ASME PWHT Requirements

[Sharik]

Can anyone tell me why an ASME Section I steam drum, P1 material, 1" thick requires PWHT and yet an ASME Section VIII, Div. 1 pressure vessel, built of the same 1" thick, P1 material using the same WPS, does not require PWHT?

 

[bmoorthy]

Did you compare for P4 material?

Yes there are differences amongst the different codes, these differences are largely due to different design Philosophy/design principles, if you check the allowable stress for a given material for a given temperature, you may get the essential difference.

If you check BS 5500(PD5500), appendix then you would realise if necessary impact test is carried out at design reference temp after necessary forming, and the values meet the requirement specified the PWHT could be waived. This kind of provision is not there inSec VIII div 1.

If you do a comparative study amongst various design standards you may find the required thickness for a given equipment would vary, if you get the reason for this you would also get the reason for your querry.

Did you check ANSI B 31.3 and compare the PWHT requirement with ASME?

 

[Sharik]

Yes, ASME B31.3 is the same as ASME Section 1. For P1 materials, PWHT is a requirement for base metals greater than 0.75".

Looking at ASME Section II, Part D for SA-516 70, there is no difference in the stress values for Section VIII, Div. 1 and Section I. The only difference is in the MAWT. Which still doesn't affect the PWHT requirements.

This is something which has puzzled me for years.

 

[GenB]

PWHT for sec I power boilers is some how mandatory due to the process work/environment where the vessel and components are used, mostly fire, the commitee at at sec I
and sec VIII do not agree in having the same for both sections. They are different designs and that is why the PWHT on sec I will always be there.(some times even if it is not need for PWHT) you must comply with the Code.
Another: note that the explosive danger that the steam possesses makes the engineers at ASME keep it mandatory.
Same is for sec VIII Lethat vessels! Hope it helps.

ER

 

[Sharik]

I don't mean to belabor the point and I do realize we have to comply with the requirements of the ASME Codes, but I'm trying to understand the rationale behind the requirements. Most items in the ASME Codes are logical but this seems to be an exception.

In regards to the explosive power of steam, it shouldn't matter if the steam is generated in a Section I power boiler with a 1" thick carbon steel steam drum, or if the steam is generated in a Section VIII, Div. 1 unfired heat exchanger with a 1" thick carbon steel exchanger shell.

As for Section I being fired, wouldn't you almost think that a 1" thick carbon steel superheater header would pretty well stress relieve itself while it is in service, assuming it is in or near the gas flow?

Interesting stuff.

 

[GenB]

There is no rationale in the design. the main thing is that sec viii commitee does not talk the same language with the sec I.
It may not make sense or it does not make sense at all but it is my belief that the word POWER BOILER is a stronger word than the only Pressure vessel. Fired boilers behave diferent as to unfired boilers.
Also look at NDE, sec I is more stringent as well, even if some times is not needed at all.
and yes it will PWHT at that hi-temp, but before that it will aneal as well and becomes brittle and crack, so PWHT will be needed befor putting in service.
I will have to take a look at the PV sec Viii to see about the NOT-needed PWHT for unfire systems on 1" thick vessel.
I was under the impression that if the vessel is for steam service it has to be PWHT.

I Hope it makes some sense.
ER