Pressure Loss in a fluid system 2

[GregDesign]

I've always been confused by this. Is there any loss in pressure if you start with a supply pressure of say 1200 psi for hydraulic or 120 psi pneumatic and you want to have the correct pressure for you actuator, say a cylinder? In fluids they teach you all these losses in energy due to elevation, velocity, and expansions and contractions like elbows. I never had to use these but from my understanding pressure is resistance to flow, so if you have elbows and tees in you piping system, does pressure increase or flow decrease? Confusing.

 

[desertfox]

Hi Gregdesign

If you start with a pressure of 1200psi as the supply pressure it cannot increase above that, pressure can be likened to voltage in an electrical circuit,electrical current analogous to flow, the resistance comes in related to friction and restrictions within your pipe line.
So the losses in energy you refer to are exchanges from potential energy to kinetic energy.
Now the more bends and lengths of pipes you use in the circuit will increase the resistance you need to overcome to maintain a given flow rate.
So if you increase the resistance you will need to increase the pressure to maintain that given flowrate.
Go to the site i posted it may help

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html

desertfox

 

[budt]

GregDesign asked:
"so if you have elbows and tees in you piping system, does pressure increase or flow decrease? Confusing."

If you think seriously about that question and there is an infinitely powerful prime mover driving a 100% efficient Fixed Volume Pump at a constant RPM supplying the circuit at 10 GPM with no other possible flow path except the "piping System" it is obvious that pressure will have to increase to overcome the resistance since flow cannot decrease with this example scenario.

Pumps produce Flow and only flow. Resist that flow and pressure increases. So does the pump make the pressure increase or does the Resistance to the pumps flow make that increase.

I've seen grown men argue that one all day about like arguing which came first, "The Chicken or the Egg?"
Set up your own experiment with a pump driven by an electric motor that has a Pressure Gauge, Flow Meter, a Relief Valve and a Ball Valve, in that order, all rated for the pressure you desire on its Outlet Port. Connect the Inlet line of that pump to a tank and the Outlet from the Ball Valve back to the same tank.

Now, with the Ball Valve fully open turn the electric motor on and check pressure and flow. It should read almost no pressure since resistance is extremely low, if all the components are rated at the flow you chose, and full pump flow. You can run this circuit as long as you want and pressure will never increase UNLESS you start RESISTING THAT FLOW by closing the Ball Valve. In fact you can set pressure at any level by the amount of restriction you set with the Ball Valve up to but not exceeding the Relief valve setting.

Many pumps have been changed unnecessarily when a pressure gauge reads at or near Zero when the problem actually was an unrestricted flow path to tank was wide open. That is why I always spesify a Flow Meter at the pump outlet, before the relief valve, on any circuit I design.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[rmw]

In the system you describe, as flow increases pressure decreases due to some or all of the factors you mention. In a static system with no flow, you can have 1200 psi at your actuator inlet. But as the cylinder begins to move due to the effects of pressure on its piston and the fluid flows as it moves, the pressure lowers in your system.

Once the cylinder reaches the end of its stroke and the flow ceases, the pressure in the whole system will once again rise to 1200 psi.

rmw