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reservoir and tank in series

reservoir and tank in series

reservoir and tank in series

(OP)
I have a reservoir of water with a total head of 100m. I have a pump connected to the tank with a pipe in series at the bottom (i.e H=0m).  The pump has a head of 50m for this sized pipe.  Would I be able to pump the water 150 m from the bottom (where the pump is)?

In other words would I be able to add the heads together when they are in series.  

Assume no friction in the pipe...

RE: reservoir and tank in series

What I understood from your post is that, there is a tank with water column height of 100 meters. Pump suction is connected to the bottom outlet of the tank and you have to pump this water to a height of 150 meters at a fixed flowrate. Right?

Assuming constant level of water in the tank (at 100meters height, assuming no friction in the pipeline, no entry losses etc., fixed flowrate and in ideal condition the answer is yes.

Some questions.
1. But how will you maintain the water level in the tank?
2. What is the application?
3. How can there not be friction in pipeline?

Regards,


RE: reservoir and tank in series

Building on Quark's good reply, there will be no flow.  There is only enough head for the elevation change and none available to change the velocity from 0 in the reservoir to any velocity corresponding to flow.

The acceleration loss should be taken into account when sizing the suction line of positive displacement pump, where the flow is started and stopped with high frequency.

RE: reservoir and tank in series

Agreed with Quark.  Pmureiko's comment on flow may or may not be applicable, depends on the pump's ability to delivery a total developed head of 50 meters refers to the operating point or the shut-in (dead head) point on the curve.  I think quark's assumption is the same as mine, that 50 meters is the TDH at rated flow.

I'm also assuming we're talking about a centrifugal pump, not a positive displacement, so acceleration losses wouldn't need to be a consideration.

RE: reservoir and tank in series

I think the 100 meter head on the reservoir will spin the pump backwards making it act like a turbine or a pump until the reservoir head drops to 50 meters at which time the pump starts to be a pump making the pump head & remaining head on the reservoir additive. Remember the system curve & the pump curve must intersect. However the point of pump maximum gpm is when its developed head is zero.

RE: reservoir and tank in series

lilliput1,

You are right about the pump acting as a turbine, but only on the condition that it is not operating and that the piping between the reservoir and pump and pump to discharge are sufficiently open.

However, when the pump is started (this should be done only against shut-off head, and with the pump at standstill), it will add the pump differential head to the suction head. The best way to explain this is to replace the 100 m of suction head provided by the reservoir with a pump providing 100 m of head.  In effect the performance will be similiar to the 2 pumps operating in series.  In other words, add the suction head to the differential head.

RE: reservoir and tank in series

Kawartha,
That is a good analogy and I see your point. I wonder though how long can a pump run dead headed. Think it can go through the start sequence you set up without it cavitating? A recirculation line from the discharge (upstream of the shut discharge valve) back to the pump suction should help prevent cavitation.

RE: reservoir and tank in series

lilliput1,

Just a couple of other points that I'd like to make.

1) When the pump is not operating, it would be necessary to have the pump discharge valve or reservoir valve closed to prevent pump rotation.  Although pump rotation would be in the normal operating direction, pump speed could exceed the design speed of the rotating element and could cause seal and bearing damage.

2) Keep in mind that it's only during start-up that the discharge valve would be in the closed position.  Of course the pump suction valve must be open.  The discharge valve could also be opened just before or during pump start without the possible damage that could be caused by Item 1 above.

3) When you raise the issue about cavitation, I assume that you are referring to low flow recirculation and not due to inadequate NPSH available.  Because of the very high suction pressure, and assuming a relatively low suction energy pump, I don't think recirc. cavitation would be a problem.  Even if some cavitation occurs, time frame of operating at shut-off head is short.

RE: reservoir and tank in series

It is hard to tell if this is a theoretical or actual pump problem.  According to the Bernoulli Equation / mechanical energy balance there is no energy for friction or allowance for acceleration losses.  So how can flow start even for a centrifugal pump?  Once flow is establish it is not a problem, so don't turn that pump off once you get it going.

From a practical standpoint it is only significant when working with positive displacment pumps.

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