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Steam Pipe Thermal Expansion 1

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280474

Mechanical
Aug 18, 2001
35
Dear All,
Thanks for all response to my previous thread although I feel sad about the funny thread. I believe you all have missunderstood for what I asked for.
I have another problem to be discussed. I have my steam line which has been experienced corroded problem frequently. If I partially replace the existing carbon steel line with stainless pipe (especially for the vertical line), what would be happened related to thermal expansion. I have calculated the delta length as the result of expanded stainless pipe. I cannot add any expansion belows or expansion bent on that line to absorb the expansion due to limited space. Do you guys have any idea..?? (hope this thread is not funny)

regards.
 
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Monel should suit you it wont corrode in steam service and has much the same thermal expansion rate as carbon steel...not cheap though, but may not bemuch more expensive than SS
Loo up Specialist alloys web site for info

 
Without knowing the configuration, there's really no way to give a good answer. It's going to depend on things like the routing, what equipment the line connects to (nozzle loads, etc.) You really ought to have a true stress analyst take a look at the changes you are proposing. But, I can tell you this much, going from carbon to stainless is going to give you a good 30%+ more expansion and it is a rare thing that I've seen a change to stainless work w/o routing changes to increase flexibility.

But, before you go to stainless, you probably ought to get someone to see what's going on with your steam system. Carbon steel is routinely used for this service. I've you're getting high corrosion, I would have to think there is some other factor. If you don't know what's causing the corrosion, changing to stainless might make things worse.

In short, I think you need to have your company contact an engineering company to examine the issues and recommend a solution.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.
 
280474:
I agree with stressguy. You should contact a good chemical treatment company to do an analysis of your condensate and boiler feedwater.
The O2 and CO2 content of water should be checked. Chemicals could be added to the feed water to remove harmful O2 and CO2 which both cause corrosion.

The deaearator should also be checked for proper operation. The deaeratior is designed to remove dissolved gasses like O2 and CO2. Proper chemical treatment and Deaeration should solve the problem.

One additional word. Check all sources of contamination of the feedwater, steam, and condensate. Check the steam and condensate return system. If there are vacuum breakers which allow air into the system the air can add O2 and CO2 which will react with the water and cause corrosion. Chek the system for other means of contamination. Are there heat exchangers which use chemicals that if they got into the steam system would cause corrosion?
 
I agree with stressguy and Dlandissr. You should have a chemical analysis of the water. One more word, if the existing system could not drain the condensate completely, there would be some kind of corroded problems on the bottom of the pipe. Before you use S.S., you have to check all the possiblities of corrosion.
 
280474:

There's nothing funny about having to go through a steam line corrosion problem and suffer the constant steam leaks. StressGuy has gone directly to the the problem: you are suffering corrosion, not necessarily over-stress of the pipe. However, in going to a stainless material, you're going to exhibit more expansion growth (the k for stainless is higher than that for carbon steel) just like he warns you. But this doesn't necessarily mean that you will require an expansion joint. The strength of the stainless may be higher than that of a comparable carbon steel or you can compensate with expansion loops (omegas) instean of joints.

One thing is for sure, you should have a competant and experienced engineer do a stress analysis on the piping anyway. This will tell you what the mechanical limitations are on the installation. Your corrosion problem may be due to other factors - as mentioned by the other thread contributors. And don't forget that your stainless solution is more prone to chloride stress corrosion, so you have to have your steam quality known to begin with. I've always found one rule of thumb that works with routing steam piping: OFFSET, OFFSET, & OFFSET wherever you can. In other words, use as many elbows to offset your piping and allow it to expand with a minimum of stress.

One more thing to mention: have you considered that you may have errosion (& not corrosion)? If you are running 2-phase under very high velocities, this is a possibility but not necessarily the answer. Just another thing to check out, however.

If you have good, normal steam quality, conservative velocities, and allow for expansion with offsets you should have not problem with normal carbon steel piping for steam up to 400 oF. You haven't told us if it is saturated, superheated or wet.

Art Montemayor
Spring, TX
 
Not much of an authority, however having come from a similar problem I can tell you what we did to correct the problem. D.A. tank was performing to capacity (however, grotesquely under rated for the demand). Therefore sulfite was added as an oxygen scavenger. Secondly and most importantly, morpholine was added to the condensate. There is a problem with using morpholine, however, it strips pipe internals. If the age of your steam piping is marked by rust molecules holding hands DO NOT USE. Furthermore, after cost shopping, Nalco and Betz Entec were WAY OUT of the ball park so my company went with ECOLAB. (Never heard of them until my present company.) Hope this helps.
 
Guys,

Thanks for all good responses. I'll try to carry out the chemical anaylis to ensure the corrosion root cause prior to apply material subtstitution.
Again, thanks.
 
FYI regarding expansion of steam lines. When I worked in power plant construction, besides loops they used a technique known as the "cold pull". This was used on the main steam line which operated at 2500 psig and 1005F. The "cold pull" was a physical pulling together of two intermediate ends (somehwhere around mid-point) of the line to a point where they could then be welded. The pull was many tons of force. The object as I understand it was that by time the line came up to operating conditions the expansion would be compensated for and there would be minimal longitudinal stress. Obviously the "cold pull" gap was professionally calculated and I guess it would also have some bearing on imparting minimal pipe stress to the turbine from the steam line.
 
Also, stainless has its own set of problems . 304 and 316 suffer chloride stress corrosion cracking, and there can also be problems associated with the dissimilar metal weld CS to SS.

The source of chlorides does not need to be the proces fluid ( steam)- it can also be environmental source( dust or soil may have chlorides).
 
Dear All,

This is to describe more detail related to my previous thread (Steam Pipe Thermal Expansion):

We are currently having an errosion corrosion problem on 2" Boiler superheater tubes. Several efforts were done to avoid water carry over which is suspected as the cause of errosion corrosion problem on the boiler superheater tubes. Since there is no satisfied results can be achieved from the chemical treatment, we are now evaluating to partially upgrade the exisiting tube material (SA-213 T-22 Alloy Steel) with SS 316H. The existing erroded alloy steel tube with 3m long will be replaced by similar layout of 3m long SS tube including its bent.
By considering the thermal expansion, do you guys have any suggestion related to the effects that could be occured on the SS Tube (especially on weld between existing alloy steel and New SS tube)..? The operating temperatur of the tube is 550 deg. Celcius.

Regards.
 
several problems can arise when a ferritic T22 tube is replaced with an austenitic SS ( 304 or 315 H) tube.

a) tube theraml expansion- the existing supports might not allow sufficient flexibility for the added growth of the SS tube.

b) The dissimilar metal weld between T22 and 304H has a high failure rate for most weld styles. The only weld style that works for this combination uses incolnel weld filler, a wide weld pass, and uses a welding machine ( orbital welder) to make the weld to ensure good weld penetration. If a manual welder were to attempt the weld, he would turn down the amps to avoid splatter, but that would prevent good weld penetration. The main issue is that a high cyclic shear stress is developed in the weld during each cycle ( shutdown to ambient then heat up to 550 F) , and the location of maximum shear stress occurs in the midwall of the weld HAZ. The creep cracks originate at the midwall and are not discoverable except by angle ultrasonics. The only reliabel is weld style is as described above.

c) you can no longer acid clean the superheater once you place austenic tubes in the superheater.

 
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