Sunday, March 6, 2011

Insights to ASME pressure vessel design

I started my career in 1980 as process vessel designer in CE-Natco, US company famous in process design of various types of onshore or offshore process vessels, such as 2 or 3 phases oil/gas/water separators, gas dehydration with glycol regeneration, flare system, oil dehydrators, steam boilers, etc....  Apparently now the company is owned by Cameron, ref web....Cameron_Natco   Petrobras, Brazil state-owned is one that  has many of Natco topsides ( glycol gas dehydration package ) installed in the semi-submersible platforms even to-date, like P51,52 and 56.
I have gained quite a bit of knowledge and work experience in the 80s' at CE Natco and understood ASME Section VIII Div 1 vessel calculations where it is somewhat much easier compared to Div 2 design which is seldom in the market needs unless very high internal pressure. It is normally advisable and better to use Div 2 when internal pressure exceed 1500psig in terms of thickness savings and other kind of advantages, eg. nozzle re-pad which will be difficult and this is done with using special forged nozzles with thick nozzle wall to compensate for the opening loss of metal. Also with the thick wall vessel, post weld heat treatment will be mandatory after the complete vessel is being fabricated to stress relief the heat affected zone and remove the stress locked. Lately I noticed there was introduction of Div 3 and this is "alien" to me as I have not seen this reference presently in my job on offshore rig engineering at current organization. In the old days, design calculations and drafting were all done manually but now with availability of design and drafting softwares, it is now much more easier for pressure vessel designers as well as drafters. Less thinking job, leaving it to the computer.

The ASME International Boiler and Pressure Vessel Code establishes rules of safety governing the design, fabrication, and inspection of boilers and pressure vessels, and nuclear power plant components during constructions. The objective of the rules is to provide a margin for deterioration in service. Advancements in design and material and the evidence of experience are constantly being added by Addenda. Originating in 1914, the ASME Boiler and Pressure Vessel Code is now adopted in part or in its entirety, by all 50 states and numerous municipalities and territories of the United States and all the provinces of Canada.
The Code is kept current by the Boiler and Pressure Committee, a volunteer group of more than 950 engineers. The Committee meets regularly to consider requests for interpretations, revision, and to develop new rules.
In the formulation of its rules and in the establishment of maximum design and operating pressures, the Committee considers technological advances including materials, construction, methods of fabrication, inspection, certification, and overpressure protection.

INTERPRETATIONS
ASME issues written replies to inquiries concerning interpretation of technical aspects of the Code. The Interpretations for each individual Section will be published separately and will be included with the update service to that Section; up to the publication of the 2013 Code. Interpretations of Section III, Divisions 1 and 2 will be included with the update service to Subsection NCA. Interpretations are not part of the Code or Addenda.

CODE CASE SUPPLEMENTS
Code Cases clarify the intent of existing requirements or provide, when the need is urgent, rules for materials or constructions not covered by existing Code rules. Cases will appear in the applicable Code Cases book: "(1)" Boilers and Pressure Vessels or "(2)" Nuclear Components. Supplements will be sent automatically four times per year to the purchasers of the Code Cases books up to the publication of the 2013 Code.

Vessels such as steam boilers, air compressors, storage tanks, accumulators and large pipes are subjected to internal fluid pressure which is uniformly distributed. All the above mentioned vessels are classified as cylinders or spheres.


THIN CYLINDER:
If the ratio of the thickness to the internal diameter i.e. t/d is less than about 1/20, the cylinder is assumed to be thin cylinder.

THICK CYLINDER:
If the ratio of thickness to the internal diameter i.e. t/d is greater than 1/20, the cylinder is assumed to be thick cylinder.

STRESSES IN CYLINDERS:
The following stresses are :
CIRCUMFERENTIAL OR HOOP STRESS:
The stress which acts tangent to the circumference and perpendicular to the axis of the cylinder is called circumferential or hoop stress. It is denoted by fh.

LONGITUDINAL STRESS:
The stress which acts normal to circumference and parallel to the axis of the cylinder is called longitudinal stress. It is denoted by fl.

RADIAL STRESS:
The stress which acts in a direction perpendicular to the internal surface is called radial stress. It is denoted by fr. Radial stress is very small as compared to fl and fh in case of thin cylinder and is therefore ignored.


I have compiled a set of slides and extracted some pictures from web and I thank those who has directly or indirectly contributed to the information in this slide presentation and if there is any violation of copyright, do notify me and I shall have no reservation in removing the content immediately from the presentation. There are some slides that have hyperlink to other external files which will not be readable below, I apologise for this inconvenience, however should you need more information, do drop me a note  :-
Pressure Vessel Choong





8 comments:

Reign226 said...

Div 3 is uses extensive CAE analysis on key parts of the design and offers even better savings in terms of the having thinner walls than Div 2. Learnt this during the ASME talk =P.

I believe it is a recognition that computer simulations have reached a level of maturity that is now recognized in this standard. However its use is only practical in very high pressure designs (cant remember figure now but higher than Div 2, naturally).

It makes ever larger vessel designs more and more practical, in tandem with the march of progress by technology.

Choong Kim Whye said...

i do not think div 3 will be commercialise any sooner. div 2 is already very complicated and the PE from america side already struggling to do the tedious design calculations. Lucky with softwares, but got to understand the basic and these professional engineers know, garbage in garbage out if anyone using software does not understand the basic.

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