Solving a Tricky Fluid Mechanics Problem

[adzp]

Hi.

i am trying to brush up on my fluid mechanics for when i do it next year at uni.

do any of you guys have any idea how to solve the follow question?

'A suction pump moves water from a reservoir to a holding tank. The system is designed in such a way that the suction pump is inclined at an angle of 10degres from the horizontal. The operating envolope specifies that the pump cannot transport water if any gases are present iun the suction pipe. Gases are released from the water when the pressure falls below 30% of atmospheric pressure. If tyhe water is transported at a velocity of 1.8m/s in the suction pipe, and assuming the water in resiviour is at rest, determine the length of pipeline from the reservoir to the holding take'

hope some one can help. this is a tricking one for me.

thanks
adzp

 

[FredGarvin]

In a real world application, this is more like a calculation of NPSHa in a pump system. This has to do with the pressure dropping below the liquid's vapor pressure.

In your case, the Bernoulli equation is what you are looking for. You know pretty much everything except the final elevation. Since you know that the pipe is at a 10° inlcline, you could calculate the delta Z and back out the line length from that.

 

[adzp]

Since you know that the pipe is at a 10° inlcline, you could calculate the delta Z and back out the line length from that.

how would this be done? i have the forumula for bernoulli;s in front of me but can't really see how to use it for this question?

thanks

adzp

 

[FredGarvin]

The two locations would be the reservoir and the end of the pipe at the tank you are transferring to.

V1 =0 (given in the problem statement)
V2 = 1.8 m/s
P1 = 14.7 psia (atmospheric pressure assuming the reservoir is open to atm)
P2 = .7*P1
Z1 = 0 (use as a referernce)
Z2 = CALCULATED VALUE

Give that a try. The rest will be basic trig.

 

[adzp]

what if the resivior is closed to atm?

would it then be a different figure?

 

[adzp]

so

would it be

0 + 14.7/w + 0squared/2g = Z2 + 10.29/w + 1.8squared/2g + loss of head

woudl this be it?

 

[adzp]

is 'w'

pg? therefore 1000 x 9.81?

adzp