Process Equipment and Plant Design
Corrosion allowance in the design of pressure vessel/chemical equipment is not necessary, if

Both B & C
Wall thickness is > 30 mm
Plain carbon steel and cast iron parts are used
Material of construction is high alloy steel

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Process Equipment and Plant Design
Triangular pitch tube layout as compared to square pitch in a shell and tube heat exchanger

Facilitates comparatively easier external cleaning because of large clearance
Permits the use of more tubes in a given shell diameter
Both B and C
Permits the use of less tubes in a given shell diameter

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