How To Calculate Water Velocity In Pipe? / Water Velocity Calculation

How To Calculate Water Velocity In Pipe? / Water Velocity Calculation

 

Water velocity is a measure of the speed of water flowing through a closed pipe system. Water velocity can be determined using a simple formula:

V = Q/A

Where,

V = velocity

Q = Flow rate

A = cross-sectional area of pipe

 

 

 

All units must be the same when using this formula.

1)  If the flow rate is measured in cubic meters per second, then the area should be in square meters

2)  if the flow rate is measured in cubic feet per second, the area should be in square feet.

 

The cross-sectional area of a pipe can be determined using the formula:

 

A = πr²

 

The area A is equal to π times the radius r squared.

For Example, Given Data

let’s find the velocity of water given a flow rate of 25 cubic feet per second and a 1 ½” diameter pipe.

Step – 1: find the cross-sectional area of the pipe.

To convert inches to feet = divided by 12

Diameter in feet = 1 ½″ ÷ 12 = 0.125′

Radius = 0.125′ ÷ 2 = 0.0625′

Area = πr²

Area = π × 0.0625²

A = π × 0.00390625

Area = 0.0122718463 sq/ft

 

Step-2: Calculate the velocity.

 

V = 25/0.0122718463

V = 2037.18 CFT/S

 

Alternative Method To find Velocity

The above formula works best when the flow rate is measured in the form of a standard length unit cubic form. But things are a little tricky when the flow rate is measured in gallons per minute or liters per minute.

In that case, start by converting the flow rate to cubic feet per second or cubic meters per second.

Alternatively, if the flow rate is measured in gallons per minute, the following formula can be used to calculate water velocity.

NOTE: 1 Gallon is equal to 3.785411784 liters.

 

V = 0.408 × QD²

Therefore, the velocity v is equal to 0.408 times the gallon flow rate per second, and the pipe diameter is divided by D in inches square.

For Example, Given Data

let’s find the water velocity given a flow rate of 20 gallons per minute and a 1 ½″ diameter pipe.

 

Cross-sectional area of the pipe.

V = (0.408 × 20)/1.5²
V = (0.408 × 20)/2.25
V = 3.6267 ft/s

So, the velocity of water inside a 1.5″ pipe with a flow rate of 20 gallons per minute is equal to 3.62 ft/s.

 

Maximum Flow Velocities in Water Systems

The velocity of water in pipes and tubes should not exceed a certain limit

Fluid flow velocity in water systems should not exceed certain limits so as to avoid noise and damaging breakage of pipes and fittings. The table below can be used as a guide for maximum velocities:

Application	Maximum Velocity
	(ft/s)	(m/s)
General Water Service	3 - 8	0.9 - 2.4
Tap water (low noise)	1.6 - 2.3	0.5 - 0.7
Tap water	3.3 - 8.2	1.0 - 2.5
Cooling water	4.9 - 8.2	1.5 - 2.5
Suction boiler feed water	1.6 - 3.3	0.5 - 1.0
Discharge boiler feed water	4.9 - 8.2	1.5 - 2.5
Condensate	3.3 - 6.5	1.0 - 2.0
Process water	5 - 10	1.5 - 3
Pump discharge	5 - 10	1.5 - 3
Pump suction	3 - 8	0.9 - 2.4
Heating circulation	3.3 - 9.8	1.0 - 3.0

 

 

ALSO READ: How To Calculate Flow Rate Of Water In A Pipe / Calculating Water Discharge In A Pipe 

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