1 basics of equipment design

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Process Equipment Design-I

(UCH505)

By

Rakesh Kumar Gupta,

Asst. Prof., Chemical Engg.

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Time table: L-3, T-1

Course outcomes:

• Knowledge of basics of process equipment design and important

parameters of equipment design

• Ability to design internal pressure vessels and external pressure vessels

• Ability to design special vessels (e.g. tall vessels) and various parts of

vessels (e.g. heads)

• Knowledge of equipment fabrication and testing methods

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Syllabus (Refer www.thapar.edu)

Design Preliminaries: Introduction, General design procedure, Equipment classification, Design

codes, Design considerations, Design pressure, Design temperature, Design stress, Factor of

safety, Design wall thickness, Corrosion allowance, Weld joint efficiency factor, Design loadings,

Stress concentration, Thermal stress and Criteria of failure.

Design of process vessels under internal pressure: Thin wall vessels, Cylindrical vessels, Tubes,

Pipes, Spherical vessels, Design of heads and closures such as different heads, Nozzle, Flange

joints, Gaskets, Types & design of non- standard flanges and Bolts.

Design of process vessels under external pressures: Introduction, Determination of safe pressure

against elastic failure, Circumferential stiffeners, Spherical shells, Pipes and tubes under external

pressure.

Design of tall vessels: Introduction, Equivalent stress under combined loadings and Longitudinal

stresses.

Design of support for process vessels: Introduction, Different types of supports, Design ofsupports.

Design of thick walled higher pressure vessels: Introduction, Stresses and theories of elastic

failure.

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Equipment fabrication and testing: Welding joints, Inspection and Non-destructive testing of

equipment.

Design of some special parts: Introduction, Expansion joints and its design, Expansion loop in

piping system, Design equations for expansive forces in pipe lines, Shafts and Keys.

Storage tanks: Introduction, Classification of storage tanks, Filling & breathing looses, Design of

liquid and gas storage tanks.

Text Books:

1) Bhattacharyya, B.C., Introduction to Chemical Equipment Design, Mechanical

Aspects, CBS Publishers and Distributors (1998).

2) Joshi, M.V. and Mahajani, V.V., Process Equipment Design, Macmillan India Limited

(1997).

Reference Books:

1) Brownell, L.E. and Young, E.H., Process Equipment Design, Wiley Eastern India

Limited (1991).

2) I.S.: 803 – 1962, Code of practice for Design, Fabrication and Erection of vertical

Mild steel cylindrical welded oil storage tanks.

3) I.S.: 2852-1969, Code for unfired pressure vessel.

4) Bhandari, V.B., Design of Machine Elements, Tata McGraw-Hill Publishing Company

Limited (2002). 4


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Evaluation scheme

S. No. Items Marks

1 Mid Semester Examination 30

2 End Semester Examination 45

3 Internals

a) Two Quizzes 9 each

b) Tutorial 7

Total 100

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Chapter – Design Preliminaries

To be covered:

• Introduction to equipment design

• Basic design parameters:

Design pressure, Design temperature, Design stress, Factor of safety, Design

wall thickness, Corrosion allowance, Weld joint efficiency factor

• Other design parameters:

Design loadings, Stress concentration, Thermal stress and Criteria of failure.

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Introduction

•

New chemical plants are being continuously set up and expanded. Thisinvolves both technical and economical evaluations.

• Phases of design: 1) Equipment design and 2) Process design

• This subject presents methods and procedures adopted in designing of

process equipments.

• The emphasis is on size of equipment, choice of material of construction

and strength considerations.

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Stress

• Definition: It is intensity of internally distributed forces that resist a

change in form of a body due to any loading forces.

• Dimension: Force /Area

• Application types: Static stress, Impact stress, Fatigue stress, etc.

• Simple and combined stresses [stress(es) act in one/ multiple directions]

• Types of simple stresses: Tensile, Compressive , shear stress and bending

stress

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Tensile stress:

E = stress (σ) / strain (e)

Compressive stress:

E = stress (σ) / strain (e)

Shear stress:

Bending stress:

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Thermal stresses

• Type A: The stresses are produced due to

temperature difference at different parts

of the body (in absence of any external

restriction) (e.g. high pressure vessels)

• Type B: Whenever expansion or contraction that would normally resultfrom heating or cooling of body is prevented, stresses are developed that

are also one type of thermal (temperature) stresses.

Bar under heating

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• Consider a straight bar is uniformly heated from temperature T1 to T2.

• We know, developed strain in bar if there were no restrictions,

e = α (T2 – T1) (α: Coefficient of thermal expansion)

• But, resulting thermal axial stress in bar in presence of restrictions can be

defined as,

σ = – (stress calculated assuming body without restrictions)

= – E.e

= – E α (T2 – T1) (σ: compressive stress)

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General design procedures

• Specifying the problem

• Analyzing the probable solutions

• Applying chemical process principles and theories of mechanics (satisfying

conditions of the problem)

• Selecting materials and stresses to suit processing conditions

• Evaluation and optimizing the design

• Preparation of drawing and specifications

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Equipments Classification

Equipments can be classified in three category, to emphasize certain commonfeatures which require similar design procedures.

• Pressure vessel groups: cylindrical / spherical vessels (e.g. reactor, heat

exchanger, distillation column, storage vessels, etc.)

• Structural groups: equipments/ components to sustain dead loads

(vessels), satisfy elastic and structural stability

• Group involving rotational motion: equipment/components with

rotational motion (e.g. centrifuges, agitators, etc.), consider torques anddynamic stress also in design

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Parameter to be known (for satisfactory design)

• Material selection

• Corrosion prevention

• Stress created due to static and dynamic load

• Elastic instability

• Combined stress and theories of failure

• Fatigue

• Brittle fracture

• Creep

• Temperature effects

• Radiation effects

• Effects of fabrication methods

• Economic considerations

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Poisson’s Ratio (μ)

• A block subjected to axial tension (in x) is elongated in the axial direction

(x), but at the same time it undergoes contractions in lateral directions (y

and z).

• Define, μ = lateral strain (compressive) / axial strain (tensile)

• This is found to be constant with in elastic limit for a given material (μ = 0.3

for structural and pressure vessel steel)

• Same phenomena occurs, under axial compression with same value of μ,

μ = lateral strain (tensile) / axial strain (compressive)15

•

Consider a 2-D sheet of infinite thickness under tensile loads (stresses) intwo sides,

• Tensile strain in x-direction due to stress σx = exx = σx /E

(E: modulus of elasticity)

Similarly, tensile strain in y-direction due to stress σy = eyy = σy /E

• Tensile strain in y- direction would cause lateral compressive strain in x –

direction due to stress σy (even in absence of σx). So, lateral strain in x-

direction due to stress σy,

Lateral compressive strain (exy) = - μ . axial tensile strain (eyy)

So, exy = - μ . eyy = - μ (σy /E)

σx

σy

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• Total tensile strain in x- direction,

ex = exx + exy = σx /E – μ (σy /E) ……… (1)

•

Similarly, tensile strain in y- direction will be,ey = eyy + eyx = σy /E – μ (σx /E) ……… (2)

• Solving equation (1) and (2) we have,

σx = (ex + μey)E / (1 – μ2) and σy = (ey + μex)E / (1 – μ

2)

• Similarly, we can also have (for 3D block under tensile stresses),

ex = exx + exy + exz= σx /E – μ (σy /E) – μ (σz /E)

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Criteria of elastic failures

• Equipment (made of ductile material) is elastically failed at yield point.

• Equipment (subjected to combined stress) has some resultant stress/

strain.

• Equipment’s failure occurs as resultant stress/ strain reaches to yield point

stress/ strain condition in that material.

Following four different theories are used to define elastic failure of any

equipment:

1) Maximum stress theory:

As any of resultant normal stress reaches to yield stress under simple

tension, equipment will elastically fail.

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Mathematically, σRN. max = σyield

Including factor of safety (for practical purpose),

σRN. max = σyield / fos

2) Maximum strain theory:

As any of resultant normal strain reaches to yield strain under simple

tension, equipment will elastically fail.

Mathematically, eRN. max = (σyield / E)


eRN. max = (σyield / E. fos)

3) Maximum shear theory:

As any of shear stress reaches to half of yield stress under simple tension,

equipment will elastically fail.

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Mathematically, τmax = (σyield / 2)


τmax = (σyield / 2. fos)

4) Theory of constant energy distortion:

Elastic failure of equipment occurs when the elastic strain energy

required for distortion of material reaches the energy required to

produce yielding under simple tension/ compression.

This criteria has to satisfies equation as,

(σ1 – σ2)2 + (σ2 – σ3)

2 + (σ3 – σ1)2 = 2 σy

2

Where, σ1, σ2 and σ3 are resultant normal stresses.


(σ1 – σ2)2 + (σ2 – σ3)

2 + (σ3 – σ1)2 = 2 (σy / fos)

2

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Design codes and standards

Codes:• Guidelines for safety design of process equipments

(based on safety first principle)

• Defined equations suggest preferred dimensions of process vessels and

structural components.

• Very economic in large scale manufacturing

• Different codes widely used in India:

IS: 2825-1969 (codes for unfired pressure vessel), BS 1500, ASME section

VIII, etc.

Standards:

• Preferred size, material properties and compositions, etc.

• Advantages:

1) Use of standard size allows easy integration of component with plant.21

2) Standard size is cheaper than other.

• Disadvantages: standard size impose constraint to the designer and is not

necessarily economic one.

• See Appendices for various codes and standards

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Design Pressure

•

Maximum working pressure: Maximum (limiting) gauge pressureexpected in equipment under any operating conditions of the process.

• Design pressure: The pressure used in design calculations for purpose ofdetermining the minimum thickness of any vessel/ its component.

(As per IS:2825-1969, Design pressure < 200 bar)

• Design pressure should be properly specified, as the cost of vesselincreases with design pressure.

• For internal pressure vessels (if static head is less than 5% of maximum

gauge pressure), (static head = ρgh)Design pressure = 1.05 (Maximum working pressure)

(5% extra is taken to consider time lag of control devices)

Otherwise, Design pressure = Maximum working pressure + static head

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•

Example: Maximum working pressure in an internal pressure vessel = 5atm and liquid (water) height in vessel = 2 m

Here, Static head of liquid = ρgh = 103 *9.8*2 = 19.6 kPa = 0.19 atm

Static head (0.19 atm) < 5% of Max. working pressure (5 atm)

So, design pressure would be 5.25 atm (as 1.05*5).

• For external pressure vessel with atm pressure outside and vaccum (P

atm absolute) inside,

Design pressure = (1 – P) atm or more (upto 1 atm)

• For external pressure vessel with outside pressure more than atm andinside as 1 atm,

Design pressure = Maximum external gauge pressure + 5% more

• For external pressure vessel with outside pressure more than 1 atm (P0:

maximum external gauge pressure) and vaccum (P i atm absolute) inside,

Design pressure = 1.05 P0 + (1 – Pi) or more (upto 1.05 P0 + 1 atm)

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Design Temperature

•

It is important to find allowable stress value of the material ofconstruction which is temperature dependent.

• Determination of design temperatures:

a) For unheated parts: highest temperature expected of stored material

b) For indirect heating equipments (by steam, hot water, etc.): highest

temperature of heating media or 10˚C higher that max. temperature of

any equipment part

c) For direct heating (by means of fire, flue gas, electricity, severe

exothermic reaction, etc.) equipments: 20˚C more than the highest

temperature of stored material (if equipment is shielded) and 50˚C more

than the highest temperature of stored material (if equipment is

unshielded)

• Minimum design temperature: 250˚C 25

Design stress and factor of safety

• Correct stress distribution in equipment to ensure adequacy of design

• Requirement of sufficient rigidity in design

• Most widely used criteria: to maintain deformation before plasticity

• Damaging stress: least stress that will make equipment unfit for service in

normal life of operation.

• Design stress/ allowable stress/ permissible stress: The stress generated

in any equipment must be limited to a permissible value that is accepted

as safe for particular operation conditions with its controlling facility.

• The ratio of damaging stress and design stress is defined as factor of

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• If an equipment is so designed that the maximum stress as calculated for

the expected condition of service is less than some certain value, the

equipment will have a proper margin of security against damage/ failure.

Factor of safety puts such margin in designing.

• Controlling factors to decide design stress and fos: load estimation

accuracy, reliability of stress computed, uniformity of material, hazards if

failure occurs, local stress concentrations, fatigue, etc.

• For ductile materials,

Design stress = Yield stress / Factor of safety

• For brittle materials,

Design stress = Ultimate tensile stress / Factor of safety

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Wall thickness

• Design wall thickness: thickness calculated from design code equations(based on stress consideration)

• Design wall thickness gives safe wall thickness against pressure or similarloads.

• It is not concerned with fabricational feasibility and availability.

•

Minimum wall thickness required for rigid construction(specified by codesas per application)

• Minimum actual wall thickness: Standard available sheet metal thicknesswhich satisfies the design thickness requirement and takes into accountfactors like rigidity, fabricational feasibility, non uniformity of sheet metal,corrosion allowances, etc. This is minimum used in any of the equipment.

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Corrosion allowance

• Depreciation of sheet metal with surrounding environment is common

problem in chemical industry.

• Difficult to predict rate of corrosion in advance

• Some consideration is given to corrosion while deciding minimum wall

thickness.

• Experienced designer/ engineer can suggest only.

•

Guidelines to decide corrosion allowance:1.5 mm: for C steel and cast iron pressure parts except tubes (in chemical

industry with non-severe conditions)

3 mm: for petroleum or chemical industry with severe conditions

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•

For stainless steel and non-ferrous pressure parts: no corrosion allowance

• For wall thickness > 30 mm: neglect corrosion allowance

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Weld Joint efficiency factor ( J )

• Joint sections in vessel are considered to be weaker compared to the

strength of rest of plate metal due to uncertainty of quality of joint.

• Thicker wall section is required around joint section.

• Increase in material cost with use of thicker wall equipment/ equipment

with thick joint sections

• Otherwise, testing the joints and take necessary corrective actions

•

Again, fabrication cost is high to testing all the joints

• Compromise is taken between material and fabrication cost (as per

application and design codes)

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• Definition: Ratio of possible strength of welded joint with strength of bareplate (Normally, J = 0.5 – 1.0)

• Classification of welded pressure vessels (as per IS: 2825-1969): I, II, III

• Class I vessels:

Vessels to contain lethal / toxic substances

Vessels operating under severe conditions

Fully radiographic joints are needed (minor defect is even not allowed)

Generally, double welded butt joint fully penetrated

Weld joint efficiency factor = 0.9 -1.0

• Class II vessels:

Vessels for medium duty operations (most of chemical process)

Maximum wall thickness including corrosion allowance = 38 mm

Spot radiographic joints

Weld joint efficiency factor = 0.8-0.85

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• Class III vessels:

Vessels for relatively light duties

Maximum wall thickness including corrosion allowance = 16 mm

No radiographic testing requiredWeld joint efficiency factor = 0.5-0.7

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Design loadings

• Types of loading experienced by a vessel,

(a) Main loadings

1) Design pressure including static head

2) Weight of vessel and its contents

3) Wind and earth quake

(b) Supplementary loadings

1) Local stresses due to supporting lugs, ring gurders, saddles, internalstructures and connecting pipes, etc.

2) Bending moment caused by eccentricity of centre of working pressure3) Thermal stresses

4) Fluctuating pressure and temperature

• Combination of various loading should be taken to account of safe design.

• However, all loading are not working at a particular time.

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Stress concentration

• Normally, distribution of elastic stress across section of equipment is regular.

• When the variation is abrupt with in a very short distance the intensity of

stress increases greatly, this is described as stress concentration.

• It is usually due to stress raisers (local irregularities in form or discontinuity

in shape).

(e.g. holes for nozzle connections, torus in formed end, sharp corners at

junction of cylindrical shell with flat cover, etc.)

• Stress concentration factor =

True maximum stress / stress expected ignoring stress concentration

• The value for stress concentration factor lies between 3-5. This is much serious

for fluctuating load conditions.

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1 basics of equipment design

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