Water quality for fire tubes boilers

[engy74]

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

 

[ChasBean1]

Sorry about the late reply - I caught you on page 2. A deaerator will extend the life of the plant by significantly reducing oxygen in the feedwater. Trying to do this by chemical addition alone (usually sodium sulfite) will cause the sulfites to quickly turn to sulfates and increase your TDS (total dissolved solids), leading to the need for more blowdown than usual. Preheating the feedwater reduces oxygen content (thereby reducing corrosion), reduces the need for blowdown and plant makeup water, and reduces thermal shock to the boiler with introduction of warmer water. There's a reason you see deaerators in most plants.

 

[engy74]

Thanks for having responded me...but now another question related to this: I want suggest my Customer don't to install a deaerator to down both the capital cost of the plant, and the operation cost: a deaerator needs in fact more pumps, and looses a relevant quantity of steam in atmosphere (with a cost…)
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

 

[TBP]

A lot will depend on how much condensate is returned. If you get almost 100% of the condensate back, then that greatly simplifies water treatment requirements. With all of the condensate returning, there not much need for softeners or a DA.

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.

 

[pablo02]

I haven't made the conversions, but it appears your boiler is fairly typical in size for a fire-tube boiler... ours were purchased with the dearator with little extras to go with it, so I can't imagine the cost being that significant.. ours served as the head tank to the boiler feed pumps, I would assume you'll need some sort of tank to maintain your pump suction head pressure...

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...

 

[TD2K]

I'll echo the comments here that a deaerator to me is just good engineering practice. But, you could talk to a firetube vendor (Cleaver Brooks is one) and ask the question to them if they have any data they could share with you.

 

[TBP]

pablo02 - If you get the right feedpumps for the intended service, and pipe them correctly (particularly the suction side), there will be no cavitation.

 

[pablo02]

TGP -- I don't disgree, it was a packaged deal from a national boiler maker -- feedwater skid preassembled; but it was not high on my priority list to fix (my boss and I were reassigned to a step-child plant that had no prior plant engineers, let alone any supervision -- maybe you can imagine the other trials and tribulations...) -- we did well, then they laid the entire management/engineering staff, among others, off to save money; go figure???

 

[TBP]

pablo02 - I may not have worded my post very well. I understand completely. Many if not most plants have pretty much gutted their maintenance and engineering groups in order to "maximize shareholder value". Shareholders are indeed important, but many decisions have been (and are being) made looking no further out than the next quarter. There are some pretty scary operations out there.

 

[pablo02]

TGP -- not to deviate too far from the intents and purposes of this forum -- but my personal opinion is that most of the corporate officers (these days) are only interested in "maximizing 'their' shareholder value" -- my former CEO just got convicted on bank fraud...

 

[engy74]

Thanks again for Your comments.
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

 

[TBP]

Most of the heat added to the water fed to a DA gives the boiler a "running start". (Any boiler manufacturer specifications I've seen list the conditions required for the boiler to make it's rated capacity, and one of them is a minimum feedwater temp.) The feedwater must be heated from whatever it's temperature is, to boiler temp to make steam anyway. Other than some minor losses through insulation, and a little bit out the vent, the great bulk of the heat added in the DA arrives in the boiler.

 

[pmureiko]

The steam losses for the deaerator are small if properly operated. You should balance this against the cost of chemicals used for water treatement (include handling, labor, and monitoring costs), deaerator capital cost, and the cost of chemical addition facilities and early boiler tube failure. The net present value (NPV) can be calcuated and you can present an economic argument to the client. NPV is value to the shareholders.

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.

 

[pmureiko]

I made a mistake and reported the cost of steam lost from the deaerator based on 30 t/d of steam and not 30 t/hr. The corrected cost increases to $15.8 per day and $5800 per year. Still do the economic analysis for the fixed and operating cost of the chemical addition vs. the deaerator.

 

[engy74]

Thanks again to all the engineers have helped me to resolve this problem...fortunately my Customer has decided to install the deaerator (choose with I agree, after all) and then, if he will place the order to my COmpany, a new deaerator will placed in the plant..

 

[ChasBean1]

It's a good result. Trust me. Navy plants in subs don't have them because they don't have the room. Surface ships do have them. From what I've seen, the plant chemistry in a surface ship is much better with the dissolved oxygen content so much lower. It's a small energy expenditure (most of which is regained) that will buy you significantly more useful life.