Why does pressure drop along a pipe

The pipe friction coefficient is a dimensionless number. The friction factor for laminar flow condition is a function of Reynolds number only, for turbulent flow it is also a function of the characteristics of the pipe wall. Determine Pipe friction coefficient at turbulent flow in the most cases Colbrook Equation:. The solutions to this calculation is plotted vs. Following table gives typical roughness values in millimeters for commonly used piping materials.

If you have valves, elbows and other elements along your pipe then you calculate the pressure drop with resistance coefficients specifically for the element. The Moody friction factor, f , expressed in the previous equations, is a function of the Reynolds number and the roughness of the internal surface of the pipe and is given by Fig.

The Moody friction factor is impacted by the characteristic of the flow in the pipe. Table 9. If the viscosity of the liquid is unknown, Fig. In using some of these figures, the relationship between viscosity in centistokes and viscosity in centipoise must be used Eq.

Pressure drop can be calculated from Eq. The general equation for calculating gas flow is stated as Eq. For practical pipeline purposes, Eq. Gas Processors Suppliers Assn. All three are effective, but the accuracy and applicability of each equation falls within certain ranges of flow and pipe diameter.

The equations are stated next. This equation is used for high-Reynolds-number flows where the Moody friction factor is merely a function of relative roughness. This equation is used for moderate-Reynolds-number flows where the Moody friction factor is independent of relative roughness and is a function of Reynolds number to a negative power.

As previously discussed, there are certain conditions under which the various formulas are more applicable. A general guideline for application of the formulas is given next. The Weymouth equation is recommended for smaller-diameter pipe generally, 12 in. This equation is recommended for larger-diameter pipe in.

The petroleum engineer will find that the general gas equation and the Weymouth equation are very useful. The Weymouth equation is ideal for designing branch laterals and trunk lines in field gas-gathering systems. The characteristics of horizontal, multiphase flow regimes are shown in Fig. They can be described as follows:. Superficial velocity is the velocity that would exist if the other phase was not present.

Multiphase Flow , vol. DOI: Idelchik, I. Rowe, M. Fluid Mech. Login: Guest. Single-Phase Flow. The friction factor itself is dependent on internal pipe diameter, the internal pipe roughness and the Reynold's number which is in turn calculated from the fluid viscosity, fluid density, fluid velocity and the internal pipe diameter. There are therefore a number of sub-calculations that must take place to calculate the overall friction loss. Working backwards we must know the fluid density and viscosity properties, know the pipe diameter and roughness properties, calculate the Reynold's number, use this to calculate the friction factor using the Colebrook-White equation, and finally plug in the friction factor to the Darcy-Weisbach equation to calculate the friction loss in the pipe.

After calculating the pipe friction loss we then need to consider possible fitting losses, change in elevation and any pump head added. The following sections consider each calculation in turn. We now need to calculate each of the items that is required to determine the friction loss in the pipe. The links in the following list provide more details about each specific calculation:. Our Pipe Flow software automatically calculates the friction loss in pipes using the Darcy-Weisbach equation since this is the most accurate method of calculation for non-compressible fluids, and it is also accepted as industry accurate for compressible flow provided certain conditions are met.

Energy loss due to valves, fittings and bends is caused by some localized disruption of the flow. The dissapation of the lost energy occurs over a finite but not necessarily short section of the pipeline, however for hydraulic calculations it is accepted practice to consider the entire amount of this loss at the location of the device.

For pipe systems with relatively long pipes, it is often the case that fitting losses will be minor in relation to the the overall pressure loss in the pipe. However some local losses such as those produced by a part open valve are often very signifcant and can never be termed a minor loss, and these must always be included. The loss that a specific pipe fitting introduces is measured using real world experimental data and this is then analyzed to determine a K factor a local loss coefficient that can be used to calculate the fitting loss as it varies with the velocity of the fluid passing through it.

Our Pipe Flow Software programs make it easy to automatically include fitting losses and other local losses in the pressure drop calculation since they come with a pre-loaded fittings database that contains many industry standard K factors for various different valves and fittings, at various different sizes. All the user has to do is to select the appropriate fitting or valve, and then choose 'Save' to add this on to the pipe, and have it included in the pipe pressure loss calculation.

There are often many different types of components that need to be modeled in a piping system, such as a heat exchanger or a chiller. Some components may introduce a known fixed pressure loss however it is more likely that the pressure drop will vary with the flow rate passing through the component. Most manufacturers will supply a component performance curve that describes the flow verus head loss characteristics of their product.