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(OP)
If we have a pipe contains water and has friction losses of 1 bar ,, static head 2 bars ,, and it is open at its ending to atm.pressure ,, now we need the pressure at the pipe exit to be 2 bars ,, so we get a pump at rated head equals 5 bars at the required flow rate ,, now when we install the pump , i know that it responds to the system resistance which in our case is 1 bar of friction and 2 bars of static head so it reachs the exit at atm.preaaure ,, so how can we get the water at the exit at 2 bars ? i got confused

Review some fundamental concepts and try to understand pump head ....

Remember, pressure drop is a function of flow rate, pipe diameter and surface roughness

It is also important to note that the design demand for a specific liquid flow - AT A CERTAIN LOCATION, WITH A CERTAIN PRESSURE, determines the pipe size, the pump design, pump horsepower etc,,,,

You do not select the pipe first and then try to make the pump and the rest of the system suitable for the end conditions....

You cannot force the flow of an entire river through a 1 inch pipe

I suspect that you may be working for an MBA ....

MJCronin
Sr. Process Engineer

You either add a control valve to control the end pressure to 2 bar or add something like a spring relief valve set at 2 bar. downstream of either the pressure can be anything < 2 barg.

Or connect it to a system which is at 2 bar.

The possibilities are endless.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

2 bar is a residual pressure which you need to account for when you select the pump. You need to specify the total head and flow.

If the flow exists to atmosphere, the pump that you have originally sized for 5 bar and a certain nominal flow will have its operating point moving to the right of the performance curve and thus it would deliver higher flow than what is nominally specified/required.

If the flow discharge sees a resistance of 2 bar (say there is some kind of apparatus at discharge which creates resistance), your pump will work at its design point /BEP (or close) and deliver nominal flow as required.

Note: 2 bar of residual pressure is common in potable water distribution systems for communal houses, buildings, etc, especially it is a value that more or less corresponds to the triggering pressure of water boilers etc. so they can operate properly.

(OP)
Hey rotw , your answer is the most clear to me ,, ok i understand that if the exit is at atm pressure , so the pump will move on the right on its curve to produce more flow rate than required ,, now if i get a control valve it will create additional resistance so the pump pressure will be the rated one , but at this case the pressure of water at exit will still be atm pressure as the control valve will eat this pressure ,, so if we need 2 bars at the exit can we throttle the control valve more so the pressure of water after the control valve will be 2 bars but at this case the flow rate will be less than required as the point moves to the left on pump curve right ?

quote
so if we need 2 bars at the exit can we throttle the control valve more so the pressure of water after the control valve will be 2 bars but at this case the flow rate will be less than required as the point moves to the left on pump curve right ?
unquote

One note: if you throttle more, it means the valve is closing and you are creating more resistance.
There seems to be some confusion in your statement.

The question is why would you need 2 bar of residual pressure? what is the purpose?
A way to look at it is to consider 2 bar residual pressure as a form of "contingency".

When this is accounted for in the pump selection/design, then the pump will keep on delivering the nominal flow of your final consumer - as a minimum - irrespective of any loss created by a device/apparatus downstream in your system, as long as this does not exceed 2 bar.

By the way, when water leaves pipe to enter reservoir it expands! since 2 bar pressure loss is quite high for a singular physical expansion pipe/reservoir, you would indeed need to put a throttling device to kill those 2 bar - which you would do for no reason except move the operating point of the pump to the best efficiency area, which - unless it is otherwise justified - makes no sense.

Is this not another version of your previous post?

You seem to be having difficulty with this idea of pressure at the end of a pipe.

The only way to get 2 bar at the absolute end/ exit of your pipe is for something to impose it. So this could be for your example of 2 bar the bottom of an atmospheric tank of water 20m high, a cylinder with gas in it at 2 bar, or the Imber into a pump.

If it's an pwn end going to atmosphere then the end point pressure will be more or less 0 barg.

Does that help?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

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