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# Calculating Frictional Pressure Drop in Pipelines

Goutam Das
26/10/2020 0 0

Introduction

Very frequently, we need to calculate frictional pressure drops in pipelines. Many of us may have domestic pumps in our households. After we estimate the pump flow from expected consumption of water, we need to calculate the pump head for purchasing a suitable pump. The following is an example of calculation of pressure drop when the flow and pipe geomerty is known.

Input Data(assumed)

1. Pipe flow -10 m3/h

2. Fluid-Water at 250C.

3. Pipe material-commercial galvanised carbon steel (absolute roughness ε=0.15 mm)

4.. Flow geometry as below(total pipe length L=20 m).

h1=3m

h2=12m

Calculation

Step-1 Calculation of flow velocity

From pipe dimension table for 25mm NB std thickness pipe

OD (D)=33.4 mm

Thickness (t)=3.38 mm

Hence ID(d)=D-2t=33.4-2x3.38=26.64 mm=0.02664 m

Pipe flow area (A)=(π/4)d2= (π/4)x0.026642=0.000557 m2

Pipe flow (Q)=10 m3/h=10/3600=0.00278 m3/s

Hence pipe flow velocity(V)=Q/A=0.00278/0.000557=4.98 m/s

Step-2 Calculation of Darcy's Friction Factor(f)

Kinematic viscosity of water(ν)=8.92x10-7 m2/s

Reynold's number (Re)=Vd/ν=4.98x0.02664/8.92x10-7=1.9x105

Re>2000 hence flow is turbulent

For determining f we use Moody's chart s below:

# Relative roughness (ε/d)=0.15/1000/0.02664=0.005631

We noe find f from Re and ε/d as follows

Hence f=0.031

Step 3 Calculation Pipe Friction Drop

Total pipe length L=20 m

Minor losses

Minor losses are calculated by K factors. Different K factors are available in the internet(refer Crane Technical Paper 410)

a) Inlet loss-K1=1

b) Foot valve with strainer-pressure drop Δpf1=0.1 bar(assumed)=1x104 Pa

c) Elbows 900 -3 Nos, K2=3x30f=3x30x0.031=2.79

d) Gate valve -2 nos, K3=2x8f=2x8x0.031=0.496

e) Exit loss, K4=1

hf= (fL/D+K1+K2+K3+K4)V2/2g+ Δpf1/ρg (Note :ρ=density=1000 kg/m3)

=(0.031x20/0.02664+1+2.79+0.496+1)(4.982/2x9.81)+1x104/(1000x9.81)

=37.12 mwc

Step 4 Calculation of Pump Head

H=52.12 mwc

We normally take some margin (say 15 %)for future increase in pipe friction drop due to ageing and calculation uncertainties.

So

H=52.12x1,15=60 mwc

Pump details, 10 m3/h flow, 60 mwc head.

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