WATER TUBE BOILER-CALCULATION FOR EVALUATION

[falco]

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

 

[Fizza453]

483 degree C would be the outlet steam tempearture. This temperature shall not be considered as a design temperature for all tubes. 483 is a design temp for superheater tubes only. 283 degree C would be the design temp for evaporator tubes, boiler bnank tubes.So take saturation temp at 72 Kg/Com2 (that is 283) as a design temp for these tubes.



Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[metengr]

In ASME Section I code, tubes are for heat conduction and pipe convey fluids. The formulas in PG-27 are based on this definition. You have not assumed proper component design temperature for the boiler tubes. Nameplate data is for output conditions. Typically, mean wall temperature is used or one can select steam saturation temperature, whichever is more conservative.

Are you familar with boiler design or have experience to do this calculation?

 

[falco]

Well not really, What we normally were doing is based our judgement on design conditions.
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?

 

[metengr]

falco;
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/sitebuilderfiles/Mean_metal_temps.xls

 

[GenB]

Saturation temp is rather low compare to furnace temps. if you do not know the design temp you may use 700F if the MAWP is 250 psi-and check your thickness,
also depends on the type of boiler and pressure.

 

[chicopee]

For what is worth, when I prepared for my NB commission in the late 70's( for which I took my exam at the World Trade Center in NYC), the tube stress selection was based on a 700 dF temperature, however, there were other temperatures to consider which were 750dF and 800dF when using formulae in section PG27. For water tube boiler, temperature selection was a bit lower that is 600-650dF when selecting stress values for tube into steam drums.
I don't know how much things changed since then as I have been out of the loop since the 90's.

 

[davefitz]

The tubes close to the burner zone are exposed to the highest heat flux, so the face that is heated by the flame will experience a higher mid wall metal temperature than the unheated side . You would need to model the tube using a 2D finite differnce or finite element model to accuraely predict the mid wall metal temperature for the "worst tube" in the buner zone. The model includes the heat conducted from the fin to the tube, and some internal fouling may be considered.

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.