New trend and overview of pump shaft sealing device

At present, in the world, the growing awareness of environmental protection by governments and citizens in the world has prompted the implementation of more comprehensive environmental laws and regulations, which has had a tremendous impact on the various industrial sectors that use pumps. In the United States, the 1990 amendment to the Federal Clean Air Act requires the Environmental Protection Agency (EPA) to impose more specific precipitation restrictions on 189 volatile organic compounds and 454 volatile air pollutants. Many of the liquids delivered by pumps in refineries and chemical plants are considered to be the main source of these precipitates. The problem of pump leakage has drawn increasing attention. Improvement of shaft seals with sealed pumps is an important way to reduce pump leakage.

1. Overview of the pump shaft seal technology In recent years, the world pump industry has made remarkable achievements in the research and development of sealing technology, mainly in the new sealing technology, sealing products and sealing systems have made great progress in three aspects.

As a conventional seal structure of a pump, the packing seal has a narrowing field of use due to a certain amount of leakage and a poor effect at a high speed. However, in recent years, with the phase-out of asbestos ropes, some new advances have been made in the packing and sealing technology. For example, compressed packing seals are currently showing their strong vitality throughout the UK industry. The low-friction compression packing made of such synthetic fibers, ceramics and Teflon materials has better chemical adaptability, abrasion resistance, high temperature resistance and longer service life than the asbestos packing, and the price is not high. .

At present, the share of mechanical seals in the market for pump seals is increasing, and various new designs of mechanical seals have played a significant role in the growth of market share. The most prominent is the cartridge seal. This seal is easy to install and maintain, safe and reliable, but the price is quite expensive. Another compelling mechanical seal is a split seal that can be replaced without the need to disassemble the pump. There are also new structures such as hot fluid dynamic pressure seals, back-up seals, and asymmetrical metal bellows seals. In addition, tandem seals and double-faced mechanical seal structures, when used with a flushing system, prevent the leakage of hazardous materials and allow the delivery of abrasive media. It is expected that the development of mechanical seals in the future is likely to focus on the use of "upstream pumped seals" and "magnetic fluid seals" or similar techniques to improve the integrity of the seal system functions. An "upstream pumping seal" is a new concept that uses a variety of sealing face grooves to pump a small amount of leakage fluid on the low pressure side (downstream) of the sealing surface back to the high pressure side (upstream). The "magnetic fluid seal" is a new technology derived from the aerospace industry project, with no leakage and no wear characteristics.

2. Upstream pumping seal The "upstream pumping seal" is a new concept for pumping small amounts of leakage fluid on the low pressure side (downstream) of the sealing surface back to the high pressure side (upstream) using various sealing face grooves. It is a hybrid structure between a series seal and a double seal. In terms of structure and operation, it looks like a backup face seal structure with a low-pressure barrier fluid between the two end faces. In terms of function, it acts as a double-sided seal. The pumping mechanism in the main seal is such that the pressure generated in the seal creates a weak pumping capacity from the low pressure barrier zone to the high pressure medium end. The common face primary seal is replaced by a small capacity, high pressure "pump" - an upstream pump seal. This "pump" pushes a small amount of barrier liquid along the path that is normally closed by the mechanical seal and is fed into the sealed chamber. Since the pressure in the seal chamber is higher than the barrier fluid, this seal is seen as pumping upstream. Upstream pumping seals can be used in toxic and dangerous media, abrasive media and slurries, poor lubricity media, and high PV values. The notation PV is a commonly used expression for seal manufacturers and users to indicate the pressure-velocity limit of the end face material combination in a given liquid. High-speed pumps are generally used under high PV conditions. Since double-sided seals require higher buffer pressures, the use of double-sided seals can make the situation even worse. For a long time, the high-speed pump industry has been plagued by high PV values. The upstream pumping seal is essentially non-contact operation, thus completely eliminating the PV value component. Because there is no need for a barrier fluid pressure higher than the pressure in the sealing chamber, the upstream pumping seal provides an efficient way for the high-speed pump industry to get out of trouble.

The principle of upstream pumping seals is that the hydrodynamic pressure and the hydrostatic pressure are balanced. Sealed ends with snap rings, springs, and main seal rings are stationary parts and the matching rotary seal rings have spiral grooves. The groove type is a series of recessed logarithmic spirals. The unslotted portion of the spiral groove outer ring is called a seal dam. When pressure is applied, hydrostatic pressure is generated on the sealing surface, and this force is generated when the mating seal ring is stationary or rotating. Hydrodynamic pressure is only generated when turning. The spiral groove plays a decisive role when it rotates, it can act as a pressure forming device. When the barrier fluid enters the spiral groove, it is led to the outer diameter where it is subjected to the resistance of the dam. The increase in pressure allows the flexible mounting face to oscillate, thereby adjusting the seal gap. A "pump-throttle" principle works, forming a non-contact mode of operation while allowing the pumped liquid to enter the high pressure zone from the low pressure zone. The force controlling the operation of the seal is axial. The opening force is the sum of the pressure generated by the spiral groove and the pressure drop on both sides of the end face, and the closing force is the sum of the system pressure and the spring force acting on the rear face of the end face. If the seal gap is reduced due to interference, the force in the liquid film will be significantly increased. Also, if the seal gap increases, the force in the liquid film decreases. In both cases, the original gap will soon recover. Unlike pure hydrostatic seals, upstream pumping seals have both dynamic and static pressure, and therefore are related to speed and pressure. The hydrostatic seal is formed by the pressure to form a seal gap, and therefore has nothing to do with the rotational speed.

3. Magnetic fluid seal "Magnetic fluid seal" is a typical derivative technology from the NASA space program, has been more than 30 years of history. The original development was used to promote rocket fuel under conditions of weightlessness in outer space. This technique of using magnetic force to control fluids has been applied to the ground by engineering and technical personnel. The magnetic fluid consists mainly of three components: a carrier fluid (usually a low vapor pressure hydrocarbon or fluorocarbon), a surfactant (a chemical binder), and magnetic particles (minimum magnetite balls). . The surfactant binds the magnetic particles to the carrier fluid to make it a colloidal suspension, resulting in a corresponding fluid magnetic property. In the past thirty years, this simple sealing principle has been applied in numerous structures with countless magnetic fluid seals in operation. It seals the shaft of the vacuum rotary device. This type of device extends throughout the semiconductor and vacuum industries, which rely on magnetic fluid seals to provide constant quality without leakage or wear. The computer disk drive industry has installed millions of magnetic fluid seals as isolation seals between drive motors and precision storage devices. The advantages of this seal are: lower installation costs and a reliable service life.

The main components of a magnetic fluid seal include a magnetic fluid, a magnet ring, two pole pieces, and a magnetically permeable shaft or sleeve. The magnetic circuit formed by the fixed pole piece and the rotary shaft concentrates the magnetic flux in the radial gap below the pole piece according to the polarity. When a magnetic fluid is applied to a radial gap, it takes the form of a "liquid O-ring" and forms a leak-free seal around the shaft. All magnetic fluid seals have the following inherent characteristics: no external power is required; no contact, no wear; no leakage when the shaft is stationary or rotating; long and reliable service life; low torque and minimal energy consumption. Magnetic fluid seals also have a unique self-recovery feature. When too high a pressure is passed through the magnetic seal, a short period of excessive pressure in the seal area will cause a part of the magnetic fluid to be instantaneously discharged from the periphery of the shaft. During the overpressure state, the magnetic fluid remains enclosed in the seal body when the interference state ends. The seal is still re-formed with the original pressure.

New auxiliary cartridge seals based on magnetic fluid sealing technology designed specifically for pumps are a cost-effective sealing method that current pump manufacturers and users can replace magnetically driven pumps and complex double seal systems, and it is easy to use in existing The pump was refitted. Pumps sealed with this well-proven technology can meet the most stringent control regulations for precipitates. The magnetic fluid cartridge seal is an auxiliary secondary seal that combines with the main mechanical seal to form a VOC protection system. The "liquid O-ring" formed by the magnetic structure and the magnetic fluid prevents the escape of the primary seal steam leakage, thereby forming a hermetic seal around the pump shaft.

After several years of production operations, pump users agree that magnetic fluid seals have the following advantages: Simple reloading improvements can be made to existing pumps; leakage is zero under both static and dynamic conditions; lower installation and use Cost; instrumentation is simple, easy to monitor; able to reload and determine the pressure in place; low maintenance requirements; no barrier or complex sealing auxiliary system; no wear parts, maintenance significantly reduced.

The basic pump manifold seal design is designed to meet the general acceptance requirements of the entire pump industry. Cartridge seals can be applied to a wider range of fields by means of an optional cooling device (for high temperature devices) or an optional gas cleaning system (protection of sealing elements against environmental corrosion). This sealing technology also has the unique ability to accurately monitor the performance of the main seal, and can predict failures or identify problems before serious accidents occur. Since the secondary magnetic fluid seal intercepts the leakage of all the main seal steam, a simple flow meter can accurately monitor the main seal and give an alarm signal when the leak is too large. New developments under investigation include an integral high pressure liquid seal housed in a container. This seal will act as a safety device in the event of a major failure of the main mechanical seal. Another research project is to develop a unique method of returning to the suction side of the pump. The amount of leaked steam and tiny liquid from the main seal can be sealed by a secondary cartridge seal. This eliminates the need for a flare or other drain and ultimately results in a closed volatile organic recovery system.

The magnetic fluid seal technology has proven to be 100% effective for the leakage of the sealed pump leakage. With the gradual understanding and understanding of this technology by the designers and users of the process industry, the use of this sealing technology will expand. It is likely to become a cost-effective way for the industry to meet environmental sealing challenges in the future.

In China, due to the lack of environmental awareness among the Chinese people, inadequate environmental laws and regulations, and inadequate enforcement, people generally pay insufficient attention to pump leakage problems. As a result, domestic pump manufacturers have invested little in how to minimize the leakage of pumps (especially industrial pumps). As a result, the level of research and development of domestic pump shaft seals is far from the international advanced level. With China's accession to the WTO, as well as the strategic policy of prioritizing the development of the environmental protection industry in the past five years, it can be foreseen that vigorously improving the shaft sealing device of the pump and reducing the leakage of the pump will become a development trend of industrial pumps in China in the future.

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