Sewer Force Main Condition Assessment 1

[shirsch]

I am trying to assess the condition of a 24" DIP sewer force main. There have been previous failures on the 20,000 LF line and I know that replacement is unavoidable, but I am trying to determine when I will need to replace the line. There is an abundance of technology for leak detection (mainly focused on water distribution) that can be used to locate any leaks. I am looking for a technology to assess the condition of the pipe without disrupting normal operation. I have taken coupon samples at 5 locations, but this is intrusive and not feasible for the entire pipe length. Any suggestions?

 

[rconner]

What is the lining system etc. that is in the pipe? What was the nature of prior problems (internal very low pH biogenic sulfuric acid corrosion at locations where main didn't flow full for some time, or what?) What were the results of the five "coupon examinations" that have been done [was there any significant external and/or internal corrosion depth/metal removal etc. observed, and also did these observations/coupon sampling locations e.g. show any correlation e.g. with the (up and down?) profile of the line or surrounding soil strata etc.? Particularly if any are external corrosion related, have any soil and/or stray current etc. asssessments yet been conducted etc.? I'm not professing a lot of assessment expertise in this regard, but I suspect answers to these questions might help guide any further assessment needed.]

 

[shirsch]

Internal corrosion from hydrogen sulfide is the cause. The cementitious liner was gone in 4 of 5 coupons. We sampled at high points on the line. I know why and how the line is corroding, but I need to find a technique for examining the degree of corrosion, i.e. a condition assessment.

Your question about stray current is interesting however. The DIP force main is in a power line right of way and within 60' of the base of the power poles. The power line is pretty high voltage. I am not sure how high, but the structures are metal power poles (not wood). I have seen no evidence of external pipe corrosion, but could stray current reach the DIP force main? It is under 4' of soil cover.

 

[rconner]

It may not be surprising (and per much literature over many years) that cement lining and eventually the pipe itself could be attacked at “high points”, at least where there could be accumulation of hydrogen sulfide gas for any extended period in any airspace. Unless however a long length of the force main is running downhill (and with no dependable air release and/or flow control device on the end to maintain line in “full” condition), it is possible that significant attack may be somewhat local in nature (depending on topography), and perhaps could lend itself to more localized re-lining or replacement of some pipes with linings/contact more suitable for this service? I guess it should also be realized that some linings can of course also be knocked out by "coupon" removal practices, though it is interestingly is very tightly-adhered in others.

The situations however where damaging, extremely low pH sulfuric acid conditions can occur have of course been explained in many references e.g. ASCE MOP #37 (WPCF MOP #9), “Design and Construction of Sanitary and Storm Sewers” (on or before 1982) and also specifically with regard to force mains, “In some instances where air has been trapped in high places in the main and where the treatment has not been sufficient to keep the main free from sulfide all along its length, damage to the pipe has occurred”, and, “If it is not feasible to construct a system that will maintain self-oxidizing conditions throughout, the engineer should anticipate the results and take the necessary remedial steps. Where substantial concentrations of sulfide will prevail, exposed walls must be protected or be constructed of acid-resistant material.” I believe it also explained further, “Interestingly, those surfaces below the wastewater level are not affected adversely because the presence of sulfides does not produce an acid condition on the exposed portions.” (I think similar information and other preventive strategies were incidentally explained also at about the same time in e.g. ASCE MOP #69, “Sulfides in Wastewater Collection and Treatment Systems”.) [See also e.g.

http://www.epa.gov/nrmrl/pubs/600r04059/600r04059chap5.pdf

.]

Also for whatever it is worth, I think it has been explained in other more recent references that attack (due to sufficiently prolonged accumulation of air) in some force mains is some more likely to extend further on the downstream/downhill length of “high points” that than it does on the upstream side (I don’t exactly know the reason for this, unless it is perhaps the flow biasing the air pocket a little downstream?) It is for this reason that special acid-resistant linings have been specified at least some straddling the areas of high points of long force mains, and in some cases with some more length of specified special linings on the downhill side.

As to any other assessment tools/methods (and while I know some folks have looked at in-situ assessment of even cementlined pipes with various types of smart pigs etc.), I am not sure I am aware of the state-of-the-art in that regard or if there are any reputable folks claiming they can feasibly/accurately assess full-length the condition of buried, cementlined pipelines (like you I will await any further response). If you however do not have accurate as-builts e.g. to aid in further determining where the line has not flowed full (I would think the most suspect areas), I guess there might be other ways to determine the existing profile and/if and where there are gas/air pockets on the line (perhaps these might include some sort of traversing video or special types of smart balls or pigs, some of which that can reportedly even plot the spatial coordinates of a pipeline as they traverse same (and that could in turn, if accurate, tell you exactly where there are any other “high points”, now known or unknown). I guess it might also be possible to at least some less intrusively than coupons check the wall thickness anywhere on the line where it could be exposed using various carefully applied and calibrated “ultrasonic” instruments. However, this would of course require exposing at least the top of the pipeline.

About the only other thing I could add is that of course all safety precautions should be employed in all operations, including of course as I suspect you are aware that trapped hydrogen sulfide gas is a toxic gas (I think I remember many years ago a reported incident in which several workers were overcome, and some even on top of the ground outside a trench, by H2S release from I think then a very large concrete pipe forcemain).

As far as stray currents, I have no reason to suspect that this would necessarily be a problem due just to the conditions you describe, based on your coupon observations thus far and information that can be downloaded from DIPRA e.g. at

http://www.dipra.org/pdf/ACpowerLines.pdf

and

http://www.dipra.org/pdf/strayCurrent.pdf

.

 

[alexcmmi]

I have used ultrasonic guided wave technology to assess a force main before. Unfortunatley it did not provide the best results. The technology is greatly suited for welded steel pipe. Unfortunately the waves do not sufficiently pass through the gasket at each DI joint, so the signal gets lost (breaks down I suppose). Therefore you essentially are only looking at one 20 foot segment, and of the segment you need to fully expose about 5' of the pipe to place the equipment around the pipe. Not really efficient.

Ironically, the project was very similar: it was a stuctural integrity evaluation of a 5 mile, 16" to 24" commone force main, with several portions located in the RR right of way (electrical lines above), and portions located under river crossing (brackish). This FM was over 50 years old and had several breaks at differnet location for different reasons.

We did the investigation in several phases:

soil evaluation using geoprobe along the route (about 20 locations). Analyzed for resistvity, pH, redox potential, sulfides, moisture content.

external field investigation consisting at test pits at about 12 locations: visual examination of exterior, pitting, soil and backfill / pipe bedding characteristics

air release structure evaluation

historical evaluation

ultrasonic guided wave evaulation

....

Plotting the profile of the FM is definately a good start per the above posts. I know you say its interal corrosion, but its also a good idea to look at the FM / pump station hydraulics. Are the high points where the pipe is compromised subject to any surges? Are there air release measures in place? That kind of thing. If the FM is DI and your soil / backfill is good, I bet that the majority of the pipe might look just as good now as it when it was put in. It very likely is a localized problem, so replacing the whole 20000 lf would NOT be a cost effective solution...

Also, call your local DIPRA regional engineer. They are INVALUABLE for these kinds of projects. I cant praise my DIPRA regional engineer enough. Great guy, invaluable resource.

Good luck!

 

[shirsch]

alexcmmi, thanks for the helpful info. It sounds like you are in a flat coastal area. Are you in the SE? I have been looking at pricing for the ultrasonic pipe wall profiling. Do you have any cost information for this? How long ago did you use this technology?

The failure was at a high point in the system and it could be localized corrosion. Of the 6 coupons taken, 5 of them had no cementitious liner remaining. That said, we took coupons only at the high points in the line.

There are approximatley 8 air release valves on the line. There was no consistent preventative maintenance plan for these and they were found to be inoperable a few years ago. I am not sure how long they were not working, but the pipe crown was definitely exposed to some trapped gas for a signicant amount of time.

I will try to locate my DIPRA engineer. Thanks again.

 

[alexcmmi]

I am in the northeast.

This is the company we used for the testing. They are out of State College, PA.

http://fbsworldwide.com/index.html

This testing was done back in 04 I believe and it was in the range of 25K I believe for 8 - 10 locations. I could be way off on the price.

Reiterating my point, your overall evaluation needs to be multifaceted. You need to protect your client from a pricey and unecessary complete replacement if the problems are indeed just local. To do this you need to show no problems at other areas. But on the other hand, you also need to weigh the cost of potential future breaks in other locations if the pipe is not good...

And again, dont forget the hydraulics in all this. Surges? VFD controlled pumps? etc...

 

[MarshfieldTimC]

Have you tried talking to a pipeline company such as ANR, Viking Gas Transmission, etc.? I believe these type of companies regularly monitor the condition of their long pipelines.

 

[dicksewerrat]

What kind of pressures are you running in the line? There are technologies out there to rehab this. I saw a "leak detector "ball at a trade show in Nov. It was put on by Ttrenchless Technology Publication. You could try to contact them to get their vendor list.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[shirsch]

fbs international doesn't think their guided wave technology will provide the results I am looking for on a DIP line. I am currently talking to a pipe inspection company, Acuren, that regularly serves the oil pipe industry. They have a digital radiography technology that they feel can provide an accurate profile of the DIP wall at localized points. Has anyone had any experience with this digital radiography technology?

 

[dicksewerrat]

Check google for laser pipe profile. I thought I saw something there that could get a profile of the pipe wall. Maybe Blackhawk Instruments

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[LittleWheels]

The laser profile equipment I've worked with on rising main jobs had real restrictions with bend radius. They were mounted to CCTV trolleys and required emptying the pipe.

 

[shirsch]

We don't have access to the inside of the force main like you would for a gravity sewer line, so the laser profiling is not really an option. Also, we need to keep the force main active during inspection.

 

[LittleWheels]

I wasn't talking about gravity sewers, shirsch. Rising main sewers are what our part of the world calls your 'force mains'. We had to remove a section of the rising main and replace with Gibault joints to gain access. I was replying to dicksewerrat suggesting laser profiling and noting that it couldn't be done with the rising main in service.

Can you cathodically protect the ductile iron pipe? This would get around the problem of replacement and the majority of the problem of condition assessment but you would need to electrically connect across the joints.

 

[dicksewerrat]

When the pipe fails the next time, it will be out of service. Have a plan to do the laser work then. Also have a bypass pumping plan in place and a spill cleanup plan. If the line is only 4 feet deep, dig it up and sample a few low points. As it is ductile, you could use a local water deptarment's 'Hot tapping' machine to get a few coupons. Also put in a few drain valves.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com