PWHT

[eegyman5000]

if vessel requires a full PWHT according to construction drwings,should i incorporate in this process blid flanges even though they have no welds just bolted,and have large thickness which may make heating & cooling rates slow(as heating and cooling rates depends on max. metal thickness
namely heating rate=400 divided by max.base metal thickness in inches (f/hr) (as i understood)
second issue
in paragraph UCS 56 the heating rate as above in f/hr
in my country we use centegrade how can i convert the whole rate (should i divide the quantity between paranthesis equivalent to 400 f,namely 222 C by max. base metal thickness by base metal thickness in inches to get heating rate in C/hr?
or divide 400 f to and thin divby inches and then get f/hr
and convert f to c to get c/hr

 

[metengr]

Convert deg F to deg C, you now have deg C/hr. This is a ramp rate in degrees Centigrade per hour for heating or cooling for PWHT. Take this ramp rate (deg C/hr) and divide it by the nominal thickness in inches, not mm.

The actual ramp rate for PWHT for material thicker than 1 inch (or 25mm) will be more conservative because thicker material can develop thermal stresses because of temperature gradients;

[deg C/hr]/nominal thickness (inches) = PWHT ramp rate

 

[DG72]

PWHT is based on the thickness of the weld, so the blind flange thickness doesn't matter. (See UCS-56)

 

[weldtek]

Unless the customer specifically requests us to do so, I don't include non welded blinds in the PWHT.

 

[kwaliteitshenk]

PWHT means, in my opinion, Post Weld Heat Treatment. Since there is no weld there need not to be heat treated.

 

[metengr]

eegyman5000;
I re-read your post and I hope that I am not reading too much into your first question. Why would you even worry about bolted on blind flanges and PWHT because if you subjecting the entire vessel to PWHT with the blind flanges bolted on you have a more serious problem - damaged fasteners.

 

[VeryPicky]

Great thread:
some picky comments that came to my mind; and sorry if I repeate what was already said.

1. Don't put in the heat treatment furnace something that does not require heat treatment: blind flanges (and bolts); unless there is good reason to do it, which was not mention in the initial question.
2. PWHT holding (soak) temperature is based on the weld thickness, heating and cooling rates are based on thickest component, no matter if it is welded or not (for dimensional stability and stress reasons).
3. If °F/hr are converted to °C/hr don't include factor 32 in the calculation, just divide by 1.8.
4. Use common sense.

Putting Human Factor Back in Engineering

 

[weldtek]

verypicky,
One minor correction to the latter part of your point 2,
heating and cooling rates are based on the material thickness of the shell or heads, not just the thickest component. You might have a head and shell that are 2" thick and a self reinforced nozzle that is 6" thick. The nozzle thickness does not govern.
See UCS 56 (d) 2.

 

[VeryPicky]

Good point weldtek. Thank you for that.

Please remember though that the Code is a set of minimum requirements not a pressure vesel design handbook.
Good engineering practice will dictate how to make provision in PWHT procedure for the nozzle, which is substantially thicker than the shell. Disregarding a thick component when calculating the heating and cooling rates may cause inacceptable stress levels and, as result, cracking.
Another comment on PWHT from me: the same paragraph quoted by you specifies a maximum of 250°F temperature spread within any 15ft interval of vessel length. It might be difficult to control at higher heating rates calculated based on shell and head plate thickness. I have seen some 3 inch wall thick vessels with heavy nozles heated at 50°F or less due to heavy nozzles and complicated geometry.

Does a typical vessel manufacturer place the TC at 15ft intervals on the shell? Not to my nowledge.

Putting Human Factor Back in Engineering

 

[vesselfab]

"""Does a typical vessel manufacturer place the TC at 15ft intervals on the shell? Not to my nowledge. """

SOME OF US DO.

and some of us put the TC on the thicker elements to control the heat and cool rates better.

Some of even know what we're doing.

 

[weldtek]

Or place auxiliary heaters around thick components to reduce otherwise unacceptably slow heating and cooling rates.

 

[VeryPicky]

Sorry Guys. Forgive my frustration.
It was not my intent to question your abilities. Most fabricators, I had misfortune to encounter, were totally relying on PWHT contractor, who could not even provide a decent PWHT procedure. When I talked emperature gradients, they gave me that absent look.

Putting Human Factor Back in Engineering