Process Equipment Design – I (ChE 553)

ByM. K. Mandal

Overview

• Engineering design is the process of converting an idea or

market need into the details information from which a product

or system can be made

• Aim of the future development in technology is to increase the

benefits and reduce the problems

Pressure Vessel

Books:

i. Process Equipment Design

- Lloyd E. Brownell & Edwin H. Young

ii. Process Equipment Design

- M. V. Joshi

Any book, you can follow

Introduction

Chemical engg. involves the application of sciences to the

process industries where one material is converted into

another by chemical or physical means

These processes require the handling & storing of large

quantities of materials in containers

Containers for fluid under pressure is called pressure vessel

Objective: Provide a general knowledge of design requirements for pressure vessels

Application Area

used in variety of industries

Petroleum refinery

Chemical industry

Power sector

Pulp & paper industry

Food & beverage industry

Types of vessel

Open vessel: used for low value liquid Closed vessel: used for toxic, hazardous fluid

Again,

i. Cylindrical:

a) Horizontal

b) Vertical

ii. Spherical:

MAIN COMPONENTS OF PRESSURE VESSEL

Following are the main components of pressure Vessels in general

Shell

Head or cover

Nozzle

Flanged joint

Support

VERTICAL PRESSURE VESSEL

• The max. Shell length to diameter ratio for a small vertical drum is about 5 : 1

SPHERICAL PRESSURIZED STORAGE VESSEL

Design Code

Based on ASME code section VIII

ASME: American society of Mechanical Engineering• an educational tool to help engineers and managers succeed in

today’s business/engineering world

Various forces

several types of stresses may occur in cylindrical shell as

follows:

i. Longitudinal/axial stress resulting from pressure within the vessel

ii. Circumferential/tangential stress resulting from pressure within the vessel

iii. Residual weld stress resulting from localized heating

iv. Stresses resulting from superimposed loads such as wind, snow & ice, auxiliary equipment & impact loads

v. Stresses resulting from thermal differences

vi. Others, such as may be encountered in practice

Stress element

Radial stress

r

Longitudinal stressl

(closed ends)

Tangential stress θ

Hoop stress

Longitudinal stress, l

Pressure area

Internal pressure, p

Pressure area

Hoop stress

θ

Materials of Construction

i. Carbon & low alloy steel

ii. High alloy steel

iii. Cast iron

iv. Non-ferrous metals: Al, Cu, Ni, Cr & its alloy

v. Non-metals: Carbon & graphite, Glass, Rubber, Plastics etc.

Ferrous metal

Material selection factors

i. Strength: represent the capability of material to withstand

external forces

ii. Corrosion resistance

iii. Fracture toughness: it is the ability of material to absorb

energy in deformation

iv. Fabricability

v. Overall cost

Problem

10000 kg of SO2/CO2/NH3/H2S/Cl2/Propane/Butane are to be

stored at 150 kg/cm2 at 30 deg C in a suitable pressure vessel.

Design and draw the above vessel & submit a neat scale drawing

of the same.

Consider the gas follows real gas mechanism as:

PV=nZRT and L:D = 2:1

SHELL

It is the primary component that contains the pressure.

Pressure vessel shells in the form of different plates are welded together to form a structure that has a common rotational axis.

Shells are either cylindrical, spherical or conical in shape.

SHELL

Horizontal drums have cylindrical shells and are constructed

in a wide range of diameter and length.

The shell sections of a tall tower may be constructed of

different materials, thickness and diameters due to process and

phase change of process fluid.

Shell of a spherical pressure vessel is spherical as well.

Estimation of Thickness of Shell

The internal pressure in the shell gives rise to stress in the shell thickness Circumferential / tangential stress:

Longitudinal/axial stress:

Where ‘p’ internal pressure

D mean diameter = (Di+Do)/2

t = shell thickness

t

pDF

2t

a

pDF

4t

Ft> Fa

Ft is considered as design stress

The shell is generally formed by a joint in the longitudinal

direction which is considered in terms of joint efficiency

Therefore, thickness of the shell is given by:

pDt

2fJ J= joint efficiency

f= maximum allowable stress

D = (Do + Di)/2&Do = Di + 2t

So,

p Do Dit

2fJ 2

ip D t

2fJ

P= design pressure = working press. + 20% of working pressure

ipDt

2fJ p

Now, final t’ = t + C

Where C = corrosion allowance

= 10 % of t or 1 mm (minm.) or 3 mm (maxm.)

With this thickness (t’), one should estimate what should be the resulting

circumferential & longitudinal stress

No corrosion allowance for stainless steel

i. Stress in the circumferential direction due to internal p (tangential or Hoop’s stress)

'i

t '

p D tF

2t

ii. Stresses in the longitudinal or axial direction:

a) Due to internal pressure

b) Due to weight of vessel & content (vertical vessel only)

c) Due to wind or piping

i1 '

pDF

4t

2 ' 'i

WF

t D t

Where, W = wt.

(Compressive)

(tensile)

3 2 'i

M MF

z D t

(tensile or Compressive)

Where, M = Bending moment due to load z = Modulus of section

Total axial stress,

Fa = F1 + F2 + F3

iii. Stress due to offset piping or wind:

s ' 'i i

2TF

t D D t

T = torque about the vessel axis

So, combining the above stress on the basis of shear-strain energy theory,

Equivalent stress:

12 2 2 2

R t t a a sF F F F F 3F

For satisfactory design,R t

a t

F F

F F

HEAD

• All the pressure vessels must be closed at the ends by heads (or another shell section).

• Heads are typically curved rather than flat.• The reason is that curved configurations are

stronger and allow the heads to be thinner, lighter and less expensive than flat heads.

• Heads can also be used inside a vessel and are known as intermediate heads.

• These intermediate heads are separate sections of the pressure vessels to permit different design conditions.

NOZZLE

• A nozzle is a cylindrical component that penetrates into the shell or head of pressure vessel.

• They are used for the following applications.

• Attach piping for flow into or out of the vessel.• Attach instrument connection (level gauges,

Thermowells, pressure gauges).• Provide access to the vessel interior at MANWAY.• Provide for direct attachment of other equipment

items (e.g. heat exchangers).

SUPPORT

• Support is used to bear all the load of pressure vessel, earthquake and wind loads.

• There are different types of supports which are used depending upon the size and orientation of the pressure vessel.

• It is considered to be the non-pressurized part of the vessel.

TYPES OF SUPPORTS

SADDLE SUPPORT:

Horizontal drums are typically supported at two locations by saddle support.

It spreads over a large area of the shell to prevent an excessive local stress in the shell at support point.

One saddle support is anchored whereas the other is free to permit unstrained longitudinal thermal expansion of the drum.

TYPES OF SUPPORTS

LEG SUPPORT:

Small vertical drums are typically supported on legs that are welded to the lower portion of the shell.

The max. ratio of support leg length to drum diameter is typically 2 : 1

Reinforcing pads are welded to the shell first to provide additional local reinforcement and load distribution.

The number of legs depends on the drum size and loads to be carried.

Support legs are also used for Spherical pressurized storage vessels.

Cross bracing between the legs is used to absorb wind or earth quake loads.

TYPES OF SUPPORTS

LUG SUPPORT: Vertical pressure vessels may also

be supported by lugs.

The use of lugs is typically limited to pressure vessels of small and medium diameter (1 to 10 ft)

Also moderate height to diameter ratios in the range of 2:1 to 5:1

The lugs are typically bolted to horizontal structural members in order to provide stability against overturning loads.

TYPES OF SUPPORTS

SKIRT SUPPORT:

Tall vertical cylindrical pressure vessels are typically supported by skirts.

A support skirt is a cylindrical shell section that is welded either to the lower portion of the vessel shell or to the bottom head (for cylindrical vessels).

The skirt is normally long enough to provide enough flexibility so that radial thermal expansion of the shell does not cause high thermal stresses at its junction with the skirt.