Pump effect on water

[waseem19]

hi everybody , my q is really simple ,, consider a pump delivering water to downstream tank, the way i understand pump is that it left the HGL for the water so, the water would act as if it was actually lifted to certain point then move down to the downstream tank. now this means that each water drop has enough energy to take it from the pump to the downstream tank ,, so what happen if the pump stops?
surge analysis says that water will start to slowdown eventually stop and move toward the pump again , but why did the water stop????? ,
since each water drop has already passed through the pump so it must have enough energy to reach the downstream tank ,it should overcome gravity,friction or any other dissipation of energy, i know that pressure will drop near the pump ,but will that pressure cause this decceleration?

 

[BobPE]

waseem19:

You pretty much answered your own question. The EGL is what the pump is generating of which the HGL is a part of. When the pump stops, the HGL instantly reverts back to the conditions established by the static elevations of the fluid. It is this transition that induces transients. The confusion in your thinking is aht looking at water as discrete particles makes you factor out the confines of a pipe wall. This pipe wall is the improtant physical part of the system that allows the EGL and HGL to form and water to move. This of that droplet surrounded by a shell similar to a pipe, the minute you stop adding energy through the open part of the shell surrounding the droplet, the droplet will revert back to static pressure.

Good question though, it really makes you think!!!

BobPE

 

[Kawartha]

Can we apply Bernoulli's Equation to this question?

P1/S.G +Z1 + V1 squared/2g + Ep = P2/S.G. +Z2 +V2 squared/2g + Hf

Where:
P is pressure in feet of liquid,
Z is elevation in ft,
V is velocity in ft/sec
E is pump bhp
Hf is friction losses in the system in feet of liquid.

From the equation, if Ep is reduced to 0, then P1=P2, V1=V2, etc, when the moment of inertia effect is over.

The change does not happen instantaneously, due to the pump rotating moment of inertia. As noted above, the pump will continue to run based on moment of inertia (with larger pumps this can span a period of 30 seconds and more). However, P2 (at the pump discharge flange) will immediately start to drop and this pressure will be totally dependent on the moment of inertia of the rotating element. Pressure downstream from the discharge will change at the wave speed of the liquid (incompressible liquid). Wave speed can be calculated and would probably be in the multi 1000 ft/sec range.

BobPE, could you please define EGL and HGL.

 

[BobPE]

kawartha:

sorry, energy grade line and hydraulic grade line.

The inertia of the pump compared to the energy in the EGL is very small for most all applications without flywheels and weird things like that. For the sake of argument to get to what you are looking for, its instantaneous in my world lol!!!

Take care....

 

[waseem19]

Bob , i've assumed droplet for simplicity , but that drop let is surrounded by other droplets that have the same energy,so it's really a section of pipe ,
but what's really interesting is what you said about adding energy through the open part , does this mean that a water droplets maybe several kilometers away from the pump still needs the pump (energy source) to reach it's distination , if this is true then pump doesn't actullay work as i'v described them before , but they more like "pushing the water forward" ,so each second a volume of water is forced into the pipe by a certain distance causing all the water in the pipe to move exactly the same distance.
this way your thoughts are correct ,water will stop because nothing is pushing it anymore , but that means that each water droplet doesn't actually have enough energy to go where it is supposed to go , please comment on my thoughts

kawartha, the pump inertia doesn't really play any role in my world to , cause a valve on the discharge side closes immediatly after the pump does.

 

[BobPE]

waseem19:

pumps add energy via velocity of the to water by defination. Without the pipe though, you only have velocity and that can be applied to the individual droplet, but the droplet would only go as far as the velocity imparted to it. With the addition of a pipe, the velocity imparted to the water turns into potential energy of the system or EGL or pressure head, all confined by the pipe. If the pipe is open to flow, then it flows in mass via the HGL (EGL-V^2/2g). If the pump shuts down and stops adding energy, the droplet nor the water in mass that had energy added will not make it to the end since the EGL is now gone for the system.

Think of a single droplet of water introduced into a pump connected to say a 10 inch pipe open to flow. The energy is added to the droplet by the pump, without bulk water it will simply fly down the pipe as far as the velocity applied to it then stop. That is a proof state,ent that water is needed in bulk to hold the energy imparted by the pump.

take care

BobPE