Application status and analysis of mechanical seal technology in kerosene pump

Yu-jun LI, Bo-ran DU, Ji-hai JIANG*

(1Diving and Salvage Department, Navy Submarine Academy, Qingdao 266042, China) (2School of Mechanical and Electrical Engineering, Harbin Institute of Technology, Harbin 150080, China)

Abstract: The kerosene pump is an important component of the aerospace equipment, and the mechanical seal technology is the key technology to ensure the normal and reliable operation of the kerosene pump. This paper summarizes the development status of the mechanical seal technology in the field of low-viscosity liquid hydraulic pumps represented by kerosene pumps. First of all, the article introduces the basic principle of mechanical seal technology; next, from two aspects of foreign and domestic respectively, discusses the mechanical seal method used in the kerosene pump and the main products; then, according to the special working conditions of the kerosene pump, the corresponding key technology of mechanical seal is summarized; finally, the application of mechanical seal technology in the field of kerosene pump is analyzed and summarized.

Key words: Kerosene pump, Mechanical seal, Friction characteristics

1 Introduction

Under the industrial background of the 21st century, various industries gradually present cross development and the hydraulic system has been widely used in aerospace equipment. In order to meet the requirements of “low cost, high reliability, non-toxic and non-polluting” in aerospace, the engine fuel controller uses a high-pressure kerosene pumped directly from the engine’s turbopump as the working medium for the kerosene hydraulic pump, thus composing a power source of servo mechanism[1] The executive part of the hydraulic system of the servo mechanism controls the adjustable nozzle in the drive nozzle of the hydraulic cylinder, so that the engine can obtain good performance in adjusting the launching attitude of the rocket. To prevent high-pressure kerosene from leaking, fuel pumps often use mechanical seal as rotary dynamic seal equipment[2]. Hydraulic pump as the power source of the entire hydraulic system, its related design parameters directly affect the operational performance of hydraulic system and maintenance frequency[3]. Compared with traditional hydraulic oil, taking into account kerosene’s lower dynamic viscosity and poor lubrication, each friction pair in the piston pump is often in a poor operating state with boundary friction or even dry friction. The mechanical seal pair acts as the only equipment which prevents the leakage of medium in the entire plunger pump cavity, the operating state of its end is related to the reliability and working life of the plunger pump. The typical mechanical seal structure and its application in a kerosene pump are shown in Fig.1. As shown in Fig.1(a), the static ring is in a stationary state under the position of the anti-rotation pin, which is an un-compensated ring; and the dynamic ring is moved with the rotation shaft under the static torque of the thrust ring and the O-ring, which is a compensated ring. Both will occur friction and wear, cooperating to become sealing pair. Compared with Fig.1(a), the mechanical seal of the kerosene pump adopts a static structure, as shown in Fig.1(b); the axial dimension of the structure is shorter, and the static ring is a compensation ring and the dynamic ring is an un-compensated ring. Therefore, there must be differences in the mechanical analysis. At the same time, the working medium is kerosene, which deteriorates the actual lubrication environment, and both the end-face structure and the material should be improved.

1.Spring seat；2.Spring；3.Anti.rotation pin of the thrust ring；4.Dynamic ring；5.Static O.ring；6.Anti.rotation pin of the static ring；7.End cup；8.Static ring；9.End cup O.ring；10. Dynamic O.ring；11. Thrust ring；12. Drive screw；13.Shell；14.Spindle

Fig.1 Structure comparison of typical mechanical seal and shaft end mechanical seal of kerosene pump

2 Foreign research status

Before the 21st century, the development level of thepsvvalue to measure the mechanical seal technical indicators[4-5] is shown in Fig.2.

Fig.2 The international development history of the mechanical sealpsvvalue

After entering the 21st century, thepsvvalue of mechanical seals has been developed to be applicable to most related fields, exploring more structural types and improving machining accuracy has become the development direction of mechanical seals in the new century.

German company Burgmann[6-7] and British company John Crane[8-9] have international influence in the mechanical seal industry. The applicable shaft diameter of Burgmann’s products is between 6 and 200 mm; the applicable pressure is between 1 and 8 MPa; the applicable line speed is 35 m/s or less. Fig.3 shows three pusher-type mechanical seals applied to low-viscosity medium. The EK700 series in Fig.3(a) can be operated in gasoline; the LB500 series in Fig.3(b) can be operated in an aqueous medium, which has already applied in sea water pumps; the H75VN series of Fig.3(c) has good performance in light oils. John Crane has developed new types of seals, such as dry gas seals, rubber bellows seals, and secondary throttling seals, and has developed pusher-type seals for pump that can operate in clean water media.

Fig.3 Mechanical seal applied in low-viscosity medium from Germany company Burgmann

Faalleev SV[10] from Samara State Aerospace University in Russia established the elastic force equation of the sealed end face and experimentally validated the theoretical model. Czeslaw Kundera from Sumy State University in Ukraine[11] established the force control equations of the spring and O-rings acted on the sealing end face and analyzed the influence of stiffness and damping on the end face stress. Kielce University of Technology[12-13] studied the heat transformer of hydrodynamic-pressure oil film, the mathematical model was established using Fourier-Bessel series, and the temperature distribution over the seal ring was obtained. The University of Perugia[14] used remote sensing technology to measure the temperature of the seal face. The Georgia Institute of Technology[15-18] used ultrasonic devices to calculate the liquid film thickness and monitor the rupture process in 1999. In 2001, a liquid film thickness detection device was developed using a shear wave sensor. The team of Noёl Brunetiere[19-24] from Université de Poitiers in France conducted a study on the evaporation of liquid film between mechanical seal rings in 2015, choosing water as the medium. The experimental device and liquid film model are shown in Fig.4. The experimental results show that once the temperature exceeds a certain critical value, the mechanical seal ring will affect the service life due to cavitation.

Fig.4 Mechanical Seal Test Stand and Liquid Film Research Model, University de Poitiers, France

Hamada Akira[25] proposed a non-dimensional bearing characteristic coefficientμvb/ps, and calculated the critical parameters that generate the difference in lubrication; afterwards Hirao Hiroshi[26] conducted a study on thef-Gcharacteristics of the oil seal material in gasoline medium. V. A. Levin et al from Russia[27] used the Sutherland equation to reveal the dependence of gas on temperature and solved the problem of heat dissipation from gas flowing through porous media.

For the study of the friction and wear characteristics of the sealed end face, the three major research directions in foreign countries are the exploration of new materials, the research of the end face coating and the end face texture organization. Jun Tomioka of Waseda University[28-29] also studied the mechanical seal performance of the blood pump for medical use, the experimental apparatus is shown in Fig.5, using a cold water circulating flushing system. The experiment was conducted by studying the leakage of the system under three kinds of sealing ring roughness, whenRa=0.009 μm, 0.088 μm, and 0.17 μm, it is believed that the leakage under the blood medium decreases with the decrease of the roughness. Argonne National Laboratory[30-31] conducted test experiments on the coating of mechanical seal rings for pumps. The experimental device is shown in Fig.6. In this experiment, ultranano-crystalline diamond (UNCD) was coated on the SiC ring surface by microwave plasma chemical vapor deposition. After that, material-matching tests of graphite-filmless SiC and graphite-plated UNCD film SiC were performed simultaneously. By comparing the wear behavior of the two under the electron microscope, it is believed that the UNCD film can greatly increase the wear resistance.

Fig.5 Blood pump seal research device of Waseda University

Fig.6 Mechanical seal test stand of Argonne National Laboratory in America

3 Domestic research status

China’s mechanical seal industry started late. In the 1960s, with the rise of high-parameter pumps in the chemical industry, mechanical seals could be tested. After more than 30 years of vigorous development, the industry has basically rooted in China’s manufacturing industry. In the embryonic stage of the development of the mechanical seal industry, many leading companies emerged in China. Fig.7(a) is a Sichuan Riji[32] CM1B series mechanical seal, applicable to systems below 3 MPa and speeds up to 3 600 r/min. The seal is a multilevel series seal and has good sealing performance for light hydrocarbon media and is widely used in the coal chemical industry. Fig.7(b) is the C8 series mechanical seal of Dandong Clone Group. This series is suitable for water, hydrocarbons, medium and low viscosity oils. It can be applied to the environment of pressure up to 5 MPa, line speed up to 25 m/s, and the shaft diameter can be set to 14～100 mm. The series of pumps have good seal compensation performance, uniform end load, simple installation and easy adjustment. Fig.7(c) shows the Lantian N74 type mechanical seal for pump. This type of pump is suitable for working conditions with a pressure of 1.6 MPa and a line speed of 15 m/s or less and this type of single-ended, unbalanced pump is suitable for water, light oil and other medium, which are used in wastewater treatment and pesticide production systems.

In terms of theoretical research on the characteristics of sealed end surfaces in China, universities and research institutes have undertaken major research projects. Li Shuangxi from Beijing University of Chemical Technology[33] used finite element analysis to analyze the relationship between the mechanical seal stress level of the end face and the radial position for the study of the heat transfer characteristics of the sealed end face. Wang Zongtai[34] from Beijing University of Chemical Technology, on the basis of the thermal structure coupling algorithm, the temperature distribution of the end face was simulated and the influence law of heat from different sources in the sealed cavity was calculated. Peng Xudong’s team from Zhejiang University of Technology[35-38] analyzed the effect of inner taper, face groove type, thermal elastic deformation of the mechanical seal on the leakage, and the mechanism of thermal elasto-hydrodynamics widely existing between the end faces was studied. The research results were widely used in hydrostatic pressure seals of nuclear main pumps. Wang Wendong from Shanghai Institute of Materials Research[39] conducted research on the friction and wear of high-strength graphite and special alloy steels, and analyzed the influence of environmental factors. Sun Jiapeng from Harbin Institute of Technology[40] studied the friction performance of carbon graphite rings in a vacuum environment. The carbon graphite master test stand is shown in Fig.8, and a soft ring material M120B with good properties was obtained; Zhao Cunran[41] later used MMU-10 fiction wear test machine to study the friction properties of commonly used friction pair materials at different temperatures. Li Jianpeng of Donghua University[42] used a vacuum sintering furnace to prepare hard-ring commonly used material WC-Co, as shown in Fig.9, and conducted friction and wear tests to measure the wear rate and surface morphology.

Fig.7 Domestic mechanical seal product of low viscosity light hydrocarbon medium

Fig.8 Carbon graphite test stand of Harbin Institute of Technology

Fig.9 WC-Co vacuum sintering furnace of Donghua University

4 Key technology for mechanical sealing vice ends

(1) Modeling of key face features

Although the structure of the mechanical sealing vice end is relatively simple, special mechanics and functions make it involved in many disciplines such as elasticity, fluid mechanics, thermodynamics, and tribology. Key and important characteristics are chosen to conduct research and adopting appropriate theoretical equations modeling will be helpful to the smooth progress of the solution process.

(2) End face leakage check

The mechanical seal is reasonable in its force, the end face is closely attached, and the leakage in practical use is often negligible. However, considering the kerosene as the sealing medium, the dynamic viscosity is in the same magnitude order as that of pure water, which results in the greater amount of oil permeation under the same gap. Combined with the deformation effect of the end face forces and the effect of thermal deformation, the size of the face gap continuously changes in the radial direction, which makes it particularly important to check the leakage.

(3) The selection and pairing of sealing pair materials

Many domestic and foreign literatures have shown that the material selection of the friction pair is one of the most important factors to ensure the “zero leakage” and “low power consumption” of the mechanical seal. The low viscosity characteristics of the kerosene medium make it difficult to establish an oil film between the friction pairs. Choosing a material with high heat dissipation capacity and good self-lubricating ability will improve the operating conditions and improve the service life and reliability of the device, and the friction and wear test is a good way to find good materials.

(4) Selection of end face test structure

According to different indicators, mechanical seals are divided into various types. For example, the medium flow method can be divided into internal flow type and outflow type; and the relative motion relationship between soft and hard rings can be divided into rotary type and static type; the stress state of sealing ring can be divided into balanced and unbalanced, etc. The above various forms of application are different, each has its own advantages and disadvantages. The selection of the mechanical seal structure type will ultimately determine whether the test results can meet the technical requirements. Therefore, according to the working conditions and operating parameters of the kerosene pump, it should be cautious to reasonably determine the best using type.

5 Conclusion

In summary, there are many key points worthy of special attention in the research process of the mechanical seal characteristics. Only by breaking these key points one by one, the characteristics of the sealing end face can be fully analyzed and ensure that the designed mechanical seal device is suitable for coal, achieving optimal working conditions. Comparing the current situation of foreign and domestic research, it is found that the R&D focus and research priorities at home and abroad are different. At the same time, objectively speaking, the strength of foreign products and scientific research is obviously higher than that of domestic ones. It is reflected in two aspects:

First, in terms of products, foreign research started early and the manufacturing industry is more developed. Therefore, the big companies headed by Burgmann, etc., are in a leading position in the industry, with a wide variety of products, diverse application conditions, and excellent performance and quality. The development of domestic products is more than half a century later than that of foreign countries. Therefore, at the initial stage of domestic industry development, it will inevitably be influenced by foreign products. So far, many manufacturers’ products have imitated Burgmann’s and other companies, but enterprises such as Sichuan Riji and Dandong Clones have independently produced high-quality products and have great potential for development;

Secondly, in terms of scientific research, the experimental equipment abroad is excellent and complete. The research of the end face characteristics focuses on the on-line status detection of end surfaces and the research and development of micro-structure on the end face. It is mainly based on experimental analysis, supplemented by theoretical simulation, and has high reference value. Domestic scientific research is often accompanied by engineering projects to solve technical problems for research purposes, which makes it less investment in theory than abroad, experimental equipment is relatively simple compared with foreign countries, and ideal experimental conditions are usually difficult to obtain. Domestic research is usually based on simulation, followed by experimentation, and the credibility and reference significance of the research results are general. Only by improving experimental conditions and increasing awareness of innovation, the scientific research environment can be improved in the field of mechanical seals.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No.51775131), National Science and technology support program(No.2014BAF08B06), National Science and technology support program(No.2015BAF07B05).