AS-C429 Customized Single Cartridge Mechanical Seal

This seal is single seal in cartridge form made as per drawing

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Case analysis of high-risk medium pump mechanical seal and seal flushing scheme transformation

The oil slurry circulation pump at the bottom of the catalytic fractionation tower is the key equipment of the catalytic cracking unit in the refinery. The pump in a certain plant 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 is 0.2 MPa, the outlet pressure is 2.0 MPa, and the medium is oil slurry; the temperature is 340 ℃, and the bellows type mechanical seal is used. The conveying medium of the slurry pump has high temperature and high density (840~1 010 kg/m3), and contains solid particles such as catalysts (Al2O3 and SiO2) at a relatively high concentration, with a maximum concentration of up to 25 g/L. These solid particles have strong adsorption and adhere to the bellows of the mechanical seal, causing a large amount of catalyst particles to deposit in the troughs of the bellows, causing the bellows to lose elasticity, resulting in frequent leakage of the mechanical seal, and causing great hidden dangers to the long-term operation of the device.

Through the anatomical analysis of the disassembly and repair of the failed mechanical seal, it is found that the main reasons for the failure and leakage of the mechanical seal are as follows:
1.2.1. Process operation causes mechanical seal failure
During the daily start & shutdown process, the pressure of the slurry pump inlet due to operational reasons, the pressure fluctuations in the inlet of the slurry pump fluctuated large, causing abnormal phenomena such as the pulp pumps, steam erosion, and pressure, which caused a large axial force, separated the contact surface of the movement and static ring, which caused the aircraft to leak. In daily operation, the conveying medium of the oil slurry pump contains catalyst particles,and
it enters the end surfaces of the friction pair of the mechanical seal for grinding, resulting in coking, carbon deposits and solid catalyst particles inside the mechanical seal bellows, causing the bellows to lose elasticity and the sealing surface to be severely worn & thus failure of the mechanical seal device.

1.2.2. Mechanical seal failure caused during equipment installation and operation
The frictional condition between the sealing end faces of the rotary face and stationary seat 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 seal has the following failures forms:
(1) Improper installation causes mechanical seal leakage
When the mechanical seal is installed, the amount of compression is too large, resulting in severe wear and scratches on the end faces of the friction pair of the mechanical seal; the sealing ring of the rotary face is too tight, and the spring cannot adjust the axial floating amount of therotary face; the sealing surface of the stationary seat is too loose, and when the rotary face floats axially, the stationary seat will break away from the seat of the stationary seat.
(2) Mechanical seal leakage due to loss of lubricating oil film on the sealing end face
Due to the existence of the seal load on the end faces of the mechanical seal, dry friction occurs while the pump is started when the seal chamber lacks liquid, resulting in mechanical seal leakage; the medium pressure is lower than the saturated steam pressure, which causes the liquid film on the seal end faces to flash and lose lubrication, resulting in mechanical seal leakage.
(3) Mechanical seal leakage caused by mechanical seal flushing

Compared with the single mechanical seal, the tandem mechanical seal has the following advantages:
(1) It is suitable for high-risk media such as toxic and harmful, and can realize zero overflow of the medium, reducing the risk of dangerous media leaking into the atmosphere.
(2) Double protection function, when the main seal fails, the secondary seal can play the role of the main seal in a short time.
(3) Through the pressure switch and the liquid level switch, the failure of the mechanical seal can be monitored in real time.

2.2. Working principle and characteristics of API 32+53B seal flushing scheme
The working principle of the API 32+53B scheme: the external piping system provides spacer fluid for the secondary seal of the pressurized series seal, and the pre-pressurized air bag accumulator tank provides pressure to the circulation system, as shown. The accumulator provides spacer fluid (sealing fluid) for the pressurized double mechanical seal, and the heat exchanger is used to cool the spacer fluid.
Its main advantages are: suitable for high-temperature working conditions, and introduce clean flushing liquid from the outside different from self-flushing ; Take away the impurity particles at the sealing end faces of mechanical seal to clean the seal; the sealing liquid 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 above modification, the P208 oil slurry pump at the bottom of the fractionation tower of the catalytic cracking unit is in good condition, and the mechanical seal is in a stable state, which effectively eliminates the hidden danger of the high-risk pump seal leaking dangerous media being discharged into the atmosphere. Summarizing the above modifications, the following conclusions can be drawn:
(1) The mechanical seal is a double bellows type mechanical seal. When the primary seal fails, the secondary seal realizes the main sealing function. Compared with the single-end seal, the double non-pressure 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 adopt the working mode of flushing, circulation and heat convection to control the temperature and lubrication parameters of the mechanical seal, establish an ideal working environment for the mechanical seal, improve the reliability of the mechanical seal, and prolong the service life; effectively improve the operation of the mechanical seal, and provide an important guarantee for the long-term operation of the mechanical seal of the high-risk medium pump