Vessel supports: skirt, saddle and bracket supports

Why it matters

A vessel is only as safe as what holds it up. Supports carry the weight of the vessel and its contents (including the water of a hydrotest), resist wind and earthquake moments, and must let the vessel grow and shrink with temperature. Poorly designed supports cause local shell buckling, cracked welds at saddle horns, and in the worst case a column toppling in a storm.

Key ideas

Choosing a support type.

• Skirt support: a cylindrical (or slightly conical) shell welded to the bottom head of a tall vertical vessel and bolted to the foundation through a base ring and anchor bolts. Used for columns, tall reactors and any vessel with a large overturning moment. Its continuous circumferential weld spreads load smoothly into the shell.
• Saddle supports: for horizontal drums and exchangers. Usually two saddles, each wrapping the shell over a contact angle of about 120–150°. One saddle is fixed; the other has slotted bolt holes so the vessel can expand axially.
• Bracket (lug) supports: plates welded to the side of a small or medium vertical vessel that sits on a structure or platform beams, typically four lugs. Leg supports (columns welded to the shell) serve small vertical vessels standing on the floor.

Loads to combine. Dead weight (empty, operating, and hydrotest-full), wind or seismic moment, piping loads and thermal loads. For a skirt the critical cases are: maximum compression on the leeward side with the vessel full (weight adds to bending) and maximum tension on the windward side with the vessel empty (weight relieves bending, so the minimum weight is the critical one for anchor bolts).

Skirt details. The skirt has access openings, vent holes at the top (to stop flammable vapour collecting), and pipe openings for the bottom outlet; it is often fireproofed. Its top weld to the bottom head is the weak link, so a reduced weld joint efficiency is applied. The skirt is also checked for buckling under axial compression; a common elastic limit is a fraction of E·t/D, and the code or design standard gives the allowable value. For hot vessels, the skirt top runs hot while the base is cold, so a long enough skirt and sometimes a "hot box" detail reduce thermal stresses.

Base ring and anchor bolts. The base ring spreads the compressive load onto the concrete foundation, whose allowable bearing stress comes from the civil design code. The ring's outward projection bends like a cantilever under the bearing pressure, which sets its thickness. Anchor bolts, in multiples of four, resist the net uplift on the windward side.

Saddles. A horizontal vessel on two saddles behaves as a beam with overhangs. The bending moments over the supports and at mid-span are equal when each saddle is about 0.207 of the length from the end, which is why saddles are commonly placed within about 0.2L of the tangent line, close enough for the heads to stiffen the shell. Local stresses at the saddle horn and circumferential stresses in the shell are evaluated by Zick's method (adopted in codes and design manuals); wear plates and stiffening rings are added if they are too high.

Brackets. Each lug transfers its load to the shell eccentrically, producing local bending in the shell wall. Lugs are checked for local shell stresses (for example by Bijlaard's or WRC methods) and are often welded onto reinforcing pads.

Formulas

σ_w = W / (π·D_s·t_s) (axial stress in skirt from weight) σ_b = 4·M / (π·D_s²·t_s) (axial stress in skirt from overturning moment) σ_comp = σ_b + σ_w(operating or test), σ_tens = σ_b − σ_w(empty) (design checks)

• W = weight (N); M = overturning moment at the skirt base (N·mm); D_s = skirt mean diameter (mm); t_s = skirt thickness, corroded (mm); σ in MPa. Thin ring: area π·D·t, section modulus π·D²·t/4.

f_c = W / A_b + M / Z_b, A_b ≈ π·D_b·L_b, Z_b ≈ π·D_b²·L_b / 4 (bearing pressure under the base ring)

• D_b = mean diameter of base ring (mm); L_b = its radial width (mm); f_c in MPa, compared with the allowable bearing stress of the concrete (from the civil code).

t_b = L_r·√(3·f_c / f_r) (base ring thickness, cantilever projection)

• L_r = outward projection beyond the skirt (mm); f_r = allowable bending stress of the ring (MPa).

F_bolt = 4·M / (N·D_bc) − W_min / N (tension in each anchor bolt)

• N = number of bolts (multiple of 4); D_bc = bolt circle diameter (mm); W_min = minimum (erection or empty) weight (N). If F_bolt ≤ 0 the vessel is stable without uplift; nominal bolts are still fitted.

R = W / 2 (reaction at each of two symmetrically placed saddles)

Worked examples

Example 1 (standard): skirt stresses Given: skirt D_s = 2000 mm, t_s = 12 mm; operating weight 400 kN; empty weight 150 kN; wind moment at base M = 600 kN·m = 6 × 10⁸ N·mm.

1. σ_w(operating) = 400 000 / (π × 2000 × 12) = 5.31 MPa; σ_w(empty) = 150 000 / 75 398 = 1.99 MPa.
2. σ_b = 4 × 6 × 10⁸ / (π × 2000² × 12) = 15.92 MPa.
3. Maximum compression (leeward, operating): 15.92 + 5.31 = 21.2 MPa.
4. Maximum tension (windward, empty): 15.92 − 1.99 = 13.9 MPa.
5. σ_comp = 21.2 MPa, σ_tens = 13.9 MPa; both are compared with the allowable stresses (reduced by the skirt-weld efficiency, and the buckling limit in compression) from your code book.

Example 2 (GATE level): base ring and anchor bolts for the same column Given: base ring mean diameter D_b = 2000 mm, width L_b = 200 mm; 12 anchor bolts on D_bc = 2200 mm; allowable bolt stress 125 MPa; ring projection L_r = 80 mm, f_r = 140 MPa.

1. A_b = π × 2000 × 200 = 1.257 × 10⁶ mm²; Z_b = π × 2000² × 200 / 4 = 6.283 × 10⁸ mm³.
2. Bearing: f_c = 400 000 / (1.257 × 10⁶) + 6 × 10⁸ / (6.283 × 10⁸) = 0.318 + 0.955 = 1.27 MPa — well within typical concrete limits.
3. Ring thickness: t_b = 80 × √(3 × 1.27 / 140) = 80 × 0.165 = 13.2 mm → use 16 mm.
4. Bolt tension (empty): F = 4 × 6 × 10⁸ / (12 × 2200) − 150 000 / 12 = 90 909 − 12 500 = 78 400 N.
5. Root area per bolt: 78 400 / 125 = 627 mm² → choose M36 bolts (root area about 759 mm²), leaving margin for corrosion.

Common mistakes

• Using the operating weight in the tension check; the empty (minimum) weight gives the worst uplift.
• Forgetting the hydrotest case, when a column full of water may be the heaviest condition.
• Treating σ = M/Z with Z of a solid section instead of the thin ring π·D²·t/4.
• Fixing both saddles of a hot horizontal vessel, which locks in thermal stress.
• Placing saddles far from the heads, so the shell ovalises at the saddle and horn stresses soar.
• Ignoring local shell stresses at lugs and brackets.

For GATE CH

Expect conceptual questions on which support suits which vessel, why one saddle slides, and where saddles are placed, plus short numericals on axial stress in a skirt from weight and moment, saddle reactions and anchor bolt loads. Practise combining weight and bending stresses with the correct sign for each side.

Quick check

1. Which weight is used to check anchor-bolt uplift?
2. A skirt has D = 1.5 m and t = 10 mm. What is the axial stress from a 300 kN weight?
3. Why is one saddle given slotted holes?
4. About where are saddles placed on a horizontal drum?

Answers: 1. The minimum (empty or erection) weight. 2. 300 000 / (π × 1500 × 10) = 6.37 MPa. 3. To let the vessel expand and contract axially. 4. Within roughly 0.2 of the length from each end, close to the heads.