Roughness of pipe wall

Roughness of the pipe wall is an important design parameter for pressure drop calculation. The roughness depends on the type of pipe being used. Most commonly used pipe materials have roughnesses as given in Table 2.3. 

It is sometimes required to calculate the pressure drop through a control valve this is best achieved by using the coefficient of valve (CV) of the control valve. For incompressible fluids, the pressure drop through the control valve is calculated using the following equation  

Care must be taken to use the control valve CV to calculate the pressure drop. For fixed CV, with an increase in flow rate, the pressure drop increases (pressure drop increases with the square of the volumetric flow rate). In most cases, the control valve is used for a fixed pressure drop (in fact, with increase in flow rate, pressure drop through the control valve decreases). The designer must consider this fact to estimate the pressure drop through the control 

Process engineering and design using Visual Basic 

Equation 2.38 is used to estimate the flow rate for gravity flow. Normally, for gravity flow, a slope of 1 100 is maintained (0.57°). For gravity flow, pipe with a diameter 200 mm or less is designed for 50% liquid capacity, and for a diameter more than 200 mm, 75% capacity. 

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Deborah number, dimensionless roughness of pipe wall, m... 

When the Reynolds number based on tube diameter is greater than 2100, the boundary layer becomes turbulent at some distance from the inlet. The transition usually occurs at a Reynolds number, based on distance from the entrance, Rcj, of between 10 and 10, depending on the roughness of the wall and the level of turbulence in (he mainstream. As shown in Fig, 4,11, the deposition rate tends to follow the development of the turbulent boundary layer. No deposition occurs until Re is about 10- the rate of deposition then approaches a constant value at Re = 2 x 10 in the region of fully developed turbulence. On dimensional ground.s. the deposition velocity at a given pipe Reynolds number can be assumed to be a function of the friction velocity, if, kinematic viscosity, v, and the particle relaxation time, m/f ... 

In laminar flow/ = 16/Re, whereas in turbulent flow the dependence of/ on Re is a function of the specific rheological behavior of the fluid and roughness of the walls of the inside of the drill pipe (91). A number of functional relationships between / and Re have been proposed for turbulent flow. Ignoring the effects of the roughness of the surface of the drill pipe, / can be approximately related to Re by a generalized form of the well-known Blasius equation for Newtonian fluids (90, 95)... 

Absolute roughness The roughness of a pipe or duct wall, normally expressed as a dimensionless ratio of the linear measure of the internal roughness divided by the diameter. 

Calculation of pressure drops in steam lines is a time-consuming task and requires the use of a number of somewhat arbitrary factors for such functions as pipe wall roughness and the resistance of fittings. To simplify the choice of pipe for given loads and steam pressures. Figure 22.4 will be found sufficiently accurate for most practical purposes. 

The shear stress Ri at the pipe wall in the upper portion of the pipe may be calculated on the assumption that the liquid above the bed is flowing through a non-circular duct, bounded at the top by the wall of the pipe and at the bottom by the upper surface of the bed. The hydraulic mean diameter may then be used in the calculation of wall shear stress. However, this does not take account of the fact that the bottom boundary, the top surface of the bed, is not stationary, and will have a greater effective roughness than the pipe... 

For turbulent flow of a Newtonian fluid, / decreases gradually with Re, which must be the case in view of the fact that the pressure drop varies with flow rate to a power slightly lower than 2.0. It is also found with turbulent flow that the value of / depends on the relative roughness of the pipe wall. The relative roughness is equal to eld, where e is the absolute roughness and d, the internal diameter of the pipe. Values of absolute roughness for various kinds of pipes and ducts are given in Table 2.1. 

The changing character of the flow in the different regions of the turbulent boundary layer explains certain aspects of the friction factor chart. If the absolute roughness of the pipe wall is smaller than the thickness of the viscous sublayer, flow disturbances caused by the roughness will be damped out by viscosity. The wall is subject to a viscous shear stress. Under these conditions, the line on the friction factor chart... 

Turbulent flow of Newtonian fluids is described in terms of the Fanning friction factor, which is correlated against the Reynolds number with the relative roughness of the pipe wall as a parameter. The same approach is adopted for non-Newtonian flow but the generalized Reynolds number is used. 

These inlet losses are mainly dominated by frictional effects in the pipe. They are represented by the Kf factor and depend largely on the roughness of the pipe s internal wall and its diameter. They can be found in most piping books and are specific for the type of pipe ... 

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