Pump Discharging Pipe Length 5

[badr82]

Hi.....
We install a new booster pump (Lawara, 1.5 liter @ 200m head)in mountain area. The length from the booster to the end point is 6km and the discharge pipe is HDPE 63mm outside dia, 16 bar (SDR-11).
The problem was that, the water didn't reach the last point unless we set the discharging pressure @ 20bar.
What is problem??????

 

[bimr]

The recommended flow rate for a typical water transmission is 1.8 liters/sec to 3 liters/sec based on economical pumping velocities.

Pumping 1.5 liters/sec for 6 km will give 77 meters of head loss due to fluid flow.

Pumping 1.8 liters/sec for 6 km will give 108 meters of head loss.

Pumping 3.0 liters/sec will give 278 meters of head loss.

Your static pumping head = 964 - 900 = 64 meters. (Note water level at pump feeding tank unknown. You also should provide an additional 20 meters of pressure at the discharge.)

From the pump curve, it looks like the minimum pump flow is 1.5 lps@200 meters.

One would expect that you can pump approximately 1.5 to 1.8 liters per second through your pipe.

The problem on system startup seems to be that the fluid flow rate is too low. At low velocities, air bubbles will not be forced down the pipeline. One would expect that air is probably trapped in pockets where the pipeline is sloped. As the pressure is increased, the air will be forced through the pipeline.

Trapped air increases the head that the pump must overcome since the air bubbles effectively narrow the pipe area available for fluid flow.

You should evaluate the pressure rating of the HDPE in the pipe sections at lower elevations because you may be overpressuring the pipe.

You also may need a pump bypass. At the low pumping rate you will operate at, you may cause the pump to overheat and fail prematurely.

 

[BigInch]

Can you make a profile from Google Earth?

From "BigInch's Extremely simple theory of everything."

 

[bimr]

Various articles describing trapped air in pipelines.

http://www.arivalves.com/index.php?option=com_zoo&task=category&category_id=46&Itemid=29

http://www.netafimusa.com/files/literature/wastewater/Dangers-Trapped-Air.pdf

http://www.valmatic.com/pdfs/AirinPipelines.pdf

 

[badr82]

I don't thing there is an air blocking the flow, due to the number of air vents on the line.We put an air vent every 500m at every high point.
Can we increase the flow rate???
The booster has two pumps of same specifications in parallel arrangement, each has Q= 6-14 cubic meter per hour.

 

[Artisi]

you now tell us you have 2 pumps in parallel pumping into this line - is this correct?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

Hwa is thet mei thet hors wettrien the him self nule drinken

When the fluid velocity is low; (1) the air bubbles will not move down the the pipeline and (2) the air bubbles will create additional headloss because the air bubbles are effectively narrowing the pipe diameter. Your fluid velocity is only 2.6 ft/sec which is low (1.5 liters/sec flow in a 48.5 mm ID pipe).

A low flow condition is the same as a zero flow condition. The air will simply not move down the pipeline no matter how many air release valves you install.

You have said the HDPE pipeline is DR11 which means it is typically rated for 13.8 bar working pressure at 80 Degrees F. DR11 HDPE pipe has an ID of 48.5 mm. (You should confirm with the pipe supplier).

http://www.performancepipe.com/en-us/Documents/Tables 1-6.pdf

13.8 Bar - 64 meter static head - 20 meter service static head = 8.2 bar available for pumping head (83.6 meter of head). So you have 83.6 meters of head available for pumping.

If you do not overpressure the pipeline, you are only able to pump 1.45 liters per sec.

If you attempt to pump at a higher flow rate, additional pressure will be necessary from the pump to force the water through and you will overpressurize the lower reaches of the pipeline.

Pumping 1.8 liters/sec for 6 km will result in 125 meters of head loss.

Pumping 3.0 liters/sec for 6 km will result in 322 meters of head loss.

Check the calculations yourself with the online calculator:

http://irrigation.wsu.edu/Content/Calculators/General/Pipe-Velocity.php

You need to determine the water demand, recalculate the water pump requirements, replace the incorrectly sized pumps, and replace the pipeline with one that has the proper pressure rating.

 

[bimr]

headloss calculation

 

[badr82]

Mr. Artisi
Yes, I have 2 pumps. But they are not working at the same time. We operate one pump and we keep the other for emergency.

 

[badr82]

The pressure decreases as the elevation increases.
First point after the pump = 16bar
End = 1bar

 

[badr82]

I noticed that, after the pipe was pressurized after starting the pump for 3 hours and the water reached the end point, it does not required more than 16bar and may be less at the pump while it need 20bar when the pipe was empty.
I think we need to install non returning valves to avoid blasting of the pipe due to back pressure when the pump is turned off.

 

[badr82]

what type of non returning valves is preferred???

 

[bimr]

You should have check valves on the pumps.

You should also have a pump control valve as well when you are pumping against head. Pump Control Valves are designed to effectively eliminate the surges associated with the starting and stopping of the pump. Electrically interfaced with the pump motor, the valve opens and closes at an adjustable speed, providing a smooth, predictable transition of pump discharge pressure and volume into the system.

http://www.cla-val.com/pdfs/E-60-31.pdf

http://www.controlvalves.com/series/125/125.html

 

[bimr]

Regarding

"I noticed that, after the pipe was pressurized after starting the pump for 3 hours and the water reached the end point, it does not required more than 16 bar and may be less at the pump while it need 20 bar when the pipe was empty."

That is caused by trapped air in the pipe.

 

[BigInch]

Exactly it is. It is also not permitted to overpressure the pipe to 20 bars even if it is only for startup of the system. The pump will reach that pressure any time the flowrate is low as well. That just tells you when the pipe will fail.

From "BigInch's Extremely simple theory of everything."

 

[badr82]

Thanks for everybody. Really your replays were useful.
I have one last question, is the length of discharge pipe to long (6km)?..If it is, what is the preferred length?

 

[BigInch]

I didn't check it, but from the calculations done by others above, it would seem that the length, for it's diameter, is OK for the flow you want to get out of it, asuming that the flow you want to get is 1.5 l/s at the end.

I think you have startup problems, because of the profile which causes higher pressures until all air is out and the line is full of water, but without the profile info, I can't be sure.

Apparently the pump gets water to the end, and if that's the flow you need there, it works. That said, the only problem I see is the very high pressure during startup, so if the system "works" now, it may not work for long.

From "BigInch's Extremely simple theory of everything."

 

[BigInch]

Or, if you can keep the water in it, it will work forever.

From "BigInch's Extremely simple theory of everything."

 

[badr82]

I think if the discharge pipe was shorter, the booster will not need to pressurize the system as much as it do with the 6km..
To reduce the length of the pipe we have to put the booster at the nearest possible position to the end point(the water flow due to gravity for 1.5km in the pipe from the booster feeding tanks).

 

[bimr]

That is a good suggestion. Use the available head from the suction side and move the booster pump station downstream.

When pumping 1.45 liters/sec, you have about 14 m of headloss per 1000 meters.

However, you should be aware that you may have the same problems with air bubbles occuring with gravity flow on the suction side. Gravity hydraulic flow problems are more difficult to rectify than pressure flow problems.

 

[BigInch]

It will help keep high pressure off of your pipe. Right now you've probably got a 900-847 + 200 = 253 meter (about 25 Barg) discharge head. Probably way over pressured.

If you move the pump downstream, say to the low point, you'll have 90m inlet head, when starting up you will still get that discharge head 90 + 200 = 290m, (29 Barg that's why it's easier to get things going) but maybe bust your pipe too, but as flow increases, the inlet pressure will drop, say to 10m, then you'll have 10+200 = 210 m discharge head with no flow. As flow increases, maybe discharge head will drop to a discharge of 160 m or so.

I have to say I'm getting tired of looking at this still without a profile.

From "BigInch's Extremely simple theory of everything."