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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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