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High Velocity Inverts
Posted by old2969 on October 21, 2016 at 1:16 pmHello all,
Just joined back into Beer Leg/Surveyor Connect, etc from about a year hiatus.
A project may or may not come up we are looking at to obtain inverts on some very deep (30-40′), very high velocity sewer lines. From my prior experience, I’ve had level rods break and swept away, in addition to obviously not being able to find or keep it steady at the invert location due to water velocity. Any ideas for some type of device or theory? Maybe some of you hydrology folks might use something for this. And no, the lines cannot be shut off in order to get the invert (treatment plant).
The main thing is, we’ve been asked to do it because up to this point, no one else has been able to. I can’t accept the job if I’ll just end up in the same boat as everyone else.
Steve Boon replied 7 years, 10 months ago 18 Members · 24 Replies -
24 Replies
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Many years ago, we were asked to obtain invert grades on a deep sewer going into a treatment plant. We waited for the lowest flow, about 2:00-3:00 AM, and used 8′-10′ lengths of iron pipe (I can’t remember the size of pipe that we used, probably 1″) lowered down, and then another section of pipe attached, etc. until we hit bottom. The hard part was getting the connected pipe out of the manhole so we could accurately measure it as it was connected together.
Sounds mickey mouse, and it was, but it worked within +/- 0.1, which was as close as it ever could be given the conditions.
As an aside, the pipe was so large and low flow was still half full, that it was almost impossible to hold onto the iron pipe.
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Why wouldn’t a jig set on top of the rim of the manhole, and a disto or some other laser measuring device, to the top of the pipe, and then subtract the pipe size to get to FL work?
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It seems you’d want to construct a rod with the part down in the flow having the smallest cross section possible, while remaining rigid. Maybe use (say) a 1/2″ O.D. stainless steel rod?
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I may be facing a similar situation soon, though with inverts in the 20′ range. What I’m planning to try is 3/4″ aluminum rod sections – the kind that Berntsen sells for driving in the ground. They’re threaded at both ends, so will go together easily, and the narrow section should stand up to the flow better than a level rod. The one thing I’m concerned about is losing accuracy due to flex.
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BajaOR, post: 396312, member: 9139 wrote: It seems you’d want to construct a rod with the part down in the flow having the smallest cross section possible, while remaining rigid. Maybe use (say) a 1/2″ O.D. stainless steel rod?
That is what I was thinking, but maybe a 1/2″ wide by 2″ piece of steel strap. Weld that to a coupler, and then go with the threaded pipe on top.
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At 30 to 40 ft deep, you will have a difficult time determining where the top of the pipe is.
Is it out of the question for the maintenance crew to enter the manhole with specialized equipment?
I’m thinking the worker would have to have a harness on and be lowered via. a cable/winch. Obviously using fresh air line.Not familiar with doing something like this, but I’m sure it could be done.
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First of all, I would refuse to do this. Secondly, it is hydraulics, not hydrology.
The time to get record information on how the pipe was constructed is when
it is being constructed and not afterward. Once there is back fill over the pipe
that is all I see.Then the assumption that people will want you to make after you obtain the invert
elevations is to calculate the slope of the pipe between the manholes.Without seeing the pipe there is no guarantee that it was laid straight or at a constant
slope. I would not assume any liability without these assurances.
Historic Boundaries and Conservation Efforts -
Though I thought of something else. seems to me the above posts are pretty good suggestions. Something with very little drag for the lower 3 feet (or whatever the biggest pipe size is you will encounter).
My thought, though not as good, was some sort of diversion be lowered first that is very rigid, just upstream from where the rod is going to be. Then the rod behind it. Though I do not know what you could use to divert it when it is 30′ down!? -
The high-flow manholes I’ve seen wouldn’t allow diversion – the flow would take the diverter downstream, either into the pipe or blocking the pipe at the outlet. I wouldn’t want to be responsible for either occurrence.
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toivo1037, post: 396342, member: 973 wrote: That is what I was thinking, but maybe a 1/2″ wide by 2″ piece of steel strap. Weld that to a coupler, and then go with the threaded pipe on top.
The advantage of using the Berntsen rods is that they’re all the same length, so you wouldn’t have to extract the coupled rod and measure them as a unit, you’d just count the sections and multiply by length, accounting for the of length at the top and whatever point length you put on the bottom section.
The other advantage for me is that I have a bunch of 4-footers left over from another project. 🙂
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The other thing to consider is that the flow probably fluctuates considerably depending on the time of day.
I wouldn’t be putting anything down something like that without the blessing of the owner of the facility.
Anytime I am requested to measure something dangerous(which isn’t that often), I work with people that have equipment and knowledge to assist. -
Just spit-balling:
If your low-flow condition is half-full (preferably less), and the pipe material is rigid and in reasonable condition), have your confined space crew, measure diameter horizontally. Then, from crest to rim and calc the total measure-down for actual invert.
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not my real name, post: 396348, member: 8199 wrote: The time to get record information on how the pipe was constructed
I don’t agree with excluding an asbuilt and calculated, existing slope performed in this manner. It is the accepted practice in all three states I’ve worked and I have no fear of liability when the facts reported in the survey are as found in the field.
Hydraulically, that flow is moving through the pipe in its current configuration, regardless of bellies, flat-spots or (gasp) high points.
When a dog-house manhole is set midline and a new collection line is tied in, it will continue to perform in the same fashion.
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Years ago we were asked to get inverts on two sewer pipes. They were concrete one being 48″ and the other 72″. The smaller ran full with only an air turbulance area at the top. The larger ran half most of the time. We tried a number of ways, but ended putting an “L” bracket on an old one piece rod and measuring the invert on both sides and averaging the two measurements then adding the diameter. I think we came out within a tenth of the as-builts. Which satified the engineer.
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Jim Frame, post: 396357, member: 10 wrote: The advantage of using the Berntsen rods is that they’re all the same length, so you wouldn’t have to extract the coupled rod and measure them as a unit, you’d just count the sections and multiply by length, accounting for the of length at the top and whatever point length you put on the bottom section.
The other advantage for me is that I have a bunch of 4-footers left over from another project. 🙂
The 9/16″ stainless steel rods are considerably stronger than the aluminum (and more expensive). We’ve used them in other high velocity water situations. Use vice grips to be sure that they are screwed together tight for the strongest joints. The SS is tougher than the vise grip teeth so consider the vise grips a consumable. Also make sure that you get them all from the same vendor – these are sold with metric and SAE threads and they aren’t compatible with each other.
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If you’re sure of the pipe size, you could use a pipe mic to get a measurement to the top of the pipe and then subtract the difference. In the past I have done something similar to what Ruel de Castillo described. I attached about a 6 ft. piece of conduit to the level rod with duct tape and measured how far it extended beyond the end of the rod. Then just probe for the bottom and add the difference to your measurement. A thin piece of pipe isn’t affected by the flow nearly as much as the level rod.
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40′ deep wow. Sounds like well. You will need confined space training and certification.
How about measure like a well with a water tape like this link.
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leegreen, post: 396481, member: 2332 wrote: How about measure like a well with a water tape like this link.
Those things work great for measuring down to the water surface, but they wouldn’t work at all for getting the invert in a high-flow situation. The flow would grab the sensor and carry it down the pipe until you run out of tape.
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imaudigger, post: 396345, member: 7286 wrote: At 30 to 40 ft deep, you will have a difficult time determining where the top of the pipe is.
Is it out of the question for the maintenance crew to enter the manhole with specialized equipment?
I’m thinking the worker would have to have a harness on and be lowered via. a cable/winch. Obviously using fresh air line.Not familiar with doing something like this, but I’m sure it could be done.
I was working with a town sewer Dept. They had the guy tied off in a manhole where we had place a monitor for flow. Only problem the manhole had a pumpline coming in about half way up. You could hear the extra flow coming and had to crank him up a few times.
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