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# Pressure-velocity relation

08/04/2008 10:33 AM

I am confused to calculate the required flow rate from the pipeline. Q=AV is the formula for calculating Flow (fluid is water).I have pipe diameter and pressure. Let me know how to calculate velocity.Is there any perfect relation between pressure and velocity.

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#1

### Re: Pressure-velocity relation

08/04/2008 12:03 PM

There is a relationship between pressure drop and velocity. Take a look at this.

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#2

### Re: Pressure-velocity relation

08/04/2008 8:47 PM

Do a Google search "flow rate from pressure drop" and see the hits that should give you an answer.

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#3

### Re: Pressure-velocity relation

08/04/2008 11:19 PM
2
Anonymous Poster
#4

### Re: Pressure-velocity relation

08/05/2008 1:02 AM

There is no direct relation between pressure and flow.You will understand this from a simple example : In a flowing pipeline, you close a valve bringing the flow to a halt.You will continue to read pressure (usually more than what you see when there is a flow).

Flow relates to "differential" pressure read across two points along the pipeline or a valve or an orifice etc.For accurate measurement of flow with differential pressure you will also need viscosity ( for water it is simple - 1 cP),correct area of through which the flow takes place,in case of long pipe line the length,internal surface roughness,number of fittings etc.

Free software is available on net to calculate the pressure drop if flow rate,viscosity ,length and diameter of pipeline are known.

For water,the term velocity head refers to V^2/2g where V is velocity in m/sec,g is the gravitational constant= 32 m/s^2. H= head will be in metres.

Head into density = pressure.Take care of the units while calculating!!

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Guru

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#5

### Re: Pressure-velocity relation

08/05/2008 1:15 AM

The relation is the energy equation between any two points 1 and 2 on the pipe line:

(P1/w) + (V12/2g) + z1 + HP = (P2/w) + (V22/2g) + z2 + ∑HL

Where,

P = Pressure (lbf / ft2 or psf)

V = Velocity (ft /sec)

W = ρg/gc

g = Gravity Acceleration (32.2 ft/sec2)

gc = Gravitational Constant (32.2 lbm . ft / lbf . sec2)

HP = Pump Head (ft)

ρ = Density (lbm/ft3)

Z = Elevational Head (ft)

∑HL= Head losses due to friction

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Anonymous Poster
#14
In reply to #5

### Re: Pressure-velocity relation

06/15/2010 4:33 AM

To explain this we need to consider two major equations

Let us take

1. Bernoulli's equation:

P + (Rho).g.y + (1/2).(Rho)v^2= const.

2. Continuity equation:

To maintain constant flow rate

Q=A1v1 =A2v2

Considering flow is happening through a tapering pipe.

P = pressure

Rho = density

g = gravity

y = height

Q = Flow rate (Kg/cu.metre)

A1 = area of cross-section-1

A2 = area of cross-section-2

v1 = velocity at cross-section-1

v2 = velocity at cross-section-2

Section 1 Larger than Section 2

So when flow happens from section 1 to section 2 the area is decresing so the velocity will increase to maintain the constant flow rate by continuity equation.

So if you take Bernoulli's equation, to maintain constant value at one side the datum/height or the pressure has to be increased.

So Area decreases-->Velocity will increase-->In turn Pressure increases.

So velocity and Pressure are inversely proportional unless the Datum/Height is same.

Hope this helps you.

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Anonymous Poster
#15
In reply to #14

### Re: Pressure-velocity relation

07/15/2010 8:40 AM

******** So Area decreases-->Velocity will increase-->In turn Pressure increases. So velocity and Pressure are inversely proportional unless the Datum/Height is same. *********

I'm sure I'm missing something here, so pardon my ignorance: If the pressure increases as the velocity increases, how are they inversely proportional?

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#6

### Re: Pressure-velocity relation

08/05/2008 10:45 AM

What, there is no direct relationship between pressure and flow?

I have a static situation with a pump, length of pipe & a valve closed. This will give me my maximum pressure and minimal(none) flow. I fully open the valve (surface area removed), I will now have minimal pressure (hence pressure drop) and maximum flow. Sounds like a direct relationship to me. Now it can be controlled by various methods and differences minimized, but by themselves it is as direct as it gets.

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#7

### Re: Pressure-velocity relation

08/05/2008 10:52 AM

Here is another useful formula for you:

http://en.wikipedia.org/wiki/Bernoulli's_equation

Member

Join Date: Jul 2008
Location: Accra, Ghana
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#8

### Re: Pressure-velocity relation

08/05/2008 11:10 AM

Mind you there is the need to be specific with the pressure available. Is it the service provider pressure P1? Then what would be the second pressure value. Is it the Atmospheric Pressure 'P2'? Then you need to get the length of the pipe converted to head Z. The head H would also be considered if you are taking water to a higher level. But on both sides of the equations you need to see which values would go to zero. This would enable for the simplification of the equation. The velocity at the section of interest would then be obtained.

Guru

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#9

### Re: Pressure-velocity relation

08/14/2008 12:49 PM

There is no relationship between pressure and velocity.

Velocity means flow and flow is produced only when there exist some differential pressure.

You can have any pressure value with no flow (zero velocity) just think on a closed volume (a tire, a football/soccer ball...). You can increase the pressure to any value till it explodes and no flow/velocity are present.

You need to know some more parameters. Mr. Abdel Halim Galala post gave you the general flow equation. Just apply it to your case.

Kind regards

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Anonymous Poster
#10

### Re: Pressure-velocity relation

06/06/2009 8:07 AM

I am confused to calculate the required flow rate from the pipeline. Q=AV is the formula for calculating Flow (fluid is water).I have pipe diameter and pressure. Let me know how to calculate velocity.Is there any perfect relation between pressure and velocity.

Off Topic (Score 5)
Guru

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Location: Madrid, Spain
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#11
In reply to #10

### Re: Pressure-velocity relation

06/07/2009 5:57 AM

Hello,

As you can see if read previous posts, there's no relationship between pressure and velocity.

Flow rate/velocity is a function of differential pressure. You need to have one more "datum" to calculate it.

Kind regards

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Off Topic (Score 5)
Anonymous Poster
#12

### Re: Pressure-velocity relation

01/07/2010 6:44 PM

the formula for calculating velocity is distance divided by time... V=D/t

Anonymous Poster
#13

### Re: Pressure-velocity relation

01/28/2010 2:30 PM

P=(\$*(g*h)^0.5 )*V

Where P=pressure N/m^2

\$=density of material (kg/m^3)

g= 9.81 m/sec^2

h=Length of element

V= velocity m/s

Anonymous Poster
#16
In reply to #13

### Re: Pressure-velocity relation

01/19/2011 5:41 AM

how the unit shall be calculated. I calculated 'h' here but the unit is coming N^2*s^4/Kg^2*m. How it comes??

Participant

Join Date: Feb 2013
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#17

### Re: Pressure-velocity relation

02/05/2013 10:15 PM

Q= A V

P (pressure)= 1000 V^2 ..... (trust me)

P= F/A = m a / A = m V / A t = m Q / A^2 t = Q' (l/s = kg/s) Q(m^3/s) / A^2

= 1000 Q^2 / A^2 = 1000(V A)^2 / A^2 = 1000 V^2 !!

Guru

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#18

### Re: Pressure-velocity relation

08/29/2013 12:03 PM

Dear Mr. takle,

It is very simple. Convert the pressure into HYDRO STATIC COLUMN and term it as H in Feet or Metre.

Then velocity (theoritical) will be Sr.Root of 2gH, where g is the acceleration due to gravity and H is the Head in Ft. or Metre. g will be 32.2 Ft/Sec^2 or 9.81 M/Sec^2

Then Co.eff of Velocity will come into picture and use the appropriate value.

DHAYANANDHAN.S

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Guru

Join Date: May 2008
Location: CHENNAI, TAMIL NADU, INDIA.
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#19

### Re: Pressure-velocity relation

04/08/2014 12:20 PM

Dear Mr.takle,

Refer any Standard Hydraulics Book. You will find the following.

1.Bernoulli's Equation

2. POTENTIAL ENERGY = KINETIC ENERGY which describes as m x g x h = 1/2 x m x V^2

DHAYANANDHAN.S

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