Need a drain valve?

[Chris73]

Hi Eng's;
I have to deal with a creek crossing of a proposed water main. The valley to be crossed is about 6m deep and 20 m wide. I am aware of the necessity of air valves at both high points but do I actually need a drain valve at the very low point? Ok, in case of a necessary draining of the entire pipe that sack will remain full of water, but what's the disadvantage? There will be no services or anything else within that section. The thing is, since there is plenty of vegetation and bush around the creek it will be difficult to find a decent place to put a retrieveable valve anyway. And how often do you really empty a water main?! Feedback is really appreciated!

 

[BobPE]

Chris73:

No, we usually dont put specific drains on creek/river crossings since if you use and inverted siphon profile, it would not be feasible. Isolation valves on both sides of the crossing are necessary. Air valve vents should be located above flood elevation with appropriate security.

If you plan on mothballing the crossing for extended periods of time, you will not want that water to go back into the distribution system and will have to make provisions to remove the water. I have seen air removal systems that work on inverted siphon profiles, or possibly design the slope to go to a low point drain on land.

Bob

 

[nackra]

We used to just use hydrants (fire on large mains and flush on small mains) at or near high and low points on water mains to periodically "flush" mains.

 

[greggy]

Chris...
Yes, there are always times in the future a watermain may need draining for repair, inspection, etc. As another mentioned, isolation valves on each side (maybe not immediately next to, but near by) of the stream should be installed in the event of a break beneath the stream. How would you even know if there is a break beneath the stream, you ask? Well, to test the integrity of a pipe crossing, you install a 1 inch copper bypass around one of the isolation valves with a yoke to set a meter (in a curb meter pit is best). Set a meter in the yoke, tightly close the main line valve on each side of the stream, and watch the meter. If the main is tight, the meter will not move. If there is a leak, you'll see the meter registering flow. This is required in Ohio.

On the other hand, there is no real reason to build a facility to drain the main under the stream since it is usually the lowest point around, anyway. We don't here. But if you had to make it drainable, at the lowest point along the pipe under the streambed, construct a tee and a 6 inch main with a gate valve going to a sump manhole, built on one of the stream banks. If you need to drain the main, close the main line valves, open the branch valve and allow the sump manhole to fill, then keep pumping out the manhole until the pipe is dry. Make sure you close the branch valve before filling the main again!