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 (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. 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. 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