Process Design Solved Sample Problems

Sample Problem – Line sizing calculations

Sample Problem Statement -

Calculate the size of line to handle 100,000 kg/hr of water. Approximate length of this line is around 200m. This water stream is available at 5 barg pressure and 300C.

The project standards requirements are - fluid velocity < 3 m/s                                   and pressure drop < 4.5 bar/km

Consider 20 nos. of 900 elbows and 4 gate valves in the line and estimate total pressure drop.

Solution -

This sample problem is solved in following 3 steps.
Step 1.

Solving this sample problem requires determination of important physical properties of given fluid (water) at given temperature and pressure conditions.
Using EnggCyclopedia's Liquid Density Calculator, water density at 300C =993.41 kg/m3
Using EnggCyclopedia's Liquid Viscosity Calculator, water viscosity at 300C =0.81 cP

Step 2.

Use EnggCyclopedia's Pipe pressure drop calculator for single phase flow to get the pressure drop in bar/km. Different pipe sizes are tried and the results are reported in following images.

4" line size.

6" line size

The 4" line size does not satisfy limitations on fluid velocity and pressure drop / km (velocity<3 m/s and pressure drop<4.5 bar/km). The next higher size of 6" is in accordance with these limitations and hence chosen as the line size.

Step 3.

Final step of solving this line sizing problem is to calculate total pressure drop for this line size considering the fittings and bends in line. EnggCyclopedia's K factor calculator can be used for this purpose.

Contribution of fittings (K factor) to the total pressure drop = KρV2/2

= 9.5 X 993.4 X 1.532/(2 X 105) bar = 0.11 bar.

Pressure drop due to straight pipe length (200 m)  = 1.3 bar/km X 0.2 km = 0.26 bar.

Total pressure drop = 0.26 + 0.11 = 0.37 bar

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