Condensate Piping Corrosion 6

[tcampbe1]

We use process steam that cannot have chemicals in it because it humidifies medical products, but condense about 80% of our steam. The black iron condensate lines either get eaten from the inside or plug up with rust. We've replaced short runs with stainless steel with some success, but I understand boiler inspectors (Michigan) frown on the use of stainless. Does anyone know specifically why? Any suggestions or background on the use of stainless or other corrosion abating schemes would be appreciated.

 

[bvi]

The downside of stainless steel (aside from cost) is that it is vulnerable to chloride stress corrosion cracking.

 

[TBP]

The first thing to do is make sure that all of the condensate that can reasonably be returned, is in fact, coming back. Make-up water is pretty much the root of all evil, as far as water treatment issues go. Check out pretreatment equipment like dealkalizers.

Regarding the use of stainless steel, talk to the inspector yourself about what's permitted, and what is not. The amount of mis-information floating around general industrial operations regarding piping codes is huge, bordering on total.

 

[abcmex]

Condensate is corrosive due to the nature of the contaminants that are available. Condensate is terrible when:
a) Oxigen is available. Check the deareator and the oxygen scavenger added (this may be used as non-volatile product). Check for leaks in your system specially in areas like HVAC's where the temperature setpoint is below 100 deg C. You may use pumping traps (spirax sarco) or ogden pumps. If you keep the system hot (under pressure) there were no vacuum.
b) CO2 is available. Check the makeup water quality. Perhaps is better to use demineralized water.
c) assure that the condensate is returned. if you control the conductivity and pH.
d) proper vents are needed in such a system

Eliminate the source of the problem rather than giving aspirin (stainless pipe) for a heart problem (corrosion).
MFG

 

[kenvlach]

abcmex,
Comments regarding deaerator, oxygen scavenger and make-up water quality may not directly apply to this system’s problem. No mention of boiler problems. Corrosion occurs in the condensate system, downstream of heated devices and upstream of deaerator. The system may contain chambers and devices which undergo frequent heatups and cooldowns; necessitating vacuum breakers which let in air. The steam traps would discharge slugs of both air and relatively cool condensate during initial heating. Further, the condensate drainage design may be a semi-open, gravity type. In this case, an SS condensate system is altogether appropriate for avoiding corrosion.

tcampbe1,
What are the operating steam and condensate line pressures?
Are there cyclic heating cycles?
What are the flow characteristics of the condensate system (open, semi-open, closed, gravity drain to receiver, vacuum pumped)?
Does “humidifies medical products” mean addition of moisture into absorbent materials or perhaps sterilization?
Finally, is there any way chloride could attack either the inside (enter the condensate from the medical operations) or the outside of piping (e.g., soggy insulation)?
Ken

 

[abcmex]

Tcampbe1,
can you describe the condensate and Boiler water conditions (pH, conductivity, iron content,oxygen content)?

kenvlach,
I think that a condensate system without vapor conditioning (volatile treatment) may be better when is closed, if technically possible. Due to the lack of detailed information, is very difficult to give a definitive answer. Only Tcampbe1 can evaluate this.
RGS

 

[GenB]

Please give more information on: amount of steam used, pressure, type of steam boiler used,
I can suggest some options as we have supplied in the pass "clean steam generators" for the industry.
Most of the clean steamers/boilers use DI water and it is corrosive and in some cases you are allowed stainless piping, or you must use stainless piping.
Please post the info and I will find out for you.,
if you need info, I can also send it to your email box.
ER

 

[MikeONeill]

It sounds like you may be adding to your problems by replacing sections with stainless steel. Your initial problem was probably carbonic acid formation due to air in the system. Now you'll encounter galvanic corrosion as well. It may sound overly simplistic, but consider a heavier wall carbon steel pipe (this will decrease the capacity as the pipe ID will be reduced) and install strainers to blowdown the larger particles. A normal coating of oxide (rust) in the pipe will actually aid in corrosion resistance, so you only want to get rid of the larger particles.

Mike

 

[TBP]

In plants like this, carbonic acid is the result of naturally occuring alkalinity in the make-up water. It breaks down into carbon dioxide under boiler conditions, and is carried with the steam where it dissolves in the condensate, dropping the pH. Dealkalizers are one form of pretreatment that can help a lot. This type of corrosion is typically the cause of "grooving" in condensate lines in general industrial plants. Air drawn in through vacuum breakers will normally cause oxygen pitting, and if this is extreme, carbuncles can clog the return lines. If there's a temp control valve on the steam supply, then there will most often have to be a vacuum breaker to allow the condensate to drain properly. I have seen plants where the water treatment people recommended the removal of the vacuum breakers, to deal with corrosion problems in the return systems. When these were removed, the mechanical problems (like water hammer) were found to be the greater problem - as in many things didn't work very well, and some wouldn't work at all. So the vacuum breakers were re-installed. This is an application where the mechanical side requires something that causes the chemical side problems, but the chemical side just has to suck it up and deal with it.

I'm aware that in theory, galvanic corrosion should be an issue when mixing stainless and carbon steel pipe. In practice, I've never actually encountered a problem with it in this type of system. However, copper return lines can be a HUGE problem. Not always, but sometimes. The copper, in and of itself won't cause problems. However, if some other factor (like some serious carry-over from the boiler) is added to the mix, copper can be pulled-off the pipe walls, and carried back to the boiler by the condensate. There, it will plate out and set up a cell. The resulting corrosion damage can look very much like oxygen pitting.

 

[GenB]

I believe that Replacing the system with stainless steel is the most appropriate way,
Then the generator has to run on DI water to avoid pitting and
cracking of the stainless.

If clean steam is needed, you can not use c/s.
ER

 

[GenB]

Please give more information on: amount of steam used, pressure, type of steam boiler used,
I can suggest some options as we have supplied in the pass "clean steam generators" for the industry.
Most of the clean steamers/boilers use DI water and it is corrosive and in some cases you are allowed stainless piping, or you must use stainless piping.
Please post the info and I will find out for you.

ER

 

[unclesyd]

We have 2 rather large areas where all the condensate lines are 304 S/S due to the possibility of a heat exchanger leak that would lower the PH of the condensate. S/S allows use to use the condensate in the process without the possibility of Fe contamination.

One other area we use S/S condensate lines form a 300# steam system that flashes to a 60# steam system to ensure steam purity due to process considerations. In conjunction with S/S condensate lines we use S/S Steam Purifiers to do a final polish on the 60# steam (S/S) prior to use at 15# through S/S lines.

We have never had a internal problem with SCC from the steam side. The few problems with SCC have come from wet insulation (older) and this was on lines that cycle which tend to concentrate the chlorides on the hot surface.

We also tend to use S/S sections at branch connections in C/S condensate lines where the condensate comes from a large HEs’. There have been no galvanic problems in such a system.

The biggest problem we had was CO2 in a large 300# saturated steam distribution system. This did a number on the header and condensate lines as the corrosion product was a mill type scale at 98% density of iron. We knew we had gotten a lot of CO2 into the system but RT didn’t show any damage, we didn’t find it until the header blew.

If you need clean steam 100% of the time S/S is the only way to go. I have never seen a code or specification that prohibits S/S in steam if you want to pay the freight.

 

[LSThill]

unclesyd (Materials) & Team Members

Regards to Piping Classification Specification for condensate using S/S.

What material table group in ASME B16.5-2003 will you be using.

Leonard@thill.biz

http://www.thill.biz

 

[unclesyd]

Isthill,
I don't have the new ASME 16.5-2003. I will check around next week and see if anyone has the new 16.5 that I can get to and see what you are inferring.

I'm retired and prior to retirement I gave all my newer codes and specifications to some up and coming engineers so all my references are somewhat aged.

 

[tcampbe1]

Thanks for all the helpful advice.

We have 2 Burnham E-50 1347 MBH steam boilers on alternate cycle. Steam pressure is 10 - 12 psi.

The galvanic aspect of stainless steel pipe might be superfluous since the steam is used on stainless steel chambers, four of them, about the size of a shipping container. Would dielectric couplers be a possibility?

The boilers are treated and monitored but I'll need to wait a few days to get the analysis. There are no volitiles in the treatment as I mentioned earlier.

The system is vented to atmosphere after the steam traps. There is a pumped condensate return tank by the chambers and a feedwater tank and pump in the boiler room.

Steam used for humidification is not returned to the system.

 

[unclesyd]

kenvlach,
Here is a very interesting site. It's a little hard to navigate and a lot is still in German but the pictures are outstanding.

http://www.metallograf.de/start-eng.htm