Mechanical Design Of Process Equipment Objectives Select suitable material of construction Specify...
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Transcript of Mechanical Design Of Process Equipment Objectives Select suitable material of construction Specify...
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
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 corrosionSuitable 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 contentLight 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:
1. 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
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 = _________ + CSxE - 0.6P
t minimum wall thickness (in)E efficiency of joints expressed as a fractionP 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 temperaturefor 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