The most common types of pressure vessel supports are the following:
• Skirts • Columns • Saddles
Figure 19 shows these types, all of which are used at Saudi Aramco facilities. ; ; ; MEX 20203.F19 ;
Skirt Support Column Support Saddle Support
Most vertical pressure vessels are supported by skirts. Skirts are an economical design because they generally transfer the loads from the vessel by shear action. They also transfer the loads to the foundation through anchor bolts and bearing plates. Column-supported vessels are normally relatively small and light in weight. The columns permit easy access under the vessel. Figure 19 shows a column support design where the columns attach directly to the vessel and the loads are transferred by shear action. Column supports are also often used for pressurized storage vessels. Cross-bracing of the columns may be necessary to minimize lateral and torsional movements under wind or earthquake loadings. An alternate design has the support columns attached to lugs and the lugs welded to the vessel shell. The bending stiffness of the shell and its ability to adequately resist the moments must be considered in the lug support design.
Horizontal vessels are normally supported by saddles. Circumferential stiffener rings may be required at saddle supports if the vessel shell is too thin to transfer the loads to the saddles. Axial thermal expansion of a horizontal vessel is normally handled by anchoring one saddle support to the foundation and letting the other saddle support move freely. The weight load of the pressure vessel must be calculated to permit correct design of the vessel support system. Weight load calculation is necessary regardless of the type of support that is used or regardless of the vessel geometry. Wind and earthquake loads must also be determined in order to design the support system. Wind and earthquake loads are especially important in the design of vertical pressure vessel supports since the bending moments that are applied at the support increase as the vessel height increases. The calculation of wind and earthquake loads that are applied to pressure vessels was discussed earlier and in CSE 110.
Vertical Vessel
Small and medium-sized vertical vessels (1.8 - 3.6 m [6 - 12 ft.]) that are located on the ground are usually supported on uniformly spaced column supports. If a short vessel is located above ground on structural steel construction, or if a short vessel is connected by piping to a reciprocating pump or compressor, it is sometimes supported by a skirt to avoid vibration problems. Larger vertical vessels are typically supported by skirts.
Once the required weight, wind, and earthquake loads have been determined, the design of vertical pressure vessel supports includes the following:
• Determine the required support size. For columns, this determination involves selection of the appropriate standard structural steel member or pipe size which is necessary for the applied load. For skirts, this determination involves determination of the appropriate skirt diameter and thickness. The height of the support column or skirt is normally set by process design conditions and considers fluid head and flow requirements. • Determine the need for and sizing of cross-bracing for
column supports.
• Calculate the stresses in the vessel shell at the support attachment points and determine if these stresses are acceptable. Determine if additional reinforcement at the shell is needed to keep the stresses within acceptable limits.
• Determine appropriate details to use at the support base to the foundation. These details include baseplate dimensions, anchor chair details, and anchor bolt number, diameter, and location.
• Determine appropriate weld details to use at the pressure vessel and baseplate attachment points.
• Determine if other specific vessel design considerations would affect the support design. These considerations might include the calculation and evaluation of localized stresses that are caused by high temperature applications or cyclic loads.
The sections that follow concentrate on the requirements for the basic design of support columns and skirts. Discussion of the other items is beyond the scope of this course.
Column Supports
Column support design considers axial weight loads, bending moment, and shear forces in the vessel. Vessel design pressure is not a consideration since the column supports are not exposed to operating pressures. Figure 20 shows the forces and bending moment expressed as W, V, and M. The axial weight force, W, is carried uniformly by all columns. The shearing forces, V, are carried by the columns that are closest to the neutral axis (Columns B). The bending moment, M, is carried by the columns that are located away from the neutral axis (Columns A).
;;; W M V A A M W V B B A A Section A-A 20203.F20
The shearing force, VB, at the top of column B causes a bending moment in the column if no cross-bracing is used. With cross- bracing, VB is resolved into components. One component of VB
acts along the axis of the cross-bracing, another acts along the column axis, and a third acts radially on the shell. The design of cross-bracing will not be discussed in this course.
Selection of the appropriate column size, number, and position for a particular situation is a trial and error process. The following Sample Problem, in conjunction with Work Aid 4A, will be used to demonstrate the general approach to the design of column supports.
Sample Problem 7 -
Design of Column Supports
Column supports must be designed for the vertical pressure vessel that is shown in Figure 21.
The following additional information is available: Wo = 92 000 lb., vessel operating weight
WT = 208 000 lb., vessel hydrotest weight
Wc = 6 000 lb., vessel empty weight
Po = 2 000 lb., lateral wind force during operation
(based on 85 MPH wind velocity)
PT = 720 lb., lateral wind force during hydrotest
(based on 51 MPH wind velocity, 60% of design) Assume that four support columns will be sufficient and that they are SA-36 carbon steel (36 000 psi yield stress). The discussion above covers Steps 1 and 2 in Work Aid 4A, plus some information about the columns. The following continues with Step 3.
D = 48 in. 3 L 4 L = 72 in. F P1 P2 e P t = 0.5 in. H = 96 in. 192 in. a a T.L. CL column Direction of wind or earthquake x x y Db y P A A MEX 20203.F21 Section a-a
3. Calculate the bending moments at the column base and the vessel tangent line, Mb and Ma respectively. This
calculation is done for both the operating and hydrotest cases. H = 96 in. L = 72 in. Mb = P(H + L) Ma = PH Operating Case Mbo = 2 000 (96 + 72) = 336,000 in.-lb. Mao = 2 000 × 96 = 192 000 in.-lb. Hydrotest Case MbT = 720 (96 + 72) = 120 960 in.-lb. MaT = 720 × 96 = 69 120 in.-lb.
4. At this point, assumptions must be made for the column design to be used. It was already assumed that four columns could be used. This assumption is applicable to most small, vertical pressure vessels.
Next, a standard column section must be selected. An iterative process is then used to ensure that the column is not overloaded and to optimize the design. For the purpose of this discussion, we will only go through the process once to illustrate the overall approach.
Assume that the columns are fabricated using a W8 × 31 standard "wide-flange" beam. This beam has the following properties (obtained from a standard civil engineering design manual, or from an equivalent source).
A = 9.12 in.2 Lx = 110 in.4 Ly = 37 in.4 Zx = 27.4 in.3 Zy = 9.24 in.3 rx = 3.47 in. ry = 2.01 in. Depth = 8 in.
Therefore, r = 2.01 in. (least radius of gyration) e = 4 in. (half of the depth for column
orientation used)
I = Ix = 110 in.4 (x-direction is
perpendicular to wind direction for this column orientation)
∑ I1 = 2(Ix + Iy) = 2(110 + 37) = 294 in.4
5. Determine allowable compressive stress.
K1L r =
1.5× 72
2.01 = 53.73
Fa = 18 014 psi, from Figure 32
6. Determine allowable bending stress. Fb = 0.6 × 36 000 = 21 600 psi
7. Calculate the maximum axial compressive load on the leeward side of each column. See Figure 30 in Work Aid 4A.
Db = Vessel outside diameter + 2 × (Distance between
shell and column centroid) Db = (48 + 2 × 0.5) + 2 × 4 Db = 57 in. Operating Case Co= Wo N + 4Mbo NDb = 92 000 4 + 4× 336 000 4× 57 Co = 28 895 lb. Hydrotest Case CT = WNT +4MNDbT b = 208 000 4 + 4× 120 960 4× 57 CT = 54 122 lb.
8. Calculate the maximum total axial uplift load on the windward side of each column. This calculation is done for the operating conditions and with the vessel empty. See Figure 30 in Work Aid 4A.
Operating Case To = −Wo N + 4Mbo NDb = − 92 000 4 + 4× 336 000 4 × 57 To= -17 105 lb.
Since this value is negative, it indicates that the operating vessel weight will overcome the wind load that tends to overturn the vessel.
Tc= 4 395 lb.
This result indicates that the wind would tend to overturn the vessel if it is empty. The columns must be bolted down to prevent this occurrence, which is always the case.
9. The eccentric loads at the top of the columns are then calculated. See Figure 30 in Work Aid 4A.
Operating Case P10 = Wo N + 4Mao ND = 92 000 4 + 4 × 192 000 4× 48 P10= 27 000 lb. P20 = 4Mao ND − Wo N = 4× 192 000 4× 48 − 92 000 4 P20= -19 000 lb. Hydrotest Case PIT = WT N + 4MaT ND = 208 000 4 + 4 × 69 120 4× 48 PIT= 53 440 lb.
Empty Vessel Case P2E = 4Mao ND − We N = 4× 192 000 4× 48 − 6 000 4 P2E= 2 500 lb.
10. Calculate the lateral force per column for both the design and test cases. See Figure 30 in Work Aid 4A.
Operating Case Fo =PoI ΣI = 2 000× 110 294 = 748 lb. Hydrotest Case F = PTI= 720 × 110= 269 lb.
11. The axial and bending stresses in the column are now compared to allowable values. This comparison is done for both the operating and test conditions.
Note that since the tensile loads in the column that were calculated in Step 8 will typically be lower than the compressive loads from Step 7, they will not need to be checked. Also, the allowable tensile stress for the columns (typically 0.67 Fy) will typically be higher than the allowable
compressive stress. a. Operating Case Axial Compression fao = Co A = 28 895 9.12 = 3 168 psi Bending fbo = P10e Zx + 0.75F oL Zx = 27 000 × 4 27.4 + 0.75 × 748 × 7227.4 = 5 416 psi b. Hydrotest Case Axial Compression faT = CA1= 54 1229.12 = 5 934 psi Bending fbT = WTe NZx + 0.75FTL Zx + P1Te Zx fbT = 208 0004 × 27.4 +× 4 0.75× 269 × 7227.4 +53 44027.4× 4 fbT = 15 923 psi
c. Combine the axial and bending stresses and compare to the allowable values.
Operating Case
fo = fao + fbo = 3 168 + 5 416 = 8 584 psi
Hydrotest Case
fT = faT + fbT = 5 934 + 15 923 = 21 857 psi
Therefore, f = fT = 21 857 psi
The hydrotest case governs the column design. Fa = 18 014 psi and Fb = 21 600 psi
Since f exceeds the lower of Fa or Fb, we must
proceed to Step 12 to further evaluate the proposed design.
12. faT
Fa =
5 934
18 014 = 0.33, which is greater than 0.15 Fe'= 12π2E 23 K1L r 2 = 12π2× 29 × 106 23× 53.732 = 51 724 faT Fa + 0.85fbT 1−faT Fe' Fb =18 0145 934 + 0.85× 15 923 1− 5 934 51724 21 600 = 1.04
This value exceeds 1.0, and therefore, the selected beam section does not meet the design requirements. Using the next heavier 8-inch “W” section should satisfy the strength requirements for this vessel. The student is left to verify this.
13. The local stresses in the shell due to the column load would then be evaluated, and the shell will be locally reinforced as needed.
Skirt Supports
The design pressure of the vessel need not be considered in the design of a skirt support because the skirt is not exposed to the operating pressure. The allowable stress that is specified by the ASME Code for the skirt material need not be used because the skirt is an external attachment and is not part of the pressure vessel itself. The local building codes or civil engineering standards usually specify the maximum allowable tensile and compressive stresses for a steel support structure such as a skirt. These may be used in skirt design. However, from a practical standpoint, ASME Code allowable tensile and compressive stresses are normally used for skirt design.
Since the skirt is not designed for pressure, it would appear that the skirt thickness should be less than the vessel shell thickness at the attachment point if the same material is used for both components. However, the skirt tends to absorb greater bending moments that are caused by wind or earthquake loads. These increased bending moments may require a greater skirt thickness. Therefore, the skirt thickness is often the same as the thickness of the bottom portion of the vessel shell.
Failure of a cylindrical skirt under axial compressive stresses may occur due to axial column buckling or local wrinkling if the skirt is not properly designed. This type of failure is the same as overloading a structural column in compression. As with the vessel shell itself, failure is more likely to occur due to column- wrinkling that is produced by excessive combined axial compressive loads. These axial compressive loads are caused by weight, plus either wind or earthquake. Paragraph UG-23 of the ASME Code limits the maximum compressive stress to prevent failure (as was discussed earlier in this module). This ASME code allowable compressive stress basis should be followed for skirt design.
Figure 22 shows support skirts that are welded directly to the vessel bottom head or shell. A Type 1 skirt may be either straight or flared and is butt-welded to the knuckle portion of the head. A Type 2 skirt may be either straight or flared and is lap- welded to the cylindrical portion of the shell. The type of weld attachment that is used between the skirt and vessel determines
;;; ;;; Straight Type 1a ;;; ;;; Flared Type 1b 15°MAX Type 1:
Butt weld blends smoothly into head contour
;;; ;;; Straight Type 2a ;;; ;;; Flared Type 2b Type 2:
Lap weld blends smoothly into shell contour
MEX 20203.F18
Skirt Type 1a is most often used for tall vessels. The centerlines of the cylindrical skirt plate and the corroded shell plate are approximately coincident. 32-SAMSS-004 requires that this detail be used for all but hemispherical heads. If the skirt plate is thicker than the bottom shell plate, the outside diameter of the skirt is made equal to the outside diameter of the bottom shell. If the uplift caused by the imposed external moment is high, and if the anchor bolt spacing becomes too small for the required bolt size, the skirt is designed as a Type 1b. The skirt flare increases the skirt diameter at the base plate and permits the use of larger diameter and/or more anchor bolts, as required. The skirt flare also increases the skirt section modulus in going from the attachment point to the base, which makes it more resistant to the applied bending moment.
The Type 2a skirt is attached to the flanged portion of the bottom head in such a way that it does not obstruct any required inspection of the head-to-shell junction weld seam. The Type 2a skirt is more difficult to fabricate and is used mainly in situations that involve high external loads, high design temperatures, or cyclic operating temperatures. A good fit between the outside diameter of the shell and the inside diameter of the skirt is essential. A flared Type 2b skirt is used for the same reasons as a Type 1b skirt.
Horizontal Vessel Saddle Supports
Figure 23 shows a typical horizontal vessel on two saddle supports. a a θ R b A
Less effective portion of unstiffened shell
Section a - a b = Width of saddle
R = Radius of shell
θ = Saddle contact angle
A = Distance between vessel tangent line and centerline of saddle
MEX 20203.F23
The required cylindrical shell and head thicknesses are generally governed by the membrane stress that is due to pressure and are calculated using the ASME Code design equations that were previously discussed. However, the design of a horizontal vessel that is supported on saddles must proceed further through the use of procedures that are not contained in the Code. The paragraphs that follow highlight the details that must be considered. The actual design calculations for a horizontal vessel on saddle supports will typically be done with a computer program such as CODECALC, which is used by Saudi Aramco.
The most common horizontal vessel support design uses two saddle supports that are located an equal distance from the vessel midpoint. With this support configuration, the load that results from the weight of the vessel and its contents will be equally divided between the two supports, even if one support should eventually settle more than the other. If more than two supports are used, the load may not be equally divided among the supports after settlement occurs.
A horizontal vessel on two saddle supports is analyzed as a uniformly loaded beam that is simply supported. The uniform weight load produces longitudinal bending stresses in the shell at mid-span and above the saddle supports. These longitudinal bending stresses are combined with the longitudinal pressure stress and are kept below the Code allowable stress. A complication occurs at the saddle location because high bending moments occur at the location where the saddle attachment stops along the shell circumference. This location is called the "horn" of the saddle. These high localized bending moments cause localized shell deformation and reduce the ability of the shell to effectively absorb bending. This localized shell deformation must be accounted for in the calculations that are made at the horn of the saddle. Figure 23 shows the zone above each saddle support where the shell is not completely effective.
If the stresses in the saddle area are excessive, a modified saddle design is required. Saddle design modifications may include the following actions:
• Increasing the contact angle between the saddles and the shell.
• Welding circumferential stiffener rings to the vessel shell. If stiffener rings are used, they must be located either in the plane of the saddle and welded both to the saddle and vessel shell, or they must be located on both sides of the saddle and welded only to the shell. The addition of stiffeners prevents local deformation of the shell and makes the entire shell section effective in resisting the local bending moment. The ring stiffeners must be strong enough to prevent shell deformation, without being overstressed themselves or allowing the shell to become overstressed. The stiffeners may be fabricated from plate or standard structural sections, whichever is most appropriate for the specific design loads. Figure 24 shows stiffener rings located at saddle supports.
Vessel shell
Saddle support Ring stiffener
MEX 20203.F24
Single stiffener Two stiffeners
The design of a saddle support system for a horizontal vessel is a complex process that requires the calculation of several different stresses at various locations in the vessel. The paragraphs that follow provide an overview of the overall procedure and of the stresses which must be calculated.
Design of Horizontal Cylindrical Vessels on Saddle Supports
As with any other vessel, horizontal vessels on saddle supports are designed for specified internal and/or external pressure. With regard to weight loading, the vessel behaves like a beam resting on supports. L. P. Zick developed an analysis procedure for calculating the stresses that are induced in the cylindrical vessel shell due to the weight loads ("Stresses in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Supports," The Welding Journal Research Supplement, 1971). This procedure calculates and evaluates the following stresses: • Maximum longitudinal bending stress.
• Tangential shear stress.
• Circumferential stress at the horn of the saddle (that is, where the saddle is welded to the vessel shell).
• Ring compression in the shell over the saddle. • Additional stress in the head used as a stiffener.
The procedure also considers the strengthening effect of stiffening rings and the design requirements for the rings themselves. The maximum unstiffened length of the vessel between heads and the total horizontal force that acts against the horns of the saddle may also be determined.
The paragraphs that follow discuss several of these stresses (except in stiffening rings) and design details that must be determined and evaluated for the design of horizontal cylindrical vessels on saddle supports. Discussion of the specific equations that are used to calculate these stresses and their allowable limits is beyond the scope of this course. Participants are referred to Zick's paper, or other readily available pressure vessel references for additional details. As previously noted, computer programs such as CODECALC are used for these