Fire water mains in sand filled concrete trenches

[sgoel]

Dear Friends

We are evaluating use of sand filled trenches for laying of fire water mains vis-a-vis burried design.

Request following information:

1. Is this design used in refining/petchem industry?
2. how do you protect the piping from external corrosion from leaked rainwater?
3. would there be safety issues in case of a hydrocarbon seepage into the sand filled trenches?
4. Do we still need to provide a cathodic protection system even with sand filled trench design.

Thanking you in anticipation for kind input.

Regards

Sgoel

 

[b1ueshift]

1. Yes
2. Use pipe made from non-corroding material.
3. Not a safety issue, but an environmental issue.
4. No (see number 2).

 

[DavidCR]

In point 4. Are you thinking of steel pipes?

From several discussions I had, I conclude that NFPA 24 does not recommend (unless you take extraordinary measures) the use of buried steel pipe (I don´t have the current version of the 24 so I wont tell it is a "shall not"). The code goes with the AWWA standards for steel pipe, but I´m not sure if going to ductil iron or plastic is a best choice, instead of trying to accomplish an approved use of steel pipe for fire mains.

I thought that if oil and AWWA steel pipes can be buried why fire mains can´t.

I have complained about this a lot until I saw an actual case with a 16 years old buried steel pipe (8"-Sch40) with lots of corrosion and underground leaks and water mess around. The pipe was supposed to be painted with tar externally, the designers thought that it was not expected to have internal corrosion problems after 25-30 years.

 

[fel3]

There are types of steel pipe that are designed for direct burial that would work for your application. However, as DavidCR pointed out, corrosion can be as issue. It just depends on how the pipe is coated and protected. Also, ductile iron is not immune to corrosion. The standard coating on ductile iron pipe is equivalent to the "tar" that DavidCD mentioned. I'm not a big fan of this coating because it is very thin (think paint) and easily scratched.

I have designed a lot of CML/C (cement mortar lined and coated) steel pipe for yard piping at pumping stations and water tanks and for transmission and distribution mains. The piping can be had with flanges, rubber gasket joints, welded joints, etc. Steel pipe is covered in the C200 series of the AWWA (American Water Works Association standards).

The cement mortar coating creates an alkaline environment around the steel that can offset the acidic nature of many, but not all, soils. Regardless, all of the steel pipe and all of the ductile iron pipe I have designed has included a polyethylene "baggie" wrap for protection except for concrete encased yard piping; some of the transmission and distribution mains have also included cathodic protection.

I suggest you find a local civil engineer with experience using steel pipe if that's the direction you want to go. On the other hand, if conditions at your site warrant, you should consider PVC. I've designed a bunch of PVC, also, for applications ranging from municipal service to prisons to industrial sites. PVC is much cheaper than steel and ductile iron (at least in California) and is very servicable at pressures up to about 150 psi (although PVC pipe is available that is rated for higher pressures, it includes a lower safety factor so, at about 150 psi, I switch to steel or ductile). Again, a local civil engineer should be able to advise you regarding this choice.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[rconner]

It is my understanding that many decades ago (before the advent of more effective modern corrosion systems such as polyethylene wrap/encasement for ductile iron pipes and cathodic protection for steel pipes) the concept of enveloping then essentially bare early steel or cast iron pipes with otherwise thought non-corrosive granular or sand backfill, though in otherwise corrosive native soil masses was tried in probably many areas. It is also my understanding that this practice was ultimately not successful for at least very long term service in at least some places, in that sand is inherently porous and eventually allowed infiltration of smaller and corrosive soil/water infiltration into contact with the pipe. While I don’t have any experience with the concrete channel twist (it appears arguably reflecting perhaps at least some more developed thinking) you throw in here, this is nonetheless some intriguing in that (in at least in talking point) one might think depending on geometry of existing soil mass and concrete wall any corrosive trench wall soil could conceivably be more effectively denied access to the piping?

Nevertheless, if the chosen pipe fire protection piping is ductile iron pipe (or “DIP”, that unlike the small diameter bare steel referred to is widespread available with cement mortar lining that is VERY effective to prevent meaningful internal corrosion) and in your and/or Owner experience you know the soils outside the channels are corrosive, however, I would still think provision of reasonably economical polyethylene encasement per ANSI/AWWA C105/A21.5 still might not be a bad idea for same, not knowing the water or flooding/mud etc. that could get eventually into and drain or not drain out of the (I assumed open-top) trenches. While I am not aware of specific instances exactly like what you may be talking about here, I wouldn’t be surprised if someone somewhere is doing something like this, as of course concrete utility corridors or “utilidors” have long been used in the mechanical field. In a somewhat modern twist to ancient and amazingly extensive open stone aqueducts, I understand incidentally pre-cast trapezoidal concrete channel sections laid contiguously on a plastic film bed, sometimes covered with wood planks or other cover etc. to minimize evaporation and/or contamination, are now being used in areas of the Andes etc. to actually gravity-transport water.

As to a subsequent suggestion of “non-corrodible” pipes, if this happens to be code for much weaker and plastic pipes etc. it would probably behoove one to be very careful and thoroughly do your homework in this area, and maybe particularly if critical fire protection for personnel and critical facilities involving potentially high flow velocities, hydrocarbon contaminated soils, and/or “combined” potable water and fire protection services etc. are involved (see also somewhat unique vulnerabilities and liabilities mentioned at e.g.

http://deq.mt.gov/LUST/techguiddocs/techguid16.pdf

and archives of these forums).