Deaerator Vent Steam Recovery
Deaerator Vent Steam Recovery
(OP)
We are evaluating a proposal to recover most of the small amount of steam that vents with the non condensible gases from a deaerator. The feed is about 50% preheated city water and 50% recovered condensate. The deaerator runs at about 5 psig and there does not seem to be much steam coming out of the vent. The proposed recovery device contacts some of the feed water with the vent gases to recover the steam. The economics assume that 1% of feedwater is currently lost as steam. How could we accurately calculate this current steam loss. We have seen models that assume 0.5% for this loss. If this is our case it would half the proposed savings.





RE: Deaerator Vent Steam Recovery
If you have a valve, you might want to look at putting a cap on the outlet with a small hole and open the valve fully. Keep increasing the orifice till you have the flow of steam you want and then go back and calculate the steam vent rate based on the orifice size.
I'm not sure if the flow rate of the steam would be enough to justify it. Plus, since the deareator is removing dissolved gases, your exchanger will also need to vent these off the steam side to prevent gas blanketting which will further reduce the amount of heat recovered.
RE: Deaerator Vent Steam Recovery
The most common method I've seen for controlling DA vent steam is to drill the gate of a valve (perhaps 1/8"), then leave it closed. Put a valve with an un-drilled gate ahead of it to permit any on-the-run changes. If this is what you have, or can install, then publications like Spirax Sarco's "Hook-Ups" have tables that show steam flow from various sized orifices at a variety of differential pressures.
RE: Deaerator Vent Steam Recovery
How do we establish that a 1/8 inch orifice is the correct size. Don't we need to know how much steam is to be vented to do the job. How can we estimate this? Do we need to experiment?
Thanks for the comments so far.
RE: Deaerator Vent Steam Recovery
Someone doesn't make sense to me. You are running feedwater through the deareator to remove oxygen and C02 and it sounds like you are then going to take that stripping steam, with those gases and do a direct injection back into the feed water to the dearator. Sounds like the gases would just build up and up in the recyle loop. Or is this feedwater stream going to another location in the plant where these gases are acceptable?
RE: Deaerator Vent Steam Recovery
RE: Deaerator Vent Steam Recovery
Is this "patented device" significantly cheaper than a small stainless steel heat exchanger? I KNOW that will recover the heat of the vent steam, especially with cold make-up water on the other side. No "propietary device/technology" fooling around, either.
Talk to the DA manufacturer about what kind of orifice size they recommend with your model of DA, and operating conditions. If you can't get any worthwhile information, I'd try just installing an isolation valve ahead of the gate valve, and try an 1/8", and see what you get. A little trail & error should tell you what you need to know. You'll need to monitor things like operating load, and sulphite (I'm assuming you're using sulphite as an oxygen scavenger) consumption and residuals.
With a delta-P of 5 PSI (DA operating pressure to atmosphere), an 1/8" orifice will pass 7.5 lbs/hr, according to Spirax Sarco's "Hook-Ups". A 1/2" orifice will only pass 119 lbs per hour. I don't think your project will have to cost very much before the payback drifts away. I think you'll get a lot more bang for your buck just repairing/replacing a couple of failed steam traps in the plant.
RE: Deaerator Vent Steam Recovery
RE: Deaerator Vent Steam Recovery
We sure didn't have that from our packaged boilers IMO. They were about 500,000 lb/hr max capacity and the dearator was more than that as it also feed all the other steam generators on-site. All I could see you needing to vent is enough steam that the oxygen concentration is kept low in the vent stream. With a few ppm, maybe, in the feed water, that doesn't have to be much steam flow I would think.
Any other comments or am I all wet (no pun intended) on this one?
RE: Deaerator Vent Steam Recovery
Something that haunts calculations like this is that very few plants have any sort of accurate metering for steam. Most just have orifice plates (and fully half of those will have the plates installed backwards...) with no pressure compensation. Steam production & usage numbers in most plants are not much more than fiction.
RE: Deaerator Vent Steam Recovery
99.95% of the units I design have a 0.05% vent rate, which is more than adequate (in most cases) to vent the non-condensable gases and reach an outlet oxygen concentration of 0.005 cc/L and ~0.0 cc/L CO2. Exceptions come with every job, but unless there is a low delta T from the inlet to the outlet, or there is unusually high non-condensable gas content, a 0.05% vent rate is standard.
Regarding the vent orifice.... the hole is drilled into the gate to insure that the vent can never be closed. In normal operation, the gate is opened to release steam & non-condensables. The amount the gate is open is generally a trial and error proceedure that involves testing the outlet concentration of non-condesable gases. Vent rates can be calculated using an orifice equation. The most common size on a DA vent valve is 2".
Regarding the "vent condenser". External vent condensers are requested sometimes and they are always specified to be shell and tube heat exchangers. They are very effective and are also not very expensive. I would shop around if I were you... especially if the vendor is not willing to disclose how his "black box" "vent condenser" functions.
You may want to make sure the vent condenser is required. Nearly every DA in service has an internal direct contact vent condenser. Besides, the steam loss is not considerably large.
Example:
A 500,000 lb/hr (outlet) DA with 50-50 makeup (60ºF) - condensate (180ºF) inlet mixture only needs to vent 283lb/hr, which uses a 4" gate valve.
Good Luck!
jproj
RE: Deaerator Vent Steam Recovery
jproj
RE: Deaerator Vent Steam Recovery
The idea of an existing built-in condenser is worth a check. The 0.5% par for steam vented confirms our finding in the literature.
Thanks for the quality help. I will post when we have more information. This issue is part of a larger study.
RE: Deaerator Vent Steam Recovery
Be very wary of the steam numbers you're being supplied with. The error can be significant.
RE: Deaerator Vent Steam Recovery
Chase down your condensate leaks. Install returns where there are none and fix all faulty steam traps. Reducing your make up will also reduce your blow requirements. Why worry about what little is going up the stack when tons more is going down the drain?
RE: Deaerator Vent Steam Recovery