Choke flow

The exit Mach number Mo may not exceed unity Mo = 1 corresponds to choked flow sonic conditions may exist only at the pipe exit. The mass velocity G in the charts is the choked mass flux for an isentropic nozzle given by Eq. (6-118). For a pipe of finite length. 

Equation (26-95) applies for subsonic as well as choked flow. Choked flow occurs at the pressure ratio qg = q, which maximizes G.ori. To maximize differentiate Eq. (26-95) and set ... 

Choked Flow by Two-Phase Energy Balance From the energy balance, Eq. (26-88), taking= 0 (stagnation) and ( = 0 ... 

For gas-phase choked flow, the pressure ratio at the vena contracta is ... 

C /d of vaive when undergoing critical (choked) flow conditions. 

In order to avoid the need to measure velocity head, the loop piping must be sized to have a velocity pressure less than 5% of the static pressure. Flow conditions at the required overload capacity should be checked for critical pressure drop to ensure that valves are adequately sized. For ease of control, the loop gas cooler is usually placed downstream of the discharge throttle valve. Care should be taken to check that choke flow will not occur in the cooler tubes. Another cause of concern is cooler heat capacity and/or cooling water approach temperature. A check of these items, especially with regard to expected ambient condi-... 

Critical and Subcritical Flow - The maximum vapor flow through a restriction, such as the nozzle or orifice of a pressure relief valve, will occur when conditions are such that the velocity through the smallest cross-sectional flow area equals the speed of sound in that vapor. This condition is referred to as "critical flow" or "choked flow . 

Application of this procedure to inadvertently ignited safety valve discharges can involve a special problem. Certain combinations of pressure ratio and length of safety valve riser can result in choked flow, with a pressure discontinuity at the exit. The pressure of the jet then adjusts to atmospheric pressure in a system of shock waves or expansion waves over a distance of a few pipe diameters. These waves can affect the local mixing of the jet with the crosswind. Since the calculation procedure incorporates correlations for subsonic jets, it cannot be expected to be entirely accurate in this case. Nevertheless, since the wave system... 

EXP AC analyzes an interconnected network of building rooms and ventilation systems. A lumped-parameter formulation is used that includes the effects of inertial and choking flow in rapid gas transienl.s. The latest version is specifically suited to calculation of the detailed effects of explosions in the far field using a parametric representation of the explosive event. A material transport capability models the effects of convection, depletion, entrainment, and filtration of... 

Leung, J. C., 1986, A Generalized Correlation for One-Component Homogenous Equilibrium Flashing Choked Flow, AIChE Journal U32U, 10, pp 1743-1746. 

A stonewall or choked flow condition occurs when sonic velocity is reached at the exit of a compressor wheel. When this point is reached, flow through the compressor cannot be increased even with further... 

Figure 10-15. Graphic illustration of a "stonewall/ or a choked flow condition.

Tube Rupture. It is common for a heat exhanger to have a high-pressure fluid in the tubes and a lower-pressure rated shell. If there is a break in one of the tubes, the higher pressure fluid will leak to the shell, resulting in overpressure. It is conservative to assume a tube is completely split with choked flow from both sides of the break. 

The flow of a compressible fluid through an orifice is limited by critical flow. Critical flow is also referred to as choked flow, sonic flow, or Mach 1. It can occur at a restriction in a line such as a relief valve orifice or a choke, where piping goes from a small branch into a larger header, where pipe size increases, or at the vent tip. The maximum flow occurs at... 

There are two flow regimes corresponding to sonic (or choked) flow for liigher pressure drops and subsonic flow for lower pressure drops. The transition between the two flow regimes occurs at tlie dimensionless critical pressure ratio, Ter,I, which is related to tlie gas lieiit capacity ratio y via... 

Leung, J. C. and Grolmes, M. A., A Generalized Correlation for Flashing Choked Flow of Initially Subcooled Liquid, AIChEJ, 34 (4), 688-691, April 1988. 

Leung, J. C. and Epstein, M., A Generalized Correlation for Two-Phase Nonflashing Homogeneous Choked Flow, Trans. ASME—Journal of Heal Transfer 2 (2), 528-530, May-... 

The Martinelli correlations for void fraction and pressure drop are used because of their simplicity and wide range of applicability. France and Stein (6 ) discuss the method by which the Martinelli gradient for two-phase flow can be incorporated into a choked flow model. Because the Martinelli equation balances frictional shear stresses cuid pressure drop, it is important to provide a good viscosity model, especially for high viscosity and non-Newtonian fluids. 

As the gas-liquid mixture travels down the vent line, the phases will slip past each other and the fluids will accelerate. This contribution to the energy balance can be most significant for high pressure blowdown. Pressure increments are calculated and when the pressure gradient becomes infinite the flow is choked. If this occurs at the end of the pipe the assumed flowrate is the converged choked flow solution. If choked flow does not occur and the end of the line is reached at the reservoir pressure, the non-choked flow solution is obtained. 

Flammable or toxic vapors can be piped to a flare after separation of liquid is obtained. An important design problem in flare use is the very high vent rate experienced for a relatively short time, if an existing flare is used. Also back-pressure effects on the liquid separator vessel must be considered, especially if choked flow of vapor occurs downstream of the separator. 

Re ocreux, M., 1977, Experimental Study of Steam-Water Choked Flow, Proc. Transient Two-Phase Flow Specialists Meeting, CSNI, Aug. 1976 Atomic Energy of Canada 2 637-669. (3)... 

A natural gas (methane) pipeline is to be designed to transport the gas at a rate of 50,000 scfm. The pipe is to be 6 in. ID, and the maximum pressure that the compressors can develop is 10,000 psig. The compressor stations are to be located in the pipeline at the point at which the pressure drops to 100 psi above that at which choked flow would occur (this is the suction pressure for the compressors). If the design temperature for the pipeline is 60°F, the compressors are 60% efficient, and the compressor stations each operate with three stages and interstage cooling to 60°F, determine... 

That is, as P2 decreases, the mass velocity will increase up to a maximum value of G, at which point the velocity at the end of the pipe reaches the speed of sound. Any further reduction in the downstream pressure can have no effect on the flow in the pipe, because the speed at which pressure information can be transmitted is the speed of sound. That is, since pressure changes are transmitted at the speed of sound, they cannot propagate upstream in a gas that is already traveling at the speed of sound. Therefore, the pressure inside the downstream end of the pipe will remain at P 2, regardless of how low the pressure outside the end of the pipe (P2) may fall. This condition is called choked flow and is a very important concept, because it establishes the conditions under which maximum gas flow can occur in a conduit. When the flow becomes choked, the mass flow rate in the pipe will be insensitive to the exit pressure but will still be dependent upon the upstream conditions. 

Although Eq. (9-17) appears to be explicit for G, it is actually implicit because the friction factor depends on the Reynolds number, which depends on G. However, the Reynolds number under choked flow conditions is often high enough that fully turbulent flow prevails, in which case the friction factor depends only on the relative pipe roughness ... 

Under isothermal conditions, choked flow occurs when... 

Just as for isothermal flow, this is an implicit expression for the choke pressure (P ) as a function of the upstream pressure (Pi), the loss coefficients (J] Kf), and the isentropic exponent (7c), which is most easily solved by iteration. It is very important to realize that once the pressure at the end of the pipe falls to P and choked flow occurs, all of the conditions within the pipe (G = G, P2 = P, etc.) will remain the same regardless of how low the pressure outside the end of the pipe falls. The pressure drop within the pipe (which determines the flow rate) is always Pt — P when the flow is choked. 

Natural gas (CH4) is transported through a 6 in. ID pipeline at a rate of 10,000 scfm. The compressor stations are 150 mi apart, and the compressor suction pressure is to be maintained at lOpsig above that at which choked flow would occur in the pipeline. The compressors are each two stage, operate adiabatically with interstage cooling to 70°F, and have an efficiency of 60%. If the pipeline temperature is 70°F, calculate ... 

When the gas velocity reaches the speed of sound, choked flow occurs and the mass flow rate reaches a maximum. It can be shown from Eq. (10-45) that this is equivalent to a maximum in YX 2, which occurs at Y = 0.667, and corresponds to the terminus of the lines in Fig 10-21. That is, XT is the pressure ratio across the valve at which choking occurs, and any further increase in X (e.g., AP) due to lowering P2 can have no effect on the flow rate. 

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