7 replies to this topic

[rubensvm]

Dear All,

I am sizing a pump to transfer water from a river to a reservoir, attached is a scratch of the system. My costumer demands that the lines should be made of high density polyethylene (HDPE). I am sizing the pump discharge line to have a velocity of about 1,7 m/s, to minimize head loss due to friction. Although the elevation difference between the pump discharge and the final destiny (water reservoir) is just 15 m, there is a hill in between then where the maximum elevation of 25 m is achieved. Normally, to size the pump head required, I would calculate the head loss due to friction plus the elevation difference.

I have the some doubts and hope you could clarify the following points:

1) At steady state operation, with the line filled with liquid, the governing elevation difference is de 15m, correct?

2) But, when the line is empty (during the startup) the pump will have to fill up the line, and, in this case, the pump head required could be greater than the first case and should the selected head, correct?

3) Every time the pump will be turned down, the siphon effect would drain almost completely the pump discharge line, isn’t it? So, when the pump restarts, it would have to fill up the line again. This should be avoided? If yes, what are the options?

Hope you could help me.

Rubensvm

Attached Files

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[fallah]

rubensvm,

 

1) Yes, provided that suction head is to be considered around zero...

 

2) Yes, at start up the required head would be around 25 m...

 

3) Once the pump turned off the check valve at pump discharge line would prevent the discharge line to be drained through the pump; but the discharge line can be totally drained into the reservoir due to siphon effect which could be prevented by installing a vacuum breaker at the high point of the discharge line...

[Bobby Strain]

I would provide sufficient pump head to deliver design flow to a destination of 25 meters. It would be embarassing to buy a smaller pump that couldn't make it up the hill. If your customer doesn't agree, then she can instruct you to install a lower head pump. Then you are no longer responsible for a bad decision made by your client. But, be certain your management knows who the customer is.

 

Bobby

[Rahimzadeh]

Dears:
I think syphone effects will not happen ,because the B point is 15 m higher than the river water level; therfore syphone effects must be forget

[fallah]

I think syphone effects will not happen ,because the B point is 15 m higher than the river water level; therfore syphone effects must be forget

 

Mr. Rahimzadeh,

 

You're right, I did respond without considering the special case for which the details has been attached by OP to the first post...

 

In fact, for a siphon to flow continuously, the height of relevant upper leg must be less than atmospheric pressure and the outlet of the down leg must be below the surface of the source reservoir (here river)...

 

Then in the case submitted by OP, siphon effect will not happen after pump turn off...
 

[Art Montemayor]

I agree with Bobby Strain's recommendation(s).  This application, in my experience, is not as simple as it seems.  There are other factors to take into account such as:

• For this system to work continuously and without any flow problems, steady state flow conditions must be ensured.  Is the pump's discharge system meant to be 100% liquid (water) full?  If so, then this means that the design must incorporate some means of establishing this criteria and maintaining it.  I don't understand how you expect the water velocity to aid in controlling the flow.  Please explain.
• The reason for the discharge piping to be full is to avoid a tendency to establish free-fall of the water from the 25-meter high point down to the 15-meter elevation.  The 100% liquid full condition can be met by designing the discharge piping such that it's friction and size maintain the piping full.  When the discharge piping is 100% full, the water at the 25-meter level will flow to the 15-meter discharge under the influence of gravity.  This is called a head "recovery" and the Bernoulli equation defines this.  Normally, it is often wise to disregard this recovery and accept the conservative value when considering pump work.  Theoretically the pump is only doing work to raise the water 15 meters, and not 25.  But taking credit for the recovery is risky due to the flow friction, flow obstructions, and possible air problems.  
• There could be an accumulation of air or non condensables at the top of the highest pipe elevation and there should be a vent valve installed at the top of the highest pipe location.  Additionally, the pump HAS to pump water initially to the 25-meter height- every time it starts up.  So the power requirements have to take this into consideration.
• A drain line directly after the pump's discharge check valve should exist in order to evacuate the 25 meters of discharge line to the highest point (and possibly to prime the pump).
• Presumably the pump suction issues are resolved.  It is not shown if the pump is at, below, or above the river's suction point.

Other potential issues are not given - such as the conditions for the target discharge and the pump's operation:  if the pump is a centrifugal type, then the design flow rate must be controlled in order to avoid it operating "off the curve".  Is a back pressure control valve employed?   If so, where?  Or is the ultimate outlet to the reservoir to "wide open"?

[rubensvm]

Dear All, Thanks for the answers.

 

Mr. Falla and Mr. Rahimzadeh, I understand your point. The end of the discharge line is open to the atmosphere, and is not submerged, so every time the pump is stopped, air would enter the line.  Then, the water contained in the line between the top of the hill and the water reservoir would be drained, but the line between the pump discharge and the top of the hill would be water filled. Is this correct?  In this case, I still should consider installing a vacuum breaker at the highest point?

 

Mr. Montemayor, the velocity is not relevant to control the flow, just to calculate the head friction loss, so the idea was to size the discharge line so that it had low pressure drop due to this mechanism (friction).

 

The idea is to work with the line 100% filled with water, so I will have to put a vent valve at the highest point of the line to avoid air, thanks to remind me. I will also consider installing a drain valve downstream of the check-valve.

 

Just to clarify, the water capitation will be almost 2 meter bellow the river bed so that the pump would be always submerged.

 

Again, thank you all for the answers. Best regards.

 

Rubensvm

[fallah]

 

Mr. Falla and Mr. Rahimzadeh, I understand your point. The end of the discharge line is open to the atmosphere, and is not submerged, so every time the pump is stopped, air would enter the line. Then, the water contained in the line between the top of the hill and the water reservoir would be drained, but the line between the pump discharge and the top of the hill would be water filled. Is this correct? In this case, I still should consider installing a vaccum breaker at the highest point?

 

 

rubensvm,

 

After pump shut off the water contained in the line between the top of the hill and the water reservoir would be drained but for the line between the pump discharge and the top of the hill there would happen two situation depending on considering vacuum breaker at the high point or not:

 

-If vacuum breaker is considered, mentioned line would be water filled...

-If vacuum breaker isn't considered, the upper part of mentioned line till the elevation equal to the elevation of discharge outlet can be drained due to vacuum generated and the pipe can normally tolerate the generated vacuum condition...