Water quality for fire tubes boilers
Water quality for fire tubes boilers
(OP)
I have to define the BOP for a plant in which there is a fire tube boiler for 12 t/h of saturated steam at 10 bar-g.
I have decided, for economical reasons, don't to put in the plant a deaerator: in fact italian norms suggest, for these boilers, (fire tube boilers, FBT), a qualit of the water that you an reach using a simpler chemial threatment.
Do you think is this a good choice ?
Could You explain the reasons for which, using a fire tube boiler, isnt't necessary the deaerator?
Thanks a lot.
Engy 74
I have decided, for economical reasons, don't to put in the plant a deaerator: in fact italian norms suggest, for these boilers, (fire tube boilers, FBT), a qualit of the water that you an reach using a simpler chemial threatment.
Do you think is this a good choice ?
Could You explain the reasons for which, using a fire tube boiler, isnt't necessary the deaerator?
Thanks a lot.
Engy 74





RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
obviously this choice (no deaerator) has an high cost in term of chemicals consumption: then I have thought to preheat (using waste heat available) the feed water to 82°C, so reducing the solubility of the O2 in the water.
In this way the consumption of chemicals goes down, and in the worst case we may place only a more economical atmospheric vessel (NOT a deaerator) to reduce O2.
What do You think? is this vessel necessary, or – only increasing the temperature of the feed water – the content of O2 diminishes and the O2 goes to the upper part of the circuit?
Have a good day, and best regards
Engy74
RE: Water quality for fire tubes boilers
The plume of steam out of a DA vent is typically small. It usually looks like much more than it really is.
I don't see how having a DA necessarily adds a pump. If you have any amount of condensate returning, then a tank will be required to receive this, plus any make-up water. (Mixing undeaerated make up water with hot condensate in a carbon steel tank is to invite serious oxygen corrosion.) You'll require a pump to push water into the boiler whether you have a DA or not. With a properly designed and installed DA, you can dump the condensate in, and run the make-up water through a level control valve. Add an oxygen scavenger to the storage section, and there should be no oxygen corrosion.
Firetube boilers will corrode just like any other boiler. If you feed it water full of dissolved oxygen, it'll pit-through, and fail nicely. What they will withstand, better than water tube designs, is scaling. Efficiency will suffer, and the stack temperature will increase, but the water is all around the tubes to take the heat away. In water tube designs, the water is inside the tubes, and it's relatively easy to block a tube with scale. Once a tube in a watertube boiler is plugged, there's no water circulating through it to take the heat of combustion away from the metal, and it will burn out and fail in short order. Under similar water side fouling conditions, a firetube will take much more abuse than a water tube.
RE: Water quality for fire tubes boilers
our only problem was that our returning condensate was so close to boiling temp that the pumps frequently cavitated under high demand... I was trying to find ways to extract more heat in the system when I left the company...
overall, I agree with the comments 'chasbean' and 'tgp' made -- I think it'll be in your customer's best interests to consider the dearator -- at least consider having the vendor give you a quote with and without to really see the difference in costs...
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
First: all the steam produced if lost in the process (approx 30 t/h: 12 t/h from the HRSG, 18 t/h from other conventional boilers)
Second: I need two pumps: 1 to feed the deaerator from the threated water storage (at ground level) to the deaerator (which may be placed at approx 8-10 m above the ground)the other to feed the boilers
Third: extimated steam consumption from deaerators: 2kg/steam for each t of water (by Thermal and nuclear power stations, Sterman, Tevlin, Sharkov, Mir Publisher): the steam cosumption has a cost as I have to heat it, to pruduce it(in a demi plant or an osmosys) to lost it in atmosphere.
Forth: I agree with You, from a technical point of view, the deaerator is a good choice (and probably, if You consider the cost for the chemical threatment of the water to reduce O2, also from an echonomical point of view... !)but I have proposed to my Customer this solution (no deaerator) and I want find all the arguments that are necessary to convince him that it may be a good solution....!
Best regards averyone, and have a good day!
Engy74
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
I calculated the cost of steam lost from the deaerator as $0.66 per day, $240 per year. This is neglibile and I expect way less than the cost of your total chemical cost. Sorry Nalco. This used your numbers of 2 kg steam lost / ton, 30 ton steam /day, and I assumed $5 / MMBtu for energy cost.
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers
RE: Water quality for fire tubes boilers