Vertical vessel skirt with opening design 3

[Spoonful]

Hi All,

To design a vertical vessel's skirt, my basic thought of approach with be check below against allowable,

stress = W/ A +/- M R/ I

Where
M due to external load, wind, seismic.
W weight (operating, hydro)
R radius of skirt
A cross section area

But if there is an opening on the skirt, ie. to allow drain nozzle come out with an elbow. This opening weakens the skirt. I have seen people put an ring at the opening to reinforce this opening. The question is do we assume after reinforcing as there is no opening? and how do you work out the amount reinforcement required? Same amount of replacement area compensated as for nozzle opening reinforcement? if so, at what are the limitations on distance from the opening can be used as reinforcement?

Thanks in advance

Regards
Spoonful

 

[TGS4]

Two comments. First, the failure mode of skirts is buckling (they are under compression). Therefore, you need to ensure that your allowable is based on compressive stress.

Second, regarding openings, I would apply the same area-replacement rules as you would for nozzles. The "ring" as you called it is a good idea - you have added a pseudo-nozzle.

 

[jtseng123]

This is a basic area and section modulus problem to resist dead weight and moment from wind or seismic.

You cut out an opening, the NA shifted, the cross sectional area changed. You can recalculate sectional modulus, then calculate the bending stress from moment, and combine with axial stress from dead weight, and then compare with buckling allowable stress.

When you add a sleeve to reinforce the opening, the NA, area and modulus change again. You can draw a cross sectional view, calculate the area, locate the NA and modulus (this is basic practice in mechanics of material text book), then calculate the stress again. Tedious, and already built in commercial program.

Keep in mind the projection of sleeve is only several inches in and out. So the full area can be used as reinforcement.

 

[racookpe1978]

I thought Magyesy's standard pressure vessel handbook has these skirt problems already solved for various sized towers with various sized holes under various wind and seismic loads for various loads and metals?

 

[Spoonful]

I got these 10th ed of Magyesy's handbook, it has only got on single page on vessel skirt, and it is only based on the same method of W/A + MY/I method, has no mention of opening.

i found in AS1210, it suggests use the same method for calculating reinforcing area as for vessel nozzles.

 

[pennpiper]

The whole subject of Vessels, Vessel Orientation and Vessel Skirts is a complex and very interesting subject. It was one of my favorite parts of plant design. I also developed Vessel training courses and taught Vessel Orientation.
To add my two cents to this post I will offer the following.
a.) Vessel Skirts come in two basic sizes: Small - those that are too small for a person to crawl inside.
and
Large - those that are too big to allow access from outside thus requires a person to crawl inside.

b.) Small Vessel Skirts will normally have an outlet lines from the bottom of the vessel and through the skirt, a drain from the bottom of the vessel and out the skirt. Small vessels skirts will have one or two Skirt Vents up near the joint between the skirt and the bottom of the vessel head. Small Vessels skirts will also have one or more Hand Holes to allow for cleaning, maintenance (painting)and inspection.

c.) Large Vessel Skirts will have one or more large outlet lines (pump suction, reboiler line, etc.) from the bottom of the vessel and through the skirt, a drain from the bottom of the vessel and out the skirt. Large vessels skirts will normally have four Skirt Vents up near the joint between the skirt and the bottom of the vessel head. Large Vessels skirts will also have one or more man size Access Holes to allow for confined space entry for cleaning, maintenance (painting)and inspection.

d.) All of these openings will tend to weaken the the skirt. There are three methods available to compensate for this weakening of the shell.
1- Reinforce the openings as previously discussed.
2- Use a thicker plate for the Skirt so reinforcement is not required.
3- A combination of slightly thicker plate and reinforcement.
What is the most cost effective?

prognosis: Lead or Lag

 

[fegenbush]

pennpiper,

I disagree that a thicker skirt will be able to eliminate the need for reinforcement for an opening or a skirt that is anything other than small. If the skirt has an opening, the neutral axis will be shifted as jtseng123 stated. This is true regardless of skirt thickness. With the NA shifted, the dead load is now eccentric at the level of the opening. This would introduce a bending moment that would need to be resisted along with the wind/seismic moment. If this combined load is acceptable, due to the thickened skirt, you are covered, but it will most certainly need to be checked.

There is also the issue of the stress riser leading to local buckling of the skirt, but I will not go into further details on that.