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 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

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

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 nickel-chromium 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 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.

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 500oC Inconel - 76% Ni, 15% Cr, 7% Fe High temperature acidic service Temp. up to 900oC

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

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

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)

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

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

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

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

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

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)

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 + 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

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