higher upstream static pressure!

[naifmbo]

is it possible that in an inclined discharge line from the pump that the static pressure in the lower level(50 m from pump discharge) will be higher than that in the higher level (pump discharge)? if it is ,how water will move from low to high pressure?.

thanks.

 

[Yorkman]

When you say static pressure are you refering to the line pressure when the pump is off?

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[BigInch]

Static pressure always increases with lower elevation in a closed piping system. In fact, you must check your pipe pressure allowable at the lowest point in the system. That is where you will usually find the highest pressures, even if the pump is on, but it does depend on how much static pressure you gain with elevation decrease verses dynamic (flow) loss per foot of pipe when running.

It is not the static pressure that moves the water from point to point in a closed piping system. In fact, static pressure plus flowing pressure loss is what must be overcome with your pump when you turn it on. If the pump is properly sized, it will add pressure (head) sufficient to overcome the static differential pressure and the pressure losses due to the liquid flowing in the pipe.

 

[25362]

To naifmbo, the answer to your query is: yes ! Water in a pipe can move from lower to higher pressures. It is mechanical energy that moves the fluid, not just pressure.

Mechanical energy in frictionless flow (remember Bernoulli's expression for incompressible fluids) has three components: kinetic energy per unit mass (V[sup]2[/sup]/2), potential energy per unit mass (Zg), and flow work (pressure multiplied by volume (=1/[ρ]).

If we divide all these components by g, we get the dimensions of length or height. V[sup]2[/sup]/2g is called velocity head, Z is called height, and p/([ρ]g) is called pressure head.

BTW, [ρ] is density; V is velocity; p is pressure; g is acceleration of gravity.

Thus water can flow from point 1 to point 2 against a higher pressure if the other components of mechanical energy in point 1 are sufficiently greater than those in point 2 so as to overcome the negative pressure difference.

 

[Yorkman]

Like these guys are saying the differential is being created at the pump suction verses discharge.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[BigInch]

25362

Oh how I wish you didn't say, "Water in a pipe can move from lower to higher pressures."

I've been biting my tounge, but I do believe I'll let it go.

 

[25362]

Of course, the conclusion is counterintuitive.

Conservation of energy would show up with flow in a horizontal (constant potential energy) pipe composed of two parts of different diameters.

When water flows from the smaller diameter to the larger diameter section, and discarding friction, the lower velocity (kinetic energy) in the enlarged portion converts into pressure energy. As a result gages installed on both would show that the flow goes from lower to higher pressures.

 

[BigInch]

No, its not that. Its that "move from lower to higher pressures", doesn't sound right when you do a derivation of the continuity and momentum equations for pipe flow. The derivation begins with the pressure equivalents of the total Bernoulli energies at both the upstream point and the downstream point. When those pressures are drawn on a free body diagram of a differential length of fluid in the pipe, its obvious (and intuitive) that fluid always flows from high pressure to low pressure.

I decided to let it ride, because it is true that pressure gauge readings would indeed agree with your statement.

 

[blackwed]

How many/which of the three energy components--velocity head, pressure head and static head--in Bernoulli's equations contribute to the readings on the gauges in the 2-different-diameters-of-pipe example? A gauge clearly does not indicate total energy content since the readings go down in the higher velocity section and then back up in subsequent lower velocity section.

DB

 

[25362]

Very good question. Pressure gages in the mentioned example measure what is called static pressure (taps are at the pipe wall).

Velocity head can be evaluated from stagnation pressure, aka total pressure, aka dynamic pressure, as measured by "impact" tubes, as with the Pitot, and equals the difference between this pressure and the static pressure.