Water-tube boiler economizer contains water on flue gas side
Water-tube boiler economizer contains water on flue gas side
(OP)
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!
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!





RE: Water-tube boiler economizer contains water on flue gas side
RE: Water-tube boiler economizer contains water on flue gas side
RE: Water-tube boiler economizer contains water on flue gas side
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 .
RE: Water-tube boiler economizer contains water on flue gas side
Steven C
Senior Member
ThirdPartyInspections.com
RE: Water-tube boiler economizer contains water on flue gas side
rmw
RE: Water-tube boiler economizer contains water on flue gas side
RE: Water-tube boiler economizer contains water on flue gas side
RE: Water-tube boiler economizer contains water on flue gas side
RE: Water-tube boiler economizer contains water on flue gas side
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?
RE: Water-tube boiler economizer contains water on flue gas side
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.
RE: Water-tube boiler economizer contains water on flue gas side
RE: Water-tube boiler economizer contains water on flue gas side
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.
RE: Water-tube boiler economizer contains water on flue gas side
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.
RE: Water-tube boiler economizer contains water on flue gas side
RE: Water-tube boiler economizer contains water on flue gas side
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
RE: Water-tube boiler economizer contains water on flue gas side