Erection Of Pressure Vessels

The designer of pressure vessels and similar equipment will ultimately become involved in the movement, transportation, and erection of that equipment. The degree of that involvement will vary due to the separation of duties and responsibilities of the parties concerned. It is prudent, however, for the designer to plan for the eventuality of these events and to integrate these activities into the original design. If this planning is done properlv, there is seldom a problem when the equipment gets to its final destination. Conversely there have been numerous problems encountered when proper planning has not been done.

There is also an economic benefit in including the lifting attachments in the base vessel bid and design. These lifting attachments are relatively inexpensive in comparison to the overall cost of the vessel and minuscule compared to the cost of the erection of the equipment. The erection alone for a major vessel can run into millions of dollars. If these attachments are added after PO award, they can become expensive extras.

There are also the consequences to life, property, and schedules if this activity is not carried out to a successful conclusion. Compared to the fabricated cost of the lifting attachments, the consequences to life, property, and schedule are too important to leave the design of these components and their effect on the vessel to those not fully versed in the design and analysis of pressure vessels.

In addition, it is important that the designer of the lifting attachments be in contact with the construction organization that will be executing the lift. This ensures that all lifting attachments meet the requirements imposed by the lifting equipment. There are so many different methods and techniques for the erection of vessels and the related costs of each that a coordinated effort between the designer and erector is mandatory. To avoid surprises, neither the designer nor the erector can afford to work in a vacuum. To this end, it is not advisable for the vessel fabricator to be responsible for the design if the fabricator is not the chief coordinator of the transport and erection of the vessel.

Vessels and related equipment can be erected in a variety of ways. Vessels are erected by means of single cranes, multiple cranes, gin poles, jacking towers, and other means. The designer of the lifting attachments should not attempt to dictate the erection method by the types of attachments that are designed for the vessel. The selection of one type of attachments versus another could very well do just that.

Not every vessel needs to be designed for erection or have lifting attachments. Obviously the larger the vessel, the more complex the vessel, the more expensive the vessel, the more care and concern that should be taken into account when designing the attachments and coordinating the lift. The following listing will provide some guidelines for the provision of special lifting attachments and a lifting analysis to be done. In general, provide lifting attachments for the following cases:

• Vessels with L/D ratios greater than 5.

• Vertical vessels greater than 8 ft in diameter or 50 ft in length.

• Vessels located in a structure or supported by a structure.

• High-alloy or heat-treated vessels (since it would not be advisable for the field to be doing welding on these vessels after they arrive on site, and wire rope slings could contaminate the vessel material)

• Vessels with special transportation requirements.

At the initial pick point, when the vessel is still horizontal, the load is shared between the lifting lugs and the tail beam or lug, based on their respective distances to the vessel center of gravity. As the lift proceeds, a greater percentage of the load is shifted to the top lugs or trunnions until the vessel is vertical and all of the load is then on the top lugs. At this point the tail beam or shackle can be removed.

During each degree of rotation, the load on the lugs, trunnions, tailing device, base ring, and vessel shell are continually varying. The loads on the welds attaching these devices will also change. The designer should evaluate these loadings at the various lift angles to determine the worst coincident case.

The worst case is dependent on the type of vessel and the type of attachments. For example, there are three types of trunnions described in this procedure. There is the bare trunnion (Type 3), where the wire rope slides around the trunnion itself. While the vessel is in the horizontal position (initial pick point), the load produces a circumferential moment on the shell. Once the vessel is in the upright position, the same load produces a longitudinal moment in the shell. At all the intermediate angles of lift there is a combination of circumferential and longitudinal moments. The designer should check the two worst cases at 0° and 90° and several combinations in between.

The same trunnion could have a lifting lug welded to the end of the trunnion (Type 1). This lug also produces circumferential and longitudinal moments in the shell. However, in addition this type of lug will produce a torsional moment on the shell that is maximum of 0° and zero at 90° of angular rotation. The rotating lug (Type 2) eliminates any torsional moment.

There is one single lift angle that will produce the maximum stress in the vessel shell but no lift angle that is the worst for all vessels. The worst case is dependent on the type of lift attachments, distances, weights, and position relative to the center of gravity.

The minimum lift location is the lowest pick point that does not overstress the overhanging portion of the vessel. The maximum lift location is the highest pick point that does not overstress the vessel between the tail and pick points. These points become significant when locating the lift points to balance the stress at the top lug, the overhang, and the midspan stress.

The use of side lugs can sometimes provide an advantage by reducing the buckling stress at midspan and the required lift height. Side lugs allow for shorter boom lengths on a two-crane lift or gin poles. A shorter boom length, in turn, allows a higher lift capacity for the cranes. The lower the lug location on the shell, the shorter the lift and the higher the allowable crane capacity. This can translate into dollars as crane capacity is affected. The challenge from the vessel side is the longitudinal bending due to the overhang and increased local shell stresses. All of these factors must be balanced to determine the lowest overall cost of an erected vessel.

Steps in Design

Given the overall weight and geometry of the vessel and the location of the center of gravity based on the erected weight, apply the following steps to either complete the design or analyze the design.

Step 1: Select the type of lifting attachments as an initial starting point:

Lift end (also referred to as the "pick end"):

a. Head lug: Usually the simplest and most economical, and produces the least stress.

b. Cone lug: Similar to a head lug but located at a conical transition section of the vessel.

c. Side lug: Complex and expensive.

d. Top flange lug: The choice for high-pressure vessels where the top center flange and head are very rigid. This method is uneconomical for average applications.

e. Side flange lug: Rarely used because it requires a very heavy nozzle and shell reinforcement.

f. Trunnions: Simple and economical. Used on a wide variety of vessels.

Tail end:

c. Choker (cinch); see later commentary.

Tailing a column during erection with a wire rope choker on the skirt above the base ring is a fairly common procedure. Most experienced erectors are qualified to perform this procedure safely. There are several advantages to using a tailing choker:

• Saves material, design, detailing, and fabrication.

• Simplifies concerns with lug and shipping orientations.

• May reduce overall height during transportation.

There are situations and conditions that could make the use of a tailing choker impractical, costly, and possibly unsafe. Provide tailing lugs or a tailing beam if:

• The column is more than about 10 ft in diameter. The larger the diameter, the more difficult it is for the wire rope to cinch down and form a good choke on the column.

• The tail load is so great that it requires the use of slings greater than about l'A in. in diameter. The larger the diameter of the rope, the less flexible it is and the more likely that it could slip up unexpectedly during erection.

Step 2: Determine the forces T and P for all angles of erection.

Step 3: Design/check the lifting attachments for the tailing force, T, and pick force, P. Step 4: Design/check the base ring assembly for stresses due to tailing force, T. Step 5: Determine the base ring stiffening configuration, if required, and design struts. Step 6: Check shell stresses due to bending during lift. This would include midspan as well as any overhang. Step 7: Analyze local loads in vessel shell and skirt due to loads from attachments.

Allowable Stresses

Per AISC: Tension

= 0.5Fy on effective net area = 0.45Fy for pin-connected members


(for short members only) Ft = for structural attachments: 0.6FV

- for vessel shell: 1.33 x ASME Factor "B"


= other than pin-connected members: 0.4FV = fillet welds in shear:

E60XX: 9600 lb/in. or 13,600psi E70XX: 11,200 lb/in. or 15,800 psi


Fh = 0.66Fv to 0.75FV, depending on die shape of the member



Shear and tension:

Tension and bending:

Fa Fb

Note: Custom-designed lifting devices that support lifted loads are generally governed by ASME B30.20 "Below the hook lifting devices." Under this specification, design stresses are limited to Fv/3. The use of AISC allowables with a load factor of 1.8 or greater will generally meet this requirement.


An = net cross-sectional area of lug, in.2 Ap = area, pin hole, in.2 Ar = area, required, in.2 As = area, strut, in.2 or shear area of bolts C = lug dimension, see sketch D0 = diameter, vessel OD, in. D! = diameter, lift hole, in. D2 = diameter, pin, in. D3 = diameter, pad eye, in. Dsk = diameter, skirt, in. Din = mean vessel diameter, in. E = modulus of elasticity, psi fr = tail end radial force, lb fL = tail end longitudinal force, lb fs = shear load, lb or lb/in. Fa = allowable stress, combined loading, psi Fb = allowable stress, bending, psi Fc = allowable stress, compression, psi Fp = allowable stress, bearing pressure, psi Fs = allowable stress, shear, psi Ft = allowable stress, tension, psi Fy = minimum specified yield stress, psi

I = moment of inertia, in.4 Jw = polar moment of inertia of weld, in.4 K = end connection coefficient Kl = overall load factor combining impact and safety factors, 1.5-2.0 Kj = impact factor, 0.25-0.5

Kr = internal moment coefficient in circular ring due to radial load, in.-lb Ks = safety factor

Kt = internal moment coefficient in circular ring due to tangential load, in.-lb Ls = length of skirt/base stiffener/strut, in. M = moment, in.-lb Mb = bending moment, in.-lb Mc = circumferential moment, in.-lb ML = longitudinal moment, in.-lb Mt = torsional moment, in.-lb

Nb = number of bolts used in tail beam or flange lug N = width of flange of tail beam with a web stiffener

(N = 1.0 without web stiffener) nL = number of head or side lugs P = pick end load, lb Pe = equivalent load, lb PL = longitudinal load per lug, lb Pr = radial load, lb Pt = transverse load per lug, lb Rb = radius of base ring to neutral axis, in.

r = radius of gyration of strut, in. Rc = radius of bolt circle of flange, in. Su = minimum specified tensile stress of bolts, psi tb = thickness of base plate, in. tg = thickness of gusset, in. tL = thickness of lug, in. tp = thickness of pad eye, in. ts = thickness of shell, in. T = tail end load, lb Tb = bolt pretension load, lbs Tt = tangential force, lb W] = fillet weld size, shell to re-pad w2 = fillet weld size, re-pad to shell w3 = fillet weld size, pad eye to lug vv4 = fillet weld size, base plate to skirt w5 = uniform load on vessel, lb/in. WE = design erection weight, lb WL = erection weight, lb Z= section modulus, in.3

a — angular position for moment coefficients in base ring, clockwise from 0° P — angle between parallel beams, degrees a = stress, combined, psi (Tb = stress, bending, psi Op = stress, bearing, psi crc = stress, compression, psi crcr = critical buckling stress, psi aT — stress, tension, psi r = shear stress, psi tt = torsional shear stress, psi 9 = lift angle, degrees 9-q — minimum bearing contact angle, degrees 6>n = sling angle to lift line, horizontal, degrees 0V — sling angle to lift line, vertical, degrees

Pressure Vessel Vertical Tower

Miscellaneous Lifting Attachments

Crc Handbook Contact Angle


Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


  • jukka-pekk kantee
    What is the mening vessal nozzale pad pin hole?
    3 years ago
  • Melba
    Why lifting trunnion directly welded to vessel?
    2 years ago

Post a comment