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5.1.1 Backwash Pump Design

This document discusses the design of a backwash pump system for a water filtration system. It describes how variable speed drives can be used to control backwash flow rates. The document then shows calculations for sizing pipes, estimating head losses, and selecting an appropriate centrifugal pump based on the required flow rate of 60m3/h and total head of 9.66m. Based on these specifications, a 5kW pump with a 120mm impeller is selected to fit the 50mm diameter piping and meet the system head requirement.

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Willard Musengeyi
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446 views6 pages

5.1.1 Backwash Pump Design

This document discusses the design of a backwash pump system for a water filtration system. It describes how variable speed drives can be used to control backwash flow rates. The document then shows calculations for sizing pipes, estimating head losses, and selecting an appropriate centrifugal pump based on the required flow rate of 60m3/h and total head of 9.66m. Based on these specifications, a 5kW pump with a 120mm impeller is selected to fit the 50mm diameter piping and meet the system head requirement.

Uploaded by

Willard Musengeyi
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
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5.1.

1 Backwash Pump Design


Filter backwash systems will function properly only when pumps are correctly sized and
accurately controlled during runtime. One of the ways to achieve this is the use of variable
speed drive for backwash flowrate control. In backwash systems, the function of the VSD and
PID tuning is to run the pump within the acceptable range with minimum fluctuations in
flow.

Variable speed drives (VSDs) are advantageous due to low operation costs potential. To
achieve the energy saving goal, the drive speed should vary between 50% and 75% of full
speed for most of the runtime. The relationship between pump speed, flow, and head is given
by the affinity laws as follows:

Figure 4.1: Backwash Pump Sizing

System specifications: Flowrate – 60m3/h


Static Head - 8m
 Determining Preliminary Pipe Size
Discharge=MeanVelocity of Flow × Pipe Cross sectional Area
π d2
Q=v ×
4
The guideline for the velocity of water should be selected in the range of 1m/s to 3m/s.

4Q
∴ d=
√ πv

4 × 0.0167
∴ d=
√ π ×3

∴ d=0.049 m

 Pipe Size Section


Using these parameters, the pipe with nominal diameter 50mm is selected.

 Estimating Friction losses due to fittings

Elbow: Quantity – 1 (Regular flanged 90o)

From appendix A4; K = 0.28

v2
H f (elbow )=K
2g
12
H f (elbow )=0.28 ×
2× 9.81

H f (elbow )=0.014 m

Valve: Quantity – 2 (Flanged Gate Valves)

From appendix A4; K = 0.09

v2
H f (valve)=K ×2
2g

12
H f (valve)=0.09 × ×2
2 ×9.81

H f (valve)=0.0092 m

Reducer: Quantity – 2 (Bell-mouth Inlet)

From appendix A4; K = 0.05

v2
H f (reducer)=K ×2
2g

12
H f (reducer)=0.05× ×2
2 × 9.81

H f (reducer)=0.0051m

 Estimating the pipe frictional losses ( H f ) using the Hazen-Williams Empirical


Formula

Pipe material – Stainless steel, welded and seamless

Hazen William’s Coefficient (C) -100

Pipe internal diameter (d) – 0.05m

Pipe length (L) – 12m


Flowrate (Q) – 60m3/h

10.67 × L ×Q1.852
H f ( pipe)=
C 1.852 ×d 4.87

10.67 ×12 ×0.01671.852


H f ( pipe)=
1001.852 ×0.05 4.87

H f ( pipe )=0.133 m

Total head losses ; H f ( total )=H f ( pipe ) + H f (elbow )+ H f (reducer ) + H f (valve)

H f ( total )=0.133+ 0.0051+ 0.0092+ 0.014

H f ( total )=0.161m

Therefore, Total head (dynamic) at design conditions:

H= (10−0.5 ) +0.16
∴ H =9.66 m

 Pump Selection
Figure 4.2: Pump Selection Chart

From the calculations, the pump with specifications 65-50-125 is selected from the pump
selection chart above. Using these specifications from the preliminary pump selection, the
critical system parameters are selected from appendix A3. These parameters are shown on the
table below.

Pump Type Power Impeller Size Pipe Nominal System NPSHR


Required (mm) Diameter Head (m) (m)
(kW) (mm)
Centrifugal 5 120 50 9.66 3

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