AS-C431 Customized Cartridge Mechanical Seal With Metal Bellow Design

This is metal bellow type cartridge mechanical seal made as per drawing

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What are the forms, causes and solutions of thermal oil pump mechanical seal failure
Difficulties are posed by high-temperature, low-temperature and easy-to-vaporize medium pump seals（mechanical seals） in the petrochemical industry. This article mainly talks about the form, cause and solution of the mechanical seal failure of the hot oil pump.

1. What are the characteristics of high temperature pumps in the petrochemical industry?
Typical high-temperature pumps in the petrochemical industry (hereinafter referred to as hot oil pumps) are the fractionation tower bottom pumps in each device. For example, atmospheric fractionation towers in atmospheric and vacuum distillation units, vacuum fractionation towers, fractionation towers in catalytic cracking and delayed coking units. The fractionation tower bottom pumps in these devices have the following characteristics: generally 360~380°C; low pressure: the inlet pressure of the pump is 0.1~0.3MPa, and the inlet pressure of the vacuum tower bottom pump is negative pressure (-0.03~ -0.02MPa). For double-supported pumps, the pressure at the sealing chamber at the outlet end is slightly higher, which is 0.6~0.9MPa; the viscosity of the medium is high: the viscosity at the working temperature is (0.06~2)×10¯4㎡/s (6~200 centistokes); The medium contains solid particles. Some are catalysts (such as catalytic slurry pumps); some are mechanical impurities, and some are generated coke. In addition, there is a certain degree of corrosion. These characteristics increase the difficulty of solving the shaft mechanical seal of the thermal oil pump.

2. What are the main forms and reasons for the failures of the thermal oil pump mechanical seals?
The main form of failure of the mechanical seal of the hot oil pump is as follows: the sealing surface is worn out which means that When the rotary face is made of tungsten carbide & the stationary seat is made of graphite, and annular grooves emerge on the surface of the stationary seat; When the rotary face is hot-fitted on the stainless steel ring seat with a tungsten carbide ring, the tungsten carbide ring will become loose; Scale is formed between the rotary face and the shaft sleeve, and the rotary face cannot float; the sealing ring of the rotary face is worn or flanged (when made with 4F-V-shaped pads): the stationary seat is out of position, etc.The main reason is high temperature. High temperature vaporizes the medium in the sealing end faces of the mechanical seal; high temperature reduces the performance of the graphite ring; high temperature makes the cooling water scale; The high temperature makes the hot-packed tungsten carbide ring loose; due to the high temperature, the hot oil pump is easy to evacuate, causing the stationary seat of the mechanical seal to be out of position, and the rotary face of the sealing ring to have defects such as flanging. The presence of mechanical impurities will also aggravate the wear of the sealing surfaces of the mechanical seal.

3. How to reduce the temperature of the hot oil pump seal
The high temperature of the hot oil pump is the main reason for the failure of the mechanical seal, so it is very important to find a way to reduce the temperature of the seal. 1) The common method for domestic pumps and seals to reduce the temperature is to set a cooling water jacket around the seal cavity and pass cooling water for cooling. This method can reduce the temperature in the sealed chamber of the mechanical seal to 200~250°C. Due to the smaller radial size of the sealing cavity of the mechanical seal of domestic pumps, the method of installing a cooling coil in the sealing cavity is generally not used; the method of cooling the shaft sleeve is also not used, which will complicate the structure. The measure that Some factories adopt which is adopting water cooling around the stationary seat of the mechanical seal is advisable. 2) If it is a single-end seal, injection flushing can be used. It is an effective method to use clean, low-temperature candles (about 100°C) from the outside, which not only reduces the temperature, but also improves the working environment. 3) Quenching water is used outside the gland. In order to prevent water scaling, softened water at about 60°C can be used. In addition to lowering the temperature, it can also prevent the sewer pipe from being blocked.

4. How can the hot oil pumps adopt double mechanical seals to achieve good results?
Two single sealing faces form a double mechanical seal, and a sealed chamber is formed between them to separate the medium in the pump from the atmosphere. There must be flushing facilities in the sealed room of the mechanical seal, and circular flushing is generally used.
Each single mechanical seal can be an ordinary mechanical seal or a bellows type mechanical seal. The circular flushing method can be achivevd by installing a small impeller on the shaft to circulate the sealing liquid and then return to the sealing chamber after cooling; Thermosiphon flushing can also be adopted to use the density difference flow caused by the temperature difference of the mechanical sealing liquid, and then returning to the sealing chamber after through external cooling Chamber; Circular flushing can also be achieved with an external supply of flushing fluid.
The structure of the double mechanical seal is complicated, the installation is difficult, and it is rarely used. According to the survey of 24 petrochemical plants and oil refineries in the United States, among the high-temperature pump shaft seals, only 4% of the pumps use double mechanical seals, and the service life is generally 1.5 to 2 times that of single mechanical seals.

5. How to apply"hard to hard"sealing materials on the mechanical seal of the thermal oil pump
Most of the general sealing starts from the method of improving the environment, turning the harsh working environment into an acceptable environment. The so-called"hard to hard"seal starts from another angle and improves the material grade of the friction pair of the mechanical seal so that it can adapt to harsh working conditions and achieve the purpose of sealing. The commonly used friction pair material is tungsten carbide, and some materials such as silicon carbide are used. The sealing structure is still the original ordinary mechanical seal. Tungsten carbide has high hardness and strength, and its performance has little change below 400 °C. The solid particles in the medium cannot wear the tungsten carbide, the smoothness of the sealing surface will not be damaged, and good sealing performance of the mechanical seal can be maintained. In addition, tungsten carbide has good thermal conductivity, and its coefficient of linear expansion is also very small, only 1/4~1/3 of that of ordinary carbon steel. Good thermal conductivity and small linear expansion coefficient enable tungsten carbide to work at high temperature, with small thermal deformation, and the flatness of the sealing end surfaces of the mechanical seal will not be damaged, which is also conducive to maintaining good sealing. This kind of friction pair of the mechanical seal is especially suitable for media with high viscosity and a small amount of solid particles. It can work well without flushing, but the gland needs to be cooled by low-pressure steam or hot water

When using tungsten carbide sealing ring for the mechanical seal, it can be of integral structure or inlaid structure. The price of the sealing ring of the inlaid structure is relatively low, but there is still a possibility of loosening under high temperature, so attention should be paid to the selection of ring seat material for the mechanical seal. When CAP42 is selected as the ring seat material, there will be almost no loose defects below 400°C, but it should be noted that CAP42 is not corrosion-resistant in some media. Although the price of integral tungsten carbide is high, it is still advisable because of its long service life.