Most of the chemical products are toxic, flammable and explosive. In the event of product leakage, in addition to causing certain economic losses, it will inevitably cause pollution to the environment and even cause harm to personnel. Due to the characteristics of the above chemical industry, the seal in chemical equipment It must be reliable and have certain corrosion resistance to ensure long-term use. In order to meet the complex environment of actual pump mechanical seal application, multi-structure, multi-material and multi-combined seals are widely used in the chemical industry, according to site requirements. New processes, new concepts, and new standards are constantly emerging. At present, mechanical seals are widely used in petroleum-related products, and with the improvement of product technology and energy conservation requirements, the application prospects of mechanical seals will be more extensive.

 

1 Mechanical seals for different conditions of use

 

In the petrochemical industry, due to the complexity of fluid mechanical seals, the temperature, pressure, journal and equipment speed of liquids in chemical equipment are different, so it should be considered from the aspects of structure, materials and ancillary equipment, such as petroleum. In the rotating equipment of chemical equipment (such as pumps, compressors, stirred tanks, etc.), the most demanding parts of the working conditions are mechanically sealed. The axial force is received like a thrust bearing, and the heat generated is derived like a heat exchanger. It has been difficult to achieve static sealing in high temperature, high pressure, and easy corrosion. It is more difficult to achieve dynamic sealing on a sealing surface of several millimeters wide during rotation. The mechanical seals are described below according to different environments.

1.1 High temperature

 

In the case of mechanical seals in high temperature environments, when the temperature exceeds 200°, general non-metallic materials such as rubber cannot be used as seals. In this case, considering the influence of thermal changes, the seals should be subjected to stable heat treatment in the same structure. The thermal expansion coefficients of the seals are nearly equal, and a metal bellows structure of stamped or welded type is used.

 

1.2 low temperature

 

Under low temperature conditions, the general material will be cold and brittle. Generally, factors such as fatigue strength, impact and expansion coefficient should be considered. The low temperature will also cause the auxiliary seal of the gasket to age, causing the seal to lose its elasticity and cause fluid. Leakage, sealing under low temperature conditions generally consider the following aspects:

1) Replace the auxiliary sealing ring with a metal bellows. Under the condition of ultra-low temperature, the metal bellows of stainless steel is often used to not only maintain excellent elastic compensation, but also eliminate mechanical vibration caused by various factors and make the sealing fit. Maintain a good state of exercise;

2) Prevent the bellows from fatigue. Under ultra-low temperature conditions, the plasticity will also decrease, but the hardness will increase. If the bellows is placed in the rotary motion, the fatigue will be accelerated, and the mechanical sealing period will be greatly shortened. In the case of a static structure;

3) Isolated from the atmosphere, the sealed chamber is isolated from the atmosphere to reduce the cold damage and improve the anti-cavitation performance. Another function of the insulation is to prevent the water surface from generating friction on the friction surface.

 

1.3 High pressure

 

If the specific pressure of the end face caused by the high-pressure medium exceeds the range of its adaptation, the liquid film will be destroyed, causing heat and abrasion. The high pressure will easily deform and break the friction pair, thus causing the mechanical seal to fail. Therefore, to solve the seal of the high-pressure medium, it is necessary to Suitable choices for structural form, material, cooling and lubrication. 1) The selection of the balance coefficient β value, the end face specific pressure is an important indicator to evaluate the sealing performance and wear resistance. It is advantageous to use the minimum end face specific pressure allowed by the seal to improve the service life; 2) Multi-end seal, multi-end is a kind The series type sealing device utilizes the pressure drop of the front and rear sealed chambers to reduce the pressure on each sealing surface. With such a decompression method, it is possible to seal the high pressure fluid, but in actual work, pressure control is required. These seals have some difficulties with auxiliary devices such as lubrication systems. 3) Adopting the balance type, for the high-pressure mechanical seal of the pump equipment with axial force balance disc, the balance disc can be used for throttling, so that a small amount of high-pressure fluid flows through the balance disc, and the pressure drops and enters the balance disc. The balance chamber is then in contact with the seal, and the balance tube is connected to the intersection of the pump in the balance chamber, and the fluid entering the balance chamber is pumped to the inlet of the pump, at which time the fluid pressure of the balance chamber drops to approximately the inlet pressure of the pump, thus sealing The problem is easy to handle.

2 Application of new technology and new structure in chemical mechanical seal 

 

2.1 Zero escape sealing technology 

 

The so-called zero-emission sealing technology means that the leakage and escape of the process fluid in the process equipment is equal to zero, that is, no process fluid escapes. The sealing technology that studies zero escape is the zero-emission sealing technology. Generally, the zero-escape seal adopts a dry running seal, which can have a contact dry running seal or a non-contact dry running seal. Under normal circumstances, the zero escape seal has certain auxiliary measures, including control and metering instruments, various valves, Main seal leakage monitoring and alarm device

A fluid blocking system is employed in a zero-escape sealing system in which a blocking fluid with a pressure greater than the process fluid pressure and atmospheric pressure is injected into the sealed chamber, allowing a small amount of blocking fluid to pass through the inner seal because the blocking fluid pressure is slightly greater than the process fluid pressure Leaking into the process fluid side, there may also be a small amount of blocking fluid leaking into the atmosphere through the outer seal. 

 

In order to achieve zero escape, a gas clogging system or a gas damper system must be used for hazardous media and hazardous media, and therefore, a seal that requires dry operation in a gas clogging system or a gas damper system is required as a zero escape seal. 

 

If the medium is a non-hazardous or non-hazardous inert gas or air, a single seal can be used. However, for hazardous media or hazardous media, double seal, cascade seal or multi-machine seal can be used to achieve zero escape.

2.2 Container seal 

 

Using the container concept in transit, a container-type seal is produced. The rotating and stationary parts of the seal are pre-assembled as a whole before installation. After inspection and pressure test, they are placed on the inner shaft of the shaft seal box, with the set screws. Fixed on the shaft, tighten the cap bolts, remove the assembly positioning piece, and the mechanical seal can be in a flat working state. 

 

3 Summary 

 

Many equipments in the petrochemical industry use mechanical seals. The use of mechanical seals is an effective way to solve run, run, drip, and leak, improve environmental protection, improve work efficiency, and reduce production costs. It is one of the essential means to realize industrial modernization in the petroleum industry. The application of new technology and new standards has brought not only economic benefits, but also new contributions to the surrounding environment and environmental protection. According to the special requirements of the chemical industry, this paper expounds the mechanical seal under different working conditions, and according to the practical application, puts forward a new technology of mechanical seal, which has certain guiding significance for the research of mechanical seal technology.