livestock watering with varying elevations, what is working pressure? 1

[elizasue1]

I have a client that wants a livestock watering system with 4 troughs controlled by float valve with 3/4" orifice opening. The well is a baseline with elevation zero, one trough is 10 ft higher at 356 ft distance, the other three troughs are 35 ft lower at 800 ft, 50 ft lower at 1560 ft, and 60 ft lower at 2150 ft. I have no information about the pump other than 10 gpm is the believe output and the system utilizes a bladder tank with a 50-30 psi switch. I need to get 5 gpm at the trough to supply the herd. My office is along the coast, so I normally do not work with such elevation differences.

Question 1: how do I determine the working pressure at the well with these elevation differences?

Question 2: if my working pressure at the well is below 30 psi, will my system still deliver water to the troughs?

 

[zdas04]

One thing that confuses me is when you say "I need 5 gpm at the trough to supply the herd" is that 5 gpm at each trough or 5 gpm total at all the troughs together? If it is the latter then the problem is solvable. If it is the former then your pump is 1/2 the size it needs to be.

You didn't say what size pipe you are using, so I'll guess it is 2-inch ID. The flow rate required for 2-inch to run full is (10.2 gpm * ID^2.5)=59.7 gpm. So even at 5 gpm times four troughs your line will not be running full. The consequence of this (very normal) condition is that you do not get any benefit of the pressure recovery on downhill portions of the line. Your pump will have to overcome friction, plus the hydrostatic gradient of 10 ft of elevation change (i.e., about 4.3 psi). Friction is dependent on pipe ID and pipe roughness.

I've thought about this system for a bit, and I don't see how the bladder does anything while any of the trough valves are open. Your flow resistance is 4.3 psi plus some small friction number unless your pipe is tiny or you have a distribution orifice in the main line that you didn't mention. A 10 gpm pump will tend to pressure up the system with all the trough-valves closed, and maybe with one trough valve open it would still try to fill the system. A second valve opening would put the downhill portion on a vacuum.

After the first trough the line will tend to be on a vacuum while the fill valves are open, but you need to make sure that if all the fill valves are shut that the pipe MAWP can tolerate 145 ft of head (about 63 psig).

David

 

[cvg]

not knowing what the slope of the lines are, there is no way to say that the pipe is not flowing full. However, with the line pressurized to 50 psi, I would doubt that you will have less than full flow.

It sounds like the pump is sized to fill the tanks at perhaps a lower rate than 5 gpm. However, most of the time only one trough at a time would be filling so you should have plenty of capacity. I wonder why 5 gpm is so important? With a float switch, the trough will re-fill during the day at whatever flow rate the pump and tank can provide. Do the cows drink more than 5 gpm?

Working pressure of the well pump is approximately equal to the pressure at the tank.

 

[BigInch]

They probably only need 10 gp_day.
But maybe there's 7200 cows, or
3600 that don't wake up at night to get a drink.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

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

 

[cvg]

ok, nice answer - but how many cows could possibly line up at one time at a trough to drink? Does it take more water to wash down hay then oats? Or are they grazing grass? Are they bulls or lactating cows? How big are the troughs? How big is the pressure tank? And who picked out the 10 gpm pump? Why 10 and not 20 gpm?

http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex801

 

[msquared48]

It's not the drinks that are the demand. It's the enemas.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BigInch]

Ah hah. Now we're getting to the real questions.

Being that there is some distance and elevation change involved, my guess is they're eating grass, or hay. Milk cows these days are pretty much kept in a barn and feed lots are usually flat.

And looks like about 3 acres, 7, 8, 6 around each trough, total 24 acres, so I don't see how they can get 3600 or 7200 head in 24 acres. So, there's the real question.

OK, so it must be dry country if we have to give them so much water with a pump and we assume that carries 10 head/acre that's 240 head.

We'll let them sleep at night, so that's 720 minutes available for pumping.

240 head * average 10 gallons/day = 2400 gallons/day
The lowest possible flowrate is 0.3 gpm. But we know that cows go in a heard, so they'll all be near the water troughs at the same time, and they'll probably all drink at the same time, I'll just say probably 3 times a day, and we'll say during daylight hours, say 8 hours of daylight.

So that's 240 cows drinking 3.3 gallons, 3 times over an 8 hour period. Let's see, 2.67 hours to recharge 792 gallons = 4.94 gpm Hummmm. THERE'S YOUR 5 GPM?

OK, so the case must be that all 240 cows travel in a heard around the circuit passing the 4 troughs each day. If we give them 5 gpm at each trough, all the cows can drink their fill, if they can drink at 5 gpm. I think that's too fast for a cow to drink, so some cows won't get a drink at this trough. We'll have to add an extra trough, and have 4 troughs now, for those that miss a drink at the last trough. The thirsty ones will be first at the next trough, so they'll all survive if we have 4 troughs at 5 gpm.

I'd say 800 gallon (ea.) troughs would do pretty well.
2' x 2.5' x 20'

Well, not bad for a first pass. We'll have to check if enough cows can get side/side at a 20' trough so 3/4 of the 240 can get a drink within 2.67 hours, but other than that, it sounds more or less reasonable.




**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

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

 

[elizasue1]

I would like to thank everyone that replied to my question. Your comments were very helpful.

 

[cvg]

observation: If the hydro tank is large enough, it should be able to provide greater flowrate than the pump. Theoretically, it could provide whatever the maximum flow rate through the pipe and orifices is - given that you have the 30 - 50 psi pressure. That is, until the hydro tank drains completely, and then flow will be limited to 10 gpm. So, larger hydro tank and larger stock tanks will improve the efficiency of the system.