ASME SEC I - BOILER DRUM THICKNESS CALCULATION

[mancini]

I have worked in a boiler drum calculation with openings parallel to vessel axis, and concerning the ASME Sec I PG 27.2.2 and PG-52 I would like to know if somebody can give me some information about:

1) In the calculation of the drum thickness (cylinder), may I consider the efficiency 1.00 and use the excess of thickness on shell to reinforce the openings and complementary also reinforce the nozzle itself?

2) When I have a pattern of openings on shell, must I adopt the effiency as informed in the PG-52 (usually less than 1.00)?

Thanks.

 

[prex]

1) Yes, you may use PG-32/PG-39 for the compensation of openings (however don't see the difference between reinforce the openings and reinforce the nozzle). Also I don't understand why you want to do this: can you be more specific?
2) The same as above: PG-52 is alternative to PG-32.

prex

http://www.xcalcs.com

Online tools for structural design

 

[mancini]

Prex,

As we have a limitation on the drum weight the idea is to minimize it adopting efficiency E=1.00 on calculation of the drum (by PG-27.2.2) and checking the openings reinforcements but not adopting the efficiency from PG-52 (on the drum shell we have openings in a definite pattern and the openings center distances are more than 2 times the opening diameter). With this method we understand that we will minimize the drum weight, so the main point is:

As we have a definite pattern of openings on shell, is it correct the consideration to not use the efficiency from PG-52? This efficiency from PG-52 would be less than 1.00 and would increase the shell thickness.

Thanks

 

[davefitz]

To minimize the drum weight one would use "nozzle reinforcement" for all nozzles larger than those that are inherently reinforced due to their small size.

Nozzle reinforcement requires full penetration welds plus thicker nozzles, while use of shell reinforcement would allow simply low cost partial penetrations welds ( type Z).

The add'l advantages of nozzle reinforcement are:
a) thinner shell thickness allows much faster startup times; the thermal stress during a particular thermal transient is proportional to the square of the shell wall thickness, so reducing the shell thickness by 40% allows one to double the allowable ramp rate.
b) the allowable loading on the nozzle is substantially increased with nozzle reinforcement, as would be shown by finite element analysis- the shell will also deflect to distribute local stresses when nozzle reinforcement is used.
c) The local thermal stresses close to the nozlle to shell weld are lower due to the correspondinlgy similar wall thicknesses.

There are available orbital robotic welders that can reduce the cost of the full penetration weld - I believe Babcock Hitachi markets one.

 

[mancini]

davefitz,

Ok, this is our main idea to minimize the shell thickness using E=1.00 and to reinforce the nozzles (lets say to increase the nozzles thickness where it will be necessary).

Thanks

 

[prex]

I confirm: as also implied by davefitz, you will use PG-52 efficiency only if you choose (it's your choice) to reinforce per PG-52, otherwise go with PG-32 only.
I wouldn't expect much difference if you don't use neck material as reinforcement, but I'll leave to you the detailed calculation.
BTW I don't agree with davefitz on the requirement of full penetration welds for using the neck as reinforcing material, but this is perhaps not your concern at the moment.

prex

http://www.xcalcs.com

Online tools for structural design

 

[mancini]

Prex,

OK, we will follow the PG-32 requests. In our case, as we have a hugebig drum OD 86.6 in ; 1755 psi, material SA 302 Gr B, we have a significant shell thickness difference.

BTW anyone have some special recommendation on pressure vessels fabrication with the material SA302 Gr B?

Thanks.

 

[davefitz]

prex:
most full penetration welds are exempt from needing extra analysis of load carrying path , per PW-15.1.6 For the case of partial pen welds, you can use the thicker nozzle for credit toward compensation but "sufficient weld shall be provided, on either side of the opening, to develop gthe required strength of the nonintegral compensation thru shear or tension in the weld, whichever is applicable".

It is true that one can use the partial pen weld if add'l analysis proves it is adequate for the assumed loads, and if the boiler is a base loaded unit that is an option. However, section I takes no consideration of cyclic fatigue or corrosion fatigue interaction, both of which are present on cycling boilers and consideration of these damaging factors would likely rule out the use of partial pen welds for nozzle reinforcement.

 

[GenB]

1) In the calculation of the drum thickness (cylinder), may I consider the efficiency 1.00 and use the excess of thickness on shell to reinforce the openings and complementary also reinforce the nozzle itself?

I will try:

A= When you calculate the required opening compensation, the formula allows the extra T of the shell towards the compensation required. the shell eff increases.


2) When I have a pattern of openings on shell, must I adopt the effiency as informed in the PG-52 (usually less than 1.00)?
A= Yes, you have to calculate the required Eff.
ER