Mechanical Design Of Process Equipment Objectives Select suitable

Mechanical Design Of Process Equipment

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

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

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

Material of Construction (Cont’d) Final Selection • Based on economic analysis which would include – Material cost – Maintenance cost

Commonly Used Materials of Construction • Metals • Polymers or Plastics • Ceramic Materials

Metals • Carbon steels • Stainless steels • Specialty alloys

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

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

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

Stainless Steels (Cont’d) Main Types of Stainless Steel § Type 304 – 18% Cr & 8% Ni § Type 304 L – low carbon version to improve welding of thick plates § Type 316 – Mo added to improve corrosion resistance in reducing conditions and at high temperature.

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

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

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

Plastics (Cont’d) Major Limitations: • Moderate tempeature and pressure applications (T < 100 o. C; P < 5 atm. ) • Low mechanical strength • Only fair resistance to solvents

Plastics (Cont’d) Main Classes: 1. Thermoplastic – can be reshaped 2. Thermosetting – cannot be remoulded Thermoplastic • Polyethylenes (low cost; T < 50 o. C) • Polypropylene ( T up to 120 o. C) • Polyvinyl chloride ( T 60 o. C)

Plastics (Cont’d) Thermosetting - good mechanical properties (T 95 o. C) - 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

Plastics (Cont’d) Polytetrafloroethylene (PTFE) Ø Known under the trade names of Teflon and Fluon ØCan be used up to 250 o. C – highest for all plastics ØResistant to all chemicals except fluorine and molten alkalies

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

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

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

Wall Thickness For cylindrical shells Px. Ri t = _____ + C Sx. E - 0. 6 P 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)

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

Design Temperature • Max. operating temperature + 50 o. C • Max. allowable working stress (S) – function of temperature for carbon steel = 13, 700 psi (T<350 o. C) • Joint efficiency (E)– defines quality of weld joint –Range 0. 85 to 1 –Common value = 0. 85

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