Nozzle flow coefficient

Flow nozzles are commonly used in the measurement of steam and other high velocity fluids where erosion can occur. Nozzle flow coefficients are insensitive to small contour changes and reasonable accuracy can be maintained for long periods under difficult measurement conditions that would create unacceptable errors using an orifice installation. 

Introducing the nozzle flow coefficient of 0.95 and using field system of units, Equation 4-117 becomes... 

This noncoherent publishing by manufacturers of K (nozzle flow coefficient) and A (nozzle flow area) is leading to a lot of confusion. Just remember that the capacity is direcdy proportional to l< X A, and manufacturers can show any K and any A as long as K X A is equal to or less than the ones certified by the National Board. 

K, nozzle flow coefficient of the valve M, molar weight of the gas (kg/kmol)... 

Ideal (Frictionless) Flow in Nozzles The flow path in well-formed nozzles follows smoothly along the nozzle contour without separating from the wall. The effects of small imperfections and small frictional losses are accounted for by correcting the ideal nozzle flow by an empirically determined coefficient of mscharge. The acceleration of a fluid initially at rest to flowing conditions in an ideal nozzle is given by ... 

Figure 2-17. Flow coefficient C for nozzles. C based on the internal diameter of the upstream pipe. By permission, Crane Co. [3]. Crane reference [9] is to Fluid Meters, American Society of Mechanical Engineers, Part 1-6th Ed., 1971. Data used to construct charts. Chart not copied from A.S.M.E. reference.

Flow coefficient for orifices and nozzles which equal the discharge coefficient corrected for velocity of approach = C / (1 - p" )V2... 

Figure 2-24. Flow coefficient for nozzles and orifices (from Brown, p. 226, Fig. 3-19).

The foregoing equations are based on flow coefficients determined by calibration with air. For application with other gases, the difference between the properties of air and those of the other gas must be considered. The gas density is incorporated into the equations, but a correction must be made for the specific heat ratio (k = cp/cv) as well. This can be done by considering the expression for the ideal (isentropic) flow of a gas through a nozzle, which can be written (in engineering units ) as follows ... 

The capacity of the relief system can be obtained from a two-phase flow calculation for nozzle flow. If the flow is not choked, then the Omega method (see Annex 8) or suitable computer code must be used to calculate flow capacity. For choked flow a larger range of methods may be applicable, e.g. ERM for vapour pressure systems (see 9.4.2) or Tangren et al. s method for gassy systems (see 9.4.3), together with the application of a discharge coefficient. The capacity can then be obtained from ... 

Giffen, E. and Crang, T.F. (1946). Steam flow in nozzles velocity coefficient at low steam speeds. Proceedings of the Institution of Mechanical Engineers, 155, 83-92. 

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