Analysis of Instantaneous Collision Process of Axial Flow Check Valve Closing

When the pump stops suddenly, the axial flow check valve will shut down suddenly under the action of the medium pressure differential force, and the impact load between the disc and the valve seat will produce when the valve closes, which will have a certain impact on the structural strength and life of the check valve. In this paper, the display dynamics is used to simulate the impact process of the disc and the valve seat when the valve is completely closed. The dynamic response of the disc and the seat, the velocity of the disc and the energy change during the collision are obtained. The research has certain guiding significance for the practical application of axial flow check valve.


Introduction
Axial-flow check valve is an automatic valve that depends on the pressure difference of medium to realize opening and closing. When the pump stops suddenly, the check valve will act quickly under the pressure difference of medium itself to prevent the driving equipment in front of the valve from reversing due to the backflow of medium. It is widely used in long-distance pipeline system [1] . In case of emergency, the medium in the pipeline behind the valve flows back quickly, which makes the pressure behind the valve greater than the pressure before the valve, so that the disc moves towards the closing direction. with the increase of the pressure difference of the medium, the speed of the disc reaches the maximum at the moment of closing, colliding with the sealing surface of the valve seat, causing some damage to the sealing surface and reducing the service life of the check valve. In severe cases, the whole pipeline system can not run normally [2] . Sibilla S etc. used dynamic grid technology to simulate the opening and closing process of axial flow check valve, and obtained that the parameters such as disc size and spring stiffness have important influence on its dynamic response [3] ; YAMASHITA Akihiko etc. studied the characteristics of rapid braking and closing of the check valve when the pump stops suddenly, and obtained that the impact acceleration will be generated when the valve clack hits the valve seat and the water hammer caused by low pressure [4] . Junfeng Wang studies the impact force between the impact hammer and the impact table in the impact testing machine, and simulates the impact moment with LS-DYNA software, and obtains the dynamic response characteristics and laws of the whole system [5] ; Weiqun Xu analyzed the dynamic response characteristics of simply supported beam under arbitrary impact load, taking Dengzhou Bridge as an example, ANSYS/LS-DYNA software was used to simulate and analyze the collision moment between automobile and bridge, and the stress-strain law during collision and the deformation  [6] . It is difficult for researchers to measure the impact directly because of its short action time and high destructiveness. In this paper, the impact process between the valve disc and the valve seat at the moment when the axial flow check valve is completely closed when the pump stops suddenly is studied by using the explicit dynamic numerical simulation method.

the basic concept of impact movement
Impact is a process in which the impact load instantly acts on the system in millisecond time, causing sudden changes in the stress and motion state of the system. Impact load is an external load that changes arbitrarily with time, and impact process is a series of dynamic changes of the system with time. Impact dynamics mainly studies the change of stress wave and the dynamic response of each component in the system. The research in this paper belongs to the problem of system dynamic response.
Non-persistent collision is the effect of system constraints in the collision process, and once the collision ends, the constraints are automatically eliminated. For this kind of problems, the dynamic response results of the system after collision are related to the motion state of the system before collision, the recovery coefficient and collision impulse in the collision contact process, this paper studies this kind of collision problem [7] .

Explicit dynamics
According to the classical mechanics theory, the dynamic equation is: In which: is the model quality matrix; is the model damping matrix; [ ] K is the model stiffness matrix; is the load vector; is the infinitesimal displacement vector; is the infinitesimal velocity vector and is the infinitesimal acceleration vector. Explicit solution method, also known as closed solution method, is calculated by matrix multiplication in each step, which does not need balanced iteration, has fast solution speed, and occupies little memory. When the time step is small enough, convergence problem generally does not occur, so it is suitable for solving transient collision problems [8] .
ANSYS LS-DYNA is a world-famous software for solving nonlinear dynamics, which can simulate and solve various complex problems, such as collision and explosion of various nonlinear structures, and has a variety of contact calculation models, good parallel solving ability, grid adaptive ability and etc. [9] .

Establish a numerical model
The 3d model of the assembly is established by Solidworks software, as shown in fig.1, the check valve is mainly composed of seven parts as shown in the figure. The inlet and outlet diameters of the valve body are 252 mm, the total length of the cavity body is 622 mm, the front and rear lengths of the valve seat are 156 mm and 462 mm respectively, and the medium in the valve is water. Under normal working conditions, the medium goes in and out from the left, and the pressure in the valve is high on the left and low on the right. When the pump is stopped, the high-pressure medium flows backwards, so that the valve clack impacts the valve seat to generate impact load during the closing process.
Meshing platform is used to mesh the model in ANSYS Workbench, the grid diagram of check valve collision calculation is shown in Fig.2, with 22,322 nodes and 93,480 units.

Dynamic response results of valve body
It can be seen from Fig.3 and Fig.4 that the stress mainly occurs at the sealing surface of the valve seat because the disc collides with the sealing surface of the valve seat at the moment of closing. At t=0.5 ms, the stress on the valve body reaches the maximum 289.53 MPa. At t=0.5 ms, the disc is separated from the sealing surface of the valve seat, the collision end, and the stress of the valve body drops rapidly to 16.874 MPa. In the period after reaching ms, the stress and strain value of the valve body changed little, and

Dynamic response results of valve clack
According to fig.5 and fig.6, when t=0.25ms occurs, it is distributed in a 120 array around the sealing surface of the disc, and the stress of the disc reaches the maximum 155.68MPa. At t=0.5ms, 1.75ms and 2ms, the corresponding maximum stresses are 64.405MPa, 62.409MPa and 53.424MPa in turn. The occurrence position of the maximum stress continuously moves from the sealing surface to the center of the disc, and then from the center of the disc to the sealing surface of the disc, which is the result that stress waves are propagated, reflected and unloaded many times in a short time.  It can be seen from fig.7, At t=0.5 ms, the collision is completed, the valve disc is no longer in contact with the valve seat. The valve disc bounces back due to the collision obstruction, because of the energy loss during the collision, the speed of the valve disc changes from the initial speed of -3.624 m/s to 3.11 m/s. After 0.5 ms, the speed remains unchanged at 3.11 m/s, because the change law of the medium force on the right side of the valve disc has not been obtained, so numerical simulation is conducted.  6. Conclusion 1)Through numerical simulation of the dynamic response of the whole collision process between the valve disc and the valve seat, the stress of the valve body and the valve disc reaches the maximum value at t=0.25 ms, and the whole collision process lasts for 0.5ms.

system energy absorption and conversion
2) The velocity of clack after collision is 3.11m/s through numerical simulation of display dynamics.
3) In the process of collision, due to the action of damping and friction, there is energy conversion in the collision system, which is about 1.3J, and the energy conversion is irreversible.
In a word, the research in this paper has certain guiding significance for the practical application of axial-flow check valve when the pump is stopped.