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