* MECHANICAL DESIGN BASICS OF TALL COLUMNS

**Column is a vertical pressure vessel which physically separates a mixture into two or more products. Process consists that mixture of different boiling point products is boiled and the more volatile components comes out of the mixture.

The process and operation of a column depends on Internals. Column Internals are broadly divided into two parts:

Part-1: Trays.

Part-2: Packing.INTRODUCTION

**Mostly Used trays are as follows

1.Valve Tray

2. Sieve Tray

3.Bubble cap tray .

4. Dual flow Tray

5.Baffle Tray

6.Chimney Tray

7.Cartridge Tray

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** Different Types of Packings

1.Rasching Ring ( 1st Generation).

2.Pall Ring ( 2nd Generation).

3.Cascade Mini Ring ,Metal Tower Packing (3 rd Gen)

4. Rasching Super Ring.

5.Wire Mesh Packing.

6.Wire Guaze Packing.

7. Grid Packing

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Sr. NoDescriptionTraysRandom PackingStructured Packing1Pressure DropHighLowLower2Fouling ServiceYesNo No3Feed Point FlexibilityYesDifficultDifficult4Liquid Hold upConsiderableSmallSmall 5CleaningEasyDifficultdifficult6CostLowLow/MediumHigh

**Minimum column diameter for trayed columns is typically 0.75m;otherwise packed columns are used.

Change in column diameter effect the vapor velocity for given vapor flow rate.

Mechanical Design of Tall ColumnsDefinitionA vertical vessel with height to dia ratio >10Design basis and criteria Process column is normally designed as self-supporting type. The self supporting type of process columns are designed as a cantilever beam i.e. a fixed support at one end in the form of a cylindrical or conical shell called Skirt, with a base ring resting on concrete foundation and fixed firmly by anchor bolts embedded onto the foundation.

**Design LoadsInternal or external design pressureSelf weight including attached piping, platforms, ladders, nozzles & manholes, operating fluid, insulation, fireproofing, welded internals, removable internals and other structural attachments.Loading due to external attachment or equipment.Wind and seismic loads.Other loads like thermal stresses, cyclic loadings etc.

**Design Load conditionsERECTION CONDITION: Column (in uncorroded and ambient temperature) erected on foundation, without insulation, platforms, ladders, trays etc., but with welded attachments plus full wind or seismic load on column.OPERATING CONDITION: Column (corroded) under design pressure and design temperature including welded items, trays, removable internals, piping, platforms, ladders, reboilers mounted on columns, insulation, fire-proofing, operating liquid etc. plus full wind on insulated column or seismic load.HYDROTEST CONDITION: Column (corroded in ambient temperature) in vertical position under test pressure including welded items, platforms, ladders, reboilers mounted on column, fire proofing, full of water plus 33% of full wind load on uninsulated column. Seismic load is not considered.EMPTY CONDITION: Column (corroded in ambient temperature) with welded items, trays, removable internals, piping, platforms, ladders, reboilers mounted on column, insulation, fire proofing plus wind on insulated column or seismic load. Internal/external pressure and operating fluid are not considered.

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Wind Design as per IS 875: 1987Basic wind speed (Vb) is based on peak gust velocity averaged over a short time intervals of about 3 sec.time at 10 m height above mean ground level in open terrain (category 2) and 50 yrs return period. This can be taken from site data or code.Design wind speed Vz = K1*K2*K3*VbWhere, K1: Probability factor/Risk coefficientK2: Terrain height and structure size factorK3: Topography factorDesign Wind Pressure Pz= 0.6* Vz^2 N/sq.mWind load =Ae*Cf*PzWhere, effective area Ae =effective dia.(De) * height (uniform wind) Cf = shape factor (=0.7 as per EDB)

Computation of Projected Area Effective Vessel Diameter;De = ( Vessel OD + twice insulation thickness) X Kd For Kd

The principal parts contributing to the total wind load are;Vessel shell OD with twice the insulation thickness, if any.Adjusted platform area.Caged ladder.Piping- largest pipe in top third of column running to ground levelVessel OD including insulation (mm)Coeff. Kdless than 914 (36)1.5914-1524(36-60)1.41524-2134(60-84)1.32134-2743(84-108)1.2more than 2743(108)1.18

Seismic Design as per IS 1893: 2002Response spectrum method is used for Columns.For a project site following parameters are required;Z = Seismic Zone factorI = Importance factor R = Response reduction factorSa/g = Average acceleration coefficient (from acceleration spectra curve-normally 2% damping is considered)Horizontal seismic coefficient Ah =Z I (Sa/g)/ 2RSeismic Base shear, Vb = Ah *Wo

Allowable stress for combined Loading

Shell thickness calculation for combined Loads:The tangential stress t due to the pressure is given by; t = PD/2t SaWhere, D = mean corroded shell diameterP = Design Pressuret = Corroded thickness of the shellSa = Code allowable stress, reduced by joint efficiencyThe unit force ; lt = PD/2 Sa*t (Kg per linear cm)The combined stress in longitudinal direction L On the windward side;L = (PD/4t) + (4M/ D2 t) (W/ D t)Kg/cm2l = (PD/4) + (4M/ D2) (W/ D)Kg/linear cm.and the shell thickness is

t = [(PD/4) + (4M/ D2)-( W/ D)] / Code allowable stress

On the leeward side;L = (PD/4t) - (4M/ D2 t) (W/ D t)Kg/cm2l = (PD/4) - (4M/ D2) (W/ D)Kg/cmThe maximum compressive stress in the shell is induced at the bottom tangent line on the leeward side when the internal pressure is equal to the atmospheric pressure;L = - (4M/ D2 t) (W/ D t) Code allowable stress

Or for the vacuum vessels;L = -(PD/4t) - (4M/ D2 t) (W/ D t) Code allowable stressMaximum axial buckling can occur locally at section when L reaches critical buckling stress.(Refer UG23 (b) and (d)) Code allowable stress (tensile or compressive) for wind/ seismic loading) are increased by 1.2 times.

Support Skirt Design :Maximum longitudinal stress due to the external moment M and weight W at the base is;L = (W/ Dsk tsk) +/ - (4M/ Dsk2 tsk)If vessel is tested in vertical position, the longitudinal compressive stress at the base L = (WT/ Dsk tsk) Support skirt thickness; based on maximum stress in the skirt-to-head weld.tsk = [(W/ Dsk) + (4M/ Dsk2 )] / E * Code allowable stressWhere; E = weld efficiency (consider 0.55 for skirt butted to knuckle portion)Skirt thickness tsk should be satisfactory for the allowable column deflection (H/200); usually tsk for tall towers is chosen not less than the corroded bottom shell section plate thickness.Support skirts for large diameter vessels, which have to be stress relieved in the field in a vertical position, must be checked to determine whether the thickness will withstand the weight under high temperature conditions.

Flared skirts are used for very tall columns for high moments.If a large access or pipe opening is located in the skirt shellL = +/- M/tsk[(/ Dsk2/4) (Ydsk/2)] W/(Dsk-Y)tskWhere, Y is opening.If L is too high then the opening has to be reinforced.Skirt thickness required to withstand reaction due to bolting is also checked.

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Anchor Bolt Design :Ab = Bolt tensile stress areaM = Overturning moment at base due to wind or earthquakeW = weight of the vesseld = Bolt circle diameterdo = Outside diameter of the base ringdi = inside diameter of the base ringN = total no. of anchor bolts in multiple of 4ZL= d2/4, linear section modulus of the bolt circleC = d, circumference of the bolt circlex = distance of an anchor bolt from the neutral axisSa= allowable design stress for the anchor boltsF = Uplift force per bolt due to the outside moment M

The maximum tension on the bolt circumference per linear cm. is; T = (M/ ZL)-(W/C) = (4M/d2)-(W/d)If 4M/d2 is larger than W/d there is a positive uplift force inducing tension stress, with magnitude depending on the distance x spanned by half the anchor bolts. The max. force F on the bolt at distance x=d/2 from the neutral axis;F=Td/N=(4M/dN)-(W/N) per boltAnd the required bolt area is;Ab = [(4M/d)-W]/NSa

**Calculation of static deflection of columnDeflection at the top of the column is calculated by adding the individual deflections of various sections due to wind load, shear load & moment at the end of each section as explained;

Deflection at top of the column is restricted to 6/100feet.This is required for proper functioning of trayed column

**Vibration Analysis :Period of vibration is calculated by;

Vibration Analysis : Criterion , as recommended by Zorrila :W/LDr2 20 Vibration analysis MUST be performed20 < W/LDr2 25 Vibration analysis SHOULD be performed 25