CHW Closed Loop Piping Corrosion - How much is too much? 1

[PagoMitch]

All,

We are designing some new construction projects at hospital which has had poor - if any, CHW closed loop piping treatment for the last 15 years. Long story. On a related note, we have been told that at some time in the past 10 years they hired a local Contractor to "acid flush" the loop piping - their words. To me, this sounds...misguided at best. Our firm had nothing to do with this effort.

For a few projects in the last maybe 5 years, we have installed commercial water filters (36"H x 12" diam), with 20-micron internal socks at AHU's. These seem to protect the coils (no coil blockages since then on units with the filters), but require literally monthly changeout due to iron oxide particles - as fine as flour when crumbled in your hand - clogging the filters. See attached. Note that as of right now, they have NOT had any pinhole leaks in this system.

I have finally convinced then to get a non-destructive ultra-sonic tester, and start measuring some pipe thicknesses at various areas - in particular at junctions that should be at a higher velocity.

Piping is 8" S40 ERW steel, with a wall thickness of 0.322". The question is: How much reduction in pipe thickness is "acceptable" - 10%? 20%? Surely not 25% - before becoming a big problem? The system is about 2000 ft in length, operates at about 50 psi, and to replace it will prove...difficult. Since the CHW Piping was installed in main corridors, they have subsequently installed dozens of electrical conduits parallel to and below the piping, effectively boxing it in. In a great many areas, you cannot even see the piping.

Once the pin hole leaks start, they are essentially doomed to initiate replacing the system. But even a 25% reduction due to corrosion would still leave almost 0.25" of wall thickness.

Any thoughts on how bad is "oh cr@p!" bad?

On a related note, these guys:

https://easywater.com/commercial/pr...C Closed,well as removes, insulating deposits

have made a pitch to install their system to "clean up" the 8" steel piping. While their photographic before-after results are impressive, I have serious concerns that their system does not "clean up" the system to the point of causing the very problem (pin hole leaks) that we are trying to avoid. I would not want to play any part in having to replace their existing piping due to our "good intentions" to resolve their decades-in-the-making problem.

TIA.

 

[Heaviside1925]

OP,
It looks like you are from the US. I would evaluate the piping with B31.1 to determine the wall thickness required at the pump's deadhead pressure. You could categorically recommend replacement or not based on the evaluation. Pin hole leaks are one problem but based on the description of the routing, if a valve was unintentionally shut too quick, I could see a much larger failure, especially in a hospital.

 

[georgeverghese]

Am not a water treatment specialist - here is my take :
You've got a tough job of first sloughing away the iron oxide / sulphide layer on the pipe walls under which the bacteria are hiding. Then replace with inhibited demineralised water (DM water) as the circulating medium.
Use all of the following on a batch basis:
biocide - hypochlorite or glutaraldehyde or similar
dissolved oxygen scavenger - hydrazine or amm bisulphite
corrosion inhibitor
Keep a nitrogen pad on the CHW expansion drum, and avoid contact with oxygen - check what seal flush plan you've got on the recirc pumps. Use only inhibited DM water as top up at the expansion drum. Analyse the recirc water quality for residual hypo concentration and for dissolved O2 and pH.

 

[TugboatEng]

Film forming additives can't protect under rust or dirt. If you do intend to use treatment do acid flush the system. If there are any oils or hydrocarbons in the system you need to caustic flush dirst. There are buffered phosphoric acid based cleaners for this application. They won't remove significant metal during the flushing cycle. If any leaks develop, they were already there but obstructed by rust.

Once treated you may start seeing powdery growths around fittings and seals. These were already leaking but now leave residue from the treatment.

Don't let anyone tell you that proper cleaning and treatment caused the leaks. I get this way too much in my industry.

 

[PagoMitch]

Thanks for the quick responses gents.

Heaviside19125 - guilty as charged. I was off in my pump data - it is 70' head, appx 30 psi. We are also at 40F design temp. My min shutoff head is 115 psi. Using piping manufacturer pressure tables (primarily because most HVAC firms don't deal with ASME B31.1 all that much), assuming 0.125" corrosion I would be left with a wall of 0.197". About 0.05" larger than 8" Schedule 10 at 0.148". The piping would still be rated at over 900 psi, according to these guys:

https://www.engineersedge.com/fluid_flow/steel-pipe-pressure-ratings.htm

That said, that would assume that the corrosion is uniform. From my limited experience in other piping systems, that has not been the case. Higher corrosion at higher velocity points, and much lower in dead ends. So to rephrase my original question: if the AVERAGE corrosion is 0.125", what would be the expectation of the min and max values? Personally, my little alarm bells would begin ringing once the wall thickness reaches 0.125". I would begin to suspect the pipe integrity at hangers, especially in the event of seismic activity. The facility location is about equal to Los Angeles in its seismic potential.

georgeverghese/TugboatEng - Great info! I have been trying to get them to hire a water treatment company for...a.very.long.time. They are not in the US, and trying to get almost any technical expertise to visit the site is...difficult, expensive, and apparantly some folks think easy to exploit. I recently tried to get a contractor to visit for a technical MRI Chiller startup. One week on site, with a 6-hour flight from Honolulu. After direct expenses they wanted...FIVE HUNDRED DOLLARS AN HOUR. I told them to pound sand, and instead am sending (2) of their staff to the US east coast for factory training for a week, at a 50% savings over these mercenaries.

"Don't let anyone tell you that proper cleaning and treatment caused the leaks. I get this way too much in my industry."
I am reassured to hear this; but in HVAC-land for building CHW and HHW piping, I have heard multiple horror stories of the opposite in my 4 decades in this crazy business.
Multiple versions of "The system had poor water treatment, we brought in a vendor, they treated the water, and then we got pin hole leaks." I would think there has to be some truth to this?
On this project, the risk/reward ratio is not enticing. If the odds of leaks were only 1 in a 100, that would still have us - or the vendor - potentially on the hook for a several million-dollar piping replacement project. Thinking about the engineering for that project gives me nightmares.

 

[georgeverghese]

ERW steel pipe and fittings / valves must be dirt cheap, I suspect. Ideally this should have been all in SS316. But then, this will never happen in the HVAC business.

 

[pierreick]

Hi,
Why don't you ask specialist of water treatment to have a look at your system?
Reputable contractors are offering services to clean system (chemical cleaning, acid cleaning and then passivation). I did it several times on jacket of crystallizers
My 2 cents,
Pierre

 

[LittleInch]

Closed loop systems, in general, are not wildly corrosive as the oxygen gets all used up quite quickly. So long as you don't empty and reflush with fresh aerated water very often, or at all, then after 6 months or less you've got a pretty inert fluid. Of course some corrosion will occur and if you've got high velocities in some paces, then you could get erosion.

If you drain and refill, you really need to add corrosion inhibitor and possibly oxygen scavenger to the fill water. And don't drink it.

A 20 micron filter might be a bit too fine, hence you're picking up really fine material which won't block anything, but gunges up your filters. you could try installing something more like backwash filters on the main loop or even a cyclone filter, but then you need constant new water entering which would need inhibiting. The link you added seems to be just that - a water treatment plant removing all the black gunge.

Or use a magnetic filter and clean it out on a regular basis? e.g.

https://www.getchemready.com/product/magstrainer/

or search commercial magnetic water filter. I've got one on my heating system at home and the water whenever I drain a radiator comes out crystal clear. I clear the filter once a year.

In terms of your question, it depends on the amount and more crucially length of a corrosion defect.

A short length ( say 15-20mm) you can have metal loss of up to 80% and its still ok. A longer area, say 200 to 300mm and it might be down to 25 to 35% metal loss.
But also your min design thickness due to pressure will be very small. Most of the metal is just there to hold the pipe and its contents up between supports, not to contain pressure.

An 8" pipe can get down to paper thickness in a small area and not leak at 90 psi. Question really is what is your max pressure at the lowest point in the system as a closed one will have low pressure at the highest point. What pressure do you maintain the loop at at the highest point?

Yes, anyone / thing -coming in new risks the "it was fine last week, now its leaking" scenario. If you're using chemicals or increased velocity to remove corrosion products, you risk lifting the final barrier preventing a leak. Usually better just to arrest any corrosion and clean up the water to prevent blockages.

I would install a mag filter centrally to try and remove as much of your existing corrosion products as possible to reduce the individual filter cleaning time, maybe drain 10% out and replace with 10% inhibited water with some oxy scav and then monitor the critical points where you can get increased erosion such as tees and constrictions. I wouldn't worry about anything less than 2mm/ 0.1" but you can always wrap it in some sort of external GRE bandage or split sleeve.




Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Heaviside1925]

OP,
LittleInch's suggestions are spot on for closed loop CW systems. All removing the pipe wall corrosion does is expose more metal to be corroded, the best scenario is protecting equipment from clogging and damage as suggested. That is not to say, cleaning and treatment are not important but make sure you understand what any process that interacts with the corrosion is actually doing to the integrity of the piping itself.
The only reason I suggested B31.1, is just to provide a way of working through the problem. I understand it's not common in commercial HVAC land but my default mode for establishing a design basis if not provided one, is working through a standard so at least I have one.
As far as, at what average thickness the alarm bells should go off? For pressure, the link you provided, I assume that is burst pressure (but I don't know that), since you are dealing with average thickness, I would think a 1.5-2 safety factor would be applied. I would also view things in term of rate. This gives you the ability to communicate to the customer that based on the current corrosion rate, the piping will need to be replaced in X number of years, then the customer can make the decision as to what makes the best sense in terms of replacement vs leaving things as is.
As far as seismic, I would assume that pipe design was done using a corrosion allowance, but this is something that you should verify. As you noted, this could be more of a concern than leaks.