Deaerator Pressurization

There should be a document somewhere entitled "Process Description", in which the Process Engineer will have already answered this question and many others.

The merest hint of whatever it contains has been withheld from the forum and it cannot be seen from here.

It won't work as well.

Deaerator - Wikipedia

..."A deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feed water to steam-generating boilers. In particular, dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion."...

http://sajidarl.blogspot.com/2012/02/deaerator-is-device-that-is-widely-used.html

Needs more information on the process, we design deaerators for dairy plants for degasing raw milk on the milk intakes.

Lactose joke

Lol... maybe

If this is anything to do with steam (as opposed to anything to do with paint or fertiliser) then it may well be that the equipment's operation is geared to 128degC operation as opposed to 113degC, as that is the saturation temperature of the water in the boiler at whatever pressure the boiler operates. In that case the item that is of interest is the ability of water to dissolve oxygen, which decreases with increasing temperature. If the temperature of the water is to be raised above 100degC for the purposes of reducing the dissolved oxygen, then keeping a pressure on it is the only way to go.

http://che.sut.ac.ir/People/Courses/100/Steam%20Table.pdf

Therefore if the operating pressure were lowered so that the operating temperature is 113degC instead of 128degC, then there will be more oxygen in the feedwater by comparison, which either needs more scavenging chemistry to knock out, or lends itself to a faster rate of deterioration of the boiler structure owing to the corrosion potential that dissolved oxygen represents.

Makes sense?

Engage a boiler Operative at the facility in conversation if more information is needed on the particular installation and why it is operated the way it is.

No, it doesn't make sense. It's a great many years since I worked on boiler feed deaerators, and then only peripherally, so I don't know offhand how the optimum operating pressure is determined, but it isn't to give the steam saturation temperature required to provide, by reducing solubility, the typical oxygen content needed by the boiler.

Water at 100°C in contact with air at 1 atm has an oxygen solubility ~ 3 mg/l (my estimate, I can't find any data, but I doubt it's far out). Boiler feed water is typically 7 μg/l out of the deaerator, and then chemical scavenger is added.

If the water temperature is 128°C, and the atmosphere is pressurised with nitrogen (to prevent boiling) to give same oxygen partial pressure as at 100°C, solubility is lower, but not by much.

The deaerator works because the flow of steam strips the oxygen - the oxygen partial pressure in the steam bubble is much lower. That's why you can practically completely deaerate water by boiling in a pan on the hob.

http://cr4.globalspec.com/thread/118228/BOILER-INTERVIEW-QUESTIONS-AND-ANSWERS-PART-1

Dear Mr.pradeep4u,

The idea of keeping 2.5 Kg/cm^2 (absolute) is to obtain the Temperature of Feed Water as 115 Deg.Celsius.This is very essential to drive out the Dissolved Oxygen and other gases from the Feed Water to the Boiler.

The condensate coming out of the condenser will be around 45 Deg.C depending upon the vacuum. While pumping, air ingress takes place.

Less pressure means, less temp. of feed water and more dissolved oxygen in feed water. To get rid of dissolved oxygen, more temp. is required and hence more pressure. Vide Steam Tables.

At ambient Temp say 40 Deg.C the Oxygen absorbed by water will be high and remains as dissolved oxygen. When the Temp.rises the dissolved oxygen is expelled out and at 105 Deg.C almost 100% of dissolved oxygen is expelled out. Inside the Boiler Drum, the Temp. will be corresponding to the Saturation Temp. for the given pressure. The oxygen will remain inside the boiler drum (as there is no exit out) will react with boiler pressure parts and corrosion is developed and promoted. Higher the temp. higher the corrosion in exponential proportion.

The De-aerator is having an arrangement to vent out the dissolved oxygen and gases, ensuring practically zero level dissolved oxygen, thus removing the risk of corrosion.

At 20 ^oC (room temperature) and standard atmospheric pressure (sea level), the maximum amount of oxygen that can dissolve in fresh water is 9 ppm. If the water temperature is below 20 ^oC, there may be more oxygen dissolved in the sample. Generally a dissolved oxygen level of 9-10 ppm. When the temp. increases, the oxygen dissolved will reduce – see the graph

(Percent saturation is the measured dissolved oxygen level divided by the greatest amount of oxygen that the water can hold at that particular temperature and atmospheric pressure, then multiplied by 100. Fish growth and activity usually require 5-6 ppm of dissolved oxygen.)

Open the link below to see the concentration of Oxygen in relation to the Temp. At near Zero Deg.C, the Dissolved Oxygen is 14.62 ppm, as the temp. increases the ppm comes down

http://www.mainevlmp.org/wp-content/uploads/2014/01/Maximum-Dissolved-Oxygen-Concentration-Saturation-Table.pdf

(Two graphs not getting pasted which will show Temp. Vs. Dissolved Oxygen in ppm)

DHAYANANDHAN.S

No! Please read #7.

It's not (mainly) the reduced solubility at higher temperature that removes the oxygen. It's the stripping action of the flowing steam.

If you were up a mountain, where due to lower atmospheric pressure the water boiling point is say 50°C, you could still deaerate water by boiling in a (non-pressurised) vessel.

Perhaps you'll find your answer here.

I always though the higher (assume steam) deaerator pressure conserves energy, keeps your feedwater hotter, the economic operating pressure depends on your condensate pressure and heat content. If you have condensate returning at 150C, don't cool it off in atmospheric receiver by expanding the steam, to feed to your bolier feedwater pump.