False Heads for a Vessel

[gayaz]

I have a dilema

I need to design a Vessel 7FT Dia Vertical Vessel with a Flat Bottom Head. When I run calculations its asking me for a Flat Head of 4" Thk. Of course I can't get that so easy.

The other choice i have is to do a false head to the tank.
I tried looking every place for information to design a false head but could not find any.

Can any one of you suggest where I may be able to find information to design the false head.

Apperciate your help

Thanks
gayaz

 

[jte]

7 ft diameter and only 4" thick? Must not be very high pressure. More details like design pressure and temp and materials would be useful. Also, some background on why this thing has to be flat bottom and what code you are designing to. Why not just build a standard vertical vessel with 2:1 heads on a skirt?

jt

 

[JStephen]

Can you just use an F&D head on the bottom, with a skirt to support it?

On API-620 tanks (less than 15 PSIG pressure), you're allowed to use a flat bottom supported on a slab, with the tank held by anchor bolts- I don't suppose ASME has an equivalent construction.

You could design with stiffened flat plate- but don't know to what extent ASME would cover or allow this type of construction.

 

[gayaz]

Hi there,

The design presure is 85 PSI. Material is SA 516-70. Design temp is 125 F.

I am not exactly sure why the bottom has to be flat. I guess this is used for water filtration (ion exchange) and needs a flat surface to support. this is my guess

Irrespective of the use. Have any of you guys designed something like this before?

I spoke to my AI and he recalls seeing this design. If I get more information i will pass it on.

If any of know how to do the design false head please let me know

Thanks

Mo.

 

[tmdixon]

Are you using UCS-34 to design your flat head?

 

[JStephen]

Can you put a flat plate inside the vessel, but have the plate vented so it doesn't support any significant pressures?

 

[rzrbk]

The reason for the flat bottom head needs to be confirmed with the purchaser.

If flat support of some filter media is the concern, options could be:
-A screen above the dished head(Johnson or mesh type) supported underneath by cross beams.

-Fill the bottom dished head with ceramic balls or other support to above the tangent line. In this case a smaller screen would be needed on the outlet/drain nozzle

 

[unclesyd]

We have 6 of double bottom Ion Exchange Tanks that take in all the above recommendations. They are ASME designed (?60 psig @ 150°C)? SS tanks with F&D heads with pipe support legs. There is a flat plate welded to the flange section of the bottom head. The flat plate hold about 40 3" strainers.

The flow is generally generally from the bottom to the top but is reversed on regeneration. The pressure is monitored across the bed and sight ports allow a check on the resin bed height. increases.

We have had one incident where the wrong resin was used and expanded and the flat plate was made concave. It was repaired by cutting window in the plate and jacking against the bottom head until the plate was again reasonably flat, it’s still inservice.

Anecdotal:
In 1959 one of these vessel was destroyed by a deflagration (nitric acid + organics). The loss of this vessel proved the fundamentals of the ASME code are pretty good. No weld or nozzle failed, all the failures were in the shell plates. This reaction is so fast that there were no shear lips on the plate.

 

[sbush]

A flat plate inside a flanged and dished head is typically support on concentric rings. This plate is solid not perforated with holes or strainer. A collection sparger is positioned top of the plate. Unfortunately, I cannot help you with the design calculations.

S. Bush

http://www.water-eg.com

 

[chicopee]

If your calculation for an unstayed flat head thickness is correct,then consider welded stays or thru-rods connecting flat head to flat head making the plate thicknesses a lot less.

 

[rmw]

Your flat head thickness comes down fast with the use of external stiffeners (ribs). Does your design permit the use of those?

rmw

 

[GenB]

I am familiar with the design of those bottoms, they are quite complicated and you have to be willing to pay engineering design for it,
There have been a few incidents/accidents on the use of flat bottoms, so be very carefull in the calculations,

 

[johnnymist2003]

Hi all,

I was involved in the design of Ion Exchangers at one stage. In these designs the bottom plate was supported on pipe "legs" inside the vessel welded onto pads attached to the lower dished end. The dished end was a semi-ellipsoidal design. The plate was also welded into the shell. Just a word of caution, these exchangers were used in cyclic service due to pressure & temperature cycling and there had been numerous failures (of the internal plates) on previous designs. We were required to design the vessel in accordance with ASME VIII, Div.1, then carry out a fatigue analysis of the entire vessel in accordance with ASME VIII, Div.2. We used FEA for the cyclic design evaluation. Remember also to take the hole efficiency into account in order to account for the holes drilled into the flat plates.
John

 

[GenB]

johnnymist2003,
RE: "legs" inside the vessel
In these designs there have been a few accidents of pierced saE heads by the legs. So care shall be observed during the calculations and analisys.
ER