ERW versus Seamless Boiler tubing

[MJCronin]

To all,

I would like to know the circumstances and decision criteria for selection of "seamless" boiler tubing over ERW ( electric resistance welded) type.

We have a request for a replacement district heating boiler with a MAWP of 250 psig and an operating pressure of 175 psig. The boiler is expected to do limited cycling. These are not very demanding conditions. Is seamless tube necessary ? There is no superheater in this boiler.

Please,......no vague memories or opinions..... I am only interested in references to books, texts, websites or technical papers.

What are the circumstances in which seamless tube would be positively required ?

I have posted this question elsewhere and have received very vague and limited responses.

Thank You


MJCronin

 

[davefitz]

I do not think there is a single "textbook" decision that covers all cases.

The ERW tubing has a stress concentration at the seam weld, and this stress concentration becomes problematic under the following scenarios:
a) high pressure ( 2400 psig+) drum boilers waterwall panels, where the operating surface metal temperature exceeds 750F, there will be a reduced creep life at the weld HAZ
b) waterwall panels which have a high rate of radiant heat transfer ( spot factor in excess of 85,000 BTU/hr /ft2) , such as oil fired units, which may have a steady temperature differential between the hot face surface temp and the cold side avg metal gtemp in excess of 75 F
c) any waterwall panel which experiences circulation upsets during startups or drum water level uncontrolled events such that individual tubes may occasionally have experience dryout such that the tube -to tube temperature differential may temporariy exceed 100F, causes a fatigue failure to occur at the seam weld dur to membrane wall stresses.

For a 250 psig boiler with good control of drum water level and good water wall cirulation during startups you might be OK with ERW tubing, but make sure the tubing shop ceritifies the calibration of the UT instrument used to continuously check weld integrity.

 

[weirweb]

I agree that these operating requirements are not very demanding. Can you tell me what OD and wall thickness the tubes are? Without a superheater, corrosion and expandibility should not be a problem, allowing for even re-design of the tube arrangement. Heat transfer should vary between types of tube.

 

[davefitz]

For a 250 psig drum boiler, the probable wall thickness selected would be based on your shop's min wall required for welding membrane walls together, usually about 0.17" MW, as this is greater than the min wall required for pressure retention for a 250 psig unit.

Generally if the boiler is a large unit ( over 60 ft tall), a 3" OD tube is standard, but if the unit is smaller and/or you have a reliable means of calculating the circulation parameters for ensuring natural circulation, a 2.5" OD tube can be used.

If the unit used forced circulation, smaller tubes can be used, but again, one needs a reliable calculation procedure to ensure proper circulation.

 

[deanc]

ASME Sec I PWT-10,PFT-12