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Boiler Blowdown Tank Capacity 1
- Thread starter code1
- Start date
the ( US) national board of boiler inspectors NBBI used to have a free program that would size the blowdown tank, but it was removed from circulation a few years ago.
As I recall, the objective was to maintain the tank pressure below 15 psig; the tank vent was sized for the max steam flow generated during the assumed blowdown event. This key item needs to be checked in the specific case of interest, as the scope of supply of this long vent line is often not by the same supplier as the tank- if this long line is undersized or if a muffler is added, it would contradict assumptions of the generic NBBI calculation.
The NBBI calc assumes you are only blowing down one steam drum at a time. For a conventional Rankine cycle that makes sense, but for a triple pressure combined cycle plant it might not make sense. The amount of sat liquid blown down is from drum hi-hi level to drum lo- lo trip level, so you will need to define drum length and diameter. The assumed drum pressure during the blowdown event is ( ??? ) psig. The choked 2-phase flow in the blowdown drain from steam drum to BD tank may use some approximation of the 2 phase choked flow curves presented in the appendix of the hard copy of the ASME steam tables .
The BD tank vent pipe can be modeled using the approx equations known as Fanno flow, simialr to the safety valve vent stack sizing presented in the appendix of B31.1 or sect VIII div 1, but extended for larger values of fL/d.
As I recall, the objective was to maintain the tank pressure below 15 psig; the tank vent was sized for the max steam flow generated during the assumed blowdown event. This key item needs to be checked in the specific case of interest, as the scope of supply of this long vent line is often not by the same supplier as the tank- if this long line is undersized or if a muffler is added, it would contradict assumptions of the generic NBBI calculation.
The NBBI calc assumes you are only blowing down one steam drum at a time. For a conventional Rankine cycle that makes sense, but for a triple pressure combined cycle plant it might not make sense. The amount of sat liquid blown down is from drum hi-hi level to drum lo- lo trip level, so you will need to define drum length and diameter. The assumed drum pressure during the blowdown event is ( ??? ) psig. The choked 2-phase flow in the blowdown drain from steam drum to BD tank may use some approximation of the 2 phase choked flow curves presented in the appendix of the hard copy of the ASME steam tables .
The BD tank vent pipe can be modeled using the approx equations known as Fanno flow, simialr to the safety valve vent stack sizing presented in the appendix of B31.1 or sect VIII div 1, but extended for larger values of fL/d.
code..
Please be more specific...you ask: "What figure should I use for boiler emergency blowdown flow?"
What does "figure" mean ?
Are you interested in massflow rates ? Is this intermittent or continuous blowdown ? Not all blowdown systems are alike...as davefitz has stated, a lot has to do with the type of boiler you have and how you intend to operate it.
I have heard of boilers having continuous and intermittent blowdown systems, what is "emergency" blowdown ?
-MJC
Please be more specific...you ask: "What figure should I use for boiler emergency blowdown flow?"
What does "figure" mean ?
Are you interested in massflow rates ? Is this intermittent or continuous blowdown ? Not all blowdown systems are alike...as davefitz has stated, a lot has to do with the type of boiler you have and how you intend to operate it.
I have heard of boilers having continuous and intermittent blowdown systems, what is "emergency" blowdown ?
-MJC
- Thread starter
- #4
I should be more specific:
Yes, I am interested to know if a industrial norm/ engineering practice/Code requirement exists for the 'figure' of the blow down mass flow rate during 'emergency' cases.
A vendor has proposed to size the emergency blowdown based on 10% of the BFW flow. The boiler is fired on municipal solid waste that have a fluctuating heating value. In this case the total steam/ BFW flow is ~ 100ton/hr.
Continuous blowdown and/or discontinuous blowdown is to maintain the required BFW water quality.
Emergency blowdown in this case is when steam drum water is blown down to maintain a drum water level below that of the HH level due to the highly variable heating vaule of the fuel.
Yes, I am interested to know if a industrial norm/ engineering practice/Code requirement exists for the 'figure' of the blow down mass flow rate during 'emergency' cases.
A vendor has proposed to size the emergency blowdown based on 10% of the BFW flow. The boiler is fired on municipal solid waste that have a fluctuating heating value. In this case the total steam/ BFW flow is ~ 100ton/hr.
Continuous blowdown and/or discontinuous blowdown is to maintain the required BFW water quality.
Emergency blowdown in this case is when steam drum water is blown down to maintain a drum water level below that of the HH level due to the highly variable heating vaule of the fuel.
- Thread starter
- #6
Shmulik,
Thank you for providing the information. The sizing of the vessel and the vapor/ condensate nozzles, layout etc I am able to perform.
I just need to know if there are any good engineering practice/ code jurisdiction to say that "...for emergency blow down or upset situations, take a figure of X% of the some mass flow (e.g. the BFW flow).
Is it up to the boiler designer to specify/ advise a certain blow down percentage? Can the Owner disagree and specify a lower figure?
I am asking because I am venturing into unfamiliar territory, so thanks in advance.
Thank you for providing the information. The sizing of the vessel and the vapor/ condensate nozzles, layout etc I am able to perform.
I just need to know if there are any good engineering practice/ code jurisdiction to say that "...for emergency blow down or upset situations, take a figure of X% of the some mass flow (e.g. the BFW flow).
Is it up to the boiler designer to specify/ advise a certain blow down percentage? Can the Owner disagree and specify a lower figure?
I am asking because I am venturing into unfamiliar territory, so thanks in advance.
-
1
- #7
Hi code1,
I suppose I've got your intention. Don't even dare trying to overcome boiler over-pressure by increased water blowdown! It may cause unlimited low water level and consequently irrevocable damage to boiler structure due to heating surface overheat.
Boiler over-pressure shall be dealt by properly sized pressure safety / pressure relieve device connected to steam phase. I would suggest you read the relevant tutorials regarding safety valves at the following link:
Boilers blowdown amount shall be determined acc. to feedwater quality only:
Regards.
I suppose I've got your intention. Don't even dare trying to overcome boiler over-pressure by increased water blowdown! It may cause unlimited low water level and consequently irrevocable damage to boiler structure due to heating surface overheat.
Boiler over-pressure shall be dealt by properly sized pressure safety / pressure relieve device connected to steam phase. I would suggest you read the relevant tutorials regarding safety valves at the following link:
Boilers blowdown amount shall be determined acc. to feedwater quality only:
Regards.
code..
Now we understand..!!
You are interested in "emergency" blowdown for your rather unique type of boiler and your unique way of controlling drum level.
It just so happens that I have worked on blowdown systems for waste-to-energy plants. We sized the systems for two conditions.....continuous and intermittent blowdown, just like many other boilers.
We also had trash/waste of various calorific values. But the control of drum level was done by the sopisticated control of the Von Roll grate speed (rate of trash burning and heat generation) and close control of the feedwater flowrate.
I do not believe that an "emergency blow" of the steam drum is the best way to control your system....but thats my opinion only.
Also, I completely agree with Shmulik. This is an especially bad way to control overpressure
-MJC
Now we understand..!!
You are interested in "emergency" blowdown for your rather unique type of boiler and your unique way of controlling drum level.
It just so happens that I have worked on blowdown systems for waste-to-energy plants. We sized the systems for two conditions.....continuous and intermittent blowdown, just like many other boilers.
We also had trash/waste of various calorific values. But the control of drum level was done by the sopisticated control of the Von Roll grate speed (rate of trash burning and heat generation) and close control of the feedwater flowrate.
I do not believe that an "emergency blow" of the steam drum is the best way to control your system....but thats my opinion only.
Also, I completely agree with Shmulik. This is an especially bad way to control overpressure
-MJC
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