PAP Lecture 1.

The objectives :

1. The roles of chemical engineers responsibility

while designing the processes and equipments

2. The roles of Inherently Safer Design

3. Review on

1. Materials / chemicals storage refer to TBS

course

2. Thermodynamics Phase equilibrium refer to

chemical engineering thermodynamics 1st

course

The objectives (cont)

4. Design Variables

5. Equipment Sizing (short cut methode of

equipment sizing)

6. Material Construction Selection

7. Mechanical design of equipments

a. Equipment under internal working pressure

load

b. Equipment under external working pressure

load

The objectives (cont.)

8. Vessel stability ( tall tower/vessel stability )

9. LOPA ( Layer Of (Equipments) Protection

analysis

The chemical engineers scope of work in

industrial fields

1. Process engineering field process

engineers

2. Operation engineering field

operation engineers

3. Utility / Fasility engineering field

fasilities engineers

4. Managerial field plant manager

Chemical engineers Responsibilities

while designing the process or the equipment

1. Responsible to comply the design intention for

example equipment designed should comply the

design capacity

2. Responsible to the safety of the equipment while

operation / start up and intended shut down

“fail safe operation philoshopy “

3. Responsible to the safety of the “environment”

minimizing/eliminating the consequences of the

accident / process failure realization

decreasing the likelihood/the frequency of the

accident realization using the accurate/suitable

of layer of protection (control systems) for the

equipment designed

Responsible to comply the design

intention

The steps or squences while designing the equipment /

process :

1. Should be familiar / full recognize about what the basic

principle / the fundamentals around the equipment

2. Equipment sizing

1. Using the best practices short cut method

calculation (using soft wares or empirical equation

ertc.)

2. Using the “Chemical Engineering Tools “ to

provide the “design equations” selecting the

design variable sizing the equipment

3. Applying the engineering sense , comparing the size of

the equipment found/calculated that it is pratically

possible / useable

The safety of the equipment designed


Fail ??? fail operation

There will be process parameters deviation

increasing / decreasing of flow

rate, temperature , pressure or

composition

Safe or unsafe????

Safe operation (while being fail) , mean no

accident realization , no equipment damage or

minor damage under the acceptable

consequences / risk associated

The safety of the equipment designed


What should you need to ensure the fail safe

operation philosophy??

Your equipment should be completed with

1. Operator intervention procedure

2. Indicators

3. Automatic controllers

Inherent existing in something (property, potencial

hazard , quality etc ...., as a permanent and

inspasable element, quality or attribute

Safer safer than others design conducted by by other

designers or existing design (maybe)

less risk associated , decreasing in potentialy

releasing hazard both passively or actively

occuring .

Inherently safer design definition :

to conduct the design of equipment , design of process ,

design of product , concerning to eliminate or to

decrease the hazard realization rather than relying on

the hazrd control only.

1. Minimizing or attenuation

significantly reduce the quantity of hazardous material

or energy in the system or eliminate the hazard entirely if

possible

2. Substitute

replace a hazardous material with less hazardous

substance or using the hazardous chemistry with a less

hazardous one

3. Moderate

reduce the hazard of a process by handling materials in

a less hazardous form, or under less hazardous condition ,

for examples at lower pressure and temperature

4. Simplify

eliminate unnecessary complexity to make plants more

“user friendly” and less prone to human error and

incorrect operation

1. In many cases it will not clear which of several potential

technologies is really inherently safer , and there may strong

disagreement about this

2. Chemical processes and chemical plants have multiple hazards ,

and the different technologies will have different inherent safety

characteristic with respect to each of those multiple hazards

Chemical substitution ??? create the later hazards (maybe)

3. Who is to determine which alternative is inherently safer ???

requiring the implementation of inherent technology ?? who

determine that this technology is inherently safer ???

4. Some technology choices which are inherently safer locally , but

more globally could increase the hazard

Thinking about the opprtunity of

implementation of inherently safer design

1. Engineer should think of this at all times in everything they do

it should be way of life for those designing and operating

chemical plant , technologies etc.

1. R&D engineer inherently safer process based on those

chemistry

2. Design engineer / process engineer inherently safer plant

design using the selected technology and process

minimize the size of equipements containing the hazardous

chemical make the plant design “user friendly”

3. Plant operation engineers and operator develop the the

inherently safer operating procedures look for opprtunity

for enhancing inherent safeety in existing facilities

4. Operator look for inherently safer ways to do all of the

tasks involves in day to day operation

STORAGE VESSEL CONTAINMENTS:

1. LIQUID PHASE

1. NON VOLATILE LIQUID

2. VOLATILE LIQUID

2. GASEOUS STATE PHASE

1. UNDER PRESSURE STORGAE

2. ATMOSPHERIC STORAGE

3. LIQUIFIED GASEOUS

1. CRYOGENIC STORAGE SYSTEM

2. NON CRYOGENIC STORAGE SYSTEM

STORAGE CONDITION 1.

STORAGE VESSEL CONDITIONS:

1. LIQUID PHASE

1. NON VOLATILE LIQUID

AT AMBIENT TEMPERATURE AND AMBIENT

PRESSURE STORAGE TANK

2. VOLATILE LIQUID

AT AMBIENT PRESSURE , BELOW ITS BOILING POINT

AT AMBIENT TEMPERATURE , ELEVATED

PRESSURE

2. GASEOUS STATE PHASE

1. UNDER PRESSURE STORGAE

UNCONDENSED CONDITION

PRESSURE VESSEL

2. ATMOSPHERIC STORAGE GAS HOLDER

STORAGE CONDITION 2

(CONT.)

1. LIQUIFIED GASEOUS STATE

1. CRYOGENIC STORAGE SYSTEM

AMBIENT PRESSURE AND AT CRYOGENIC

TEMPERATURE AND AT ITS BOILING POINT

(SATURATED LIQUID AT SLIGHTLY OBOVE ITS

BP)

ELEVATED PRESSURE , CRYOGENIC

TEMPERATURE AND AT ITS BOILING POINT

2. NON CRYOGENIC STORAGE SYSTEM

AT AMBIENT TEMPERATURTE AND HIGH

PRESSURE AND AT ITS BOILING POINT

(SATURATED LIQUID)

AT ELEVATED TEMPERATURE AND ELEVATED

PRESSURE AS SATURATED LIQUID

STORAGE CONDITION 3

(CONT.)

EMPIRICAL EQUATION

USED TO DEFINE THE STORAGE CONDITION FOR

SATURATED LIQUID

ESPECIALLY FOR LIQUIFIED GAS STORAGE BOTH AT

ELEVATED OR CRYOGENIC TEMPERATURE

Antoinne Equation

T

BAP 0ln

Where :

P0 = vapor pressure , psi or cmHg or other unit

A , B = Antoinne constante , according to the liquified gas stored

T = storage temperature , liquid temperature being stored , K

or R

STORAGE SYSTEM / LAY OUT

1. STORING LIQUID UNDER AMBIENT CONDITION

1. OUT DOOR STORAGE

2. USING VERTICAL SILINDRICAL TANK , FLAT BOTTOM AND

DOOME TYPE TANK , EQUIPPED WITH P/V VALVE

CONNECTING TO FLARING SYSTEM (FOR FLAMMABLE

LIQUID) , LI OR LC (FOR CONTINOUS PUMPING SERVICES)

2. STORING LIQUID AT ITS BOILING POINT (AT ELEVATED TEMP.

AND PRESSURE)

1. OUT DOOR STORAGE

USING SPHERE VESSEL (ESPECIALLY FOR THE MOST

VOLATILE LIQUID, LNG OR LPG ETC.)

USING VERTICAL SILINDRICAL VESSEL BLANKETTING

WITH HEAT INSULATION MATERIAL WITH THE TYPE OF

DOUBLE WALL VESSEL OR SINGLE WALL VESSEL

NEED INSULATION TOO THICK

1. STORING LIQUID AT ITS BOILING POINT (AT ELEVATED

TEMP. AND PRESSURE

1. INDOOR SYSTEM STORAGE USING BURRIED

HORIZONTAL VESSEL, EQUIPPED WITH P/V VALVE AND

LI OR LC FOR CONTINOUS PUMPING SERVICES

2. STORING GASEOUS STATE

1. GASEOUS STATE COMPRESSIBLE FLUID

2. TO ATTENUATE THE VOLUME OF GAS BEING STORED

USUALLY USE STORAGE SYSTEM AT ELEVATED

PRESSURE BUT THE GAS NOT YET CONDENSED

USED SMALL SIZE VESSEL LESS THICKNESS OF

VESSEL SHEEL.

3. STORING UNDER AMBIENT PRESSURE GAS HOLDER ,

NEED LARGE DIAMETER OF GAS HOLDER , USING

DOME HEAD/ROOF SMALL SIZE OR USED FLOATING

ROOF

STORAGE SYSTEM / LAY OUT

(CONT. 2)

ENSURING THE SAFETY OF THE

EQUIPMENT WITH INHERENTLY

LAYER OF PROTECTION

LI LC LLAI

PC

DRAINING

SYSTEM

SATURATED

LIQUID

HHPAI

LI LC LLAI

DRAINING

SYSTEM

SUBCOOLD

LIQUID

P/V VALVE

LI LC

LLAI

LI LC

LLAI

SATURATED

LIQUID

PC

SUBCOOLD

LIQUID

DRAINING

SYSTEM DRAINING

SYSTEM

HHPAI

ENSURING THE SAFETY OF THE

EQUIPMENT WITH INHERENTLY

LAYER OF PROTECTION

P/V VALVE

Phase equilibrium

liquid

vapor

Equilibrium means :

1. Rate of vapor condenses = rate of liquid

boils

2. Equilibrium occurs depend on pressure ,

temperature and composition

3. The temperature and pressure of each

phase are equal

Tvapor phase = T liquid phase

Pvapor phase = Pliquid phase

4. At spesific composition , equilibrium

occurs at soecific P, T , means that the

phase composition at equilibrium state

depend on T,P

F(Xi, T, P) = 0

Xi = f (T,P)

T = f(Xi, P)

Note :

P = total presure

suppress the

equilibrium

ROULT & DALTON LAW

compliance only for ideal solution

SATURATED

LIQUID , Xi ---n **

3

*

2

*

1

1

*

....... i

i

itotal

pppp

pp

o

ii

totalii

px

pyp

*

Additional notes;

Yi = mole fraction of component i in the vapor phase

Xi = mole fracti on of component i inthe liquid phase which in

equilibrium with its vapor phase

Pio = pure vapor pressure of component i at equilibrium

temperature f(Teq )

you can estimate using Antoinne equation

o

ii

totalii

px

pyp

*

PHASE EQUILIBRIUM

total

i

i

ii

total

i

i

i

p

p

constmequilibriuphase

x

yK

p

p

x

y

0

0

.

How to estimate Ki, eq ??

1. Estimate pure vapor pressure of i

component , using Roult. & Dalton

Law

2. EstImate pure vapor pressure ,

using EOS ( Equation of State for

example : PENGROBINSON OR

REDLICH AND KWONG

3. YOU CAN ALSO USE

EQUILIBRIUM PHASE CHARTS

THAT WRITTEN IN ALL

TEXTBOOK OF CHEM . ENG.

THERMODYNANAMICS

APPLICATION OF PHASE EQUILIBRIUM IN

PROCESS DESIGN PHASE

1, ESTIMATION OF DUE POINT TEMPERATURE OF

CONDENSABLE GAS MIXTURE THE DEW POINT IS

DEFINED BY :

AT SPECIFIED TOTAL PRESSURE OF THE MIXTURE ,

Ptotal defined

the dew point of this mixture is that the

temperature at which the vapor will start to

condensed , so the state of the mixture should be

in saturated vapor

At this dew pint , the vapor composition follow

this equation.

0,1 iy

2. ESTIMATION OF BUBLE POINT TEMPERATURE OF

LIQUID MIXTURE

THE BUBLE POINT IS DEFINED AS FOLLOW : THE

TEMPERATURE WHICH THE MIXTURE WILL START

TO EVAPORATE

AT THIS CONDITION SHOULD

a. The liquid mixture in saturated liquid state

b. THE SUM OF PARTIAL VAPOR PRESSURE OF EACH

COMPONENT (i) BE EQUAL TO TOTAL

PRESSURE OF MIXTURE SYSTEM

IN PRACTICE FOR ESTIMATING THE BOTTOM

CONDTION OF THE LIQUID FLOW OUT FROM THE

REBOILER

0,1 ix

PROCESS CONDITION DESIGN

FOR COLUMN DESTILATION

RULES OF THUMB /

SEQUENCES

1. CHOOSE THE VARIABLES DEFINED BY DESIGNER

a. product composition DESIGN intention ( top product or bottom

product)

b. Using the mass balance around the column calculate the

existing calculated composition

2. CHOOSE THE DESIGN VARIABLE

a. INTERNAL REFLUX RATIO (Lo/D) (1,20 – 1,4) Rmin

b. Number of plate (for existing column) the design work is to

optimize the existing column by varying the process condition

3. CHOOSE THE APPROPRIATE AVAILABLE SOURCE OF UTILITIES :

a. STEAM / FUEL AS HEATING MEDIUM

b. COOLING WATER / AMBIENT AIR/ REFRIGERANT AS COOLING

MEDIUM

RULES OF THUMB / SEQUENCES (CONT.)

4. CHOOSE THE PREFERENCE DESIGN PARAMETER TO OBTAIN

COLUMN / PROCESS CONDITION

a. OPERATING PRESSURE DEFINED to aim the minimum

thickness of shell vessel the operating temp calculated will

define the utility need it would be that the utility beyond of the

available one

b. Operating temperature both at the top and the bottom

according to the maximum achievable for the available utility

in the plant top operating temp rely on to the existing /

avaliable cooling medium , if cooling water used , so the top

condition would be = 35oC + 10oC (as thermal approach)

bottom operating temperature rely on to the heating medium

available , if steam used , the maximum available steam (from

boiler) = 200oC , the maximum temperature achieved by this

steam = 200 – 30 (as thermal approach) = 170 oC.

5. CHOOSE THE PREFERENCES OF USING CONDENSOR (TOTAL/

PARTIAL) AND ALSO THE REBOILER TO PROVIDE VAPOR

REFLUX TO COLUMN.

Total condensor

Bottom product at its

boiling point

destilate product at its

boiling point

Partial

reboiler

Heating

medium

Condensate

accumulator

DISTILATION COLUMN USING

TOTAL CONDENSOR AND

PARTIAL REBOILER

Conventional

distilation column


boiling point

Partial

reboiler

Heating

medium

Partial condensor

Vapor product at its

dew point

Condensate

accumulator DISTILATION COLUMN USING

PARTIAL CONDENSOR AND

PARTIAL REBOILER

Conventional

distilation column

Total condensor


boiling point

destilate product at its

boiling point

Condensate

accumulator

Vapor reflux

Flue gas

Fuel

medium

Total reboiler

(furnace)


TOTAL CONDENSOR AND

TOTAL REBOILER

Total condensor


boiling point

Vapor reflux

Flue gas

Fuel

medium

Total reboiler

(furnace)

Partial condensor


dew point

Condensate

accumulator


PARTIAL CONDENSOR AND

TOTAL REBOILER

Bottom product

at its boiling point

Partial

reboiler

Heat pump assisted

DISTILATION COLUMN

with PARTIAL

REBOILER

SATURATED

VAPOR

SUPERHEATED

VAPOR

COMPRESSOR

DESTILATE product


Saturated liquid at

high pressure

Throtling

valve

Destilate

separator

No heating

medium need

Bottom product


(liquid)

Partial

reboiler

Heat pump assisted

DISTILATION COLUMN

with PARTIAL

REBOILER and vaporas

top product

SATURATED

VAPOR

SUPERHEATED

VAPOR

COMPRESSOR

Saturated liquid at

high pressure

Throtling

valve

Destilate

separator


dew point at

column pressure

HEAT BALANCE

F = D + B

F.Xi,F= D.Xi,D + B.Xi,B

F.HL,F= D.(QCD+HL,D )+ B.(HL,B – QR,B)

MASS BALANCE

QRB = QR/B

REBOILER

B , Xi, B,

HL, B

D , Xi, D, HL, D

Lo , Xi,,O

HL,,O

Total

condensor

Sat`d vapor

Sat`d liquid

Sat`d vapor

QC, D = QC /D

F , Xi,F,

HL,F

CONVENTIONAL

DESTILATION COLUMN

Design equations

TOP SECTION

NTH

plate

N=1

LN , XiN

, HL, N

GN+1 , Yi,N+1 ,

HG,N+1

D , Xi, D,

HL, D

Lo , Xi,,O

HL,,O

Total

condensor

RO = LO/D

Xi,O = Xi,D

HL,O = HD

TTOP = TBP,D

Sat`d vapor

Sat`d liquid

H, enthalpi = f (T,P,Xi)

GN+1 = LN + D

GN+1.Yi,N = LN.Xi,N + D.Xi,D

Yi,N = (LN/GN+1).Xi,N + (D/GN+1).Xi,D

(GN+1)/D = LN/D + 1

= RN + 1

(LN/GN+1) = RN/(1+RN)

Yi,N = {RN/(1+RN-1)}Xi,N + {1/(1+RN)} Xi,D

NTH

plate

N=1

LN , XiN

, HL, N

GN+1 , Yi,N+1 ,

HG,N+1

D , Xi, D,

HL, D

Lo , Xi,,O

HL,,O

Total

condensor

RO = LO/D

Xi,O = Xi,D

HL,O = HD

TTOP = TBP,D

Sat`d vapor

Sat`d liquid Heat balance :

HG,N+1={RN/(1+RN)}HL,N+{1/(1+RN)}(HD+QCD)

Yi,N = {RN/(1+RN+1)}Xi,N + {1/(1+RN)} Xi,D

Mass balance :

RN = (LN/D) R0 USUALY USED AS

DESIGN VARIABLE

N = 0 means represent to total condensor

Sat`d liquid = BOTTOM PRODUCT

GM = LM+1 – B

RM = GM/B

STRIPPING SECTION

GM Yi,M = LM+1 Xi, M+1 - B Xi,B

Yi,M = (LM+1/GM) Xi, M+1 - (B/GM) Xi,B

GM .HG,M = LM+1.HL, M+1 – B(HL,B - QRB)

HG,M = (LM+1/GM).HL, M+1 - (B/GM).(HL,B - QRB)

HG,M = {(RM+1)/RM}.HL,M+1 - (1/RM).(HL,B - QRB)

Yi,M = {(RM+1)/RM} Xi, M+1 - (1/RM) Xi,B

M =2 QRB = QR/B

M =1

Mth PLATE from

bottom

LM+1 , XiNM+1,

HL, M+1

GM , Yi,M,

HG, M

B , Xi, B,

HL, B

SATURATED

VAPOR

• The mechanical design of a pressure vessel can proceed

only

after the materials have been specified. The ASME Code

does not state what materials must be used in each

application

• user to specify the appropriate materials for each

application considering various material selection factors

in conjunction with ASME Code

Material Selection Factors

The main factors that influence material selection

are:

• Strength

• Corrosion Resistance

• Resistance to Hydrogen Attack

• Fracture Toughness

• Fabricability

Strength

• Strength is a material's ability to withstand an imposed force

or stress.

• Strength is a significant factor in the material selection for a

particular application.

• Strength determines how thick a component must be to

withstand the imposed loads.

• The strength properties depend on the chemical

composition of the material.

• Creep resistance (a measure of material strength at elevated

temperature) is increased by the addition of alloying

elements such as chromium, molybdenum, and/or nickel to

carbon steel. Therefore, alloy materials are often used in

elevated temperature applications.

Corrosion Resistance

• Corrosion is the deterioration of metals by

chemical action.

• A material's resistance to corrosion is probably

the most importantfactor that influences its

selection for a specific application.

• The most common method that is used to

address corrosion in pressure vessels is to

specify a corrosion allowance.

• A corrosion allowance is supplemental metal

thickness that is added to the minimum thickness

that is required to resist the applied loads.

• This added thickness compensates for thinning

(i.e., corrosion) that will take place during service.

Resistance to Hydrogen Attack

• At temperatures from approximately 300°F to 400°F, monatomic

hydrogen diffuses into voids that are normally present in steel.

• In these voids, the monatomic hydrogen forms molecular

hydrogen, which cannot diffuse out of the steel.

• If this hydrogen diffusion continues, pressure can build to high

levels within the steel, and the steel can crack.

• At elevated temperatures, over approximately 600°F, monatomic

hydrogen not only causes cracks to form but also attacks the

steel.

• Hydrogen attack differs from corrosion in that damage occurs

throughout the thickness of the component, rather than just at its

surface, and occurs without any metal loss.

• In addition, once hydrogen attack has occurred, the metal cannot

be repaired and must be replaced.

• Hydrogen attack is a potential design factor at hydrogen partial

• pressures above approximately 100 psia.

Fracture Toughness

• Fracture toughness refers to the ability of a material to

withstand conditions that could cause a brittle

fracture.

• The fracture toughness at a given temperature varies

with different steels and with different manufacturing

and fabrication processes.

• It is especially important for material selection to

eliminate the risk of brittle fracture since a brittle

fracture is catastrophic in nature.

• It occurs without warning the first time the necessary

combination of critical size defect, low enough

temperature, and high enough stress occurs.

Fabricability

• Pressure vessels commonly use welded

construction.

• Therefore, the materials used must be weldable so

that individual components can be assembled into

the completed vessel.

B. Maximum Allowable Stress

• Maximum allowable stress is the maximum stress that

may be safely applied to a pressure vessel component

• The design of a pressure vessel must ensure that these

internal stresses never exceed the strength of the vessel

components.

• Pressure vessel components are designed such that the

component stresses that are caused by the loads are

limited to maximum allowable values that will ensure safe

operation.

Note that the allowable stresses at temperatures between

-20°F and 650°F are the same as the allowable stress at 650°F

for each material presented

Carbon Steel Plates and Sheets

Mechanical design

• The mechanical design of a pressure vessel begins with

specification of the design pressure and design temperature.

• Pressure imposes loads that must be withstood by the

individual vessel components.

• Temperature affects material strength and, thus, its allowable

stress, regardless of the design pressure.

All pressure vessels must be designed for the most severe

conditions of coincident pressure and temperature that are

expected during normal service.

Normal service includes conditions that are associated with:

· Startup.

· Normal operation.

· Deviations from normal operation that can be anticipated

(e.g., catalyst regeneration or process upsets).

· Shutdown.

Pressure

Operating Pressure

The operating pressure must be set based on the

maximum internal or external pressure that the pressure

vessel may encounter.

• Ambient temperature effects.

• Normal operational variations.

• Pressure variations due to changes in the vapor

pressure of the contained fluid.

• Pump or compressor shut-off pressure.

• Static head due to the liquid level in the vessel.

• System pressure drop.

• Normal pre-startup activities or other operating

conditions that may occur (e.g., vacuum), that should

be considered

in the design.

Design Pressure

• The specified design pressure is based on the maximum

operating pressure at the top of the vessel, plus the margin that

the process design engineer determines is suitable for the

particular application.

• A suitable margin must also be provided between the

maximum operating pressure and the safety relief valve set

pressure.

• This margin is necessary to prevent frequent and unnecessary

opening of the safety relief valve that may occur during normal

variations in operating pressure.

• The safety relief valve set pressure is normally set equal

to the design pressure.

• Pressure vessels, especially tall towers, may have liquid

in them during normal operation.

• The maximum height of this liquid normally does not reach

the top of the vessel.

• The liquid level that is required for design is specified by

the process design engineer.

Temperature

Operating Temperature

• The operating temperature must be

set based on the maximum and

minimum metal temperatures that the

pressure vessel may encounter.

• The operation and vertical length of

tall towers, and the presence of

liquid in the bottom section,

sometime result in large temperature

reductions between the bottom and

top of the vessel.

Design Temperature

The design temperature of a pressure vessel is the maximum

fluid temperature that occurs under normal operating

conditions, plus an allowance for variations that occur during

operation.

Mechanical Design for Internal Pressure

1. in a cylindrical shell under internal pressure

this is an idealized stat

2. the ASME Code Formulas have been modified to

account for nonideal behavior.

Summary of ASME Code Equations

Typical types of closure heads.

Elliptical Heads

• The 2:1 semi-elliptical head is the most commonly used

head type high pressure vessel used

• Half of its minor axis (i.e., the inside depth of the head

minus the length of the straight flange section) equals

one-fourth of the inside diameter of the head.

• The thickness of this type of head is normally equal to the

thickness of the cylinder to which it is attached.

Torispherical Heads

• A torispherical (or flanged and dished) head is typically

somewhat flatter than an elliptical head and can be the same

thickness as an elliptical head for identical design conditions

and diameter.

• The minimum permitted knuckle radius of a torispherical head

is 6% of the maximum inside crown radius.

• The maximum inside crown radius equals the outside diameter

of the head.

Hemispherical Heads

•In carbon steel construction, hemispherical heads are

generally not as economical as elliptical or torispherical

heads because of higher fabrication cost.

• The required thickness of a hemispherical head is normally

one-half the thickness of an elliptical or torispherical head

for the same design conditions, material, and diameter.

• Hemispherical heads are an economical option to consider

when expensive alloy material is used.

Design for Internal Pressure

A. What are the minimum required thicknesses

for the two cylindrical sections?

DESIGN INFORMATION

Design Pressure = 250 psig

Design Temperature = 700° F

Shell and Head Material is SA-515

Gr. 60

Corrosion Allowance = 0.125"

Both Heads are Seamless

Shell and Cone Welds are Double

Welded and will be Spot

Radiographed

The Vessel is in All Vapor Service

Cylinder Dimensions Shown are

Inside Diameters

Solution

1. The required wall thickness for internal pressure of a cylindrical

shell is calculated using the following equation

2. Since the welds are spot radiographed, E = 0.85 (from Figure 4.5)

3. S = 14,400 psi for SA-515/Gr. 60 at 700°F (from Figure 3.2)

P is given as 250 psig.

For the 6 ft. - 0 in. shell, calculate r (including corrosion allowance)

r = 0.5D + CA = 0.5 x 72 + 0.125 = 36.125 in.

t = 0.872 in. required including corrosion allowance

For the 4 ft. - 0 in. shell, calculate r (including corrosion allowance)

r = 0.5 x 48 + 0.125 = 24.125 in.

t = 0.624 in. required (including corrosion allowance)

B. For the same vessel, what are the minimum required

thicknesses for the top and bottom heads?

Solution

1. Since both heads are seamless, E = 1.0.

2. Top Head - Hemispherical head (Equation from Figure 4.6)

r = 24 + 0.125 = 24.125 in.


3. Bottom Head - 2:1 Semi-Elliptical Head

D = 72 + 2 x 0.125 = 72.25 in.


Design for External Pressure and

Compressive Stresses

• Pressure vessels are subject to compressive forces such as

those caused by dead weight, wind, earthquake, and internal

vacuum.

• The failure type of this vessel is due to elastic instability,

which makes shell weaker in compression than in tension

(due to under internal working pressure)

• In failure by elastic instability, the vessel is said to collapse or

buckle.

• These basic principles also apply to other forms of shells as

well as to heads and to compressive loads other than external

pressure.

Overview

• The critical pressure that causes buckling is not a simple

function of the stress that is produced in the shell.

• An allowable stress is not used to design pressure

vessels that are subject to elastic instability.

• Instead, the design is based on the prevention of elastic

collapse under the applied external pressure

• This applied external pressure is normally 15 psig for

full vacuum conditions.

• The maximum allowable external pressure can be increased

by welding circumferential stiffening rings (i.e., stiffeners)

around the vessel shell

Shells

• The allowable external pressure of a cylindrical shell is a

function of material, design temperature, outside diameter,

corroded thickness, and unstiffened length.

Heads

• The allowable external pressure of a head is a function of

material, design temperature, outside radius, head depth,

and corroded thickness.

• The head thickness is increased as required to achieve

the required external pressure.

DESIGN INFORMATION

Design Pressure = Full Vacuum

Design Temperature = 500° F

Shell and Head Material is

SA-285 Gr. B, Yield Stress = 27 ksi

Corrosion Allowance = 0.0625"

Cylinder Dimension Shown is Inside

Diameter

External Pressure Calculation

The vendor has proposed that the wall

thickness of this tower be 7/16 in., and

no stiffener rings have been specified.

Is the 7/16 in. thickness acceptable for

external pressure?

Solution

1. First, calculate the unstiffened design length, L, and the

outside diameter, Do, of the cylindrical shell, both in inches.

L = Tangent Length + 2 x 1/3 (Head Depth)

The tangent length = 150 ft.

Since the heads are semi-elliptical, the depth of each head is

equal to ¼ the inside diameter of the shell.

Head Depth = 48 /4 = 12 in.

L = 150 x 12 + 2/3 x 12 = 1,808 in.

Do = 48 + 2 ´ 7/16 = 48.875 in.

Next, determine the ratios L/Do and Do/t.

Accounting for the corrosion allowance,

t = 7/16 – 1/16 = 6/16 = 0.375 in.

Do/t = 48.875 / 0.375 = 130

L/Do = 1808 / 48.875 = 37

2. Determine the value of A using Figure 4.12 and the calculated

Do/t and L/Do.

Note: If L/Do > 50, use L/Do = 50.

For L/Do < 0.05, use L/Do = 0.05.

Factor A

Calculate maximum allowable external pressure for the value of t, psi.

Where

E = Young's modulus of elasticity at design temperature for the

material, psi. Do not confuse this parameter with the weld joint

efficiency, E, that is used elsewhere.

Figure CS-1

E = 27 x 106 psi from Figure CS-1 (Figure 4.13) at T = 500°F

PAP Lecture 1.

Documents

Transcript of PAP Lecture 1.