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Function of an accumulator with a pump

  1. Apr 14, 2015 #1
    Hello,

    I'm performing some simulations of apump with a hydraulic accumulator. I don't understand clearly the results. In a closed loop, i connected a pump with a Valve in order to create a pressure drop. The results are a linear increase of the pressure in the circuit and the pressure drop is only created by the valve. Is the pressure increase realistic ? The pump efficiency is 100% I don't know how the pressure could increase with only a valve in the loop. I added a hydraulic accumulator at the entry of the pump and i have now a constant pressure of 1 bar. The accumulator has changed the behavior of the circuit, but i don't clearly understand why. Someone have an idea ?

    Thank's a lot.
     
  2. jcsd
  3. Apr 15, 2015 #2

    Baluncore

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    Sorry about the delay.
    In a hydraulic circuit it is normal to have a reservoir tank at atmospheric pressure. The pump takes fluid from that tank and pushes it through the system. If you have a flow restriction valve then the pump output pressure will rise. You have a stable system so there is no advantage in adding an accumulator to the system since there is no dynamic flow rate. Does your simulation assume pump input is from a tank at 1 Atm ?
    Connecting an accumulator to the pump input would be pointless since the pressure will remain at 1 Atm and so there will be no fluid to accumulate.

    What sort of pump do you have, constant flow volume ? constant pressure ?
    What software do you use to simulate hydraulics ?
    What hydraulic system are you trying to simulate ?
    Please post a diagram of your system with your refined question.
     
  4. Apr 15, 2015 #3
    I understand that to mean the pump outlet is directly connected to the pump inlet with a valve positioned somewhere in the line.
    .
    If so, here is my take on what is happening.
    You can ask yourself some questions, simply such as,
    "What does a pump do?"
    Answer - It adds energy to the fluid.

    What might be a consequence of this additional energy?
    The fluid temperature may increase?

    And? If the rise in temperature is not compensated quickly enough as heat flow to the environment?
    As a fluid's temperature increases, it expands.

    And?
    If enclosed in a rigid system of pipes the pressure of the fluid will increase.

    How does an accumulator minimize the pressure buildup?
    YOUR answer here ____________________________________
     
  5. Apr 15, 2015 #4
    Thank you for the answer.

    The pump inlet is a fixed volume of fluid with an initial state variable at 1 atm. I have a centrifugal pump in the circuit. As 256bits mentionned the energy added by the pump has rised the temperature of the fluide and the pressure. I have confused a 100% efficiency with no heat transfer and i didn't pay attention to the enthalpy transfers to the fluid. You mentionned that we use accumulator for dynamic flow rate. Is that for the start of the pump until we get a permanent flow rate ?
     
  6. Apr 15, 2015 #5
    Thank you for the answer.

    I had a look and indeed there is an enthalpy transfer to the fluid. I have checked some documents about accumulators. What i understand is (please correct me if I'm wrong):
    The accumulator follows the Boyle Mariotte law and is composed of a fluid and gas separated by a barrier. So if the pressure of the fluid rises, in order to keep it at a fixed value, we have to increase the volume of the fluid. For the gas, the results will be an increase of the pressure and the volume will be smaller. In an accumulator, do we have a constant temperature (Boyle Mariotte law is valid at T constant if I'm right), the temperature of the gas compressed doesn't rise ? I understand that the fluid can be assimilated as incompressible, and if the fluid volume inside the accumulator increases at a specified volume, at a constant temperature we can have the same pressure.

    I found 2 types of equation : for isentropic process and adiabatic process. How can we 'define' if the process is fast or low ? It depends on the application ?

    Thank you for your help.
     
  7. Apr 15, 2015 #6

    Baluncore

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    An accumulator is attached to the high pressure side of the pump. The fluid volume in the accumulator that compresses gas comes from the fluid volume in the vented tank. The top of the tank contains air at 1 Atm. An accumulator usually contains dry nitrogen at an initial pressure. Think of an accumulator as a limited volume gas bubble trapped in the system.

    It is the thermal time constant of the gas in the accumulator that determines fast or slow.

    If hydraulic fluid flows slowly into the accumulator, then as the gas in the accumulator is compressed it will rise in temperature. That heat will then be lost by radiation from the accumulator gas. The opposite process occurs as fluid pressure falls and heat is absorbed from the walls of the accumulator.

    Fast cyclic changes of accumulator gas pressure will result in cyclic heating and cooling of the gas. That will be more efficient since less time is available for heat diffusion between the gas and the wall of the accumulator.
     
  8. Apr 23, 2015 #7
    Sorry I didn't have internet this last few days.
    Thank you for your explanations I understand cleary how that works now. However u have 2 last questions.Is The gas pressure value always equal to the pressure of the fluid ? When the nitrogen gas is compressed, some fluid flows in the accumulator. How occur the heat transfer between the fluid stacked in the accumulator and the fluid that flows through the pipe ?
     
  9. Apr 23, 2015 #8

    Baluncore

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    A hydraulic accumulator contains a flexible bladder that contains the nitrogen. That bladder is pre-charged to an initial pressure. Fluid will not start to enter the accumulator until the fluid pressure rises above the gas pre-charge pressure. Gas and fluid pressure will then be the same while there is fluid in the accumulator.

    Heat generated by gas compression is usually small compared with the thermal mass of the accumulator and oil. Heat is lost to the fluid and through the walls of the accumulator. Accumulators are not usually provided with additional cooling since heat is both generated and absorbed as the accumulator is cycled. The accumulator is usually fitted with a 'T' connector at the bottom so it can be close to the hydraulic circuit. There is a tidal flow of hot oil between the accumulator and the cooled fluid in the system.
     
  10. Apr 23, 2015 #9

    stewartcs

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    Heat transfer in accumulators is a tricky thing actually. If you charge or discharge the accumulator slowly (assuming you're using reasonably low system pressure), then you can use the Ideal Gas Law and some other process like Isothermal to get a reasonable answer on the relationship between the pressure and volume of the accumulator. If your system pressure is too high, then you'll need to use real gas data as the Ideal Gas assumptions break down at high pressure (usually after 3,000-psig you would see unacceptable errors in most engineering applications).

    If you are charging or discharging rapidly, then you should be using real gas data also, but now you should assume it is an adiabatic process since there wouldn't be enough time for heat transfer to the environment to occur (ok in reality it's not perfectly adiabatic but close enough for engineering work). This essentially means that you model the pressure volume relationship assuming the entropy of the gas is constant (i.e., isentropic even though it's irreversible). This leads to a significantly different behavior of the accumulator performance.

    Moreover, the type of precharge gas effects the performance as well. Using Helium instead of Nitrogen can actually improve the performance of the accumulator.

    Hope this helps.

    CS
     
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