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Process industry environmental protection scheme(differential pressure5)
Feb 29, 2020

2.8 fluid velocity

Flow rate is not a fluid property, but it can be used to predict the behavior of moving fluid and determine the application of differential pressure flowmeter. In a word, velocity is the rate of change of the position of the object relative to the reference object, that is to say, the velocity and direction of motion of the object are specified. In hydrodynamics, velocity defines the velocity of a fluid particle relative to a stationary reference, such as a pipe. When a fluid flows around an object or through a pipe, the viscosity of the fluid results in a velocity profile. If there is no viscosity, the fluid velocity in the cross section of the pipe is uniform. Even the minimum viscosity will cause the shear stress between adjacent fluid particles to produce uneven velocity profile in the pipe, in which the velocity on the pipe wall is zero and the velocity on the pipe centerline is the maximum, thus forming the flow.

The fluid flowing in the tube also defines the velocity field. People have done extensive research on the flow of the fluid in the pipe. Knowing the flow rate and the properties of the fluid can predict the flow field.

In other words, when the mass flow rate (mass unit / unit time) is divided by the density under the reference conditions, if the pressure and temperature conditions during the flow are adjusted to the reference conditions, the flow rate is equal to the volume occupied by the fluid. If the flow pressure is 146.96 psia, the temperature is 68 ° F, and the reference conditions are 14.696 psia and 68 ° F, the value will increase tenfold when the mass flow is converted to the standard volume flow (divided by the density of the fluid at the reference conditions). The mass remains constant, but in order for the pressure to remain at 14.696 psia, the volume occupied must be 10 times the flow condition.

For a given amount of liquid, the mass is constant, but the volume changes with pressure and temperature

Mass flow measurement is preferred for most gases and liquids, while volume flow is acceptable for stable liquids. The flowmeter suitable for mass flow includes multi parameter differential pressure flowmeter or Coriolis flowmeter. Volume flowmeter includes differential pressure flowmeter, turbine flowmeter, vortex flowmeter, electromagnetic flowmeter or variable section flowmeter.

2.10 isentropic index

When the gas flows through the limiting position in the tube, the density changes with the change of pressure. It is assumed that gas expansion is an isentropic process. For some types of flow restriction, the effect of density change on flow can be determined theoretically, while for other types of flow restriction, it can be determined empirically. The related fluid property is gas isentropic index (called K or G), which is mainly a function of temperature. It is common practice to determine K at nominal temperature and use this value for all flows. The typical range for K is between 1.0 and 1.4.

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