Studies on leakage characteristics and efficiency of a fully-rotary valve energy recovery device by CFD simulation
Introduction
Water scarcity has become one of the most serious global crises for overwhelming population growth, industrialization and contamination [1]. Desalination is a realistic and promising technology among all the possible measures to alleviate the stresses on water supply [2], [3]. Seawater reverse osmosis (SWRO) develops rapidly and has become a leading desalination technology [4], [5], [6], [7]. As a water purification process by membrane, SWRO is energy-intensive in principle [8]. However, the rapid development of SWRO benefits greatly from isobaric energy recovery devices (ERDs) that could significantly reduce the specific energy consumption from 8 kWh/m3 to 2 kWh/m3 for desalination plants [9], [10]. Isobaric ERDs based on positive displacement principle can recover energy that is otherwise wasted directly from high pressure brine (HPB) to pressurize feed low-pressure seawater (LPS) [11]. In SWRO desalination plants, operation pressure usually ranges from 5.5 to 6.8 MPa and the HPB pressure is just 0.2 to 0.3 MPa lower than the operation pressure [12], [13], [14]. However, the feed LPS pressure is very low and usually ranges from 0.1 MPa to 0.3 MPa which is far lower than the operation pressure [13], [15]. That is to say, isobaric ERDs have to operate under large pressure difference all the time. Therefore, leakage occurs inevitably under the pressure difference from high pressure portion to low pressure portion within these ERDs. Leakage is inevitable and always an unwanted phenomenon since leakage reduces recyclable HPB flow rate and thus leads to a reduction in efficiency and a waste of energy. Moreover, severe leakage will cause the failure of the energy recovery process for isobaric ERDs, because the high pressure portion and the low pressure portion could not be maintained separately if leaking is too severe. Furthermore, leaking does harm to the long-term ERD performance due to local cavitations produced by leakage [15]. Consequently, great efforts have been taken to deal with leakage problems since the appearing of isobaric ERDs.
Various sealing structures have been designed for different isobaric ERDs to reduce leakage and improve ERD performance. Leakage should be limited strictly to obtain required ERD performance and the accepted leakage ratio (the ratio of leakage rate to ERD treatment capacity) is usually around 2% in application [13], [15], [16]. For example, as the commercially representative products, Double Work Exchanger (DWEER), SalTec DT and PX pressure exchanger (PX) have been used in SWRO plants worldwide for last several decades and all the three types of ERDs adopt different sealing structures. Coaxial pistons within LinX valve which is the core equipment of DWEER reciprocate in the cylinder [17]. Seals by accurate mating between coaxial pistons and the cylinder are used, and the leakage ratio of DWEER is about 1.8% [13]. A rotor rotates in the rotating valve which is a key device of SalTec DT. Tightly radial clearances between rotor and casing provide required seal performance and the leakage ratio is about 2.2% [16], [18]. For PX, a rotor rotates in a sleeve with high speed and the axial clearances between the rotor and end covers have to be well designed to obtain a leakage ratio of 1.7% [13], [19]. New types of sealing structures have also been proposed by researchers. Song developed a reciprocating fluid-switcher ERD which was based on unique open/close plate design to improve seal performance and the leakage ratio was lower than 1.7% [20]. Wang introduced a single-cylinder ERD with sealing plates which could achieve seal by internal flow pressure difference of the ERD and the measured leakage ratio was about 2% [21]. Wu investigated a rotary ERD which is similar to PX, then made textured grooves on the surfaces of two end covers to improve dynamical seal performance. Results indicated that leakage rate of the textured device was half smaller than that of the un-textured one [22].
Notably, all the above studies are conducted by experiments which show comprehensive ERD performance besides leakage characteristics. Therefore, it is necessary to investigate the leakage characteristics of the specific ERDs with novel sealing structures. Compared with experimental research which is limited by costs, machining and so on, the numerical simulation can not only do systematic and detailed analyses on leakage characteristics, but also lay the foundation for ERD design. However, few open literatures about leakage studies of isobaric ERDs by numerical simulation can be found. Qi et al. investigated the leakage characteristics of a fluid switcher that is the key device of a new ERD with different leakage clearances and HPB pressures by numerical simulation. Their results showed that leakage rate presented a linear relationship with HPB pressure and a polynomial relationship with leakage clearance. Moreover, the ERD energy recovery efficiency was also discussed based on the simulation results [23]. However, the effects of geometric parameters of the fluid switcher on leakage were not studied except the leakage clearance. What's more, the leakage of check valve nests which are essential to that ERD is not considered causing an over-estimated efficiency. In our earlier work, a new type of isobaric ERD named fully-rotary valve energy recovery device (FRV-ERD) with novel sealing structures was developed and the hydraulic performance was tested [24]. In order to understand and control the leakage, the leakage characteristics of fully-rotary valve (FRV) that is the key device of the FRV-ERD are studied numerically. The influences of pressure difference and all geometric parameters of the FRV sealing structure (leakage clearance height, radius of FRV inlet/outlet, diameter and length) are investigated systematically. In addition, the FRV-ERD performance and required clearance height under different conditions are also presented and discussed.
Section snippets
Principle of the FRV-ERD
As shown in Fig. 1, FRV-ERD consists of only two identical FRVs and connecting pipelines. As the key device of the FRV-ERD, the FRV is composed of a rotor and a stator. The rotor mainly composed of two symmetric semi-cylinders can rotate inside the stator as shown in Fig. 2 which illustrates the structure of FRV1. Two cavities that are formed after the rotor and the stator are assembled together are connected with two connecting pipelines. There exist two operation phases alternating
CFD model
Fig. 3 shows a three-dimensional geometric model of the sealing structure. The shape of the geometric model is like a curving cuboid with a fairly small height. All the geometric parameters of the sealing structure have been marked in Fig. 3 including length L, rotator diameter D, radius of FRV inlet/outlet Ri and clearance height h. The design parameters are as follows: L = 120 mm, D = 100 mm and Ri = 13 mm, and to study its effects, the clearance height h is set at 0.010 mm, 0.015 mm, 0.020 mm, 0.025 mm
Theoretical verification
A theoretical verification has been conducted to verify the reliability of the CFD simulation results. To do so, a laminar flow between two stationary parallel circular discs with a known analytical solution has been solved by the above-stated numerical model. As shown in Fig. 5, the model of two stationary parallel circular discs is an extreme case of the model stated in this paper. The analytical leakage formula for the flow shown in Fig. 5 is described by Eq. (3). The corresponding
Pressure contours of sealing structure
In order to investigate the pressure distribution in the sealing structure comprehensively, pressure contours of the sealing structure with the designed parameters including forward leakage and reverse leakage are studied. For simplicity, only the results of the clearance height of 0.030 mm are provided. The inlet pressure is set at 6.1 MPa and the outlet pressure is 0.1 MPa. As shown in Fig. 7, the red color refers to high pressure section while the blue color represents low pressure section. The
Conclusions
Leakage characteristics which directly affect the efficiency are of great importance for isobaric ERDs. In order to provide theoretical foundation for FRV-ERD design, leakage characteristics, efficiency and ERD performance for the increased treatment capacity are studied and discussed.
The results show that leakage rates of forward and reverse leakage for the designed sealing structure with the same clearance height are almost identical and present a good linear relation with pressure
Acknowledgements
This research was supported by the 14th science and technology foundation of Beijing University of Technology (No. ykj-2015-12187) and the Beijing Municipal Science & Technology Plan Project (No. Z111100058911006).
References (25)
- et al.
Novel design and operational control of integrated ultrafiltration — reverse osmosis system with RO concentrate backwash
Desalination
(2016) - et al.
Energy-efficient reverse osmosis desalination: effect of retentate recycle and pump and energy recovery device efficiencies
Desalination
(2015) - et al.
Analysis of the influence of the configuration in ERD retrofitin two-stage SWRO trains
J. Membr. Sci.
(2016) - et al.
State-of-the-art of reverse osmosis desalination
Desalination
(2007) Seawater reverse osmosis with isobaric energy recovery devices
Desalination
(2007)- et al.
SWRO core hydraulic system: extension of the SalTec DT to higher flows and lower energy consumption
Desalination
(2007) - et al.
SWRO core hydraulic module — the right concept decides in terms of energy consumption and reliability part II. Advanced pressure exchanger design
Desalination
(2004) - et al.
SWRO with ERI's PX pressure exchanger device — a global survey
Desalination
(2008) - et al.
Functionality test of an innovative single-cylinder energy recovery device for SWRO desalination system
Desalination
(2016) - et al.
Employing groove-textured surface to improve operational performance of rotary energy recovery device in membrane desalination system
Desalination
(2015)
Theoretical investigation on internal leakage and its effect on the efficiency of fluid switcher-energy recovery device for reverse osmosis desalting plant
Chin. J. Chem. Eng.
Development and experimental studies on a fully-rotary valve energy recovery device for SWRO desalination system
Desalination
Cited by (18)
Groove parameters optimization of rotary excitation control valve using computational fluid dynamics coupled with response surface method
2023, Ain Shams Engineering JournalNumerical and experimental research on the integrated energy recovery and pressure boost device for seawater reverse osmosis desalination system
2022, DesalinationCitation Excerpt :Fortunately, seawater reverse osmosis (SWRO) desalination was confirmed to be a successfully and preferred technology for extracting pure water from seawater or brackish water due to its high efficiency, modularity, and easy maintenance compared to other desalination technologies [1–5]. The energy recovery device (ERD) is an indispensable component of the SWRO systems and can significantly decrease the consumption of energy by recovering the pressure energy from the rejected high-pressure brine [6–10]. The rotary energy recovery device (RERD) is a category of isobaric ERD following the positive displacement principle, which could directly transfer energy from brine under high pressure to seawater under low pressure [11,12].
Development and operational stability evaluation of new three-cylinder energy recovery device for SWRO desalination system
2021, DesalinationCitation Excerpt :Recently seawater reverse osmosis (SWRO) desalination has developed into a mainstream technology that can alleviate water crisis [5–7]. One of the main reasons for the rapid development of SWRO technology is the appearance of isobaric energy recovery device (ERD), which can efficiently recover the hydraulic energy in high pressure (HP) brine and significantly reduce the energy consumption of desalination system [8–10]. Although isobaric ERD can reduce the energy consumption of desalination system, their flowrate and pressure stability are also crucial to the stable and safe operation of SWRO system.
Sealing reliability modeling of aviation seal based on interval uncertainty method and multidimensional response surface
2019, Chinese Journal of AeronauticsCitation Excerpt :Yan et al.11 deduced the calculation formula of the seal gap and calculated the value of leakage rate under certain operating conditions. Zhou et al.4 and Liu et al.12 analyzed the pressure difference on both sides of the O-ring, and the influence of the geometrical parameters on sealing performance by simulation and numerical methods, respectively. Liang et al.13 and Hu et al.14 studied the variation of sealing performance under the combined effect of rubber material properties and geometry of the sealing structure.