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Thermal waterhammer: steam sparger
2

Thermal waterhammer: steam sparger

Thermal waterhammer: steam sparger

(OP)

Have you ever experienced strong vibrations in water tanks provided with steam spargers?

The sparger is made of several holed pipes forming an hexagon on the bottom of the tank. The holes are 6 mm and were drilled on the top of the pipes

Low pressure waste steam from batch processes is used to heat water from 75oC to 95oC.

The tank itself is 8m diameter and 9 m height. Cold water comes from top and hot water is pumped from the tank bottom.

Level can change a lot, because heated water is transferred to a second hot water tank (reservoir)only when there is steam to heating it in the main tank. Cold water, on the other hand, is continually supplied to the main tank

A pair of chicanes inside the tank were installed to avoid pumping cold water. These chicanes are so located:
 - lower one: a horizontal circular ring at 2.5 m from the bottom, inner diameter 6 m and outer diameter 9 m
 - higher one: a horizontal disk at 3.0 m from the bottom,

A set of column is placed to support the chicanes. Some radial nervures (flat bars) were welded to the lower chicane.

Well, bangs can be heard and the tank is shaking a lot.

Internal inspection of the tank showed that some nervures detached from the chicane. One has even gone to rupture!

Well, I had heard of collapsing effects of steam bubbles and thermal waterhammer, but I was quite surprised with the intensity.

Do you think a small injection of air to the steam could cause a benefical cushioning effect?

Do you think the chicanes should be drilled to let eventual confined steam to flow to the upper part of tank, so avoiding hammer effects against such chicane?

Other ideas??

Thanks

fvincent

RE: Thermal waterhammer: steam sparger

Just an idea:

Use multiple spargers in parrellel as follows: each sparger to be oriented vertically, and the sparger would be co-avial with a larger diameter  flow nozzle. The cold water would rise thru the annular space at the bottom of the flow nozzle, mix with the steam bubbles from the sparger, and exit the top of the flow nozzle.  This procedure breaks the warming process into 2 parts; first within the annular space , second as the hot water ejected from the flow nozzles mixes with the tank water.

RE: Thermal waterhammer: steam sparger

(OP)
thanks davefitz

It is an idea for future however

As for immediate modifications:

Is it probable that large steam volumes might be forming just above the flat chicanes and either abruptly condensing or suddenly escaping so that in both cases water impacts against the surfaces? And if yes:

 - Do you think that air injection (in the steam flow) could reduce the magnitude of such thermal waterhammer effects?

 - Do you think that drilling some holes in the chicane or just removing them could minimize the problem, by allowing steam to leak upwards thru such holes ?

regards

fvincent

RE: Thermal waterhammer: steam sparger

The problem is the steam, the collapsing bubbles create this problem, I doubt that any of the methods will help you.
You may:
reduce the size of the holes (sparger) and limit the steam pressure; this may reduce the size of the bubbles and consecuently noise and your heating capacity.
Install eductors (ventury type) inside of the vessel.

Kind regards
Check the link:

http://www.process-heating.com/CDA/ArticleInformation/f...

RE: Thermal waterhammer: steam sparger

(OP)
Thanks for your suggestions, abcmex

However I have to take in account that steam pressure is only 1 bar g and the hydrostatic pressure at the bottom of tank is 0.8 bar g. So only 0.2 bar is left as "driving force".
Moreover the steam generation is not constant, depending on heat release in a batch process of vegetable oil hydrogenation. Steam mass rate changes abruptly from zero to up to 12 t/h

Venturi injector might not operate very well under these conditions, don't you think so?

For future modifications I think however that I could explore solutions like the one you have kindly suggested.

My present concern is how to diminish the strength of the thermal waterhammer.

I am not convinced of the efficacy of air injection to promote some cushioning of implosions, either... But I would like to hear more about other experiences on this solution..

As to eliminating the chicanes, I think they are submitted to strong impacts because non condensed steam bubbles will accumulate under them. When these bubbles (coalesced) finally condense or escape water would impact violently against the metal. That could explain why many of the nervures are so damaged.

I would appreciate a lot your comments.

thanks

fvincent

RE: Thermal waterhammer: steam sparger

Spirax Sarco used to publish a series of booklets on a variety of steam & compressed air topics, and I only have 3 of them. One is "Direct Steam Injection - Heating Liquids With Live Steam". It indicates that a larger number of smaller holes are better than fewer larger holes. It has an chart that shows 1.6mm, 2, 3 4 & 5mm holes. It doesn't show 6mm. "The holes should be as small as possible." "Note that the total area of the holes is almost always much less than the cross sectional area of the pipe in a well designed system." It goes on to say "Holes should generally be equally spaced, and drilled just below the centre line on one or both sides of the sparge pipe."

There are a couple of sizing rules of thumb for the holes:

1/ A 1/8" hole will pass 8 lbs/hr, with an 8 PSI differential."

2/ A 1.6mm hole will pass 1.6 kg/hr, with a 1.6 bar differential."

Regarding the mixing of air to reduce noise, I once saw this recommended in a very old engineer's book (1880's). I've never had the occasion to try it, but as I recall, there was a lot of excellent information in this particular book, so I'd be surprised if it didn't help.

RE: Thermal waterhammer: steam sparger

I concurr with abcmex's opinion on the mechanism of steam bubbling. As an analogy we could envisage a kitchen boiling pot, in which, at the beginning of the heating process we actually hear the implosion of the rising steam bubbles when they coalesce upon meeting with colder water.

Some thoughts: to improve heat transfer -with less bubble implosion effects- one should aim at larger specific surfaces, namely, smaller bubbles.

From gas-liquid dispersion studies we learn that to keep within the "bubble flow" regime range for a tank of more than 2 m diameter, the bubble velocity should be lower than 4 cm/s. Velocities higher than 5 cm/s might bring the sparging steam into the "churn-turbulent" range generating bubbles much larger than their usual 3-8 mm in diameter.

For the sake of good operation, beside doing what abcmex has recommended, one thing to do would be to keep the tank level and the steam pressures at constant values, and, as you suggest perforate the chicane as a last resort. About the chicanes themselves, apart from removing them altogether as you are considering, is it possible to lift them up so as to make their distance from the sparger larger than 2.5 m ?

As for the possible air cushioning effects I believe that your suspicion is right; steam being lighter (less sp. gravity) than air would tend to collect nearer the lower side of the chicane anyway.

Good luck.

RE: Thermal waterhammer: steam sparger

(OP)
Thank you both TBP and 25362,

The mention to a book dated 1880 was quite amazing and shows us that we are not inventing the wheel...

The proposition about the localization of the hole (only below center line) will certainly reduce dangerous behaviour inside the steampipe (sparger).

I think that adding this to a possible reduction of holes diameter as indicated by you and by 25362 could result fine.

As to level and pressure stabilization, 25362, I can say that the tank is atmospheric and a certain variation of level takes place because water is pumped from (heated water from the bottom) and to (cold water to the top). The variation of level associated with these flowrates however is quite low, because steam injection takes place during 10 to 20 min each four hours.

The air segregation from steam however physically reasonable would probably never happen due to high turbulence and high diffusity, don't you think?

Thanks all



fvincent

RE: Thermal waterhammer: steam sparger

Just below the lower chicane the water turbulence may have subsided, and given the needed residence time, steam bubbles may preferentially attach to it.

Besides, on heating (atmospheric) water, dissolved air would be released. Neither this effect nor any steel (weakening) corrosion effects have been noticed or reported.

Whether the internally diffused-and-heated air may help in breaking the steam bubbles into smaller units is unknown to me. The easiest thing would be to give it a try. Do you agree ?

RE: Thermal waterhammer: steam sparger

(OP)
Thanks, 25362

Yes, I do agree with you!

The best I can do now is just trying some air-to-steam mixture (near 1% of air) and check the results.

regards

fvincent

RE: Thermal waterhammer: steam sparger

I do not think that the air injection will help but will add rust/oxigenand pitt the steel.
Your problem may be in the operating cycle, large amount of steam is used at once creating cold condensate, then replenish with cold water(make-up water). you may want to add some heat to this water. as it is done with deaerators. heat the incoming makeup water. also isolate the return condensate, return water should drop into the steam side instead of the water side.
'hope this helps.
ER

RE: Thermal waterhammer: steam sparger

I initially thought about corrosion problems from adding air/oxygen as well. But if a carbon steel tank is being used to heat raw water, corrosion will be pretty spectacular, whether air is added or not.

RE: Thermal waterhammer: steam sparger

(OP)
Corrosion IS a problem indeed... but not for the tank which was made out of SS. The sparger and the chicanes, however, are made out of CS and so, as you've pointed out, do undergo severe corrosion. That will be corrected in the near future, I hope.

Regards

fvincent

RE: Thermal waterhammer: steam sparger

I can not believe it, corrossion was just a comment!
ER

RE: Thermal waterhammer: steam sparger

(OP)

Never mind, generalblr: it was just a comment of yours not a witch spell... Corrosion was happening before it!!

Would you suggest to replace CS pieces by SS, or do you think epoxy painting could prevent this kind of corrosion?

regards


fvincent

RE: Thermal waterhammer: steam sparger

I'll bet that by the time you fool around with a coating on carbon steel, it will be cheaper just to go with stainless steel from the beginning.

RE: Thermal waterhammer: steam sparger

(OP)

TBP, I guess you are correct, but the 'beginning' is over a long time ago... So, what are my options now? What is feasible?
regards

fvincent

RE: Thermal waterhammer: steam sparger

I guess I didn't word that very well. You have an existing stainless tank, but some misc bits are carbon steel, and corroded. I meant that if it were my installation, and it's time for replacing any carbon steel components that are corroded, I'd simply go with SS for anything in contact with the water, rather than fool with coatings on CS. I've never been a big fan of coatings.

RE: Thermal waterhammer: steam sparger

fvincent--The "overpressure" due to the collapsing bubbles is due to the size of the bubbles and the ambient pressure surrounding the bubbles.  Making the holes in the sparger smaller would help, so would interducing the steam at a higher elevation in the 9 meter tall tank to reduce the ambient pressure. For a rapid condensation collapse to occur, the water must be subcooled with respect to the steam at least 20oC--probably more.  This might supply another motivation for introducing the steam toward the top of the tank where the water is warmer.  

Introducing air into the steam is also an idea as it cushions the bubble collapse. (Don't know what a "chicane" is so no comment there).

Hope you let us know what works.  kirsner@kirsner.org  

RE: Thermal waterhammer: steam sparger

Mr. Kirsner! I was thinking about "condensation induced water hammer" when this string started :)

RE: Thermal waterhammer: steam sparger

You should take a look at the Komax tank heater.  I was instrumental in getting this type heater installed in place of steam spargers.  We had gone through the whole routine, different size holes. shaped holes, up and down holes, and many others.   Upon installation of these heaters the vibration and noise vanished and to my knowledge there have been no failures.  I included 2 other manufacturers of direct heating equipment that also might be of interest.

http://www.komax.com/gen-shscepter.html

www.hydro-thermal.com

http://www.pickheaters.com/dsi.htm

RE: Thermal waterhammer: steam sparger

It looks that you want to take a cheaper approach.
I would find a good glass lining Company as it is done in hot  water tanks made of c/s,
it is far better than  epoxy.
ER
 

RE: Thermal waterhammer: steam sparger

(OP)
-> kirsner,

I must have badly translated the Portuguese word 'chicana' to 'chicane'. I actually meant 'baffles'...

The tank, as I've mentioned in the first text, is continually fed with return warm water at 75oC.

From time to time, a massive flow of steam comes from batch hydrogenators (where H2 is added to vegetable oils) and is injected by means of the sparger in the bottom of the main tank.

Heated water at 95-98oC should concentrate in the bottom of this tank, from where it is pumped to a second hot water tank. This second tank continually feeds many heat exchangers, which for their turn give back warm water to the main tank, and so on.  

The mentioned baffles are intended to prevent intense mixing of warm water (75oC) fed at the top of the tank and heated water (95-98oC) at the bottom.

As to the reason for the loud and intense bangs, I suspected that they are related to some accumulation of steam under the baffles due to poor contact of bubbles with water and/or due to the low distance between the sparger and the baffles (only 2 m).

This steam pocket accumulated under the baffles could undergo a rapid condensation or suddenly escape upwards, passing through the space between the two baffles. In both cases water moving downwards  - to fill the free space left by the steam pocket - would hit violently the baffles.

In case the reason for the bangs is thermal waterhammer, I was betting that air could reduce the intensity of the strokes.

In any case, I think the design of the baffles is to be blamed, at least partially. Do you agree?

Thank you for your answer.
regards


-> unclesyd,

Thanks for the suggestion. I will consider other concepts, too. regards

-> generalblr

Glass lining seem to be a good option, indeed. Thanks




fvincent

RE: Thermal waterhammer: steam sparger

I'd be very surprised if the steam bubbles could travel 2m to accumulate anywhere. I think the steam bubbles collapsing pretty much as they exit the sparge line. Since the heat source is exhaust steam from a process, does the pressure vary? This, in combination with some fluctuation in water temperature in the tank may explain why some "bangs" are bigger than others.

Check out Wayne Kirsner's website for the article on condensation induced water hammer. There are some very impressive forces at work.

RE: Thermal waterhammer: steam sparger

fvincent--I think your theory makes good sence:  steam bubbles are coalescing under the baffles, then as they grow large, escaping to float up into the 75oC water where there's enough subcooling to make them violently collapse.

If you do inject non-condensable gas into the steam entering the tank, what about using nitrogen instead of air to avoid all the corrosion worry?  

wayne kirsner (kirsner@kirsner.org)

RE: Thermal waterhammer: steam sparger

(OP)
TBP,

The total steam produced each hydrogenation batch is close to 1.65 Gcal (close to 6.4MMBTU). The mass of water below the baffles is close to 65t (or 143,000 pounds)
Water is initially at 75oC (167oF). In order to reach 100oC (212oF), almost 1.64 Gcal of heat must be added to the water

The numbers are quite close. so that any excess of steam injected will not condense anymore

So even if we have a very homogeneous heating process some excess of steam will reach the baffles.

When you consider that heating will unlikely to be so homogeneous then some hot columns of water and bubbles moving upwards induced by densities differences with surrounding water may exist and it seems likely to have more steam reaching the baffles before condensing.

Am I correct?

regards

fvincent

RE: Thermal waterhammer: steam sparger

(OP)
Kirsner,

I am glad my approach seems at least verisimilar! I will consider now to remove all baffles.

Thank you very much for your opinion
regard

fvincent

RE: Thermal waterhammer: steam sparger

fvincent & Wayne Kirsner - I was thinking of a more traditional arrangement in which the steam would be throttled-back as the tank came up to temperature. If the steam continues to be fed to the tank, then I would have to agree that eventually, a water temp would be reached where the steam bubbles would no long condense, but rise, and collect under the baffles.

RE: Thermal waterhammer: steam sparger

TBP--I'm not sure you weren't correct in your original post.  At any rate, I think your solution makes good since too, i.e., install a thermostat that controls steam supply thru the sparger that throttles it off as low-tank water temperature reaches the point at which it no longer condenses the small bubbles leaving the sparger.  It will be interesting to see what fvincent finally decides to do and if it works. Thanks for your input, I often read your posts.

wayne kirsner (kirsner@kirsner.org)

RE: Thermal waterhammer: steam sparger

fvincent,

We have had a similar experience in the nuclear industry -- large dynamic events caused by steam discharge through spargers into a cool water pool.

The solution for the nuclear industry was to recognize that such events would occur and then design and analyze the affected systems assuming the dynamic events would continue to occur in the future.

So,  after the expenditure of a significant amount of money and research, analytical methods were developed to predict the loads but no solution was found that would make the events go away.

Research with condensation induced waterhammer in pipelines has consistently shown that the introduction of air will significantly reduce the waterhammer magnitude.  So, your idea of air injection has some merit. However, because of the geometry of the bubble collapse in the water tank it's hard to say if air introduction will be beneficial.  If it's easy to add air, you could do your own test to see if there is any beneficial effect.

TREMOLO.

RE: Thermal waterhammer: steam sparger

(OP)
Tremolo, thanks for your input.

Thanks all who contributed to this discussion.

I hope the first set of modifications I have proposed to my client can be sufficient to decrease the intensity of the bangs:
 - inject air (worth testing at least, as you all suggest)
 - remove the baffles

Additionaly I have proposed:

 - replace remaining carbon steel pieces with SS pieces (to stop water contamination)

I will let you know whether/when these modifications succeed or not.

Thanks again
Regards
fabio
fabio_vincent@figener.com.br

fvincent

RE: Thermal waterhammer: steam sparger

The solution with adding air into steam is a werry effective way to avoid bangs from the process, I was more asking what is the right way to say it direct translate from danish is boiling air but I'm not shure is this the right way to say it

RE: Thermal waterhammer: steam sparger

(OP)

Ebru, directly from Kobenhavn to Sao Paulo, it was a clear and sound message. Have you already used air to avoid such bangs? Can you describe your case, please?

By the way, since the start of this thread our client has not evolved the implementation of any solution we prepared. Now in December, things may go further again.

Thanks

fvincent

RE: Thermal waterhammer: steam sparger

I have worked with several steam spargers where we used 5mm holes. They worked almost completely silently except when ambient temperature water is being heated. If your water temperature is >70C you should not get "loud bangs".

My vote for the most likely cause is that when the water is very hot the steam bubbles do not condense and you are forming larger bubbles under the baffles. When these bubbles grow sufficiently to rise around the edge of the baffle you have got a much bigger problem.

If you are going to remove the carbon steel baffles to replace them with SS, first try running the vessel with no baffles at all. Please keep us informed of how your trials go.

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