Mechanical Design

1
Mechanical Design

• Of
• Process Equipment

2
Objectives

• Select suitable material of construction
• Specify design temperature and pressure
• Calculate wall thickness

3
Material of Construction

• Mechanical and physical properties
• Corrosion resistance
• Ease of fabrication
• Availability in standard sizes
• Cost

4
Material of Construction (Contd)

• Preliminary Selection
• Selection Charts
• Literature
• Previous experience
• Advise from materials supplier
• Advise from equipment manufacturer
• Advise from consultants

5
Material of Construction (Contd)

• Final Selection
• Based on economic analysis which would include
• Material cost
• Maintenance cost

6
Commonly Used Materials of Construction

• Metals
• Polymers or Plastics
• Ceramic Materials

7
Metals

• Carbon steels
• Stainless steels
• Specialty alloys

8
Carbon Steels

• Most common engineering material
• Advantages
• Inexpensive
• Good tensile strength and ductility
• Available in a wide range of standard
  forms and sizes
• Easily worked and welded

9
Carbon Steels (Contd)

• Limitations
• Corrosion resistance not good
• External surface need painting to prevent
  atmospheric corrosion
• Suitable for use with
• Most organic solvents
• Steam, air, cooling water, boiler feed water
• Concentrated sulfuric acid and caustic alkalies

10
Stainless Steels

• Most frequently used corrosion resistant
  materials in the chemical industry
• High chromium or high nickel-chromium alloys of
  iron
• chromium content must be gt 12
• Nickel added to improve weldability and corrosion
  resistance in non-oxidizing env.

11
Stainless Steels(Contd)

• Main Types of Stainless Steel
• Type 304 18 Cr 8 Ni
• Type 304L low carbon version to improve welding
  of thick plates
• Type 316 Mo added to improve corrosion
  resistance in reducing conditions and at high
  temperature.

12
Stainless Steels(Contd)

• Limitations
• Intergranular corrosion or weld decay possible in
  reducing environment
• Stress cracking can be caused by a few ppm of
  chloride ions

13
Specialty Alloys

• Monel 67 Ni, 33 Cu
• Better corrosion resistance than SS
• No stress-corrosion cracking in chloride
  solutions
• Temp. up to 500oC
• Inconel - 76 Ni, 15 Cr, 7 Fe
• High temperature acidic service
• Temp. up to 900oC

14
Plastics

• Provide corrosion resistance at low cost.
• Main advantages
• Excellent resistance to weak mineral acids
• Tolerate small changes in pH, minor impurities or
  oxygen content
• Light weight, easy to fabricate and install

15
Plastics (Contd)

• Major Limitations
• Moderate tempeature and pressure applications (T
  lt 100oC P lt 5 atm.)
• Low mechanical strength
• Only fair resistance to solvents

16
Plastics (Contd)

• Main Classes
• Thermoplastic can be reshaped
• 2. Thermosetting cannot be remoulded
• Thermoplastic
• Polyethylenes (low cost T lt 50oC)
• Polypropylene ( T up to 120oC)
• Polyvinyl chloride ( T ? 60oC)

17
Plastics (Contd)

• Thermosetting
• - good mechanical properties (T ? 95oC)
• - good chemical resistance (except strong
  alkalies)
• Examples
• Phenolic resins filled with carbon, graphite,
  silica
• Polyester resins reinforced with glass or
  carbon fibre to improve strength

18
Plastics (Contd)

• Polytetrafloroethylene
• (PTFE)
• Known under the trade names of Teflon and Fluon
• Can be used up to 250oC highest for all
  plastics
• Resistant to all chemicals except fluorine and
  molten alkalies

19
Rubber Lining

• Metal surface lined with rubber to provide
• Cost effective solution for corrosion control and
  abrasion resistance e.g. acid storage, steel
  pickling
• Why rubber?
• Able to bond strongly to various surfaces
• Good combination of elasticity and tensile
  strength

20
Ceramic Materials

• Provide high temperature corrosion resistance
  and/or thermal protection (up to 2000oC)
• Ceramic or refractory materials metal oxides,
  carbides and nitrides
• Used as either solid bodies or coatings
• Glass mostly used in glass lining

21
Pressure Vessel

• What is Pressure Vessel?
• Any vessel which contains fluid above 15 psi (or
  103 kPa)
• Examples reactors, distillation towers,
  separators
• ASME Boiler and Pressure Vessel Code contain
  rules for design, fabrication and inspection

22
Wall Thickness

• For cylindrical shells  
• PxRi
• t _________ C
• SxE - 0.6P
•    t minimum wall thickness (in)
• E efficiency of joints expressed as a fraction
• P maximum allowable internal pressure (psig)
• Ri inside radius of the shell, before corrosion
  allowance (in)
• S maximum allowable working stress (psi)
• C allowance for corrosion (in)

23
Maximum Allowable Internal Pressure

• Maximum pressure it is likely to be subjected in
  operation
• Normally taken as relief valve set pressure 10
  above the normal working pressure
• Add hydraulic head in the base of the vessel to
  the operating pressure
• For bioreactor, consider steam pressure for
  sterilization

24
Design Temperature

• Max. operating temperature 50oC
• Max. allowable working stress (S) function of
  temperature
• for carbon steel 13,700 psi (Tlt350oC)
• Joint efficiency (E) defines quality of weld
  joint
• Range 0.85 to 1
• Common value 0.85

25
Corrosion Allowance

• Additional thickness added to allow for material
  lost by corrosion and erosion
• Usually based on experience
• For carbon and low-alloy steel use a minimum of
  2.0 mm
• For more severe conditions increase to 4.0 mm.
• No allowance for SS and other high-alloy steels