At present, the growing awareness of environmental protection among governments and people around the world has led to the implementation of more comprehensive environmental regulations, which have had a huge impact on various industrial departments that use pumps. In the United States, the 1990 Federal Clean Air Act Amendments of 1990 required the Environmental Protection Agency (EPA) to enforce more specific limits on precipitates for 189 volatile organic compounds and 454 volatile hazardous air pollutants. Many fluids delivered by process pumps in refineries and chemical plants are considered to be the main source of the above precipitates. The problem of pump leakage has drawn more and more attention. Improve the sealed pump shaft seal device is an important way to reduce pump leakage. 1. Overview of pump shaft seal technology In recent years, the world's pump in the sealing technology research and development has made remarkable achievements, mainly in the new sealing technology, sealing products and sealing system has made great progress in three aspects. Packing seal as the traditional seal structure of the pump, due to its a certain amount of leakage and poor performance at high speed and other issues, the use of shrinking. However, with the gradual elimination of asbestos rope in recent years, some progress has been made in packing seal technology. For example, compression packing seals are now showing great vitality throughout the UK industry. These low friction, compressed fillers made of synthetic, ceramic, and PTFE materials have better chemical compatibility, abrasion resistance, high temperature resistance and longer life than asbestos packing, and are not expensive . At present, mechanical seals occupy a larger share in the pump seal market, and various newly designed mechanical seals play a significant role in increasing market share. The most prominent is the cartridge seal. This seal easy to install and maintain, but also safe and reliable, but the price is quite expensive. Another striking mechanical seal is the split seal, which seals can be replaced without removing the pump. Then there is a new structure of hydrodynamic fluid pressure seal, backup seal and asymmetrical metal bellows seal. In addition, tandem and double-ended mechanical seals, when used in conjunction with a flushing system, prevent the release of hazardous materials and allow the transport of abrasive media. It is expected that the development of mechanical seals in the future is likely to focus on the use of "upstream pumping seals" and "magnetic fluid seals" or similar technologies to improve the integrity of the sealing system function. "Upstream Pumping Sealing" is a new concept that utilizes a variety of sealing facet flow channels to pump small amounts of leakage fluid from the low pressure side (downstream) of the sealing face back to the high pressure side (upstream). The "magnetic fluid seal" is derived from the aerospace industry new technology, with no leakage and no wear characteristics. 2. Upstream Pumping Sealing Upstream Pumping Sealing is a new concept that uses a variety of seal facet slots to pump small amounts of leakage fluid from the low pressure side (downstream) of the seal face back to the high pressure side (upstream). It is a hybrid structure between tandem and double-ended seals. In terms of construction and working principle, it looks like a backup end seal with a low pressure barrier fluid between the two end faces. In function, it functions as a double-ended 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 to the high pressure medium end. The common end face main seal is replaced by a small capacity high pressure "pump" - upstream pumping seal. This type of "pump" pushes a small amount of barrier fluid along the path normally enclosed by a mechanical seal and into the sealed chamber. Because the sealed chamber is at a higher pressure than the barrier fluid, this seal is considered to be pumped upstream. Upstream pumping seals are suitable for use in toxic and hazardous media, abrasive media and slurries, lubricants, and materials with high levels of PV. The symbol PV is a common expression used by seal manufacturers and users to represent the pressure-velocity limit of a face material combination in a given fluid. High-speed pumps are typically used under high PV conditions and double-ended seals can make the situation worse due to the higher buffer pressure required for double-ended seals. For a long time, the high-speed pump industry has been plagued by high PV values. The upstream pumping seal is essentially a non-contact operation and therefore completely eliminates the PV component. The upstream pumping seal provides an effective way for the high-speed pump industry to get out of trouble because it does not require a barrier fluid pressure higher than the seal chamber pressure. The principle of the upstream pumping seal is the equilibrium between hydrodynamic pressure and hydrostatic pressure. There are retaining rings with snap rings, springs and main sealing rings which are stationary parts, with a matching spiral groove on the rotating sealing ring. Grooves are a series of recessed logarithmic spirals. The unslotted part of the spiral groove outer ring is called the seal dam. When pressure is applied, hydrostatic pressure is created on the sealing surface and this force is generated when the mating seal is at rest or rotating. Hydrodynamic pressure generated only when turning. The spiral groove plays a decisive role when it turns, and it acts as a pressure-setting device. As the barrier fluid enters the helical groove, it is led to the outer diameter where it is damped by the dam. The increased pressure allows the flexibly mounted end face to move, adjusting the seal gap. A "pump - throttle" principle at work, forming a non-contact mode of operation, at the same time make the pumping liquid from the low-pressure zone into the high-pressure zone. The force that controls the seal operation is axial. The opening force is the sum of the pressure generated by the helical groove and the pressure drop across the end face. The closing force is the sum of the system pressure acting on the rear of the face and the spring force. If the sealing gap is reduced due to interference, the force in the liquid film will be significantly increased. Also, if the seal gap increases, the force within the liquid film will decrease. In both cases, the original gap will soon be restored. And simple hydrostatic type seal is different, the upstream pump seal both dynamic and static pressure, so with the speed and pressure are related. The hydrostatic seal is formed by the pressure seal gap, and therefore has nothing to do with the speed. $$ Div $$ 3. Magnetic Fluid Seal "Magnetic Fluid Seal" is a typical derived technology from the NASA space project that has been in existence for more than three decades. Initially developed to propel rocket fuel in the event of weightlessness in outer space, this technique of using magnetic force to control fluids has been extended to the surface by engineers and technicians. The magnetic fluid consists essentially of three parts: a carrier fluid (usually a low vapor pressure hydrocarbon or fluorocarbon), a surfactant (a chemical binder), and magnetic particles (tiny magnetite balls) . Surfactants bind the magnetic particles to the carrier fluid to make it a colloidal suspension, resulting in a corresponding fluid magnetism. In the past three decades, this simple sealing principle has been applied in many structures, with numerous magnetic fluid seals in operation. It seals the shaft of the vacuum swing device. Such devices are found throughout the semiconductor and vacuum industries, which rely on magnetic fluid seals for leak-proof, wear-free characteristics to provide consistent quality. The computer disk drive industry has installed millions of magnetic fluid seals for isolated seals between drive motors and precision storage. The advantages of this seal are: lower installation costs and 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 hub. The magnetic circuit formed by the fixed pole piece and the rotating shaft concentrates the magnetic flux in the radial gap under the pole piece according to the polarity. When a magnetic fluid is applied to the radial gap, it assumes the form of a "liquid O-ring" and forms a leak-free seal around the shaft. All magnetic fluid seals feature the following inherent features: No external power required; no contact, no wear; no leakage when the shaft is at rest or rotating; long and reliable service life; low torque and minimal energy consumption. Magnetic fluid seal also has a unique self-healing features. When too much pressure is applied through the magnetic seal, a short period of overpressure in the seal area will cause a portion of the magnetic fluid to be momentarily discharged from the shaft all around. During overpressure conditions, the magnetic fluid remains enclosed within the seal and when the interference condition is over , Still with the original pressure to form a seal again. A new cartridge seal designed specifically for pumps based on magnetic fluid sealing technology is a cost effective method of sealing that pump manufacturers and users can now replace magnetically driven pumps and complex dual seal systems and is easy to use in existing The pump re-modified. Pumps sealed with this proven technology meet the most stringent deposit control regulations. Magnetic Fluid Cartridge Seal is an auxiliary secondary seal that combines with the primary mechanical seal to form a volatile organic compound protection system. The "liquid O-ring" formed by the magnetic structure and the magnetic fluid prevents the primary seal vapor from escaping to create a hermetic seal around the pump shaft. After several years of production run, the pump users agreed that the magnetic fluid seal has the following advantages: Simple retrofitting improvements to existing pumps; Zero leakage under both static and dynamic conditions; Lower installation and use Costs; Instrumentation is simple and easy to monitor; Ability to reload and measure pressure in situ; Low maintenance requirements; No barrier fluid or complex seal assist system; No wear parts, significantly reduced maintenance. The basic pump cartridge seal structure is designed according to the general acceptance requirements of the entire pump industry. Contained with an optional cooling device (for high temperature applications) or an optional gas purification system (protecting the sealing element from environmental corrosion), cartridge seals can be used in a wide range of applications. This sealing technology also has the unique ability to accurately monitor primary seal performance and to predict or identify problems before a serious accident occurs. Since the secondary magnetic fluid seal blocks all the primary seal steam leaks, a simple flow meter can accurately monitor the primary seal and give an alarm when the leak is excessive. New developments under study include an integral, high-pressure liquid seal 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 with the primary seal escaping steam and small liquid leaks sealed by a secondary cartridge seal. This will eliminate the torch or other drain and eventually form a closed volatile organic recovery system. Magnetic fluid sealing techniques have proven to be 100% efficient for leaks that seal pump leaks. With the gradual understanding and understanding of this technology by process industry designers and users, the use of this sealing technology will continue to grow. It is likely to be a cost-effective way for the industry to meet the challenge of environmental sealing in the future. In China, due to people's lack of awareness of environmental protection, environmental laws and regulations are not sound enough, coupled with the implementation of the force is not enough, people generally pay attention to the problem of pump leakage. So that the domestic pump manufacturers on how to minimize the pump (especially industrial pumps) leaks less investment. Resulting in domestic pump shaft seal device research and development level compared with the international advanced level there is a big gap. With China's accession to the WTO and the strategic policy of giving priority to the development of the environmental protection industry in the recent five years, it is foreseeable that to vigorously improve the shaft seal device of the pump and reduce the amount of pump leakage will become a trend of development for industrial pumps in the future.
