tube damage in steam heat exchanger

[alexchisholm]

We were recently doing a routine inspection on a heat exchanger in which LP steam is passed through the tubes to provide the reboiler heat to stream of hydrocarbon liquids. We were surprised to find, however, that there was substatial damage to the inside of the tubes and are a little confused as to why this has occured.
the initial theory was that as the tube bundle was largely oversized, this led to a great deal of tube flooding, and when the hot steam contacted the colder water sitting in the tubes, some description of cavitation ensues, causing the damage. the tube bundle has therefore been redesigned, and will be monitored. Was just wondering whether anybody has enountered similar problems in an exchanger and whether there is any solution?

Thanks for your help

Alex

 

[steam4]

Alex,

1) Do you have a vacuum breaker on the steam side of the heat exchanger ?

2) Does the condensate drain by gravity after the steam trap, or does it go up ?

Dave

 

[unclesyd]

What type of damage?

What is the tube material and temperature?

I assume this is a horizontal reboiler.

 

[alexchisholm]

firstly, thanks for responding.

steam4:

1)I am not entirely sure what you mean by a vaccuum breaker, but i do not think we have one. Was the reason for asking this for checking whether the steam may be condensing at sub-atmospheric pressures, and possibly pulling in air through loose flanges etc? we checked the pressure on the steam supply, which was about 45 lbs, so that is not a problem.

2) The condensate goes up (approx 20 ft) after the exchanger, under pressure, as it is transported about 150m to an atmospheric tank

unclesyd:

It is pitting damage that has been observed

Tube material is carbon steel and the temperature of the steam is about 150 degC, so that would be the maximum temperature of the tubes.

Yes, this is a horizontal reboiler

 

[unclesyd]

Check to see if you have boiler water feed carryover. This can happen periodically when a small amount of the boiler feed waster treatment chemicals are carried out of the boiler and through the system. Most of the time this problem will make its self known by deposits around leaks. The type damage noted in the system will depend on the BFW treatment. If this is true, it needs to be corrected for many reasons.

O2 and CO2 both can cause pitting in condensate. When you have damage resulting from either you should check the steam drum for similar damage.

As to steam4's question about the vertical direction of the condensate after the HE has merit. I have experience with problems originating with condensate damage though the damage being relatively smooth erosion corrosion damage with the upstream edge some times being sharp, not pitting as you describe. This normally occurs in piping.
One way to get around this condition is to use a condensate discharge pot on the condensate discharge line. This will keep the HE drained of condensate. I assume you are using the height of the condensate line to keep some slight pressure on the HE. All our low pressure systems were at least 25# psig. We had no essentially atmospheric condensers.

What is the duty cycle on this HE?
Where was the pitting in the tube, top, bottom, or all around?

 

[hacksaw]

How are you keeping the condensed steam drained and isolated from the atm. condensers? Control valve on condensate level?

 

[steam4]

alexchisholm,

My question concerning the vacuum breaker was brought on by an interesting comment that you made in your first post:

"the tube bundle was largely oversized"

If this is true, your heat exchanger is able to produce the desired temperature rise with much lower pressure steam. In fact, it's possible for the control valve to be almost closed, with the result being that the heat exchanger is operating in enough vacuum to flood a significant portion of the tubes. This is where a spring loaded vacuum breaker will open and let a sufficient amount of air come into the steam space to allow the condensate to drain by gravity. The air also inhibits your heat transfer ability somewhat, which offsets an oversized heat exchanger.

Now, in order for the vacuum breaker to help, the condensate has to be able to drain away by gravity.

Two things can never peacefully coexist in a steam system.......a control valve....and a rise in the condensate line !!

Now...can someone help me off this soapbox !?!

Dave

 

[alexchisholm]

Right, again thanks for the help!

unclesyd:

as you mentioned, i think the issue of draining by gravity etc would lend itself to more equal/abrasive erosion of the matrials, whereas i am told that we certainly experienced pitting damage.

the exchanger is in continuous use for an expected cycle of 4 years, and i am told that that it is offline for a maximum of once evry two years.

As for the whereabouts of the piting on the tubes, unfortunately the inspection report has only supplied us with 'some tubes were seen to have pitting damage', which is a little vague to say he least!

hacksaw:

The intention was to have a control on the level in the exchanger, using the condesate outlet stream. However, it sems that the control valve on the outlet was too small to alow this, and remains fully open, along with its associated bypass valve.

It is thought that level is therefore "controlled" by the steam pressure required to 'push' the condensate to the atmospheric tank downstream. ie the higher the flood level, the higher the temperature and hence the steam.

steam4: Sorry, I know what you mean now about the vacum breaker, but we do not have one of them

A few general points i have been considering...

I thought that this damage may have occurred due to the presence of air that is in the system before start up (this is not purged with anything before use), but have ben assured that the amountof air that this would introduce is not sufficient to cause the observed damage.

I thought that there may be an air contamination in the steam system, but there is no similar damage to any other reboilers using the same supply, so this is unlikely.

The chances of air being taken from the atmospheric tank back into the exchanger are very slim, as they are separated by about 150m of pipe. All indications suggest that the HE operates above atm, and so this wuold involve the travelling of air against the pressure gradient.

Thanks again!

 

[hacksaw]

put a pressure gage on the steam chest of your reboiler, you are pulling air into the system.

 

[unclesyd]

hacksaw's steam guage would be a great help. Make it a duplex gauge which will tell if you are in going into vacuum. The guage will also let you know if the pressure in the exchanger is varying enough to cause flashing of the condensate.
I still think that a condesate pot with level control below the reboiler level is the way to go.

Again I say check any leaks to see if you have any any sign of a deposit. I say this because contaminated steam is often overlooked.

 

[alexchisholm]

hacksaw: we do have a pressure guage, which shows we are certainly not operating at vacuum (abuot 45 psi), hence the confusion.

unclesyd: That seems a very good idea (to drop the condensate drum), but there is a feeling that this may not help. It may be that the design of the condensate lines leaving the exchanger/drum lead to poor hydraulics. This means that the exchanger will flood even after moving the drum down, as the level is almost determined by the pressure required to push the condensate to the atmospheric tank. ie the level is increased, reducing the surface area of the coils available for condensing heat transfer, and thus the pressure increases.

Thanks again

Alex

 

[unclesyd]

Alex,
There is a product called a (?steam condensate pump?) that I've seen but never been associated with. I think it is/was made by a stem trap manufacturer and make for low pressure steam. I'll look around for some information. Someone out there should have some information.

All our condensate pots had both a level and a (pressure) control valve. Exactly what the control scheme of the pot and steam chest was I don’t know right now. The level in the pots was approximately 2-3 ft below the bottom the bottom of the exchanger.

Again I have to say that I’ve never seen steam condensate just pit a C/S steel heat exchanger tube unless the condensate was contaminated. Have your utilities people check their steam with an open mind. It’s too easy to say we have the best steam or water in the world.

I’ll get back to you if I can find more information concerning any of the above. Let the forum know how you resolved the problem.

 

[jay165]

It's called a Pressure Powered Pump and it's made by Spirax/Sarco (or whoever bought them out)

 

[unclesyd]

Thanks "jay165"!
I've been scared to mention any manufacturers or suppliers, they ain't there no more.

 

[steam4]

alexchisholm,

Here is the Armstrong website for the pressure-powered condensate pump:

http://www.armstrong-intl.com/products/condpumps/cp.php3

Dave