Water-tube boiler economizer contains water on flue gas side 2

[ashaari]

Hi all. Appreciate your help to check whether my assumption is correct.

I have a water-tube gas-fired boiler, producing 40tons/hour of steam at 40 barg, 385 degC. During boiler startup, I noticed that there is water that is drained from the economizer. Sometimes the amount looks a lot until I can see mist coming out from my stack.

My question is, where is the source of water? My guess is it's the result from combustion (C + 2H + 2O2 + N2 --> CO2 + 2H20 + N2). Is my assumption correct? Next question, why only at economizer?

Thanks!

 

[davefitz]

because that is where the tube leak is. check you rate of feedwater makeup ( ie demineralized water makeup)- if there is no tube leak, the makeup rate woul donly be 0.5% of steam flow; with a tube leak, the makeup rate could be 20%.

 

[ashaari]

But during normal boiler operating load, my total feedwater input is just 0.2 - 0.5 ton/hour higher than my steam output...that's why I didn't consider tube leak. I only observed the condition i mentioned during startup...

 

[davefitz]

If the feedwater temperatrue entering the economizer ( or the economizer temperature prior to startup) is below 212 F, then you will condense water vapor formed from combustion.

I recall a case of a boiler explosion caused in 1961 (NJ) due to the use of a horizontal shaft rotary air heater, while using very low combustion air flow ( less than currently required 25% MCR air flow)during startup. The startup fuel was natural gas. The cold air heater condensed the water vapor, and this condensation filled the air heater hopper and air heater baskets with water. The water blockage prevented continuous flow of secondary air to the burners , which led to the filling of the furnace with fuel gas which then caused the explosion. Ever since then, NFPA requires a min startup air flow of 25% MCR , sufficient to be measurable across a standard flow element and sufficient to unblock passages taht are filled with condensed water .

 

[eyec]

I can see mist coming out from my stack

i agree on the tube leak, else how is the "mist" getting into your combustion string?

Steven C
Senior Member
ThirdPartyInspections.com

 

[rmw]

When you start up, your entire boiler back end is cold (ambient temperature). Until it heats up above 212F (100C) it will condense the water in the flue gas. Depending on the temperature of the water being fed into the economizer (if you are waiting on DA steam from the boiler warm up, the water in the economizer tubes is cold too) until the whole back end warms up, it will continue to condense the water in the flue gas and that water will collect in low places or run out of low points (hopefully). As the back end begins to warm up, the water collected on the economizer tubes and fins and the duct walls and lying in the low collection points will begin to vaporize. You will see water vapor in the stack at this point and it will continue until the collected water is boiled off. If the ambient is cold enough, you may see water vapor well into the operating cycle. If you have a DA, the FW shrinkage that you state is not unreasonable. If you do not add load soon, (in other words, you remain at start up load for an extended period) your boiler back end might not heat up at all depending on the ambient and the temperature of the water in to the economizer.

rmw

 

[ashaari]

Thanks all for your opinions. Appreciate it.

 

[chicopee]

Is gas the only fuel that you burn?

 

[ashaari]

yes chicopee, only use methane as our fuel gas.

 

[CFSE001]

To: davefitz

Thanks for the 25% MCR posting. Can you provide more information concerning this 1961 incident? Are you suggesting that this incident was the basis for the addition of 25% MCR minimum air flow to NFPA 85? If so, why wouldn't maximum air flow during the purge cycle clear the secondary air flow passages to the burner prior to light off? And, if purge air flow rates fail to unblock the required air flow passages, why would 25% MCR air flow offer any additional safety margin at lightoff? Was air flow in this case inferred by pressure instead of flow, i.e. was a false positive created due to backpressure? Can you explain how air flow passages to the burner(s) can be restricted by liquid condensation in a typical startup scenario?

 

[davefitz]

The incident occured circa 1962 at the PSEG bergen or mercer station, gas fired units. My memory of the incident is based on a repoprt I read in 1989, and it stuck in my mind. I believe 1 mort as a result of the accident.

In those days, air flow was measured by a flow venturi, and the press drop across this venturi varies by the 2nd power of the flow. At 15% MCR flow, one has only 2% of full range pressure drop across the DP flow Xmitter, and a cailbration error can easily show flow when in fact zero flow exists. Today, people use rosemount digital DP xmitters and may pretend the accuracy exists at such low DP's, but such technolgy was not available in 1962.

The horizontal axis air heater had hoppers which plugged with condensed water vapor , easy to ccur during startup prior to STG synch as feedwater temp to the economizer may be only 100F, and flue gas temp to the airheater gas in may be below 200F.

The airheater and its hopper plugged with water; the burners or air registers were not plugged. Air could not continuously flow thru the plugged air heater baskets.

 

[CFSE001]

With the exception of 7.6.2.2.6.1 in NFPA 85-2007 for fluidized-bed boilers can you cite the requirement for 25% MCR min startup air flow?

 

[davefitz]

cfse001:

see NFPA 85B ( 1989) par 2.6.1.c, 5.1.4.2, and chap 3 definition of "purge rate".

The above referenced paragraphs require the air flow rate be continuoulsy maintained above the "pureg rate", and chap 3 definition of purge rate explicitly states a min of 25% MCR air flow.

 

[chicopee]

The leak from the economizer during startup is not unusual. The economizer is the relatively cold side of the boiler. Particularly during high humidity and warm atmospheric conditions, moisture will condense on the economizer.
I used to inspect boilers and one such W.T. boiler would exhibit this condition. This boiler would burn #6 oil and the soot residue would be moisture ladden similar to syrup running down the lengths of the tubes during these inspection periods.
So if it is not a leaking tube as mentioned in the other posts, suspect the above scenario.

 

[CFSE001]

davefitz: Please see my new posting so that the discussion is directly relevant to the applicable thread.

 

[DIRTYDAVE]

BACK IN 1960 IT WAS RECOGNIZED THERE COULD BE BURNER INSTABILITY WHEN STARTING UP ON NATURAL GAS. WHY? 2 OPINIONS BASED ON TESTS RUN BY ONE BOILER COMPANY IN MISSISSIPPI AND ANOTHER IN NEW MEXICO. PRODUCTS OF COMBUSTION IN THE AIR HEATER BASKETS DILUTING THE AIR FLOW TO THE BURNER OR MOISTURE CONDENSING ON THE AIR HEATER BASKETS MIXING WITH THE AIR FLOW TO THE BURNERS.
2 RECOMMENDATIONS. INCREASE THE AIR FLOW AND/OR DONT START THE AIR HEATER UNTIL GAS TEMPERATURE ENTERING AIR HEATER REACHED 400 DEGREE F.
I TURNED 75 LAST WEEK SO MAYBE MY MEMORY IS SLIPPING, BUT I THINK
THE GIST OF WHAT I JUST WROTE IS CORRECT

 

[muscovy]

When burning hydrogen rich fuel, for every 1 pound of hydrogen you burn it makes 9 pounds of water due to chemical process. A good way to determine the amount of air you need is to take the higher heating value of the fuel and divide by 1300: HHV/1300. It gived you pound of air per pound of fuel.