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This post was last edited by goldliyang on 2019-10-26 at 14:57. In the design of pipeline systems, in addition to carrying out necessary design checks for static loads, it is also necessary to take into account the impact loads resulting from the instability of the medium. Here, why do we emphasize impact load rather than just load? Because only unbalanced impact loads can generate a corresponding dynamic response. A balanced static load does not produce this shock effect. As shown in the figure below, each section of pipe in the piping system may be subjected to potential impact loads. The impact load generated by the fluid exists along the axis of the pipe and acts at points where the direction of the pipe changes. In the system below, the impact force acts on the axes of pipes 2-3 and 3-4; these are also the softest areas and positions in this piping system, where the likelihood of damage is highest. Since the flow of the medium along the centerline in the pipe is generally considered a one-dimensional system, the impact load of the medium on the pipe segment is also treated as one-dimensional. As shown in the figure below, if the centerline of the pipe segment is in the X direction, then all the relevant loads in the X direction can be expressed using the following formula: m represents the mass of the pipe segment, while C and K represent its damping and stiffness respectively. dV represents the volume element, ρ is the density of the medium inside the pipe segment at that point, m_d represents the mass of that element, and p_d represents its momentum. By taking the derivative with respect to volume across the entire pipe segment, we obtain the corresponding expression. Integrating this expression gives the total momentum of the medium inside the pipe segment; taking its derivative with respect to time yields the corresponding impact load. In steady-state flow, the velocity, density, and mass of the medium remain constant; as a result, the integral terms above become zero, yielding the vibration equation we discussed earlier. At this time, the pipe segment itself does not experience impact vibration due to the flow of the medium. The flow velocity of the medium is affected by the displacement, velocity, and acceleration of the pipeline, with the vibrations of the medium interacting with those of the pipeline structure. However, in pipeline calculations, it is generally assumed that the fluid flows within a stationary pipe, and the thermohydraulic analysis of the fluid does not take into account the effects of the movement of the pipe itself. (Additional knowledge: Mass flow rate refers to the mass of fluid that passes through the effective cross-section of a closed pipe or an open channel per unit of time.) Corresponding to the volume flow rate (the volume of fluid that passes through in a given time), it can be expressed as the product of the volume flow rate and the fluid density. Volumetric flow rate is equal to flow velocity multiplied by the cross-sectional area through which the medium flows. ) After analyzing the fluid state, the fluid loads acting on the pipeline are calculated as shown in the figure below. Since sections 2-3 are in the Z direction, the fluid load exists only in this Z direction; this is a one-dimensional fluid state that is widely recognized by pipeline engineers. Thermohydraulic analysis typically provides parameters such as pressure, temperature, flow velocity, and density; therefore, these boundary conditions are used to simulate the impact load on the pipe section. Here, there are mainly two forces acting on the elbows at both ends of the pipe segment, as well as the total friction force between the fluid and the pipe wall. Pressure thrust and momentum force. The calculations for these two types of forces are as follows: Pressure thrust: Momentum force (mass flow rate multiplied by flow velocity): Friction force: It is the frictional shear stress per unit area; L is the length of the pipe section, and D is the diameter of the inner surface cross-section of the pipe. The momentum force acts on the elbows of the pipeline, and the internal pressure thrust does the same. So the tube end thrust is the sum of these two. For 2–3 pipe sections, assuming that the fluid flows from 2 to 3, the impact force generated can be calculated using the following formula: Since only transient flow generates impact loads, in thermal-hydraulic analysis, only the fluctuations from the average flow rate are typically taken into account, without considering all the terms in the aforementioned equation. Frictional force is usually ignored because it results from the overall flow rate. When dealing with traveling pressure waves, momentum is also often neglected. Having covered these basic concepts, we will now begin to discuss open and closed safety valves, as well as the methods for calculating the trip load of steam turbines.