Thickness difference btw cold finished and hot finished tubes of same OD for boiler use

[coolniks37]

Indian Boiler Regulations specifies minimum thickness of tubes for boiler use as per OD.
Pls see the attached link.

In this, a table is given which specifies the minimum thickness, irrespective of calculated thickness for design conditions.
It says, for tube OD < 32mm, the tubes should have min 2.9mm thickness (if hot finished) or 2.03mm (if cold finished)

What is the logic in this?

Again for once through or assisted circulation boilers, for OD <29mm, minimum tube thickness can be 1.62mm
Here no cold/hot finish requirements are given?

Could anyone explain this

 

[davefitz]

You should revie the fabrication specification for both types of tubing as it will advise the maximum expected variation in wall thickness; this varies depending on the finishing method.

For small bore tubes used in once thru steam generators , the primary design issue is minimizing the tube-to-tube temperature unbalance in the furnace waterwall . The frictional pressure drop through a long , small diameter tube is proportional to the -5 power of the ID, therefore variations in the ID due to the finishing method( and due to accumulations of magnetite coating) has a strongly deleterious impact on the circuit's tube-to-tube temperature difference. You must choose the fabricaton spec that has the smalest variaiton in ID, and you must also use a feedwater treatment method that minimies the accumulatio of magnetite inside the furnace waterwall tubes.

"Whom the gods would destroy, they first make mad "

 

[EdStainless]

The ID is not specified on boiler tubes, only the OD and wall.
And the tolerances are greater with hot finished tubing than with cold finished.

The inclusion of ERW tubes in interesting, I didn't read in enough detail but the welded tubes have significantly lower strength in creep limited application. They should only be used at low temperatures.

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Plymouth Tube

 

[metengr]

It would appear that the Indian regulatory body uses a standard approach regarding design use of boiler tubes based on OD, and corresponding wall thickness. I have not run a comparison between ASME Section I, PG-27 and this regulation. Further into the regulation, the Indian authority appears to use 850 deg F (454 deg C) to differentiate between time independent properties (TS) versus time dependent properties (creep), which govern determination of allowable stress values. The temperature is higher than ASME Section II uses for most carbon steels.