Pressure drop

For other uses, see Pressure Drop (disambiguation).

Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main determinants of resistance to fluid flow are fluid velocity through the pipe and fluid viscosity. The flow of any liquid or gas will always flow in the direction of least resistance(less pressure). Pressure drop increases proportional to the frictional shear forces within the piping network. A piping network containing a high relative roughness rating as well as many pipe fittings and joints, tube convergence, divergence, turns, surface roughness and other physical properties will affect the pressure drop. High flow velocities and / or high fluid viscosities result in a larger pressure drop across a section of pipe or a valve or elbow. Low velocity will result in lower or no pressure drop. ^[1]

Pressure Drop can be calculated by 2 values: the Reynolds Number, NRe (determining laminar or turbulent flow), and the relative roughness of the piping, ε/D. NRe = Dvρ/μ Where D is the pipe diameter in metres, v is the velocity of the flow in metres per second, ρ is the density in kilograms per cubic metre, and μ is in kilograms per metre-second. The relative roughness of the piping is usually known and then these two values can be cross referenced using:

http://www.fao.org/docrep/x5744e/x5744eee.gif

By cross referencing the Reynolds number with the relative roughness, the friction factor, f, is calculated. f=(ΔpπR2/2πRΔL)/(ρv2/2) Where solving for Δp will allow for the calculation of pressure drop through a pipe. ^[2]

^[3]

See also

• Pressure measurement
• Atmospheric pressure
• Voltage drop
• Darcy–Weisbach equation ( To calculate pressure drop in a channel )

References

1. ^ The Christmas Tree Farm, http://www.christmas-tree-farm.com/dripterm.html
2. ^ Geankoplis, C. J. (2003). Transport Processes and Separation Process Principles. Upper Saddle River, NJ: Prentice Hall Professional Technical Reference.
3. ^ Welty, James (2008). Fundamentals of Momentum, Heat, and Mass Transfer. United States: Hamilton Printing.