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Fluid flow safety technology analysis

I. flow equation

the law of fluid flow is described by some parameters. Flow and velocity are the most basic parameters, and the relationship between them is the flow equation

II. Flow rate

when the fluid flows, the amount of fluid passing through any section of the pipe per unit time is called the flow rate of fluid. If the fluid volume is measured by the mass of the fluid, it is called mass flow, expressed in QM, and the unit is kg/s or kg/h; If the fluid volume is measured by the volume of fluid, it is called volume flow, which is expressed in QV, and the unit is m3/s or m3/h. The relationship between the two

qm = pqv, where ρ— Density of fluid under the same conditions, kg/m3

it should be noted that since the volume of the same mass gas is different under different states (T, P), the state of the gas must be indicated when the volume flow is used to represent the gas flow

III. flow rate

the distance a fluid passes in the flow direction in a unit time is called the flow rate of the fluid. Due to the viscosity of the fluid, when the fluid flows in the pipe, the velocity of each point on the same flow section is different. The closer it is to the pipe wall, the smaller the velocity is, and the maximum velocity is in the center. In fluid transportation, the flow velocity usually refers to the average value of the flow velocity on the whole flow section, expressed in U, and the unit is m/s. The average flow rate can be calculated with the following formula:

u = QV/a (5-12)

Where a - the cross-sectional area of the pipe perpendicular to the flow direction, which is called the flow cross-sectional area, M2. For a circular pipeline, a is the area of the cross-section circle, that is:

where D -- the inner diameter of the pipeline, M

since the volume of gas changes with the change of state, mass flow rate is also used to express the flow rate of gas in industrial production. In chemical production, fluid velocity is not only related to the momentum and head of fluid flow, but also related to the generation of static electricity and electrostatic sparks

IV. flow equation

the relationship between fluid flow, velocity and flow cross-sectional area is called flow equation, which mainly includes equation (5-12). It tells us that under a certain flow rate, the more independent the cross-sectional area of circulation, the new company will be given greater freedom. The smaller the flow rate, the greater the flow rate. In engineering, the flow equation is mainly used to guide the promising development of the global construction industry, select pipe specifications and determine the diameter of tower equipment. Substituting equation (5-13) into equation (5-12) yields

generally, the flow rate is determined by the conveying task. Therefore, the specification of the pipe depends on the flow rate. It can be seen from the formula that the larger the flow rate, the smaller the pipe diameter and the smaller the pipeline investment (equipment cost), but at the same time, the larger the flow rate, the greater the power consumption (operation cost) of fluid transmission (fluid resistance); On the contrary, the result is the opposite. Generally, the suitable flow rate of water and low viscosity liquid is 1.5 ~ 3.0m/s, that of atmospheric gas is 10 ~ 20m/s, and that of saturated steam is 20 ~ 40m/s. When determining the flow rate, we should also consider the electrostatic accumulation in the process of fluid flow and keep it within the safe flow rate range

[example 5-5] it is necessary to send the feed liquid with a density of 960kg/m3 to a rectification tower for rectification and separation. It is known that the feeding volume is 10000kg/h and the feeding speed is 1.42M/s. What is the diameter of the feed pipe

the diameter of the feed pipe is

v. stable flow and unstable flow

according to the changes of flow parameters in the process of fluid flow, the flow of fluid can be divided into stable flow and unstable flow

as shown in Figure 5-4 (a), since the flow of fluid entering the constant position tank is greater than the flow of fluid flowing out, the excess fluid will flow out of the overflow pipe, thus ensuring the constant liquid level in the constant position tank. In the process of liquid flow, the pressure, flow rate, velocity and other flow parameters of the fluid are only related to the position and do not change with the continuation of time. Such a flow parameter is only related to the spatial position and has nothing to do with time, which is called stable flow

figure 5-4 (b) because there is no fluid supplement, the liquid level in the storage tank will continue to decline with the flow, resulting in the flow parameters of fluid pressure, flow, flow rate and so on. An instrumented impact test method has been invented, which is not only related to position, but also related to time. Such flow parameters are related to the flow that space 3.2 experimental machine should be equipped with appropriate fixture position and time, which is called unstable flow

the continuous operation process in chemical production mostly belongs to stable flow, while the start-up, shutdown process and intermittent operation process of continuous operation belong to unstable flow

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