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 (Cont’d)
Preliminary Selection
Selection Charts
Literature
Previous experience
Advise from materials supplier
Advise from equipment manufacturer
Advise from consultants

5. Material of Construction (Cont’d)
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. Most common engineering material
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 (Cont’d)
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 > 12%
Nickel added to improve weldability and corrosion resistance in non-oxidizing env.

11. Stainless Steels (Cont’d)
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 (Cont’d)
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 (Cont’d) Major Limitations:
Moderate tempeature and pressure applications (T < 100oC; P < 5 atm.)
Low mechanical strength
Only fair resistance to solvents

16. Plastics (Cont’d) Main Classes: Thermoplastic – can be reshaped
2. Thermosetting – cannot be remoulded
Thermoplastic
Polyethylenes (low cost; T < 50oC)
Polypropylene ( T up to 120oC)
Polyvinyl chloride ( T  60oC)

17. Plastics (Cont’d) 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. Polytetrafloroethylene
Plastics (Cont’d)
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 (T<350oC)
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