New direction in the development of pump shaft sealing devices

Currently, on a global scale, the growing environmental awareness among governments and the public worldwide has led to the implementation of more comprehensive environmental regulations, which have had a significant impact on various industrial sectors that use pumps. In the United States, the 1990 Amendments to the Federal Clean Air Act require the Environmental Protection Agency (EPA) to impose more specific emission limits on 189 volatile organic compounds (VOCs) and 454 hazardous air pollutants (HAPs). Many liquids transported by process pumps in refineries and chemical plants are regarded as major sources of the aforementioned emissions. The issue of pump leakage has attracted increasing attention, and improving the shaft sealing devices of sealed pumps is a crucial approach to reducing pump leakage.

1. Pump Shaft Sealing Technology

In recent years, the global pump industry has achieved remarkable progress in the research and development of sealing technology, with significant advancements mainly made in three aspects: new sealing technologies, sealing products, and sealing systems.

As a traditional sealing structure for pumps, packing seals have seen a declining range of applications due to issues such as a certain degree of leakage and poor performance at high speeds. However, in recent years, with the gradual phase-out of asbestos ropes, new progress has been made in packing seal technology. For example, compressed packing seals are currently demonstrating strong vitality across the entire industrial sector in the UK. Such low-friction compressed packings, made from materials like synthetic fibers, ceramics, and polytetrafluoroethylene (PTFE), offer better chemical resistance, wear resistance, high-temperature resistance, and longer service life compared to asbestos packings, while remaining reasonably priced.

At present, mechanical seals occupy an increasingly large share of the pump sealing market, and various newly designed mechanical seals have greatly contributed to the growth of this marketshare. The most prominent among them is the cartridge seal, which is easy to install and maintain, as well as safe and reliable, though relatively expensive. Another notable type of mechanicalseal is the split seal, which can be replaced without disassembling the pump. Additionally, there are new structures such as thermohydrodynamic seals, backup seals, and asymmetric formed etal bellows seals. Furthermore, when used in conjunction with a flushing system, tandem seal and double mechanical seal structures prevent the leakage of hazardous substances and allow tetransportation of abrasive media. It is expected that the future development of mechanical seals will likely focus on the adoption of "upstream pumping seals" and "magnetic fluid seals" or similar technologies to enhance the functional integrity of sealing systems. An "upstream pumping seal" is a new concept that uses various seal face grooves to pump a small amount of leaked fluid from the low-pressure side (downstream) of the seal face back to the high-pressure side (upstream). A "magnetic fluid seal" is a new technology derived from aerospace industry projects, featuring zero leakage and no wear.

2. Upstream Pumping Seals

An upstream pumping seal is a new concept that uses various seal face grooves to pump a small amount of leaked fluid from the low-pressure side (downstream) of the seal face back to the high-pressure side (upstream). It is a hybrid structure between a tandem seal and a double mechanical seal. In terms of structure and working principle, it resembles a backup face seal structure with a low-pressure barrier fluid between the two faces. Functionally, it serves the purpose of a double mechanical seal. The pumping mechanism in the primary seal operates as follows: the pressure generated in the seal creates a weak pumping capacity from the low-pressure barrier zone to the high-pressure medium end. The conventional face primary seal is replaced by a small-capacity, high-pressure "pump"—the upstream pumping seal. This "pump" pushes a small amount of barrier fluid along the path typically sealed by a mechanical seal and delivers it into the seal chamber. Since the pressure in the seal chamber is higher than that of the barrier fluid, this seal is considered to perform "upstream pumping".

Upstream pumping seals are suitable for toxic and hazardous media, abrasive media and slurries, media with poor lubricity, and applications with high PV values. The term "PV" is a commonly used expression among seal manufacturers and users, referring to the pressure-velocity limit of a face material combination in a given fluid. High-speed pumps usually operate under high PV conditions; the use of double mechanical seals can exacerbate the situation because they require higher buffer fluid pressure. For a long time, the high-speed pump industry has been plagued by high PV values. Upstream pumping seals operate in a basically non-contact mode, thus completely eliminating the impact of PV values. As they do not require barrier fluid pressure higher than that of the seal chamber, upstream pumping seals provide an effective solution for the high-speed pump industry to overcome this challenge.

The principle of upstream pumping seals relies on the balance between hydrodynamic pressure and hydrostatic pressure. The seal end with a snap ring, spring, and primary seal ring is a stationary component, which is matched with a rotating seal ring featuring spiral grooves. The groove pattern consists of a series of recessed logarithmic spirals. The ungrooved portion on the outer circumference of the spiral grooves is called the "seal dam". When pressure is applied, hydrostatic pressure is generated on the seal face, and this force is produced whether the mating seal ring is stationary or rotating. Hydrodynamic pressure is only generated when the seal ring rotates. During rotation, the spiral grooves play a decisive role, acting as a pressure-generating device. When the barrier fluid enters the spiral grooves, it is guided to the outer diameter, where it encounters resistance from the seal dam. The increase in pressure allows the flexibly mounted face to move, thereby adjusting the seal clearance. A "pump-throttle valve" principle comes into play, enabling non-contact operation while transferring the pumped fluid from the low-pressure zone to the high-pressure zone.

The forces controlling the seal operation are axial. The opening force is the sum of the pressure generated by the spiral grooves and the pressure drop across the two sides of the face, while the closing force is the sum of the system pressure acting on the rear of the face and the spring force. If the seal clearance narrows due to interference, the force in the fluid film increases significantly; similarly, if the seal clearance widens, the force in the fluid film decreases. In both cases, the original clearance is quickly restored. Unlike pure hydrostatic seals, upstream pumping seals involve both hydrodynamic and hydrostatic pressures, thus being related to both rotational speed and pressure. In contrast, hydrostatic seals form a seal clearance through pressure and are therefore independent of rotational speed.

3. Magnetic Fluid Seals

Magnetic fluid seals are a typical derivative technology from NASA's space programs, with a history of over 30 years. Initially developed to propel rocket fuel under weightless conditions in outer space, this technology— which uses magnetic force to control fluids—has since been adapted for terrestrial applications by engineers. A magnetic fluid mainly consists of three components: a carrier fluid (usually a hydrocarbon or fluorocarbon with low vapor pressure), a surfactant (a chemical binder), and magnetic particles (tiny magnetite spheres). The surfactant binds the magnetic particles to the carrier fluid, forming a colloidal suspension that imparts magnetic properties to the fluid.

Over the past 30 years, this simple sealing principle has been applied in numerous structures, with countless magnetic fluid seals in operation. They can seal the shafts of vacuum rotating devices, which are widely used in the semiconductor and vacuum industries—sectors that rely on the zero-leakage and wear-free characteristics of magnetic fluid seals to ensure consistent quality. 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 type of seal include low 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 conductive shaft or sleeve. The magnetic circuit formed by the fixed pole pieces and the rotating shaft concentrates magnetic flux in the radial gap under the pole pieces according to polarity. When a magnetic fluid is added to the radial gap, it forms a "liquid O-ring" and creates a leak-tight seal around the shaft. All magnetic fluid seals possess the following inherent characteristics: no external power required; no contact and no wear; no leakage whether 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 excessive pressure acts on the magnetic seal, the temporary overpressure in the sealing area may cause a portion of the magnetic fluid to be momentarily displaced from around the shaft. However, during the overpressure period, the magnetic fluid remains confined within the seal body, and once the disturbance subsides, it re-forms the seal at the original pressure.

The new auxiliary cartridge seal designed specifically for pumps based on magnetic fluid seal technology is an economical and effective sealing alternative for pump manufacturers and users, replacing magnetic drive pumps and complex double seal systems. Moreover, it can be easily retrofitted to existing pumps. Pumps sealed with this fully validated technology can meet the most stringent emission control regulations. The magnetic fluid cartridge seal is an auxiliary secondary seal that, when combined with the primary mechanical seal, forms a VOC protection system.The "liquid O-ring" formed by the magnetic structure and magnetic fluid prevents the leakage of vapor from the primary seal, thereby creating a hermetic seal around the pump shaft.

After years of operational use, pump users consistently recognize the following advantages of magnetic fluid seals: simple retrofitting for existing pumps; zero leakage under both static and dynamic conditions; low installation and operating costs; simple instrumentation for easy monitoring; in-situ refilling and pressure measurement capabilities; low maintenance requirements; no need for barrier fluids or complex seal support systems; no wear parts, resulting in significantly reducedmaintenance.

The structure of the basic cartridge seal for pumps is designed in accordance with the general acceptance requirements of the entire pump industry. By incorporating optional cooling devices (for high-temperature applications) or optional gas purification systems (to protect seal components from environmental corrosion), cartridge seals can be applied in a wider range of fields. This sealingtechnology also features a unique ability to accurately monitor the performance of the primary seal and predict failures or identify issues before serious accidents occur. Since the secondary agnetic fluid seal intercepts all vapor leakage from the primary seal, a simple flowmeter can accurately monitor the primary seal and issue an alarm if leakage exceeds safe limits.

Ongoing research advancements include an integrated high-pressure fluid seal installed within the cartridge. This seal acts as a safety device in the event of a severe failure of the primary mechanical seal. Another research project aims to develop a unique method to return the vapor and tiny liquid leaks from the primary seal to the pump suction side, where they can be contained by the secondary cartridge seal. This would eliminate the need for flare systems or other discharge devices and ultimately form a closed VOC recovery system.

Magnetic fluid seal technology has long been proven 100% effective in sealing pump leakage emissions. As designers and users in the process industry gradually gain awareness and understanding of this technology, its application scope will continue to expand. It is highly likely to become an economical and effective solution for the industrial sector to meet future environmental sealing challenges.

In China, due to relatively low environmental awareness among the public, incomplete environmental regulations, and insufficient enforcement, the issue of pump leakage has generally not received adequate attention. As a result, domestic pump manufacturers have invested little in research on minimizing leakage in pumps (especially industrial pumps), leading to a significant gap between the R&D level of domestic pump shaft sealing devices and the international advanced level. With China's accession to the WTO and the recent proposal of the strategic policy to prioritize the development of the environmental protection industry over the past five years, it can be predicted that vigorously improving pump shaft sealing devices and reducing pump leakage will become a development trend for China's industrial pumps in the future.