Static Head- high point vs. discharge point 3

[odools]

This should be a basic questions, but it seems that everyone has a different opinion. If I am figuring our the static head for a system curve of a pump and force main, which number is used, the high point in the force main, or the discharge elevation? For discussion, say pump on is at elev 100.0 and the highest point is 150.0 and the discharge is at 140.0. The force main does have Air release and vacuum valves at the high points.

 

[cvg]

static assumes no discharge, therefore the maximum water column would be to the high point.

 

[BigInch]

You could also have no flow at a discharge at 140 ft elevation.

If you could fill the pipe to 150 ft somehow and could maintain the siphon between the 140 ft on the upslope and the 140 ft elevation on the downslope, ONCE the siphon was activated, you would only need a pump with 140 ft head, HOWEVER you must think about how you will fill that siphon. If you want to buy a 140 ft head pump, you're going to have to fill that last 10 ft somehow. Perhaps by increasing the pump suction pressure by an additional 10 feet during initial fill (can you position a tanker 10 ft higher than normal NPSHA?), fill the siphon in the field by tanker truck, or hire a temporary pump and put it in series with your 140 ft head pump.

I recomment that you buy a pump with a curve that can reach 150 ft head at low flowrates but has a BEP head of 140 ft at your required normal BEP flowrate. With that strategy you could fill slowly to the 150 ft elevation during initial fill then, once the siphon is flowing, increase flowrate to BEP flow with BEP head at 140 ft.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[cvg]

force mains cycle on and off several times per hour. at each pump shutdown, the gravity portion of the main will drain (unless you have installed a pressure sustaining valve at the discharge point). So every time the pump starts up it will have 150 static pressure to overcome.

 

[BigInch]

Thanks. Then a 150 + a bit ft @ BEP is the way to go.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[jgailla]

Depending on where the high point is, the static plus friction head may be higher at the outfall than at the high point. We had a recent project where the high point of the force main was much nearer the pump than the outfall.
Static head was 30' at the high point and 20' at the outfall. However, total friction head was about 80'. If half of the line drains, the total head just after pump start is 30' static plus 40' friction for a total head of 70'. When the line is full under pressure, the total head is 20' static plus 80' friction for a total head of 100'.
The geometry of the line will determine the design point. If the high point of the line was at the discharge, the design head would have been 110'.

 

[head22]

Glad to find this thread. I am modeling a similar situation in WaterCAD where the high point in the force main is 50 ft and the discharge is 4 ft. WaterCAD gives me a negative pressure at the high point (-4 psi) but still gives me a flow. I'm not exactly a hydraulics expert, but I believe that the negative pressure is acceptable as long as it isn't below the vapor pressure of the fluid.?

So does this mean that it is flowing by gravity/siphon from from the high point to the discharge?

 

[BigInch]

Gravity flow means without pump power.

It must be above vapor pressure. It can't ever be less. If it is equal, vaporization occurs and the segment fills with vapor displacing any liquid.

Negative pressure may not always indicate that you have a siphon. It is possible that in a completely horizontal pipe that you get a slightly positive static pressure and most of any negative pressure being caused by a high fluid velocity, but yes, its probably a siphon of some sort.

A few bytes about hydraulic grade lines and siphons are here,

http://www.ce.metu.edu.tr/~ce374/links/eglhgl.doc

http://www.agf.gov.bc.ca/resmgmt/publist/500series/590304-5.pdf

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/