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Guest (Visitor) (OP)
2 Apr 02 16:29
A question or ten for the group.

We have a 660 Mw power boiler at our power plant with a deaerator vessel and DA storage "tank".  The DA vessel has a National Board number and a MAWP of 200 psig at 400 degrees F (so stated on the manufacturer's name plate).  The DA storage tank (located below the DA "vessel") has NO National Board number but is made by the same company as a unit.  

Is the DA vessel a Code vessel?  It seems to me that the DA vessel and the storage tank below it are both vented to atmosphere (?) but there is a safety relief valve on the upper DA vessel.  How could the DA vessel be containing pressure (200 psig?) if it is connected to the storage tank which is vented?  If the DA vessel is not pressurized why is it National Board registered and why does it have a MAWP of 400 psig?

The questions come up because I have been told that if a "vessel" is National Board registered it must have a "R" stamp company do any weld repairs.  Our DA vessel and DA storage tank need to have some cracks welded and the contractor says that we DO NOT need to have a "R" stamp organization do the work.  I have read the Pensylvania Boiler regulations but I can't see any mention of DA vessels (if they really are pressure vessels that is a different story).

I am so confused.

Thanks.
AEF (Petroleum)
3 Apr 02 0:30
Welcome to code land. Sorry, the DA vessel is a pressure vessel. Most jurisdictions are quite anal retentive about DA's as the older ones (not stress relieved) use to come unzipped and create mayhem on a regular basis. Check with your local regualtory authority to see if an R stamp holder is required to do the work. Last note, is your DA stress relieved? Are the cracks from service or leftovers from fabrication?
MJCronin (Mechanical)
4 Apr 02 12:24
AEF is on the money.... Check with the local jurisdiction

DAs are frequently designed to a much higher pressure than the pressure where they actually operate. This is because DAs are subject to internal shock ( transients) during plant upset conditions. (If the ASME vessel rules were used at the actual operating pressures, te shell and heads would be far too thin and fragile)

Because of some catastrophic failures (and loss of life) about twelve years ago, the Heat Exchange Institute (HEI) issued new "Standards and Typical Specifications for Deaerators" -1992 edition, which requires increased shell and head thicknesses and PWHT for the main seams in the DA and storage tank. NACE and TAPPI also issued guidelines on DA design due to failures in the pulp and paper industry

Try a GOOGLE search on "Deaerator failures" and deaerator design and review the results

I also suggest that you contact a technical rep at Kansas City Deaerator (www.kansascitydeaerator.com) and ask questions.

Good luck !!


MJC
jproj (Chemical)
4 Apr 02 17:39
The Deaerator and Storage Tank ARE BOTH pressure vessels and with a MAWP of 200 psig, it is most likely operating above 125 psig.  The vent to atmosphere has either an orifice plate or a gate valve with a hole drilled in the gate such that the non-condensable gases are ALWAYS vented.  

Deaerator vent lines are usually (depending on the vent rate) somewhere between 2" and 12".  I have seen larger vent sizes, but only on very large (or vacuum) units.  The vent line is not large enough to relieve all the pressure in the unit; it is only designed to vent non-condensable gases.  The normal vent rate on a pressurized DA is 0.5% of the steam flow.

The safety valve on the DA is usually sized to relieve the entire steam capacity of the unit at the design pressure.

If I were you I would contact the national board directly with your question.  An e-mail list of persons qualified to answer your question can be found at:

http://www.nationalboard.org/staff.html

Good Luck!

jproj





twjag (Structural)
6 Apr 02 16:42
You did not mention an ASME Mfr's Data Report for the subject stamped vessel. Review the Data Report. Both vessels may be covered on the same U-1 report as a multi-chambered vessel. This would be legal if the Dearator is directly connected to the storage tank, without valves to isolate the two vessels. If data report is not available from owner, you can obtain a copy (for a fee) from the National Board.

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