WATER TUBE BOILER-CALCULATION FOR EVALUATION
WATER TUBE BOILER-CALCULATION FOR EVALUATION
(OP)
Good Morning Everyone, Perhaps somebody can be of assistance with the following issues
We have a situation with a power boiler evaluation, design conditions on plate are 72 kg/cm2 and 483 C.
Furnace tubes are 3" OD.
We would like to calculate the minimum required thickness per ASME CODE SEC 1 to compare with thickness readings
FIRST QUESTION
ASME CODE SEC 1 PG27 provides 2 formulas, PG.27.2.1 and PG27.2.2 One specify for tube and the other for Pipe. I haven't found a definition on ASME CODE SEC 1 between tube and pipe however I would choose the one for TUBE (PG27.2.1), as far as I'm concern this term is address to Boilers and heat exchangers tubes. As for pipe is most adequate for whatever is outside the boiler. Any Ideas about where can we found the appropiate definition (B31.1 is broad and I feel it does not provide and application concern, that is the only one I had found at the moment)
SECOND QUESTION AND MOST IMPORTANT
This is concerning the correct application of the formulas. If you use PG-27.2.1 to the contions above mentioned you will obtain 0.233" for tube thickness
Our boiler tubes are 0.157" thick (Nominal thickness per drawing and it match the reading taken)! (Material is SA192 and at 900F you have S=5900psi per sec II)
I was aware that when calculating you have to use design conditions but it might be possible that furnace can operate at a different temperature and will reduce thickness. But what about those tubes close to burners? would iy take higher temp?
What would you think, is this boiler operating below code conditions or what are we missing something in our analysis
any suggestion will be highly appreciated
best regards
We have a situation with a power boiler evaluation, design conditions on plate are 72 kg/cm2 and 483 C.
Furnace tubes are 3" OD.
We would like to calculate the minimum required thickness per ASME CODE SEC 1 to compare with thickness readings
FIRST QUESTION
ASME CODE SEC 1 PG27 provides 2 formulas, PG.27.2.1 and PG27.2.2 One specify for tube and the other for Pipe. I haven't found a definition on ASME CODE SEC 1 between tube and pipe however I would choose the one for TUBE (PG27.2.1), as far as I'm concern this term is address to Boilers and heat exchangers tubes. As for pipe is most adequate for whatever is outside the boiler. Any Ideas about where can we found the appropiate definition (B31.1 is broad and I feel it does not provide and application concern, that is the only one I had found at the moment)
SECOND QUESTION AND MOST IMPORTANT
This is concerning the correct application of the formulas. If you use PG-27.2.1 to the contions above mentioned you will obtain 0.233" for tube thickness
Our boiler tubes are 0.157" thick (Nominal thickness per drawing and it match the reading taken)! (Material is SA192 and at 900F you have S=5900psi per sec II)
I was aware that when calculating you have to use design conditions but it might be possible that furnace can operate at a different temperature and will reduce thickness. But what about those tubes close to burners? would iy take higher temp?
What would you think, is this boiler operating below code conditions or what are we missing something in our analysis
any suggestion will be highly appreciated
best regards





RE: WATER TUBE BOILER-CALCULATION FOR EVALUATION
Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN
RE: WATER TUBE BOILER-CALCULATION FOR EVALUATION
Are you familar with boiler design or have experience to do this calculation?
RE: WATER TUBE BOILER-CALCULATION FOR EVALUATION
We were using formulas and the parameters of the plate to calculate minimum wall thickess.
One of the boilers actually is 0.250" inches thick for the wall tubes, we found very rare that only 0.017" were allowed when assuming the 0.233"
Can I ask you one question, Should you have to considere the Furnace temperature? What about those tubes that are close to burners? Should you have to consider moere temeperature other than the saturation point?
RE: WATER TUBE BOILER-CALCULATION FOR EVALUATION
You need to understand how to calculate mean wall temperature in a tube to compare with steam saturation temperature - look below
http://www.hcheattransfer.com/sitebuildercontent/s...
RE: WATER TUBE BOILER-CALCULATION FOR EVALUATION
also depends on the type of boiler and pressure.
RE: WATER TUBE BOILER-CALCULATION FOR EVALUATION
I don't know how much things changed since then as I have been out of the loop since the 90's.
RE: WATER TUBE BOILER-CALCULATION FOR EVALUATION
The highest heat flux may occur with an oil flame , peak heat flux as high as 120,000 btu/hr/ft2 based on ID area. Coal fired (PC)units may have a lower flux, perhaps 80,000 btu/hr/ft2 at the "worst tube". Since it only takes the failure of one tube to take the unit down, all tubes are designed as if they will behave as the "worst tube". The tube wall thickness may be determined iteratively if such a model is available. The designer also needs to ensure the water circulation is adequate to avoid exceeding the DNB heat flux at this peak heat flux location.
The saturation temperature may be 650 F, but he tube mid wall metal temp at teh peak heat flux zone could be as high as 900 F. The tube OD temperature facigng the flame is higher yet.
The conduction heat transfer equation used for the model is termed a harmonic DE, and the stress equation is called the bi-harmonic DE. The thermal stress distibution thru the tube can also be calculated with the same model. One can easily see that the thermal stress experienced at the tube OD is greater than yield stress , and this easily explains the occurrence of "alligator cracking" when correct boundary conditions are imposed on the fin restraint.