Boiler Economizer Tube Failure

A1: Titanium?

A2: the equipment manufacturer might have an idea or two. It's worth a phone call, surely?

Slip on cast iron fins sections and expand the tubes into the sections. This is done with economisers in sulphuric acid plants.

This is what I was trying to describe.

Dear Mr.aliShujah,

You have not mentioned the Sulphur Content of the fuel, and based on the sulphur Content, the SO2 gas and its DEW POINT TO BE ESTIMATED.

Also, You have not mentioned the EXCESS AIR used. This is also important, since the Higher Excess Air will dilute the Flue gas and Temp. of Flue Gas falls, approaching towards DEW POINT.

Once the Dew Point is estimated, the Exit Flue Gas Temp. is to be maintained about 25 Degrees above Dew Point to minimize COLD END PROBLEM. But this will reduce Boiler Efficiency.

Controlling Excess Air will help Fuel Economy.

Pl. post the Information how the problem is solved/minimized.

RAJESWARI.D

The sulfure is 4 to 5% and 3 to 5 % excess air is maintained.

<...sulfure is 4 to 5%...>

At those levels, it's a lost cause.

That's not 'furnace oil'. That's 'powdered sulphur slurry'. That material needs processing to recover the sulphur as a sell-able product!

Dear Mr. Saeed Anwar,

The Excess Air Percent of 3 to 5% appears to be too low. Sulphur Content appears to be high high and hence SO2 content will be high, as a result the cold end problem will be severe and hence corrossion. occurs and tubes are damaged.

DHAYANANDHAN.S

When we analyze flue gas the oxygen in flue gas is 3 to 5 % when boiler is running on furnace oil and SO2 is below 800 ppm. When boiler is running on natural gas the oxygen in flue gas is 2 to 3 % . These are results of Oxygen not air. We analyze following in boiler flue gas CO2 = 9.5 to 11 % . O2 = 3 to 5 , CO = 200 pmm ,SO2 = less than 800 ppm.

Dear Mr.SaeedAnwar,

Thank you for you information.

The analysis result posted by you informs the following.

1. CO 200 ppm, shows COMBUSTION is NOT PROPER and COMPLETE. This is indicates that the Air-Fuel Ratio is very poor.

2. Air-Fuel Ratio is LESS, is confirmed by very low Oxygen Level in the Flue Gas.

3. SO2 level - 800 ppm, by any standard, is VERY HIGH, and again this high level indicates/confirms Air-Fuel Ratio is VERY POOR/INADEQUATE. Correct Air-Fuel ratio will definitely keep SO2 below 800 ppm and severity of corrossion problem will come down.

Henc, the Boiler Efficiency will be poor and Fuel Cost will be very high and COMBUSTION RELATED ISSUE is to be set right first.

DHAYANANDHAN.S

What about AIR PREHEATER, which is normally after the air heater.? If Economiser itself is damaged, which has a higher temp.zone, the air heater will be severely damaged, causing air leakage and enc air/fuel ratio is affected.

What is the Flue Gas Analysis and Gas Exit Temp. at the chimney.?

DHAYANANDHAN.S

Do not have air preheaters. FLue gas analysis CO2 = 9.5 to 11 % O2 = 3 to 5 % , Exit temperature = 200 C . (after economizer towards chimney)

Within an economizer the condensation of sulfur on the tubes is driven by the water temperature inside of the tubes much more than the gas on the outside of the tubes. Think of a Coke can on a hot summer day. The condensation on the outside metal is driven primarily by the temperature of the liquid on the inside. In fact, most cold end corrosion occurs in environments where the bulk temperature of the flue gas is higher than the dewpoint of the sulfur in the gas. It is a direct contact phenomenon.

The most direct way to alleviate cold end corrosion due to sulfur in the flue gas is to raise the temperature of the feedwater inside of the tube to something approaching the dewpoint of the sulfur in the gas.

Duplex stainless materials have shown a resistance to sulfuric acid corrosion but there are no guarantees that it will last long enough to justify the added expense. In your current situation though it might be worth attempting as you already need to do repairs to the economizer and raising the feedwater temperature might not be an option.

Cast iron is a great material for this application but using that as a retrofit is difficult.

Dear Mr.bobstemen,

I agree with your views - indicating coke bottle example and it is correct.

Simultaneously the CONDENSATION also takes place when SATURATION TEMPERATURE IS CROSSED IN THE DOWN TREND.

For example, in the winter we see mist in the atmospheric air and visibility falls depending upon the intensity of the mist/fog - this is due to CONDENSATION.

DHAYANANDHAN.S

Agreed. My assumption, based on the exit gas temperature, was that the bulk gas temperature was high enough to prevent condensation until the gases found something cooler to cause the condensation. Once exposed to ambient air temperatures the condensation will take place.

Dear Mr.bobstemen,

I forgot to add one more point in my reply to my posting No.13, and now I request you to read in addition to my earlier point.

There should be a Difference of 50 Deg.C in the Feed Water Temp and the Saturation Temp. for the Boiler Pressure. If this Difference is coming down, then the NATURAL CIRCULATION of WATER INSIDE THE BOILER IS AFFECTED.

Upto 1100 psi or 77 Kg/cm^2, in side the boiler, the NATURAL CIRCULATION will take care. For further higher pressure, FORCED CIRCULATION is VERY COMPULSORY.

For pressures beyond 3208 psi or 223 Kg/cm^2 KNOWN AS CRITICAL PRESSURE at which the DENSITY OF STEAM and WATER inside the boiler is EQUAL and the value being 19.2 Lbs/CFT or 307 Kg/M^3 and ONCE THROUGH philosophy is employed, that is NO WATER CIRCULATION in side the Boiler, the water just enters the boiler and leaves as super heated steam.

DHAYANANDHAN.S

I would expect to see about 200 ppm CO as a residual value. 3% O2 indicates a good air/fuel ratio. Feed water temperature is what it is, dependant on the boiler design. Trying to increase the O2 only results in a lower back end temperature. Suggest you concentrate on tube material. All the rest is what it is. Don't get confused with theories. Try to get a better fuel.

Make tube shields from stainless steel pipe split down the middle slightly larger than the tube diameter. By using SS you can get a supply of these locally. Any sheilds made from cast iron would probably be special order and very expensive compared to SS, especially if they have to be cast upon order.

I worked on a couple of projects several years ago, replacing boiler tubes on Recovery Boilers in pulpmills.

Special composite tubes where used, carbon steel (SA210) inner tubes with 1.5 - 2 mm 304L extruded on the od. The tubes where supplied by Mesto on one project and Andritz on the other.

Here's a typical butt weld detail.