Pressure Drop Calculation
Regardless of whether the flow is laminar or turbulent, the pressure loss or pressure drop through a pipeline is described by a dimensionless similarity parameter. This so-called Darcy friction factor f:
If you reduce the diameter of a pipeline, the pressure loss can be increased to the diameter ratio powered by 5 assuming that the friction factor and volumetric flow rate are constant.
Pressure loss for turbulent flow
In addition to the pressure loss due to internal friction caused by the viscosity of the fluid, there is therefore an additional pressure loss due to the turbulence. The pressure loss is therefore greater in turbulent flow than in laminar flow.
The calculation method depends strongly on the phase, liquid, gas or two-phase mixture.
- Liquid flow known as compressible fluid due to having constant volume with change in pressure:
I. Newtonian fluid: Through the use of Darcy-Weisbach equation. In order to use the mentioned equation, the following data are required:
- Friction factor (obtained from Moody diagram having Reynolds Number and relative roughness, roughness value of steel: 0.04 mm)
- Length of pipeline
- Diameter of pipeline
- Velocity
II. Non-Newtonian fluid, like blood and ketchup,: Through the use of complicated methods
2. Gas flow (compressible flow):
known as in-compressible fluid due to change of density with change in pressure. But, if the pressure of the gas doesn’t change by more than 10 %, gas can be treated as an in-compressible fluid
For “short” lines, such as in a plant. where AP > 10% PI,either break into sections where AP < 10%PI or use:
3. Two-Phase Mixture:
If there is a slop prior to heat exchanger, the liquid accumlates there. In order to have a fluid, the gas pressure should increase high enough to overcome the pressure of liquid in the slop. This causes a pulsating stream which causes material damage.
Two-phase (liquidvapor) flow is quite complicated and even the long-winded methods do not have high accuracy. You cannot even have complete certainty as to which flow regime exists for a given situation. Volume 2 of Ludwig’s design books’ and the GPSA Data Book’ give methods for analyzing two-phase behavior.
For our purposes. a rough estimate for general two- phase situations can be achieved with the Lockhart and Martinelli3 correlation. Perry’s‘ has a writeup on this cor- relation. To apply the method, each phase’s pressure drop is calculated as though it alone was in the line. Then the following parameter is calculated:
Example:
If 1,000Ib/hr of saturated 600-psig condensate is flashed to 2OOpsig, what size line will give a pressure drop of l.Opsi/lOOft or less?
Enter at 6OOpsig below insert on the right, and read down to a 2OOpsig end pressure. Read left to intersection with 1,00Olb/hrflowrate, then up verti- cally to select a 1.5 in for a 0.28psi/lOOft pressure drop. Note that the velocity given by this lines up if 16.5ft/s are used; on the insert at the right read up from 6OOpsig to 2OOpsig to find the velocity correction factor 0.41, so that the corrected velocity is 6.8 ft/s.
4. Pipe Fittings such as elbows, tees and valves
I. Equivalent length method: allows the user to describe the pressure drop through a fitting as a length of pipe.
Head loss= f (L/D)equivalent (V2/2g)
II. K-Method
Head loss= K(V2/2g)
A natural circulation:
It means that the total pressure loss including fitting, valve, pipe length, should be equal to the pressure obtaind through static height. An estimate for steel-pipe roughness would 0,4 micrometer.(see VDI Wärmeatlas)
