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Homework Help: Hydraulics problem - calculate pressure at different points

  1. Apr 21, 2012 #1
    1. The problem statement, all variables and given/known data

    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.

    2. Relevant equations


    3. 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?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 21, 2012 #2
    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: Apr 21, 2012
  4. Apr 21, 2012 #3
    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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