Pump shaft seal device develops new heading

At present, the increasing environmental awareness of governments and people around the world has led to the implementation of more comprehensive environmental regulations, which has had a huge impact on the various industrial sectors using pumps. In the United States, the 1990 amendment to the Federal Clean Air Code requires the Environmental Protection Agency (EPA) to impose more specific precipitation limits on 189 volatile organic compounds and 454 volatile hazardous air pollutants. Many of the liquids delivered by process pumps in refineries and chemical plants are considered to be the primary source of such precipitates. The problem of pump leakage has attracted more and more attention. Improving the shaft seal with a sealed pump is an important way to reduce pump leakage.

Pump shaft sealing technology

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

As a traditional sealing structure for pumps, packing seals have become increasingly shrinking due to problems such as a certain amount of leakage and poor performance at high speeds. However, in recent years, with the phasing out of asbestos ropes, some new progress has been made in the packing sealing technology. For example, compression packing seals are currently showing their strong vitality throughout the UK industry. Low-friction compression fillers made from such synthetic fibers, ceramics, and PTFE have better chemical compatibility, wear resistance, high temperature resistance, and longer service life than asbestos fillers, and are not expensive. .

At present, mechanical seals occupy an increasing share of the pump seal market, and various newly designed 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 at a very expensive price. Another striking mechanical seal is the split seal, which can be replaced without disassembling the pump. There are new structures such as hot fluid dynamic pressure seals, spare seals and asymmetrically shaped metal bellows seals. In addition, the tandem seal and double mechanical seal construction, when used in conjunction with a flushing system, prevents the leakage of hazardous materials and allows 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 techniques to improve the integrity of the sealing system. The “upstream pumping seal” is a new concept that uses a variety of sealing face runners to pump a small amount 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 a new technology derived from the aerospace industry project, featuring no leakage and no wear.

2. Upstream pumping seal:

The upstream pumping seal is a new concept for pumping a small amount of leakage fluid from the low pressure side (downstream) of the sealing face back to the high pressure side (upstream) using various sealing face flow channels. It is a hybrid structure between a series seal and a double seal. In terms of structure and working principle, it looks like a spare end seal structure with a low pressure barrier between the two end faces. In terms of function, it functions as a double seal. The pumping mechanism in the primary seal is such that the pressure generated in the seal creates a weak pumping capability from the low pressure barrier zone to the high pressure media end. The conventional end face primary seal is replaced by a small capacity high pressure "pump" - upstream pumping seal. This "pump" pushes a small amount of barrier fluid along a path that is normally sealed by a mechanical seal and is fed into the sealed chamber. Since the sealed chamber is at a higher pressure than the barrier fluid, this seal is considered to be pumping upstream. The upstream pumping seal can be used in toxic and hazardous media, abrasive media and slurries, various media with poor lubricity, and high PV values. The symbol PV is an expression commonly used by seal manufacturers and users to indicate the pressure-speed limit of the end material combination in a given liquid. High-speed pumps are typically used under high PV values. Because double-face seals require higher buffer pressures, double-face seals can make things worse. The high-speed pump industry has long been plagued by high PV values. The upstream pumping seal is essentially non-contact running, thus completely eliminating the component of the PV value. 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 higher barrier pressure than the seal chamber pressure.

The principle of the upstream pumping seal is that the hydrodynamic pressure and the hydrostatic pressure are balanced. The sealing end with the snap ring, the spring and the main sealing ring is a stationary part, and the matching rotary sealing ring has a spiral groove. The trough is a series of concave logarithmic spirals. The unslotted portion of the spiral groove outer ring is called a seal dam. When pressure is applied, hydrostatic pressure is created on the sealing surface and this force is generated when the mating seal ring is stationary or rotating. The hydrodynamic pressure is only generated when it is rotated. The spiral groove plays a decisive role in rotation and acts as a pressure forming device. When the barrier fluid enters the spiral groove, it is directed to the outer diameter where it is subjected to the resistance of the sealing dam. The increase in pressure allows the end face of the flexible mounting to be tilted to adjust the seal gap. A "pump-throttle" principle works to create a non-contact mode of operation while allowing the pumped liquid to enter the high pressure zone from the low pressure zone. The force that controls 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 of the end face. If the seal gap is reduced due to interference, the force in the liquid film is significantly increased. Also, if the seal gap is increased, the force in the liquid film is reduced. In both cases, the original gap will soon be restored. Unlike a simple static pressure seal, the upstream pumping seal has both dynamic and static pressures and is therefore related to both speed and pressure. The static pressure type seal forms a seal gap by pressure, and thus is independent of the rotational speed.

3. Magnetic fluid seal:

"Magnetic Fluid Seal" is a typical derivative technology from NASA's space program and has been in existence for more than 30 years. The initial development was used to propel rocket fuel under conditions of weight loss in outer space. This technique of using magnetic force to control fluids has been applied to the ground by engineers and technicians. The magnetic fluid consists mainly of three parts: a carrier fluid (usually a low vapor pressure hydrocarbon or fluorocarbon), a surfactant (a chemical binder) and magnetic particles (very small magnetite balls). . The surfactant encapsulates the magnetic particles into the carrier fluid to form a colloidal suspension, thereby producing a corresponding fluid magnetic property. In the past three decades, this simple sealing principle has been applied in numerous structures with numerous magnetic fluid seals in operation. It seals the shaft of the vacuum swing unit. Such devices are found in the semiconductor and vacuum industries, which rely on magnetic fluid seals to provide a 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 the magnetic fluid seal include a magnetic fluid, a magnet ring, two pole pieces, and a magnetically conductive shaft or sleeve. The magnetic circuit formed by the fixed pole piece and the rotating shaft concentrates the magnetic flux in the radial gap below 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 have the following inherent characteristics: no external power required; no contact, no wear; no leakage when the shaft is stationary or rotating; long and reliable service life; low torque and minimum energy consumption. Magnetic fluid seals also have a unique self-healing feature. When the excessive pressure passes through the magnetic seal, the excessively high pressure in the sealing area for a short time causes 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 sealed body, and when the disturbance state is over, , the seal is still re-formed at the original pressure.

The new auxiliary cartridge seal based on magnetic fluid seal technology designed for pumps is a cost-effective sealing method that pump manufacturers and users can replace magnetic drive pumps and complex double seal systems, and it is easy to use in existing The pump was refitted. Pumps sealed with this proven technology can meet the most stringent deposit control regulations. The Magnetic Fluid Cartridge Seal is an auxiliary secondary seal that combines with the main mechanical seal to form a volatile organics protection system. The "liquid O-ring" formed by the magnetic structure and the magnetic fluid prevents the main seal vapor from escaping, thereby forming a hermetic seal around the pump shaft.

After several years of production operation, pump users agree that magnetic fluid seals have the following advantages: simple reassembly improvements for existing pumps; zero leakage under both static and dynamic conditions; lower installation and use Cost; simple instrumentation, easy to monitor; refilling and measuring pressure in situ; low maintenance requirements; no barrier fluid or complex sealing aid system; no wear parts, significantly reduced maintenance.

The basic pump-type seal construction is designed to meet the general acceptance requirements of the entire pump industry. Cartridge seals can be used in a wider range of applications by incorporating optional cooling devices (for high temperature devices) or optional gas purification systems that protect the sealing components from environmental corrosion. This sealing technology also has the unique ability to accurately monitor the performance of the primary seal and to predict failures or identify problems before a serious accident occurs. Since the secondary magnetic fluid seal intercepts all of the main seal's vapor leaks, a simple flow meter accurately monitors the primary seal and provides an alarm signal when the leak is too large. New developments under study 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 leakage of steam and tiny liquids from the main seal can be closed by a secondary cartridge seal. This eliminates the flare or other discharge device and ultimately forms a closed VOC recovery system.

Magnetic fluid seal technology has long proven to be 100% effective for leaking leaks from sealed pumps. As the designers and users of the process industry gain a gradual understanding and understanding of this technology, the use of this sealing technology will continue to expand. It is likely to be a cost-effective way to meet the environmental sealing challenges of the future industry.

In China, due to the lack of environmental awareness of the Chinese people, inadequate environmental protection regulations, and insufficient enforcement, people generally pay insufficient attention to the problem of pump leakage. This has led domestic pump manufacturers to invest 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 seal devices is far from the international advanced level. With China's accession to the WTO and the strategic direction of giving priority to the development of environmental protection industry in the past five years, it is foreseeable 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.