to a double end face. The solution is a two-stage mechanical seal (ie, a series mechanical seal). The first level sealing medium is diesel oil. An energy storage tank is installed between the two-stage seals, and isolation liquid (sealing liquid) is added to the pressurized air bag accumulator as the sealing medium for the secondary seal. When the first-stage seal leaks, the accumulator presses the isolation medium into the seal cavity of the mechanical seal so that the pumped medium cannot leak into the atmosphere. At the same time, the pressure switch on the accumulator alarms.

Its main advantages are: it is suitable for high-temperature working conditions. Different from self-flushing, clean flushing fluid is introduced from the outside; it takes away impurity particles at the sealing end face to clean the mechanical seal; the sealing fluid pressure is higher than the medium pressure to ensure zero discharge of the process medium; the system is equipped with a pressure alarm switch, which can detect seal failure in time; when the primary seal fails, the secondary seal realizes the main sealing function and improves the safety level of equipment operation.

3.Conclusion
After the implementation of the above modifications, the slurry pump P208 at the bottom of the fractionation tower of the catalytic cracking unit is operating in good condition, and the mechanical seal is in a stable state. This effectively eliminates the hidden danger of high-risk pump seal leakage and dangerous media being discharged into the atmosphere. At the same time, it provides support for pump seal maintenance with early warning, the reliability of the continuous operation of the machine pump has been significantly improved, reducing the maintenance workload in the machine repair shop and reducing maintenance costs. Summarizing the above transformations, the following conclusions are drawn:

(1) The mechanical seal is a double bellows mechanical seal. When the primary seal fails, the secondary seal realizes the main sealing function. Compared with a single seal, the double non-pressure mechanical seal can reduce the risk of circuit leakage and improve the safety level of equipment operation; (2) The function of the mechanical seal auxiliary system is to use flushing, circulation, and heat convection working methods to control the temperature and lubrication equation parameters of the mechanical seal, establish an ideal working environment for the mechanical seal, and improve the reliability of the mechanical seal; effectively improves the operating condition of the mechanical seal, providing an important guarantee for the long-term operation of the mechanical seal of high-risk medium pumps.