hello,

Appreciate if someone can tell me the difference between fired and unfired pressure vessel. Is it right to say that fired pressure vessel is a boiler and must be design to boiler code. Is there a case where fired pv is designed to Sec. VIII. thanks

Hello,

it is a boiler only, if steam is generated for the use outside of itsself. Otherwise it might also be considered as a unfired pressure vessel. Both, calculation,design for boilers or a fired pressure vessels are possible, however, you have to follow one case completely.

Fired pressure vessels are vessels which are fired electrically or by gas or fuel. You also may look for an interpretation by ASME.

Regards Jürgen

Hi,

The difference between fired and unfired vessels is the fire!

Different definitions are established in the laws and Codes of this world. In the actual case I recommend to look into the applicable boiler law at the location of installation:

1- If a vesses is exposed to heat from the combustion of fuel it is fired
2- If a vesses is exposed to radiation from the combustion of fuel it is fired
3- If a vesses is exposed to heat from a fire, or electric heating it is fired

Case 1 and 2 exclude process heat, case 3 includes!

If the vessel is fired, as per Preamble of Section I, it may be built to Section I or VIII.
If built to Section VIII, Div. 1, UW-2 addresses special requirements, and the nameplate shall be stamped "DF" under the U-stamp. The MDR will specify Direct firing as well.

Does this help?

Practical consideration from cathegorizing a pressure vessel as UB-DF or Power Boiler.
In a petrochemical plant there is the need of a massive production of energy: this is usually given by power boiler.
At the same time it is economical to recover waste heat and it usually obtained by a waste heat steam generator where process gas heats water and produces steam: the WHSG is a heat exchanger were channel is Sect.VIII and shell may be Sect.VIII (UB) but usually is requested to be Sect.I. It is usual to have a burner to stabilize the steam production: in this case the item should be a Sect.I boiler.
Other steam generators have their prime scope in the process, e.g. incinerators. They also usually have a burner to keep temperature and fire regime to serve the process.
Important to consider that UB shall be equipped with safety valves as required by Sect.I (UG-125(b) but, under the responsibility of the User (UG-125(a)(1). In this case, the engineering, not controlled by an AI, follow requirements of Sect.VIII and place a stop valve to isolate safety valve (see UG-135(d)) because these generators are included in the process and it would be a serious problem to have a shut down. The isolating valve permits to make maintenance to the safety valve without interrupting the process. It is important as well to consider that safety valves on UB press. vessel are provided to the User's responsibility not the pressure vessel manufacturer.
Sect.I does not allow any valve between boiler and safety valve.
It is common that european engineering ignore the requirements of Code stamped safety valves for boiler and press.vess. and buy them not stamped from european market. No problem for real power boiler where the Manufacturer knows its job and provides Code stamped safety valves. The problem comes when the engineering orders any drum to Sect.I, makes its own piping at field and orders to other manufacturers the safety valves (non Code stamped).
DF pressure vessels requirements for safety valves are per Sect. VIII.
Since the Code of construction is specified by the engineering, when items are not erected in USA I do not interfere with the choice but I make aware my manufacturer of the above. For the Manufacturer and the engineering the most convinient to apply is Sect. VIII.