Vessel Skirts

1. Purpose

This procedure describes the different vessel support skirts used by Inflection Point Engineering and when to use them. It also gives some consideration to metallurgy, insulation and venting.

2. Skirt Type Guidelines

2.1 Straight Skirts

Use straight support skirts for vertical vessels. Center the skirt on the shell thickness.

Inflection Point Engineering utilizes two details for the connection between a vessel and a straight support skirt.

a. Standard Drawing 3-102

Use this connection for high temperature service (typically considered greater than 700F for carbon steel), vessels over 4 inches thick, or vessels subjected to cyclic loading, such as frequent thermal cycling. This skirt connection is a conservative, safe approach and is acceptable for all vessels. However, it is expensive, and possibly difficult to fabricate. Use this connection any time the indicated conditions are present, or (because it’s the conservative approach) whenever there is doubt as to which detail to use.

The drawing detail is streamlined, with all sharp corners, notches, and other areas of potential stress concentration eliminated. Other constructions that result in the same geometry and weld contour may also be acceptable, such as the use of a “stub end”. Note that an insulation air space is included in this detail. Also note that the skirt to shell weld must be located such that non-destructive field examination from the outside is possible.

b. Standard Skirt Support

Use this connection for any vessels operating below 700F (e.g., any vessel not considered high temperature). This connection is simpler, less costly and easier to fabricate than the detail shown on Standard Drawing 3-102.

Depending upon the metallurgy used and the attendant start of the creep range, a higher cut off temperature may apply to specific vessels when the onset of the creep phenomenon for the specified metallurgy occurs above 700F.

This connection is described in Standard Specification 3-11, paragraph 5.1.m:

“Seams in supporting skirts shall be made with full penetration butt welds. Connections between straight skirts and vessel heads shall be made with a smooth flat-faced weld. The width of the weld shall be at least equal to the skirt thickness, and its height shall be approximately twice its width.”

2.2 Conical Skirts

An alternative to a straight skirt is a conical or flared skirt. Use conical support skirts when a vessel or piece of equipment is located on a tabletop or a structure. A conical skirt allows the vessel to project below the support level, which reduces the wind overturning moment on the vessel. An additional benefit of a conical skirt is that the load on the base plate is reduced since the load is distributed on a larger diameter. Typical detail is shown on Standard Drawing 3-103.

3. Metallurgy

The skirt length must be sufficient to absorb any radial growth of the vessel. The upper portion of the skirt (where the skirt meets the vessel) shall be made of the same material as the vessel shell. The remainder of the uninsulated skirt may be made of carbon steel.

4. Insulation

The skirt is insulated to minimize the thermal gradient between the shell and the skirt. It is for this reason that an air space beneath the shell to skirt connection is required. The air space allows the heat from the shell to radiate to the skirt, which keeps the temperatures of the upper portion of the skirt in the same vicinity as the connection. The reduction in thermal gradient resulting from the presence of the air space helps to reduce bending, thermal and discontinuity stresses at the skirt to shell joint. For more information on insulation, see Standard Specification 9-11.

5. Venting

Vents are required at the top portion of the enclosed skirt space to allow for the escape of any gasses as well as to promote airflow and cooling. See Standard Drawings 3-102 and 3-103 for general locations of these vents. Do not locate flanges on any of the piping that is located inside of a skirt because flanges have leak potential and are not easily accessible within a skirt. A confined space promotes dangerous concentration of leaking vapors.