Thermal waterhammer: steam sparger 2

[fvincent]

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

 

[unclesyd]

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

http://www.hydro-thermal.com

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

 

[GenB]

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

 

[fvincent]

-> 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

 

[TBP]

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.

 

[kirsner]

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)

 

[fvincent]

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

 

[fvincent]

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

 

[TBP]

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.

 

[kirsner]

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)

 

[Tremolo]

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.

 

[fvincent]

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

 

[ebru]

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

 

[fvincent]

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

 

[katmar]

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.