Dissolved O2 removal from boiler feed water
Dissolved O2 removal from boiler feed water
(OP)
Our company has signed a pledge to reduce energy consumption. I am looking at using waste steam to pre-heat the feed water to the deaerator. I undertand that there is a general belief of a maximum temperature limit- about 20 oC temperature difference between the incoming feed water and the outgoing deaerated boiler feed water has to be maintained for efficient O2 stripping with steam. I wonder why this has to be so. If the feed water is preheated to a higher temperature than normal boiling point of 100 oC, it should flash (to the extent dpendent on the temperature difference) as it comes off the sprays. This steam flashing should be as effective as using raw steam to strip the O2. It should eliminate the use of raw steam for stripping. Anyone has experience in this please comment.
Thanks.
Thanks.





RE: Dissolved O2 removal from boiler feed water
i do not believe that what is described, as i understand, will leave boiler feedwater in the bottom of the dearator - that is preheating condensate and then flashing in dearator. what will cool the steam to produce boiler feedwater in the lower portion of the dearator?
-pmover
RE: Dissolved O2 removal from boiler feed water
you have to get the energy from somewhere in the boiler to preheat the feed water to above 100 °C. This energy is released while flashing. I think it won't be that efficient.
Edwin Muller
KEMA Power Generation & Sustainables
Arnhem, The Netherlands
E-mail e.f.muller@kema.nl
Internet: http://www.kema-water.nl
RE: Dissolved O2 removal from boiler feed water
You can raise the pressure in the DA if you like to use that spare energy.
RE: Dissolved O2 removal from boiler feed water
First of all, flashing liquid through spray valves is a good way to ruin them. If you break the spray valves, you will have near zero performance in the unit, no matter how much steam flow you have. FYI, 90% of deaerators are operated at 5 psig. Saturation temperature at 5 psig is 228ºF (108.9ºC). If you are bringing the water in at 212ºF (100ºC), it's not going to flash anyway.
Secondly, pre-heating the water releases dissolved gases. I'm guessing your pipe upstream of the deaerator is not stainless steel. If it's not, the liberated gases are going to cause corrosion and eventual pipe failure (dangerous and expensive). There is a reason plants remove dissolved gases IN the deaerator .... this is one of them.
Thirdly, why are you using the waste steam in a pre-heater? A deaerator is nothing more than a direct contact heat exchanger. If you want to use the waste steam to its full potential, use it IN the deaerator.
To answer your original question, a 20ºF temperature difference between the inlet and outlet of the deaerator is desirable because it allows for enough steam flow through the unit. Steam flow through the spray and tray section disrupts the liquid film as it falls. This breaks the falling film into droplets (more surface area). The smaller these droplets are, the easier the dissolved gases are to remove (less distance for diffusion).
jproj
RE: Dissolved O2 removal from boiler feed water
I have NEVER liked this arrangement, and pointed out to those that would listen, the separate connections for make-up water, and condensate on the DA. I always thought it unlikely that a DA manufacturer would go to the expense of welding on another connection, just for the sheer fun of it. There's also the issue of taking make-up water at city/plant pressure, dumping it into a vented tank, then pumping that same water into the DA. Why deliberately lose the initial pressure, only to have to kick it up again?
I'd appreciate your thoughts on mixing condensate & M/U water in a common tank, then pumping it to the DA. Is there something I don't understand?
Thanks
RE: Dissolved O2 removal from boiler feed water
I 100% agree with you! There is absolutely no advantage (except for padding the supplier's wallet) to mixing condensate and make-up in a tank prior to the DA. Like you said... you loose water pressure, which requires you to use a pump to get the water to the DA.... more money down the drain. It is much easier to just mix the streams (in the pipe) prior to the DA inlet. Then there is still only one nozzle and you eliminate the need for a stainless steel tank and the associated pumps. Control valves can be installed to control the amount of water the DA sees and the ratio of condensate to make-up. From what I've seen, condensate is allowed to return at will and the make-up is controlled by the level in the storage tank.
The only reason I could think of for having a condensate return tank would be if you had a spray type DA where you needed to run it at near constant load / inlet temp.... but I'm not too fond of spray type DA's either!
jproj
RE: Dissolved O2 removal from boiler feed water
If you are needing a surge tank for reasons above then the make-up water should also be added in this tank with the return condensate. This helps even the temperature before entering the deaertor and letting the deaerator operate at a more consistent load. This is more critical for a spray/scrubber deaerator but also doesn't hurt a spray/tray deaerator either. Hope this helps.
RE: Dissolved O2 removal from boiler feed water
KeItSiSt - My experience with coatings on tanks, piping, etc that are subject to thermal expansion/contraction is not good. The coatings/linings I've seen eventually crack, and allow water between the coating and the carbon steel. The subsequent corrosion between the layers just goes nuts. Perhaps it's because the ones I've seen used most often are as a band-aid, after initial damage has already occured. My experience with stainless steel is that it's not that much more expensive than carbon steel, after labour costs are subtracted. And if SS is the correct material to use, there's no savings to be had by trying to make carbon steel work.
I've always run the DA normal operating level with a bit of space between it, and the overflow connection to allow for condensate pumps sending returns back in a surge. Even if the DA level goes a little high for a few minutes, and bit of water goes down the overflow, it's not normally enough to worry about.
RE: Dissolved O2 removal from boiler feed water
RE: Dissolved O2 removal from boiler feed water
On the other hand, I've seen plants with the HP condensate coming back to it's own connection on the DA have problems, even though things were connected correctly. They had some HP traps failed open, and this was keeping the DA pressurized. The PRV that was supposed to be supplying steam wouldn't open, as the DA pressure was made. They were busy pitting their storage section until they fixed the traps.
I come at this as someone who has worked in operations and maintenance for a number of years as opposed to a DA designer. I have no doubt that some plants benefit from using a surge tank, however, I have had far more problems on systems with surge tanks, than on systems without them.
One MAJOR problem I've noticed in several plants lately is that the safety valve protection on DAs is often GROSSLY UNDERSIZED. (A safety valve that would pass 1,200#/hr was protecting a DA, and a customer supplied steam PRV that was capable of feeding 8,500 #/hr to it.) People (at least one of whom was the PE who "designed" this installation) have looked at the dinky little safety valve that comes mounted on the unit from the factory, and have assumed that this is all that's required. I think that DA manufacturers would do better to not put any safety valves at all on their equipment, but leave a larger connection available. The one DA that I refer to above had the factory mounted safety installed on the only availble point on the vessel, the safety inlet was the same size as the connection to the vessel. That connection wasn't even CLOSE to being large enough. We installed an additional safety valve in the piping downstream of the steam PRV, and dropped the size of the PRV from 2" to 1".
RE: Dissolved O2 removal from boiler feed water
RE: Dissolved O2 removal from boiler feed water
Regarding a DA not being sized to handle a FULL condensate return: This is the SOLE fault of the system designer. Full flow cases must be considered and designed for (forwarded to the deaerator designer). Most DA's can mechanically handle increased condensate returns (to a point). The unit may not be able to fully deaerate under these conditions (if it was not designed to), but return condensate will likely have low non-condesables anyway.
jproj
RE: Dissolved O2 removal from boiler feed water
I'm not a big fan of bypasses on things like steam traps, and control valves. They help sometimes, but overall, bypasses have caused me far more problems than they've ever solved. The way most plants have downsized their operating crews, there's nobody left to operate a bypass valve anyway.
RE: Dissolved O2 removal from boiler feed water
My experience has primairly been with condensate from surface condensers so I have no idea if the above idea has any merit or not. I would welcome any comments ...
RE: Dissolved O2 removal from boiler feed water
I still think preheating the feed water above its boiling point(say 110 oC) and then let it flash would remove the dissolved O2 and CO2. We have a rather old deaerator made by Permutit It is basically a sparging/stripping column inside a tank(the deaerator). The feed water enters the sparging column at the bottom. It meets the steam coming off the sparger. The sparger is a perforated pipe of 3/4" holes. I am a bit taken by the simple design but it works. As such, the hot feed water entering and flashing in the current stripping column should in principle work as well. The only difference is the stripping steam is now 'self' generated from the hot feed water. To prevent premature flashing, the feed water control valve can be located as close as possible to the deaerator. If necessary, an orifice of suitable size can be installed in the line to keep the water pressure above its flashing point. And the line spec after should be stainless steel. This measures should prevent corrosion in the water line.
I like more comments on this proposal. All opinions are welcome.
Thank you.
Gugubarra
P/s The waste steam is contaminated with process solids. It can't be re-used directly. Currently, it is condensed and sent to the cooling tower.
RE: Dissolved O2 removal from boiler feed water
Subsequent injection of chemical scavengers would still bring the O2 concentration down.
Specialists say that deaerators and feed water heaters are subject to corrosion in various forms: SCC, exfoliation, erosion/corrosion, pitting, snake skin and general corrosion. AISI type 446 ferritic StSt is an alloy practically immune to all of these corrosion mechanisms.
Same sources say that even 7 ppb oxygen would cause corrosion fatigue in carbon steel deaerators, starting with pits as stress risers that ultimately promote the initiation of typical transgranular cracks. This process is enhanced by long residence times ie, contact time cycles between metal and corroder.
RE: Dissolved O2 removal from boiler feed water
The majority of deaerators in service are spray-tray type units with stainless steel tray enclosures and carbon steel head & shell. Many of these very units were installed in the 60's and are still in service!
By the way, who are your sources? It sounds like a sales pitch from a deaerator company who manufactures 446 SS packed tower deaerators.
jproj
RE: Dissolved O2 removal from boiler feed water
RE: Dissolved O2 removal from boiler feed water
RE: Dissolved O2 removal from boiler feed water
If condensate and makeup water mix in a stainless steel atmospheric surge tank before being pumped to a deaerator, is it also necessary that the pump discharge line be stainless steel? If so, why? Should the wetted parts of the pump be stainless also?
b8checker
RE: Dissolved O2 removal from boiler feed water
No, on both accounts. The surge tank needs protection because of the temperature changes that are occuring with the incoming water and the fact that the water in the surge tank is exposed to atmospheric conditions. As water temperature changes occur more or less gasses become dissolved in the water. As water cools the saturation point for dissolved oxygen (and carbon dioxide) is greater meaning the water can hold more dissolved gasses. For example, water at 0 psig and 178F will have a dissolved oxygen content of approximately 2.9 ppm and water at 0 psig and 118F will have a dissolved oxygen content of approximately 5.7 ppm. As water temperature increases it cannot hold as much dissolved gasses and is "driven" out. In a surge tank when mixing returning condensate and make-up water the temperature of the make-up water is increased releasing the gasses. Raising the water temperature increases the reaction rate. In the pump and piping the water temperature does not increase significantly alhtough some pump experts might disagree about the pump statement. However, for this application I have seen many carbon pumps used for a very long time. Although I believe attacking still takes place it is not as troublesome as in a surge tank or boiler tube because of the steady temperature, the lack of residence time, and thickness of materials involved. I do know of some installations where stainless piping and pumps were specified and supplied. In my opinion it's not worth the extra cost.