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Research on the Pressure Compensation for the Underwater underwater ISSN 0141 0814. International Journal of the Society for Underwater Technology, Vol 26, No 3, pp ??-??, 2005 TECHNOLOGY Research on the pressure compensation for the underwater hydraulic motor Technical Paper Technical Y LI and Q WANG The State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University, Hangzhou, PR China Abstract The properties of UAP Seawater can be considered incompressible at a moder- Considering the influence of underwater ambient pres- ate depth (down to about one thousand meters). The sure (UAP) on underwater hydraulic motors (UHMs), and pressure at the moderate depth h below the water sur- utilising the state-of-the-art of pressure compensation of face is underwater hydraulic systems (UHS), this paper propos- 1 ⎡ ⎛ v ⎞ 2 ⎤ ρ 2 ⎢ ⎥ es a pressure compensation technique for the UHS psea =p a + gh+ V1-⎜ ⎟ (1) 2ρ ⎣⎢ ⎝ V ⎠ ⎦⎥ where the hydraulic power unit is installed inside a deep diving submersible or a submersible drilling platform with where Psea is the ambient hydrostatic pressure Pa is the atmospheric pressure, some actuators of which are atmospheric pressure, ρ is the mass density, g the gravi- directly surrounded by seawater and others are installed tational acceleration, h the depth of the point, v the inside it. An underwater ambient-pressure-compensation local velocity, and V the velocity of the body through valve (UAPCV) has been developed. The pressure in the the water. return line of the external sub-circuits of the UHS is com- The first two items on the right of equation (1) is a pensated by this UAPCV, but not that of the internal sub- seawater static pressure. Moreover, there exists a seawa- circuits. Theoretical analysis, simulation and experimental ter dynamic pressure, which is the last item on the left results show that reliable pressure compensation can be of equation (1). ensured with a small flowrate by the UAPCV. The opera- The ocean currents result from the superposition of tional performance of the UHM is further improved after a great number of disturbances or external forces. its leakage pressure and return pressure are compensat- Basically they include atmospheric-pressure variations, ed. wind forces acting on the free surface, seismic distur- bances acting on the bottom of the sea and the Keywords: Underwater hydraulic system, pressure combined attraction of the moon and sun acting on the compensated valve, underwater ambient pressure seawater.1 When the undercurrent dynamic variation acts on the surface of underwater equipments, the 1. Introduction undercurrent flow variation is turned into pressure vari- ation acting on the external surface of underwater Underwater vehicles, such as a remotely operated vehicle equipments. (ROV) or autonomous underwater vehicle (AUV), and The speed of underwater equipments, such as underwater construction machinery, such as submarine descent and ascent, causes a hydrodynamic pressure. cable trenchers, deep diving submersibles, manned sub- The vertical speed causes a variation of seawater depth. mersibles, submersible drilling platform etc, play an The external ambient pressure of equipment changes important role in ocean exploitation research. Hydraulic with seawater depth; the horizontal speed of underwa- systems, due to their remarkable advantages, are widely ter equipment creates a hydrodynamic pressure because used to control these underwater equipments. When of a relative kinematical velocity to the seawater. The working in a certain depth, the underwater ambient pres- UAP variation affects a UHM in many respects when sure (UAP) will considerably affect the performance of the UHM is underwater. some hydraulic control components and actuators of an underwater hydraulic system (UHS). Hence, one must 2. Influence of the UAP on UHM take into account the influence of the UAP on the hydraulic components and try to reduce or remove its Influence on output torque of UHM influence by adopting some suitable techniques. The The operating schematic of a radial piston hydraulic main goal of this paper is to explore and describe an motor is shown in Fig 1.2 The UAP directly acts on an available pressure compensation method for some special external end face of a shaft 6 when it is working in the sea- underwater hydraulic motors. water environment. The other end face of shaft 6 is in 51 Li & Wang. Research on the pressure compensation for the underwater hydraulic motor sary that some suitable methods must be adopted in order that conventional hydraulic motors can directly be used in the sea- water pressure environment. Necessity of the ambient pressure compensation Traditionally, a method to solve the above problems is adopting Fig 1: Operating schematic of radial piston hydraulic motor pressure compensation for underwater hydraulic systems. housing 1 and 2. The internal chamber of the housing is That is to say, a UAP is added to hydraulic systems. connected with a leakage line or a return line of an Pressure in the leakage line of UHM and the return line underwater hydraulic system. The pressure of the inter- of UHS is slightly larger than the UAP by a constant. nal end face of shaft 6 is approximately atmospheric. Balance between the internal and external pressure of Shaft 6 is subject to an unbalanced axial force. The unbal- the UHS can be reached after being compensated. Like anced axial force acts on its main bearings as well. The this, the unbalanced axial force caused by the UAP can unbalanced axial force is variable with the seawater be removed. Any elastic deformations of the UHM depth, which causes an increase of the output torque. The shell will disappear; those sealing components prevent- output torque caused by the UAP reduces the mechanical ing contamination from external leakage can work nor- efficiency of the hydraulic motor, leads to an increased mally as well. requirement for input pressure, increases internal leakage On the other hand, using a cam lobe hydraulic and reduces volumetric efficiency. Additionally, when the motor as an example, the volume leakage consists of seawater depth is deep enough, the UAP can affect main three parts; the leakage in the clearance of the port bearing life and even the UHM life. plate, the external leakage in the clearance between pis- tons and the cylinder hole and the volume loss caused by Influence on pressure proof capacity of the the elastic compression of the fluid at the bottom of pis- UHM shell tons. Each parameter related with the structure of the The inner chamber of a conventional hydraulic motor hydraulic motor cannot change with the operating envi- is connected with the leakage line or the return line of ronment after the type of a hydraulic motor has been hydraulic systems. The hydraulic motor shell can only determined. These above-mentioned volume losses are suffer from a lower pressure than ambient hydrostatic only related to the supply pressure and the return pres- pressure. So the hydraulic motor effectively runs in a sure of the hydraulic motor.3 The volume loss caused by shallower depth. the UAP can be removed or greatly reduced if the pres- A mechanical element shaped as a vessel, such as a sure difference between the input port and the output hydraulic motor, hydraulic cylinder, control valve and so port of the UHM can be kept constant or only slightly on, can inevitably suffer an elastic deformation due to the changed after the pressure has been compensated. variation of the external UAP. The holding force of some connecting pieces can become larger or smaller due to State-of-the-art research the elastic deformation. Bolts of the shell perhaps may be Hydraulic systems of underwater machinery can be loose and lead to failure of the underwater hydraulic classified into two kinds according to their layout. In the component and seawater invasion into the UHS. first type, the whole hydraulic system is directly under- water, such as hydraulic systems of underwater vehicles Influence on sealing components (ROV/AUV), underwater working tools, underwater Some conventional hydraulic sealing components are construction machinery, such as a subsea cable trencher, used to act in only one direction. The function of the and so on (shown in Fig 2(a)). The second type is that seal is to leak-proof in one direction. A UHM is subject the hydraulic power unit is installed in the atmospheric to bi-directional pressure when it is underwater. These circumstance, some hydraulic actuators of the system sealing components will lose their functionality when are surrounded by seawater, and other actuators are in the UAP is bigger than the inner pressure, and the sea- the same environment as the hydraulic power unit; for water may invade the hydraulic system and contami- example, external hydraulic sub-circuits of manned nate the fluid. submersibles or submersible drilling platforms, etc, The UAP can affect efficiency, mechanical perform- (shown in Fig 2 (b)). ance and sealing components of the UHM to some Presently many researchers have been carrying out extent as illustrated by the above analysis. So it is neces- research on conventional pressure compensators. A blad- 52 underwater TECHNOLOGY Vol 26, No 3, 2005 hydraulic system utilises seawater as its pressure medium, which is com- patible with the oceanic working environment. This type of system can use direct seawater suction and dis- charge. The pressure of the hydraulic system is automatically compensated, which is suitable for any seawater depth. The system structure is sim- pler, has less weight and a higher effi- ciency. Response frequency and posi- Fig 2: Layout of underwater hydraulic systems tion control precision are largely enhanced but the physicochemical property of seawa- der-type pressure compensator can be installed directly on ter is different to mineral oil. The properties of seawa- the UHP reservoir to compensate for pressure, where the ter are lower viscosity, poor lubricating ability, higher hydraulic power unit is directly in the underwater pressure conductance, higher vaporisation pressure, and so on.
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