Why we are not considering the pipe diameter in static head in a headloss calculation? 1

[CHK07]

Actually for head loss calculation for a selection of a pump, we are taking the frictional loss plus static head. Here, the static head we are considering is vertical height between the suction water level and discharge water level, it is independent of the pipe size we are using. That is; for any pipes (2" or 3" or 6"), the static head is same. My doubt is how can we consider this static head independent of pipe size... Because, the weight and pressure of the water in the vertical height of the pipe is vary with pipe diameter.
Consider a situation:
I want to pump water at 22m height at 10 GPM ( the suction tank is about the pump datum level). Here we calculate head loss as 22+ friction loss in pipe. (Static head + friction loss)
Obviously, the pipe with larger dia have less friction loss.
Assume I'm getting the head loss as:
For 1" pipe = 42m
For 2" pipe = 35m
For 8" pipe = 23m
If we are increasing the diameter, the head loss
Will be low and loww.. Here why the weight of the fluid is not in the scene?
How can we consider the pump curve in this scenario?

 

[EdStainless]

The mass of the water may very with pipe size, but so does the area that it loads.
Static head is only related to height differences.
This is pump systems 101 stuff.

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P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

Because your sentence "the weight and pressure of the water in the vertical height of the pipe is vary with pipe diameter" is incorrect.

The weight of the water will increase, but the pressure won't. Pressure is force per unit area. The unit area hasn't changed so the column of water above that unit area is the same regardless of pipe diameter.

In a fluid pressure acts in all directions at any point.

The system curve is overlaid on the pump curve and where the two intersect that is your flow.

So at 0 flow you have 23m head. At flows >0 your systems curve is >23m. How much greater depends on your pipe size.

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

 

[EdStainless]

When you plot a system curve the static head is a linear offset that is applied before the dynamic factors are added on top of it.


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P.E. Metallurgy, consulting work welcomed

 

[CHK07]

Thanks for your replies guys...
Jst consider it in a large view..

Here I want to transport water through a large pvc cylinder of diameter 6m to 22m height. I'm using a 1hp pump that provides a head of 22m at 10GPM. The pump connection is just 2". I'm going reduce the cylinder bottom to 2" so that I can connect the pump at the bottom area directly.
Here for the 6m dia cylinder, the frictional loss is negligible. But do you guys think, can I do this witb a 1hp pump... I don't think so because that much quantity of the water can't push by a 1hp pump up to that height..

 

[EdStainless]

Where is the water going?
We need a sketch of the system.
The pump only sees the differential head.
How high above the surface in your tank is the destination?
And how low can the level in your tank get?
What size is the pump outlet?
I presume that is less than 2".
There is usually some benefit to keeping inlet pipe sizes as large as practical until very near the pump.

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P.E. Metallurgy, consulting work welcomed

 

[swazimatt]

think of it as two seperate items. Imagine you are lifting a box you lift it through the air and use a certain amount of energy (static energy), now you lift the box, but slide it against a rough wall at the same time the static energy is the same (lifting the same height) but you need to increase your energy to counteract the friction against the wall (static plus friction) the amount of friction could vary by material , stickyness etc (this is the impact pipe diameter has on calculating friction forces)

water pressure is a factor of height, a 1m high dam wall can hold back a small swimming pool of water just as easily as a 100km (1m deep ) dam

 

[1503-44]

You can do this with a 1 HP pump, but only at a flow rate that a 1 HP pump will allow.
Power is Weight of water x height being pumped within a given time.

Water Wright is 62.4 lbs/ft3
Height 72ft
If you try it at 1 ft3/sec

62.4 x 72 = 4493 ft-lbs in 1 sec
1 HP is 550 ft-lbs/ sec

4493/550 = 8.2 HP

Say the pump has a 50% efficiency, so you must double that power.
8.2/ 0.50 = 16.4 HP

So with only 1 HP you can lift 1 ft3/sec / 16.4 = 0.061 ft3/sec
So that's almost 0.5 gallons/sec, or about 2 liters/sec
Or 120 liters/minute.


Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Of course you can do this with a 1HP pump. But the flow is very low for such a huge pipe.

You're only pushing what the pump is putting out.

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

 

[CHK07]

THANKS FOR YOUR RESPONSES GUYS...
I got your points.. Actually I think this is the best thing that happening on the internet, for me.. This kind of active community is very rare I think...

I have one doubt balance.,

If the pump is continuously pushing water why the limit of head is there..?

if the pump have a maximum head of 22m at 0 flow, the pump is still working and pushing water even after 22m ( assume only vertical height is there). why the water is stopping there?

if the mass of the water is independent of pump head, why should it stop at a particular height?

ALSO,,
Consider a case, Im using 2 same pumps having 12m maximum head. Im taking suction from the same tank and on the discharge ends, for the one pump Im connecting 6" pipe and for the other pump, Im connecting 1/4" dia pipe only in vertical directions ( seems like 2 pipes of different sizes are vertically standing on the 2 same pumps with common suction). should the water height be 12m for the both pumps?

 

[1503-44]

Head limits a pump in two ways.

Head = Pressure: pump casing wall thicknesses or seals may experience overpressure on higher heads.

Head requires Power: Pump components, shafts, must transmit that power/torque from motor to fluid. Shaft diameters must be able to do so without shearing, impellers must not disintegrate.
Pump Motors must also have the rated power capacity to supply the power required by the pump, limited by the amount of power that can be transmitted by the pump shaft without damage.

Higher and Higher heads need more and more power and will eventually exceed the motor capacity. The pump pumps, but the fluid does not reach higher levels. Overheating often follows when the pump flow into the pipe stops and the fluid only recirculated within the pump.

At 22m your pump is expending all its energy on friction of the bearings, seals, circulating the water in the pump casing round and round and holding the water column at its 22m height. No energy is remaining to actually move any of that water along the pipe.

Yes, Both pumps will have the same 12m Head. Total system resistance head is the sum of elevation resistance + flow resistance. You have 12m of elevation resistance and no flow, so flow resistance =0. Your "system resistance" is a constant 12m, the same for both pipes. Since there is no flow, the flow resistance is also the same for any pipe diameter and it = 0. The total for any pipe diameter is always 12+0 = 12m.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."