Why is, in engineering calculations, the pressure drop generally calculated as only the frictional pressure drop ΔPf?

Generally, in engineering calculations, only the incompressible frictional pressure drop ΔPf of single-phase flow is considered, as its contribution is the greatest and it is easy to calculate. Under normal conditions, the effects of the static pressure drop ΔPs and the dynamic pressure drop ΔPn are relatively small, and their impact can be reduced by properly designing the pipes. For special cases or high-precision calculations, it is necessary to consider the effects of the static pressure drop ΔPs and the velocity pressure drop ΔPn. In practical calculations, the Fanning equation takes into account only the incompressible frictional pressure drop ΔPf of single-phase flow, as it already incorporates the effects of ΔPs and ΔPn. The calculation formula for the Fanning equation has been verified through experiments and is widely used in engineering practice, enabling it to meet the computational needs in most cases. In HG20570.7, only the incompressible frictional pressure drop ΔPf for single-phase flow is considered, possibly because the pipe design already takes into account the effects of the static pressure drop ΔPs and the velocity pressure drop ΔPn, or because the precision required for actual calculations does not necessitate considering these two types of pressure drops. .

However, for the Fanning equation, the correction coefficient term (Lλ/D + ΣK) is calculated using only the equivalent length method – that is, by determining the pressure drop per unit length of pipe, and then adding the equivalent length Le of the pipe fittings to the straight pipe length L; the value of (L + Le) is used in place of L in the correction coefficient term to calculate the actual pressure drop along the pipe. Is it reasonable not to take into account the effect of ΣK?

Your question is quite insightful. ΣK represents the sum of the resistance coefficients, including those associated with various pipe fittings, elbows, amplifiers, etc.; in the Fanning equation, empirical formulas are often used to estimate these values. It should be noted that the Fanning equation itself is an empirical formula, and its accuracy depends on the experimental data chosen as well as the parameters of the formula. Under normal circumstances, using the equivalent length method to calculate the correction coefficient term (Lλ/D + ΣK) is sufficient to meet the needs of engineering practice, as the resistance coefficients in actual pipeline systems do not exactly correspond to those in the empirical formulas used for calculation, and multiple resistance coefficients may exist in such systems, making the calculation process very complex. Therefore, the equivalent length method is commonly used in engineering practice for simplified calculations. Of course, in special cases where higher precision in calculations is required, more complex calculation methods or more accurate experimental data can be considered for use. .

This post was last edited by range_lXfxP on 2023-5-8 09:28. In the calculation sheets I’ve seen, only ΔPf is taken into account for the pipe pressure drop; the calculation formula is (Le+L)λ/D*(u^2)*ρ/2. Denoted as ΔP1, it represents the frictional pressure drop in the inlet pipe or outlet pipe ； Secondly, for the equipment and flow resistance elements, an empirical pressure drop is used, denoted as ΔPe1, which represents the sum of the pressure drops due to the equipment or flow resistance elements on the inlet pipe or outlet pipe. So, for this ΔPe1, how should the empirical values of pressure drop for various devices and resistive elements be determined? Are there any reference specifications?

The empirical pressure drop values for equipment and resistive elements usually need to be selected based on actual conditions. Generally, their pressure drop values can be referred to in relevant standards and manuals. Taking Chinese standards as an example, the \"Code for Construction and Acceptance of Water Supply and Drainage Engineering\" (GB 50242-2015) provides empirical formulas and tables for the pressure loss of pipe components, allowing for estimates to be made using the data contained therein. The American standard ANSI/HI 9.6.3, \"Centrifugal Pumps – Guidelines for Installation, Operation, and Maintenance,\" also provides empirical pressure drop values for various equipment and resistance elements. In addition, other relevant standards and manuals can also be referred to, such as \"Selection and Design of Fluid Machinery Equipment\". It should be noted that these empirical values are for reference only; in actual design and calculation, it is necessary to take into account the actual conditions as well as the applicability of these empirical values in order to conduct a comprehensive assessment. .

As can be seen from the graph, the static pressure drop has already been included. The velocity pressure drop is very small for ordinary liquid pumps and can be ignored. You can try to calculate it.