Hydraulics problem - calculate pressure at different points

  • Thread starter jet1985
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Homework Statement



Hey, stuck on this question in my hydraulics course:

Question involves lubricating oil pumped around a large piece of machinery.
Density of oil = 810 km/m3.
Viscosity = 0.01 kg/m.s throughout the system.
Flow rate is 0.140 Litres/sec
Pipe diameter = 3mm

Pressure at point A = 1.55 kPa.
Point A is 5m above the ground.

Point B is 36.5m above the ground. Question requires pressure to be found at this point ignoring minor losses and friction.



Homework Equations



Bernoulli

The Attempt at a Solution



I attempted to solve this problem using Bernoulli equation: (pA/Pg)+(VA2/2g) + ZA = (pB/Pg)+(VB2/2g)+ZB, However the result I get is negative and looks all wrong.

We were given very limited information but I assume we have enough info to answer the question. I don't see how the pressure at point A could possibly be only 1.55 kPa, given the oil is being lifted 31.5m high and 1.55 kPa is many times less pressure than even a car tyre.

The flow rate also seems unrealistically high, and the pipe diameter seems unrealistically narrow.

The question only specifies "pressure = 1.55 kPa". Is it possible they meant dynamic pressure?

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
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Something seems wrong with the numbers given in the problem. They seem unrealistic to me as well. Besides pressure issues as you say, the velocity is almost 20 m/sec. That would cause a lot of heating of the oil and large pressure drops.
 
Last edited:
  • #3
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For the purposes of answering the question, that would not matter as we have been told to ignore minor losses and friction losses. However I cannot get my head around the pressure drop.

Also, I have asked to lecturers for clarification on the question, but they have been very lazy about getting back to me... I thought perhaps they meant MPa instead of kPa.
 

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