Removable False Bottom

[fabtank]

I have been asked to help design a removable false bottom inside of a vertical pressure vessel.

I am wondering if there is anyone that has done something like this that can give any advice or recommended literature.

The vessel is 11'-0" diameter and the false bottom is to be designed for 90 psig.
The false bottom will be located inside of an elliptical head.
The idea we have now is to use multiple removable square panels supported by plates underneath.
One of the problems I have with this design is figuring out how the stress is transferred from the support plates to the outer head.
I've attached a quick sketch to better understand our design.

Any advice is welcome.

 

[pennpiper]

Why are you doing this?

prognosis: Lead or Lag

 

[fabtank]

pennpiper
Good question. This is what the customer wants.
There will be screens in the false bottom sections and the customer would like access to the lower part of the vessel for cleaning/maintenance reasons.

 

[SnTMan]

fabtank, so no 90 PSIG across the false bottom? Easy just design for the weight.

Regards,

Mike

 

[fabtank]

Mike-
Because of the screens in the plates there should be little pressure differential across the false bottom, but the customer would like this designed for 100% screen failure where all of the screens become plugged and the false bottom is exposed to design pressure + static head = 90 PSIG.

I realize that this is an unlikely design requirement and that there may be alternative designs to accomplish this, but I would like to evaluate this design more before I dismiss what the customer wants and recommend other options.

 

[pennpiper]

Fabtank,
If that is what the Client wants then the false bottom/Screen support will need to be designed/act/look just like the bottom semi-elliptical head. Possibly it would be installed in an inverted position and work better in compression.

Interesting Client requirement. Maybe it is time to tell the Client NO!

prognosis: Lead or Lag

 

[SnTMan]

fabtank, Clogged or not, I don't really see how a differential develops across the screens. Is there a path for a flow to exit below the screens, and does this path permit the downstream pressure to go to atmospheric? Can you vent the thing? It would take only a small (clog proof) hole or two to equalize.

OK, so the pressure applies, you will need to design using structural principles as dictated by the geometry. To apply to the bottom head you can perhaps treat the total load as an equvalent (additional) pressure, and again depending on the geometry, may need to account for force (stress) concentrations applied to the head. Don't know of any standard type calcs for this, FEA may be required.

Regards,

Mike

 

[racookpe1978]

How "small" do the parts and pieces of the false bottom need to be?

Example: "They have to fit in and out through a 18 x 18 inch manway, but can be up to 10 feet long." "They have to less than 6 ft x 6 ft." etc.

What will be the weight limit of the pieces? 50 lbs? 250 lbs?

Who/how are you going to lift the parts up to the manway, how do you plan on staging them outside the manway staging (a 10 foot long part won't fit on a 3 ft x 3 ft ladder platform)?

Crane available or a padeye available inside - or do people have to hand-carry them around inside the tank while installing?

 

[fabtank]

racookpe1978,
The pieces must fit through a 36" diameter manway. Weight limit of 250 lbs.
Expected for two people to hand carry the pieces inside the tank.
I believe there is plenty of room outside of the vessel.

 

[austsa]

fabtank,
I've done something very similar before. Depending on your detailed requirements, I'm aware of two options:
(1) Design a tubesheet with manway on lower dished end. The screens fit into the holes. If you do not have adequate screen area, you'll need to utilise basket screens to overcome.
(2) Design a structural framework that the screens attach to.
Note, you may not need to design for the full 90psig if the clogged screens fail prior to seeing full differential pressure.

 

[Compositepro]

You could cast concrete into the bottom head of the vessel.