THE DYNAMIC CHARACTERISTICS OF A DIRECT-ACTING WATER HYDRAULIC RELIEF VALVE WITH DOUBLE DAMPING: NUMERICAL AND EXPERIMENTAL INVESTIGATION Yipan DENG * Yinshui LIU * Defa WU * Hui LI * * State Key Laboratory of Digital Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China (E-mail: liuwater@hust,edu.cn) Abstract. Relief valve plays an important role in a water hydraulic system such as maintaining the pressure constant and preventing the system from over-pressured. Unstable operation or chatter in the system can be initiated from unreasonable dynamic characteristics of the relief valve, which will cause unnecessary loss and reduce operating efficiency in the water hydraulic system. In this paper, a direct-acting water hydraulic relief valve with double damping was designed and analyzed. The double damping consists of a damping orifice on the main spool and a damping annular gap between the valve sleeve and the main spool. Furthermore, an AMESim model of the relief valve was established to analyze the influence of the structure parameter on the dynamic characteristics and optimized parameters were obtained for the design. Based on the optimized parameters, a relief valve was manufactured and assembled. Experimental investigation were performed on the relief valve for simulation results verifying and further optimization. An improved performance was obtained when the damping orifice diameter was 0.6mm compared to 0.4mm, which exhibit a lower pressure overshot and less transition processing time. Keywords: Direct-acting relief valve, Water hydraulic, Double damping, Dynamic characteristics, AMESim INTRODUTION As one of the most important valve elements in a high pressure water pipe system, a water hydraulic relief valve can maintain the pressure constant or prevent pressure from rising too high to create security risks in the pipe. The dynamic characteristics of the relief valve, in conjunction with other components are, therefore, critical in designing a sophisticated water hydraulic system. Unexpected phenomenon will be triggered by an improper dynamic response of the valve, such as cavitation, vibration, etc. Furthermore, such phenomenon can initiate the failure of the pipe and the components. The dynamic characteristics of a water hydraulic relief valve can be mainly evaluated by responding time (t 1 ), unloading time (t 3 ), transition processing time (t 2 ) and pressure overshot ( p ). Generally, the pressure relief valve with good response characteristics should be featured with low value of responding time (t 1 ) (usually less than 100ms), unloading time (t 3 ), transition processing time (t 2 ). Additionally, criterion are applied to limiting the pressure overshot ( p ), which indicate that the quotient value of p and p n should not exceed 30%.[1,2] p p p n B C D 0.1(pn-p0) 0.9(pn-p0) 0.95(pn-p0) 1.05(pn-p0) A E p 0 t 1 t 2 t 3 0 t FIGURE 1. Dynamic response characteristics of water hydraulic relief valve. 1
There are two types of pressure control valves commercially available: direct and pilot type. The direct-acting relief valve exhibits a simple structure and high reliability is widely used in current water hydraulic systems. However, the dynamic response performance of the direct type are poor than the pilot type, which can induce vibration and noise into the water hydraulic system. More measures are taken to solve these disadvantages such as: optimizing the structure (silencing grooves, etc.) of the valve seat to improving the throttling effect [3,4], adding damping structure to the main spool, etc. The damping structure with an annular gap has been successfully applied into the relief valve which made by Danfoss [5,6]. However, practical applications have revealed that some severe vibration and noise still existed when the system pressure was high. More optimizing measurement should be taken to solve such disadvantages. [7-9] In the paper, a direct-acting relief valve with double damping was designed. Numerical analysis with an AMESim model were undertook to optimize the structure to gain better dynamic response characteristics. Based on optimized parameters, the relief valve was designed and manufactured, subsequent experimental investigation were conducted to test the practical dynamic performance, as a verifying and further optimizing means of the valve. INTRODUTION WORKING PRINCIPLE OF THE DIRECT-ACTING WATER HYDRAULIC RELIEF VALVE WITH DOUBLE DAMPING Figure 2 illustrates the working principle of a direct-acting water hydraulic relief valve with double damping. The rated pressure and flow of the designed relief valve are 14 MPa and 120 L/min, respectively. In Figure 2, a main spool with a damping orifice is installed in the valve body, the gap formed between the main spool and the valve sleeve has a damping effect. The combined effect of the damping orifice and the damping annular gap exert great influence on the dynamic response characteristics of the water hydraulic relief valve. Meanwhile, a damping chamber is connected to the outlet by the damping orifice and the damping annular gap. The force of the compressed spring maintains the return of the main spool, which could ensure the normal closing of the valve. The predetermined pressure can be adjusted by revolving the pressure adjusting handle. If the inlet pressure or system pressure exceeds the predetermined pressure of the main spring, the main spool will be pushed left to open the valve port and water will overflow through the outlet to maintain the decrease the inlet pressure. When the inlet pressure drops to a value under the predetermined pressure, the main spool will be pushed back to close the valve. During the fast open and close period of the valve, the main spool is in reciprocating motion and the amplitude decreases gradually until it is at rest. In this dynamic transition process, when the main spool moves left, the damping chamber is instantly filled with low-pressure water from the outlet, thus, the speed of the main spool increases and the pressure overshoot in inlet decreases. However, when the main spool moves right, the water in the damping chamber is discharged by a very small flow rate to the outlet through the damping orifice and the damping annular gap. As a result, the pressure in the damping chamber will rise and the motion damping of the main spool will increase. The rapid stabilization of the main spool can be achieved. Under this steady-state condition of flow, the pressures are equal in both the damping chamber and outlet, and the inlet pressure remains constant. In Figure 2, the crucial structural parameters is marked, d m and d s represent the diameter of the main spool and the valve seat, respectively. The damping parameters are characterized by the diameter (d) of the orifice, the distance (δ) of the annular gap and the contact length (L) for the main spool. The conical half-angle of the main spool is symbolized by α. FIGURE 2. Working principle of direct-acting water hydraulic relief valve with double damping. 2
NUMERICAL SIMULATION AND RESULT ANALYSIS Many structure parameters will influence the dynamic response characteristics of the water hydraulic relief valve, such as the mass of the main spool, the spring stiffness, the damping parameters, etc. The AMESim method is applied to analyze the influence of the structure parameters, furthermore, to optimize the parameters to gain better dynamic response characteristics of the relief valve. The AMESim is one kind of high-powered software for model building, simulation and dynamic analysis. This software uses model building method based on the graphical physics model. The AMESim model of direct-acting water hydraulic relief valve with double damping was built in this study, as described in Figure 3. In the model, components named BAP12 BAF01 BAP026 BAP041 MAS005 and BAP016 are connected successively to set up the simulation system. Among these, BAP026 (poppet with sharp edge seat) and BAP041 (poppet with no seat) are classified as valve port component, MAS005 (mass with friction and ideal end stops) is one kind of mass component, meanwhile, BAP12 (piston) and BAP016 (piston with spring) will play the role as a damping chamber and spring chamber, respectively. According to the structure and performance parameters of the relief valve, the simulation parameters were selected and are shown in Table 1. The parameters that need to be explored to show the effects on the dynamic characteristics of the relief valve are list below. 1) the spring stiffness (K), 2) the mass of the main spool (m), 3) the damping orifice diameter (d) and the damping annular gap (δ) The result of numerical simulation are shown below TABLE 1. AMESim model input parameters Items Value Unit Density of water 1000 kg/m 3 Bulk modulus of water 2000 MPa Dynamic viscosity of water 1.01 cp Diameter of main spool (dm) 25 mm contact length (L) 19 mm conical half-angle of the main spool 45 Degree Mass of the main spool 0.179 kg BAP12 BAF01 BAP026 BAP041 MAS005 BAP016 FIGURE 3. AMESim model of direct-acting water hydraulic relief valve with double damping. Effect of Spring Stiffness (K) on the Dynamic Response of Relief Valve The spring is an important part of the relief valve and the stiffness of which will impact both the static and dynamic characteristics of the valve. In the simulation process, the same pre-compression force was ensured while the spring stiffness and compressed length varied. As shown in Figure 4, with the increase of the stiffness, it was found that the transition processing time (t 2 ) shortened and the steady inlet pressure increased, which 3
showed better properties. With the combination of simulation result and design index such as volume and mass, the final determined value of the spring stiffness was 127.9 N/mm. Effect of Mass of Main Spool (m) on the Dynamic Response of Relief Valve The inertia of the main spool will increase with the grow of the mass, which will reduce the responsiveness of the main spool and finally decrease the dynamic response of the valve. As shown in Figure 5, the pressure overshot is larger when m is set up to 0.18 kg than a value of 0.12 kg. Additionally, the transition processing time increases with the grow of the mass. It was suggested that optimization design should be done to reduce the mass of the main spool. Effect of Damping Orifice Diameter (d) and Damping Annular Gap (δ) on the Dynamic Response of Relief Valve Figure 6 illustrates the effect of damping orifice diameter (d) on the dynamic response of the relief valve. A value of damping annular gap was set up to 0.04mm in this case. It can inferred that the diameter of the damping orifice can influence the pressure overshot and the transition processing time significantly. The pressure overshot increases as the damping effect strengthened, which can exacerbate the vibration of the main spool and undermine the dynamic response of the relief valve. Figure 7 illustrates the effect of damping annular gap (δ) on the dynamic response of the relief valve. In this case, the diameter value of the damping orifice was set up to 0.4mm while the annular gap varied. Simulation results turns out to be in accordance with what described in Figure 6. On the basis of the simulation result and with the consideration of the actual manufacturing and assembling, it was recommended that the ranges of the damping orifice diameter should be 0.4mm~0.6mm while the damping annular gap be 0.04mm~0.06mm. Subsequent experimental investigation were carried out to verify the simulation results. FIGURE 4. Dynamic response of relief valve with different spring stiffness (K). FIGURE 5. Dynamic response of relief valve with different mass of main spool (m). 4
FIGURE 6. Dynamic response of relief valve with different damping orifice diameter (d). FIGURE 7. Dynamic response of relief valve with different damping annular gap (δ). EXPERIMENTAL INVESTIGATION AND RESULT ANALYSIS On the basis of parameter calculation and simulation results listed in former chapter, a direct-acting relief valve with double damping was designed and manufactured, as shown in Figure 8. A testing system was designed to measure the dynamic characteristics of the relief valve, as shown in Figure 9. Two kinds of main spool with different damping orifice diameter (0.4mm and 0.6mm) were tested, and the value of steady pressure was set up to 12MPa and 14MPa, respectively. The test results are shown below. Figure 10 to Figure 13 elaborate the dynamic response of the relief valve under the condition that the steady inlet pressure was set as 14MPa. FIGURE 8. Photograph of direct-acting water hydraulic relief valve with double damping 5
6-1 7 6-2 2 M 3 4 5 8 1 FIGURE 9. Dynamic characteristic testing system for the relief valve 1- Filter 2- high pressure pump 3- valve 4- solenoid valve 5- throttle valve 6- pressure gauge 7- tested valves 8- flowmeter 9- tank It can be deduced from the step-up curve of the relief valve as shown in Fig.10 and Fig. 12 that the diameter of the main spool has little influence on the response time. The response time are 68ms and 67ms when the value of the d are 0.4mm and 0.6mm, respectively. However, the size of the damping orifice will exert great influence on the transition processing time. The transition processing time of the relief valve is 786ms when d is 0.4mm, but it shortens to 181ms as d increases to 0.6mm. Besides, the pressure overshot increases with the narrowing of the damping orifice diameter. Additionally, as shown in Figure 11 and Figure 13, the unloading time will prolong as the damping orifice diameter narrows. All data are classified and summed in Table 2. 9 FIGURE 10. Step-up curve of the relief valve at 14MPa when d=0.4mm. FIGURE 11. Step-down curve of the relief valve at 14MPa when d=0.4mm. 6
FIGURE 12. Step-up curve of the relief valve at 14MPa when d=0.6mm. FIGURE 13. Step-down curve of the relief valve at 14MPa when d=0.6mm. TABLE 2. Dynamic characteristics of the relief valve when the steady inlet pressure is 14MPa. d(mm) t1(ms) t3(ms) t2(ms) p(mpa) p pn (%) 0.4 68 54 786 3.56 25.43 0.6 67 67 181 2.7 19.29 In another experiment investigation when the steady inlet pressure was set as 12MPa, the results turns out to in coincide with previous conclusions, as shown from Figure 14 to Figure 17. The transition processing time of the relief valve is 979ms when d is 0.4mm, but it shortens to 264ms as d increases to 0.6mm. The pressure overshot increase from 2.7MPa to 3.37MPa as d narrows from 0.6mm to 0.4mm. All data are classified and summed in Table 3. The results of the experimental investigation is conform to what described in previous studies (doc). Since the dynamic response transfer function of the relief valve contains a second-order oscillation link. The presence of damping reduces the frequency response of the main spool, which will affect the dynamic characteristics of the valve. The damping effect strength as the damping orifice diameter decreases, so the transition processing time and the pressure overshot will increase. TABLE 3. Dynamic characteristics of the relief valve when the steady inlet pressure is 12MPa. d(mm) t1(ms) t3(ms) t2(ms) p(mpa) p pn (%) 0.4 64 45 979 3.37 28.08 0.6 63 64 264 2.89 24.08 7
FIGURE 14. Step-up curve of the relief valve at 12MPa when d=0.4mm. FIGURE 15. Step-down curve of the relief valve at 12MPa when d=0.4mm. FIGURE 16. Step-up curve of the relief valve at 12MPa when d=0.6mm. FIGURE 17. Step-down curve of the relief valve at 12MPa when d=0.6mm. 8
CONCLUSION Double damping structure was introduced into the water hydraulic pressure relief valve to improve its dynamic characteristics for preventing unstable operation or chatter in a water hydraulic system. The double damping were made up with a damping orifice on the main spool and a damping annular gap between the valve sleeve and the main spool. An AMESim model of the relief valve was established to analyze the influence of the structure parameters on the dynamic characteristics. The parameter included the spring stiffness (K), the mass of the main spool (m), the damping orifice diameter (d) and the damping annular gap (δ). Optimized parameter s value ranges were obtained for the design. Experimental investigation were carried out to analyze the dynamic characteristics of the manufactured relief valve. ACKNOWLEDGMENTS This research was supported by the financial support from the National Natural Science Foundation of China (No. 51575200 and N0. 51509097), and China Postdoctoral Science Foundation (No. 2014M560609 and No. 2015T80804 ). REFERENCES 1. Luo X, He X, Cao S, et al. Theoretical and experimental analysis of a one-stage water hydraulic relief valve with a oneway damper[j]. Journal of Pressure Vessel Technology, 2013, 135(6): 061210. 2. Dasgupta K, Karmakar R. Modelling and dynamics of single-stage pressure relief valve with directional damping[j]. Simulation Modelling Practice & Theory, 2002, 10(1 2):51-67. 3. Licskó G, Champneys A, Hos C. Dynamical analysis of a hydraulic pressure relief valve[c]//proceedings of the World Congress on Engineering. 2009, 2: 1-3. 4. He X, He H, Liu Y, et al. Numerical simulation on the dynamic characteristics of a two-stage water hydraulic relief valve[j]. Jixie Gongcheng Xuebao(Chinese Journal of Mechanical Engineering), 2006, 42(1): 75-80. 5. Maiti R, Saha R, Watton J. The static and dynamic characteristics of a pressure relief valve with a proportional solenoidcontrolled pilot stage[j]. Proceedings of the Institution of Mechanical Engineers, Part I: Journal of Systems and Control Engineering, 2002, 216(2): 143-156. 6. Taketani S, Sawa Y, Masai T, et al. A novel technique for cardiopulmonary bypass using vacuum system for venous drainage with pressure relief valve: an experimental study[j]. Artificial organs, 1998, 22(4): 337-341. 7. Dasgupta K, Karmakar R. Dynamic analysis of pilot operated pressure relief valve[j]. Simulation Modelling Practice and Theory, 2002, 10(1): 35-49. 8. Darby R. The dynamic response of pressure relief valves in vapor or gas service, part I: Mathematical model[j]. Journal of Loss Prevention in the Process Industries, 2013, 26(6): 1262-1268. 9. Aldeeb A A, Darby R, Arndt S. The dynamic response of pressure relief valves in vapor or gas service. Part II: Experimental investigation[j]. Journal of Loss Prevention in the Process Industries, 2014, 31: 127-132. 9