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Discussion in 'Irrigation' started by Wet_Boots, Oct 24, 2010.
read this mitch
What, no badgers?
Pardon me I know this is BS but I'll try anyway.
There are generally four causes of water Hammers in irrigation systems
Valve closure, uncontrolled flow velocity in empty pipes, trapped air in long runs of pipe, and for those that have them reverse flow when pumps stop.
Formulas for determining pressure surging are generally not suitable for irrigation system design. necessary data of sufficient accuracy usually cannot be determined especially as it relates to the most important factor the last 25% of valve closure time. When complete valve data is not available, control valves should be considered in the quick closure category.
Another source is air pockets in the high points of undulating water lines. If air pockets are exhausted by opening a large valve quickly water hammer can occur. Water cannot escape at a velocity comparable to the air escaping through the valve. The water will will come to a sudden stop after filling area of the former air pockets and pressure surges develop.
Zones should be compact as possible and the zone entrance should be located with consideration for the distribution of flow within the zone. The total flow into zone entrance should be divided as soon as possible to distribute the flow in different directions. This is not as important in small zones as large ones but it is a good habit to develop.
So we know air is a major problem in water hammers. We know that splitting zones as soon as possible is good irrigation. I will add try to keep all your heads on a zone at the same elevation. Not always possible but sometimes creating multiple smaller zones to maintain as much consistent elevation is smart irrigation.
Or you lucky dogs of today can use sam/prs heads and griswold valves with zone delay controllers and not worry about any of this.
Yeah, but it still doesn't explain my question. It only explains the reasoning when water is shut off.
I actually agree with boots, which was my answer already in my head. I was just seeing if there was a more sound answer.
I'd go with Boots answer as well. Probably lots of air in the laterals.
I think air in the lines would probably be a key. Water slams into air-filled lines like into a pressure tank, and the high flow causes a pressure drop in the supply line. The pressure drop could make the zone pressure close to that of the supply line pressure just upstream of the valve, and the zone valve could then be closing momentarily.
I highlighted the accurate portion of your post. Keep trying Pete .... one of these threads you might actually get it right.
Well since that is straight from Richard Choate who between him and James Watson have written the book that several generations of Texans and a few others such as Mike Leary learned irrigation from I think I'll go with that and not our local forum genius.
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Great book,taught me the basics and I've passed it on to many employees.
So we have ourselves a case of plagiarism here? Got a reference Pete?
But heck ... lets break it down since it would appear more likely this is just your attempt to put something you read into your own words and didn't quite get it right.
Now this is complete and utter BS. Have you ever heard of a model Pete?
Air in lines contributes to water hammer when it is trapped because it is compressible. Didn't we already cover this? One potential remedy for this is an air release valve at high points in the line.
In irrigation systems, downstream lines are not a closed system, and therefore, not a problem with respect to trapped air.
This is more an issue of stress on pipe and fittings due to mass & momentum and quick direction changes. That is why we put in thrust blocks.
Trapped air is an issue.
If I run a lateral line down the middle of a zone, and tee off that line to the sprinklers, I have "split the zone", yet I still have a long run of pipe.
Almost never possible, nor is is reasonable. A more apt design point might be to minimize the amount of elevation change between high and low points in a zone.