壅塞流(臨界流、阻塞流、窒息流、Choked Flow)簡介英文版教學文案

1、壅塞流(臨界流、阻塞流、窒息流、Choked Flow)簡介英文版2Restriction and Choked FlowRestrictionRestriction is mainly used to achieve controlled or restricted flow of process medium. Such as orifice, venturi tube, control valve, etc.Choked flowA flowing fluid can reach a limiting condition when passing through a restrictio

2、n, that the mass flow will not increase with a further decrease in the downstream pressure while at a given upstream pressure and temperature. This is called Choked Flow.Restriction and Choked flow in Chemical EngineeringqPositive applicationOne application of choked flow is in cracking reactors. Ma

3、intaining equal flow through parallel reactor tubes is essential to ensure even residence time in the cracking reactor. Installing a restriction orifice at the entrance of each reactor tube achieves choked flow. Provided that choked flow is maintained, flowrates are not affected by any variation in

4、reactor tube pressure drop.qNegative effectIt is usually desirable to avoid choking conditions in a liquid system, because it often accompanies cavitation, two-phase flow which can damage equipment.6Choked Flow in Gases - Mass Flow RateNote that the limited parameter in choked flow is the gas veloci

5、tyvelocity, and mass flow can be increased with increased upstream pressure (increased fluid density).When the gas velocity is choked, the equation for the mass flow rate is:F is mass flow rateC is discharge coefficientA is discharge hole cross-sectional areaP is absolute upstream pressure of the ga

6、sM is the molar mass of the gasK is Cp/Cv, the adiabatic indexR is the molar gas constantT is the absolute temperatureThe mass flow rate is primarily dependent on the cross-sectional areaarea A A and the upstream pressure Pupstream pressure P, and only weakly dependent on the temperature T. The rate

7、 does not depend on the downstream pressure at all. All other terms are constants or depend only on the composition of the material in the flow.7Choked Flow in LiquidsWhen the liquid flows through the restriction, there is a point downstream of the restriction called Vena ContractaVena Contracta. It

8、 is a place where the diameter of the stream is the least, and fluid velocity is at its maximum, pressure is at its minimum. If the liquid pressure at vena contracta is below that of the liquids vapor pressure at the prevailing liquid temperature, vapor cavities will form. When the vapor bubble form

9、ation in the restriction prevents the flow from increasing any further, then the liquid-vapor mixture flow reaches the limit condition, also called choked flow. An increase in pressure drop by decreasing the downstream pressure does not result in more flow.8Choked Flow in Liquids -Valve Max Allowabl

10、e Pressure DropIf choked flow occurs in a valve?pCalculate the max allowable pressure drop the max allowable pressure drop (Pmax) of this valvepCompare with the actual service pressure differential (P = P1 P2)pIf Pmax is less than P, choked flow will exist under the service conditions specified.We c

11、an obtain Pmax from the following relationships:where,FL is the valve recovery factor, can be found in the below tables.FF is the liquid critical pressure ratio factor.P1 is the upstream pressureP2 is the downstream pressurePv is the vapor pressure at inlet temperaturePc is the thermodynamic critica

12、l pressure Type of valve FLCage-trim globesUnbalanced0.8Balanced0.7Butterflyfishtail0.43Conventional0.55BallVee-ball, modified-ball, etc.0.4Full-area ball0.3Conventional globesingle and double port(full port)0.75single and double port(reduced port)0.65Three way0.75AngleFlow tends to open (standard b

13、ody)0.85Flow tends to close (standard body)0.5Flow tends to close (venturi outlet)0.2CamflexFlow tends to close0.729Choked Flow in Liquids -Cavitation and FlashingOnce it is known that choked flow conditions will develop within the specified valve design (Pmax is calculated to be less than P), a fur

14、ther distinction can be made to determine whether the choked flow is caused by cavitation or flashing.CavitationIf downstream pressure recovery is sufficient to raise the outlet pressure above the vapor pressure of the liquid, the bubbles will collapse, or implode, producing cavitation. Collapsing o

15、f the vapor bubbles releases energy and produces a noise similar to what one would expect if gravel were flowing through the valve. If the bubbles collapse in close proximity to solid surfaces in the valve, the energy released will gradually tear away the material leaving a rough, cinderlike surface

16、. Cavitation damage may extend to the adjacent downstream pipeline, if that is where pressure recovery occurs and the bubbles collapse. FlashingIf pressure at the valve outlet remains below the vapor pressure of the liquid, the bubbles will remain in the downstream system and the process is said to

17、have flashed. Flashing can produce serious erosion damage to the valve trim parts and is characterized by a smooth, polished appearance of the eroded surface. Flashing damage is normally greatest at the point of highest velocity, which is usually at or near the seat line of the valve plug and seat r

18、ing.Pv10Choked Flow in Liquids -Eliminate or minimize the damage of CavitationSelect the type of the valve that can split the total pressure drop across the valve into smaller portions, such as using multiplestage trims.Select the valve with hard materials.Change the system by raising pressure P2, s

19、uch as moving the valve to a location that has more static head.11SummaryChoked Flow in GasesqGas velocity across restriction in the flow path become SONIC SONIC i.e. at a MachMach number of 1qAs a rule of thumb, the Critical Pressure Critical Pressure Ratio 0.53Ratio 0.53 is applicable to a wide range of common gases.qThe mass flow rate is primarily depe