problem of long suction pipe

[tinsnano]

Dear all,

I need to place a pump far from its suction tip (river) and close to electric supply such that cost of electric cable reduce.

so i am planning to place the pump far from the river but at lower elevation

what kind of possible problem may i face

 

[LittleInch]

Lack of inlet pressure into the pump so that the pump cavitates. If the pipe goes higher than the river level at any point then you might pull a vacuum and have no flow.

To avoid low inlet pressure you might need to make the inlet pipe much bigger to the extent that it cost mote than the electric cable which would then allow you to use a smaller pipe from the discharge of the pump.

This sounds a bit like a student question? ???

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[bimr]

If you oversize the inlet pipe, you will encounter problems with sediment settling out in the pipe.

 

[tinsnano]

ok but what will happen if i do the following to overcome the problem you mentioned
1. i will let the pump be positioned well below the river elevation such that i get enough suction pressure
2. there may be either priming or foot value to prevent no flow condition since there might be highest point between the river and the pump suction

btw i am not a student . i am a chemical engineer with limited experience

 

[EdStainless]

One approach is to build a pit/intake bay at the pump location. This will assure that your pump has proper intake head and give your intake pipe a chance to keep up with flow when the pump starts.

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P.E. Metallurgy, Plymouth Tube

 

[SNORGY]

tinsnano,

Are you and your company sure that, from a TIC perspective, a long suction line will cost less than running the electric cable?

It might be true, but I find it hard to believe, unless there are other costs involved.

 

[LittleInch]

If you are that far below the river you will have to build a water proof pit!

The issue is that for an inlet line like this, the pressure at the start point is limited to atmospheric pressure ( 1 bara) and the static head difference between river level and pump inlet (10m = 1 bar)

A pump on the other hand could easily put 10bar in the discharge. The impact is on pipe size and cost which gets bigger and bigger as you get further away.

I find it very odd that a cable costs more than the bigger pipe differential, not withstanding the other issues.

What in your opinion is "far away from the river" or "elevation lower than the river"

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[tinsnano]

Dear all

for better clarification i have attached simple sketch describing the situation

by the way i have two point to mention
1. as we make the suction pipe longer we will make the discharge line shorter we just moving the point of pump location between the water source and the discharge point, so i cant see any additional cost associated with it except the pipe that goes into and out the pit

2. in my location and condition (to use 10cm diam. plastic pipe and 45kw motor for the pump using 35mm2 x3 cable) cable cost is more than pipe cost

 

[LittleInch]

Nice diagram, but needs dimensions, levels etc. Also flow rate. 45kW sounds like an high value for a 100 mm pipe.

Usually the inlet pipe is bigger than the outlet pipe.

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[tinsnano]

Assume 800m total length from river to discharge point, 40m height and 29liter/second
yes usually suction pipe are bigger (though i don't know why) in that case cost of suction pipe is more sensetive

 

[LittleInch]

Tinsnano,

We are actually trying to help but you're drip feeding information. Please mark on your diagram the height of river level high point and what pressure you need at the outlet of your pump.

Your pipe size is quite low for the flow rate. I get close to 4m/sec

You have a bigger pipe on the inlet because there is a lot less pressure available to push the fluid through your pipe.

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Also: If you get a response it's polite to respond to it.

 

[dogtop]

Tinsnano,

What kind of pump are you using?


dogtop
An Inversion of Top Dog

 

[tinsnano]

LittleInch,

sorry for the problem

1. I have again uploaded the sketch with dimension
2. I have tried to calculate the pressure just at the pump outlet and I found surprising large value 16atm which is unpractical and impossible with centrifugal pump (i guess) so i need to consider the whole case again

"dogtop" i am building and planning to use centrifugal pump with the following dimensions

impeller inlet diameter 150mm
impeller outlet diameter 500mm
impeller width 40mm
rpm 1100
impeller inlet angle 15.44
impeller exist angle 37.58

 

[EdStainless]

How about the pump rating in terms of flow rate and head, maybe a pump curve?
How much working pressure do you need at the delivery location?
I believe that you made a mistake calculating the head.
Having a suction line that works as a siphon will require a special approach to assure that it doesn't break flow. There are usually small suction pumps at such a high point and some associated equipment.

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P.E. Metallurgy, Plymouth Tube

 

[crshears]

EdStainless,

If I understand what you're saying, the "special approach" would likely include/involve dedicated aspirating equipment for the purpose of priming the intake line and pump casing. My first job out of college was in a research facility where eductors operated by compressed air and installed at the crown of the horizontal-shaft cooling water pump casings were used to prime the pumps by drawing up cooling water from an interred suction header - I mean, to reduce the pressure inside the casings so atmospheric pressure would force the water from the cooling tower sump into the casings. The eductors worked quite nicely.

Not only a foot valve but also a suction isolating valve would have to be installed between the water source and the pump suction, so that in event of pump seal or other failure without break of siphon the inflow of water could be stopped and pit/sump flooding averted.

The longer the pump suction line, the more critical it would be to start the pump with its discharge valve fully closed, and only when the pump had reached full speed to slowly open its discharge valve to establish flow; getting a mass of water very near atmospheric pressure moving without vapour lock or cavitation occurring is far more likely to be problematic than with getting the same water, but fully pressurized, moving...and even at that the definite possibility of water hammer exists.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[LittleInch]

tinsnano,

You really need to do a calculation on head loss for your pipe size and flows.

assuming your pump is half way ( 400m approx.)I get around 50m head loss for your flow rate (29 l/sec) - I'm not sure exactly what a 100mm plastic pipe is.

The problem is you only have 15m available before you start cavitating the pump.

Therefore your inlet line needs to be about 140mm ID, so probably a 150mm pipe.

This is why inlet pipes are bigger when pumping from an atmospheric source - you only have a limited amount of pressure (1bar) acting on the surface of the water. Therefore your head loss is limited and your pipe needs to be bigger than a pipe which has pressurised water in it.

Maybe that will make a difference to your cost equation?

Also think about whether you need such a small pipe and hence a bigger pump/motor and bigger cable. Maybe a bigger pipe and hence a smaller powered motor and smaller cable would cost less overall or when you add in the cost of the electricity

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

 

[tinsnano]

LittleInch,
when i say 100mm plastic pipe i mean a plastic pipe (hose) with 100mm internal diameter
Here is my calculation for pressure drop and power requirement of a system with the following information
Q(flow rate)=0.029 m3/s
height between suction and discharge point h=40m
pipe diameter Dia=0.1m

pipe lenght L=800m

then calculation

head loss due to friction

=8*f*(L/Dia)*(velo*velo)/(2*9.81)=93.4m <-----------assuming f=friction factor=0.0021
and power requirement
p=Q*(headloss*density*g+density*g*height+pressuredifference)
p=37kw

and now i agree that the pipe diameter made a big pressure drop due to pipe friction