Pressure in a pipe.

Homework Statement

For this problem use: ρwater = 1000 kg/m3

A cylindrical water-storage tank has a height of 7.9 m and a radius of 6.2 m. The storage tank is full of water but is vented to the atmosphere. The bottom of the tank is 35 m off the ground. A 12 cm diameter pipe runs vertically down from the tank and goes 1.7 m underground before turning horizontal. The water flow in the horizontal pipe is 85 L/s.

What is the water pressure in the horizontal pipe underground? (kPa)

Homework Equations

p=F/A
pascal (Pa) = 1N/m2
p=ρgd
Atmospheric Pressure = 101.3kPa

The Attempt at a Solution

I know that gauge pressure is ρ*gravity*depth. I am assuming my problem is that I am not using the appropriate height as rho and gravity are given. For heights I have tried numerous combinations of heights. I have tried 1.7, 1.7+35, 1.7+35+7.9. This is a hydrodynamics problem, so my method may be incorrect in trying to solve for the pressure.

supratim1
Gold Member
as it is vented out to atmosphere, add atmospheric pressure to it.

supratim1
Gold Member
and height is 1.7+35+7.9

This still yields an incorrect answer. I've got 14 tries on the problem thus far, that happens to be one of them. Is my methodology correct in attempting to find the pressure. I have more equations, but the only one I can clearly relate to pressure is Bernoulli's principle which the question does not provide all the variables for to my understanding.

supratim1
Gold Member
I think your approach is right. I don't see any other way. may be the answer is mistaken in the book you are referring. or may be there is something missing in the question.

It's an online homework for engineering. I don't know the answer, all I know is whether I get it right or wrong. Would it make a difference if the ground was used as a datum instead of the pipe?

The question nicely informed me that I made a common mistake and that water pressure is measured in gauge pressure. I got an answer of 505.472 after atmospheric pressure was added and I used the ground as a datum. I did 35+7.9 as one height and used -1.7 as the other. I'm assuming since it said common mistake that I am headed in the right direction with previous experiences with their random hints.

supratim1
Gold Member
gauge pressure is applicable underground too. i dont think you should subtract 1.7

If you use 1.7 as positive or negative it still yields an incorrect answer. I've tried numerous combinations of the heights and can't seem to get it right. As far as the gauge pressure is concerned, it is most definitely applicable underground. Adding atmospheric pressure gives you the absolute pressure. I believe the question wants an answer solely based off of the gauge pressure which is the depth ( or height) multiplied by gravity and rho.

Last edited:
tiny-tim
Homework Helper
Welcome to PF!

Hi Nuke787! Welcome to PF!
A cylindrical water-storage tank has a height of 7.9 m and a radius of 6.2 m. The storage tank is full of water but is vented to the atmosphere. The bottom of the tank is 35 m off the ground. A 12 cm diameter pipe runs vertically down from the tank and goes 1.7 m underground before turning horizontal. The water flow in the horizontal pipe is 85 L/s.

I know that gauge pressure is ρ*gravity*depth. I am assuming my problem is that I am not using the appropriate height as rho and gravity are given. For heights I have tried numerous combinations of heights. I have tried 1.7, 1.7+35, 1.7+35+7.9. This is a hydrodynamics problem, so my method may be incorrect in trying to solve for the pressure.

You haven't used the flow rate.

(This is a Bernoulli's equation problem)

supratim1
Gold Member
oh yes,sorry i didnt notice that!!! use that 85 L/s

Well played sir. Using Q= vA and the continuity equation v1A1=v2A2 to solve for the velocities I'm assuming. I was hesitant to use Bernouilli's as it involves an initial pressure on the right side P1 and on the left side a P2. I'm assuming I'm looking for P2. For P1 would I find it by using my afore mentioned method of height*gravity*rho?

Last edited:
supratim1
Gold Member
yes, that is P1

tiny-tim
Homework Helper
Hi Nuke787!
I was hesitant to use Bernouilli's as it involves an initial pressure on the right side P1 and on the left side a P2. I'm assuming I'm looking for P2. For P1 would I find it by using my afore mentioned method of height*gravity*rho?

Bernoulli's equation is along a streamline.

You compare P etc at one end of the streamline to P etc at the other end.

In this case, your streamline starts at the water surface, so P1 is Patm

Alright so this is where I'm at.
75 L/s= .075 m3/s
Q = vA
.075 m3/s=v2((.06m)2*pi)
v1A1=v2A2
v1*(pi*(6.2m)2*7.9m)
P1+(1/2)$$\rho$$v12+$$\rho$$gh1 = P2+(1/2)$$\rho$$v22+$$\rho$$gh2

(101.3kPa+(1/2)1000kg/m3(7.861418572*10-5m/s)2+1000kg/m3(9.81m/s2)(35m+7.9m)=P2+(1/2)(1000kg/m3)(6.631455m/s)2+1000kg/m3(9.81m/s2)(-1.7m)

How does that look, and as far as the last term, assuming that I use the ground as my datum, am I correct in making it negative?

tiny-tim
Homework Helper
Hi Nuke787!
How does that look, and as far as the last term, assuming that I use the ground as my datum, am I correct in making it negative?

(I haven't checked your arithmetic, but …)

Yes that looks right,

except isn't it 85 L/s, not 75?

and if you're using gauge pressure, then P1 = 0.

And yes, the 1.7 is negative (or you could use the water surface as your zero-level, and put the whole 1.7 + 7.9 on the same side).

That might explain why I got it wrong, I'll crunch the numbers again. It will be a miracle from god if I pass this engineering assignment. Thanks for all the help in advance.

Got it on try 26. Thanks a ton for the help guys.

Hydrodynamic pressure (P):
P=ρv²/2

Volumetric flow rate (Q):
Q=Av
(A → area; v → velocity)

Attempt to solve:

Q=85L/s→0.085m³/s

A=πr²
A=0.0036πm²

v=Q/A
v=23.61m/s

P=ρv²/2
P=278742.284Pa