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Furnace Forced Cooling

Sunday, December 03, 2017   By: Aida Hanani [969] 0 Stars
Dear all,

I have a confusion over this one issue - type of furnace cooling during emergency shutdown under low water condition.

When the water level is not visible in the gage glass, emergency shutdown of the boiler must be carried out, such as cutting off the burner, shut off feed pump, and close the main steam valve.

However, there are various opinions on types of cooling. Some said that the boiler must be forced cooling by continuing running the FD fan or ID fan, some said the fan must be stopped to allow natural cooling. This is because thermal stress imposed by forced cooling could damage the boiler / alter the metallurgy.

Could you all fine gentlemen share your experience or your opinion on this issue? Thanks.

Warmest regards

Aida

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Sunday, December 03, 2017   By: cmolanes@itba.edu.ar [50] 4 Stars
Mrs Hanani:

As per air cooling, I think that it might depend on the type of boiler. As a matter of fact, the coefficient of convection of air, and gases, is quite low, much smaller than that of steam or water, so I would say thet at first sight, cooling rate would not be so larges to put the metallurgy of steel at risk.

What has, on several occasions, been a problem, is the cutting off the water flow through the economizer, since these are normally built with finned tubes, and accumulated soot began a fire with very serious consequences when the steel was suddently left without the cooling effect of the feed-water

Claudio Molanes

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Thursday, December 07, 2017   By: Jim Watts [764] 5 Stars
Both answers are right under different circumstances.
For both heating and cooling the steam and water drum is normally the controlling metal for rate of change due to its metal thickness.
The heating rate is normally higher than the cooling rate due the ease of contracting the inside of the drum faster than the outside and cracking it.
With low water in the drum if the water cools quickly it also is possible for the bottom length of the drum to be shorter than the top half and cause a banana bend.
Natural cooling of the boiler with it boxed in is the least risky and is great if you have the time.

The problem is a forced shutdown has no time so forced cooling is used to allow maintenance quicker. This can only be used if the metal temperatures are monitored and the drum is kept at a higher than normal water water level to avoid top to bottom stress.

If the drum is empty force cooling can be used to cool the tubes but the drum is left hot and cannot be refilled until it cools. Not knowing where the drum level is very risky for cooling.

So your answer is in there somewhere but there is no rule that applies other than dont stress the drum







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Friday, December 15, 2017   By: Ross Burns [390] 5 Stars
Hi Aida,

I cannot answer your question, but I can answer the metallurgical aspect of your question, and I think Mr. Molanes and Mr. Watts have done a good job of answering your question, so let me add the following:

Boiler heating surfaces can be dry meaning steam on the steam side or water sides as long as the steel temperature is below about 750F. Above this threshold is you will change the micro-structure of the low carbon steel where the heat transfer is taking place.

My thought, is as long as the steam space is not superheated, then you are likely to be safe. It was not that long ago that field traction vertical tube boilers where designed to have a dry top tubesheet, so there was about a 1-foot to 1.5-foot dry space between the dry tube sheet and the normal operating water level to accumulate steam for normal steam usage.

Ross

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Saturday, December 16, 2017   By: Jim Watts [764] 5 Stars
Just to be clear, the boiler may be safe from having the steel change from one grade to another if kept below its service temperature as you say.
However it is not safe from having its grade remain the same but with a crack in it.
The safe rate of cooling from any temperature is dependent on the drum thickness but a figure of 40 degrees celsius per hour is about right for 4 inch drum however dont quote me this is very important to get right, new drums are expensive.

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Saturday, December 16, 2017   By: Jim Watts [764] 5 Stars
Gasp I should have said 40 degrees F

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Wednesday, December 20, 2017   By: Ross Burns [390] 5 Stars
I do not think this can be simply answered. As you know, the thermal or temperature gradient is influenced by the shape, size and localized heating or cooling. I know you used a thick drum for an example, but the results will be different for the dry tubesheet that is perhaps half an inch or so in thickness. None the less, a slow air cooling is about what I would consider safe and we would use insulating blankets to slow it right down to a really safe cooling, for as your said they have a long deliver time and are expensive. I am not familiar with the temperature gradient of 40F per inch, which maybe safe for your application. With this said, and in general, thick low carbon steels that have a delta T of 400F per inch will experience a strain of about 0.001 inch per inch which places a stress of say around 30,000psi, which is a problem when the pressure vessel also has an internal pressure adding to the stress. However, if the vessel is no live loads like internal pressure and has a yield stress of 50,000 psi then the cooling rate is safe.

The ASME piping code allows cooling rate from heat treating at a maximum of 600F or less per hour per half the maximum actual thickness of a weld. But we must do our home work not to get to close to the yield point otherwise it material will rupture, which is your point. We based the temperature that is safe but in no case great than 600F. Also, fancy steels will have a different cooling parameters.

In thin walled pressure vessels like the vertical firetube boiler the safe thermal gradient is relative the distance from the hot spot since the thickness is heated with a lower delta T. We have this problem with medium and high temperature hot water piping systems that hump when the water is thermally stratified, for we can exceed the yield strength and dangerously ripe the pipe.

It will be wrong to give a rule of thumb when working with temperature gradients per inch, unlike the temperature of microstructure change is around 750F for low carbon steel, which is used in the vertical firetube boiler.

I also cannot imagine someone adding or immediately immersing cool feedwater to a dry tube sheet that is at 600F at the steam-side for fear of some surface hardening or localized distortion of the rolled joints. I maybe over reacting, but I would at least consider investigating it metallurgically.

Ross

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Wednesday, December 20, 2017   By: Jim Watts [764] 5 Stars
The unit is 40 degrees F per hour not per inch
The value changes with metal thickness but is given as a ball park for 4 inch Drum and would be on a large boiler of 1500 psi or so .
This has nothing to do with welding limits applicable to pipes as it takes into account drum attachments like risers headers downcomers and all the other fittings moving with the drum. It isset by the boiler manufacturer and as well as avoiding cracks and bends ,avoids unsealing expanded tubes which contract readily due to thinness.

It is so far away from 600 degrees per hour that I am emphasising this number is not safe in the answer to the question.

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By: cmolanes@itba.edu.ar [50]

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