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What would be the pressure inside a shell boiler?

Friday, October 13, 2017   By: B [2189] 1 Stars
Imagine a situation where the water level drop upto a few mm from the top of bottom tube sheet of a vertical shell boiler whose capable of running dry. Im trying to understand what would be the pressure inside the boiler as the water level fall down? So eventually we have got only steam inside the shell - would the shell pressure be still at the inlet of the pump pressure?

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Sunday, October 15, 2017   By: Murphy [21] 4 Stars
Steam press inside the boiler shell has 3 outcomes. The 2 extreme conditions are when the steam cant leave the boiler shell then the boiler will over pressurize. The second is the steam in the shell is allowed to leave the boiler faster than the steam can evaporate in which case the pressure decreases to zero. In the last case the outcome will depend on the evaporation rate relative to the steam leaving the boiler shell and the boiler feed water entering the boiler. Nice question and surly someone will give another perspective.

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Monday, October 16, 2017   By: Jim Watts [764] 5 Stars
The pressure inside the boiler will remain the same as long as there is water to evaporate.
Once all the water has gone the boiler will become a superheater and surfaces in contact with the fire will heat up rapidly and the steam will also increase in temperature. If the boiler is on line the pressure will fall as no new steam is forming and the steam expansion will not be enough to keep up flow.
If feed water is still coming in the pressure will be normal but the overheating metal will be deforming till it fails and you better not be nearby.

If no water is coming in the pressure will fall and the temperature will rise till failure but hopefully not as dangerously as when water is present.

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Wednesday, October 18, 2017   By: B [2189] 1 Stars
Thanks for respond. This boiler that Im talking about is capable of running dry because the maximum gas temperature that is entering into boiler is only 387 degC which is lower than maximum temperature of carbon steel 400 degC. So there is no danger of it melting.

However, since there is no damper installed and there is no way of turning off the engine the heat source, I think the only risk would be an increase in pressure if the steam is unable to escape. What are the scenarios where the steam cant escape? And wouldnt the safety valve be enough to disperse the excess pressure as there is no other way to turn off the engine? Thanks.

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Wednesday, October 18, 2017   By: Jim Watts [764] 5 Stars
There is always a way of turning off the engine.
The boiler is only capable of running dry if the designer said so.
The fact that the tubes will not melt does not mean that the boiler will not expand to damaging conditions.
It would also be the last time the boiler could be used because if the engine cannot be stopped the boiler could not be cooled to a safe temperature for introducing feed water again.

If I were you I would find out how to turn the engine off as you might need it one day.

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Wednesday, October 18, 2017   By: cmolanes@itba.edu.ar [50] 4 Stars
Mr. B:

The situation you are posing appears to me as if it were an exhaust gas boiler linked to a Diesel engine, as we find on board ships. The temperature resembles that of the exhaust of a 2-stroke Diesel, by the way.

If there is no feed water, then there must always be some means to open the safety valve if pressure is high enough, or some discharge to an auxiliary condenser in your installation, so retaining steam in the boiler, unable to escape, seems an extremely unusual situation. With the boiler running dry, pressure should be atmospheric, and the temperature, as you say, slightly below that of the exhaust gases.

You mention the boiler to be vertical... is it a Cochran?

Claudio Molanes

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Thursday, October 19, 2017   By: Jim Watts [764] 5 Stars
Here is a link to further info on Marine boilers,


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Thursday, October 19, 2017   By: B [2189] 1 Stars
Thanks Claudio,

Yes, this is an exhaust gas boiler back of a diesel engine and on-board of ship smoke tube boiler.

Im trying to understand what happens to the water/steam pressure and temperature as the water level drops? Is there any water level that youll recommend to shut down the feedwater when the water level is dropping or itll reach a stage were steam output will become equal to water inlet? Because as the water level drops, the heat transfer should decrease and hence less steam produced, hence a new normal water level established? Obviously, feedwater is turned off when water level reaches high high 2nd high.

Yes, any excess steam is condensed through an external HX and recycled back to the boiler which should prevent from any steam building up inside boiler and hence increasing its pressure.

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Saturday, October 21, 2017   By: cmolanes@itba.edu.ar [50] 4 Stars
Hullo, Mr. B.

I am trying to understand your operators point of view, but I can say that if water level goes down is because you are needing steam, and in such quantities that you are extracting more than the water you are supplying with the pump.

If so, you should not cease feeding, because you will end up with no steam, in contradiction to the fact that you are actually needing steam. Of course, you are right to say that with less water level in a vertical smoke-tube boiler you will have less effective heating area interchanging heat, but I do not know if an equilibrium point will be eventually reached, so ignoring this, I would keep a water level near the NWL. Remember that, as was said in another reply, with no water, and with steam in the boiler, it will begin working as a superheater.

I read the DNV report on the boiler Mr. Watts indicated, but your boiler, being smoke-tube, not water-tube, should be safer in this respect in my opinion nontheless, you should avoid extreme rapid temperature changes, to avoid eventual thermal cracks or future water leaks to appear. The operating instructions should cover these circumstances.

If the tubes accumulate too much soot, you would notice a decrease in steam production, but normally soot cleaning must be done mechanically, opening the boiler with the main engine stopped, impossible whilst sailing, as far as I know.

When you mention the auxiliary HEx to condensate excess steam, I imagine you meant to say that the condensate is led to the feed-water tank, but not necesarily straight back to the boiler, so you still have complete control of the water you feed the boiler with.

I hope I am giving you a clarifying answer, or at least, a little.

Claudio Molanes

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Sunday, October 22, 2017   By: Jim Watts [764] 5 Stars
The information present is now enough for me to understand your question and your equipment.
Your Boiler has vertical fire tubes normally fully covered with Boiler water and the heat input is not under boiler control. Any steam generated in excess of steam loading is wasted in a heat exchanger and condensate returned to keep the boiler steam load matching the heat input.

Your question refers to the possibility that the feed water pump drops to say 50 percent of the Heat input and what would happen.

Firstly the steam pressure is at maximum capability when the tubes are covered and zero when uncovered. The safety valves will protect any situation that arises.
The reduction in feed water flow is no doubt going to result in the level dropping and it will stabilize at a level where the amount of steam generated is equal to the feed available. The steam excess will dissappear and the pressure will fall until the load balances the generation. The exhaust temperature to stack will rise to release the heat not being recovered.
This direction will continue if feedwater continues towards zero.
There is nothing fundamentally wrong with this condition given dry operation is allowed but it is not normal and your question is a good one because how long can you do it.
I suspect your instructions cover normal operation or no feedwater at all. At no feedwater it is best to depressure by draining as this is the least stress. Apart from fire which we will ignore at the moment. My advice would be to minimise the risk to the ship. If this means limping with low level and low steam I would do it. If the ship does not need it, I would drain to get back to a predicted mode of operation.

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Is Cp/Cv applicable to steam to calculate?
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