Pipe velocities, typical values

Two different types of fluid flow are usually considered in hydrodynamic problems (Figure 9.2.1). When the flow is smooth and steady, and occurs as if separate layers (laminae) of the fluid have steady and characteristic velocities, the flow is said to be laminar. For example, the flow of water through a smooth pipe is typically laminar, with the flow velocity being zero right at the walls (because of friction between the fluid and the wall) and having some maximum value in the middle of the pipe. The velocity profile under these conditions is typically parabolic. When the flow involves unsteady and chaotic motion, in which only on the average is there a net flow in a particular direction, it is termed... 

Table 6.7 shows typical steam velocities for various industrial and commercial applications. Use the given values as guides when sizing steam piping. 

To keep the particles in suspension, the flow should be at least 0.15m/sec faster than either 1) the critical deposition velocity of the coarsest particles, or 2) the laminar/turbulent flow transition velocity. The flow rate should also be kept below approximately 3 m/sec to minimize pipe wear. The critical deposition velocity is the fluid flow rate that will just keep the coarsest particles suspended, and is dependent on the particle diameter, the effective slurry density, and the slurry viscosity. It is best determined experimentally by slurry loop testing, and for typical slurries it will lie in the range from 1 m/s to 4.5 m/sec. Many empirical models exist for estimating the value of the deposition velocity, such as the following relations, which are valid over the ranges of slurry characteristics typical for coal slurries ... 

Now let us hold the water flow rate constant at some modest average velocity, such as 2ft/s, and slowly increase the air velocity from zero to some large value. This will cause to increase, since the j overall linear velocity is increased. However, now there will be bubbles of gas in the pipe the density in Eq. 14.1 is no longer the density of water but is the average density of the gas-liquid mixture in the vertical pipe. At low flow rates the density goes down much faster than goes up, so Pj — P decreases steadily as we increase the gas flow rate. Finally, a point is reached where further increase in the gas velocity causes to increase faster than p decreases Pj Pj will increase with an increasing gas flow rate. A typical plot of experimental data for such a system is shown in Fig. 14.2. j... 

Because of limited variation in experimental values of the Schmidt number, some uncertainty is associated with its exponent. Also, the Reynolds number is based on gas velocity relative to the pipe wall rather than the liquid surface. The above correlation and data typically lie some 20% higher than the Chilton-Coibum prediction. Wetted-wall column studies by Jackson and Caegleske and Johnstone and Pigford, employing Re based on gas velocity relative to the liquid, generally are in reasonable agreement with the Chilton-Coibum analogy. 

The approximate value of the shear rate encountered in a wide variety of circumstances found in everyday life or in industrial situations is shown in table 1. Readers may relate these numbers to their own field of interest by simply dividing a typical velocity in any flow of interest by a typical dimension. An example of this is the average velocity of a liquid flowing in a pipe divided by the pipe radius, or the velocity of a moving sphere divided by its radius. 

The Darcy-Weisbach Equation applies to a wide range of fluids, while the Hazen-Williams Equation is based on empirical data and is used primarily in water modeling applications. Each of these methods calculates friction losses as a function of the velocity of the fluid and some measure of the pipe s resistance to flow (pipe wall roughness). Typical pipe roughness values for these methods are shown in Table 3.3. These values can vary depending on the product manufacturer, workmanship, age, and many other factors. 

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