Constant in minimum tube thickness formula for boiler tubes

[coolniks37]

In Indian Boiler Reg 338, for minimum thickness of tube a formula is given. Then a constant C is added.
C=0.75 (if pressure <70kg) and C=0 if pressure >70kg
Why is this required and what happens at 70kg pressure ?

Also in ASME formula,PG27, t=formula + .005D
What is the implication of 0.005D? Why is it required?

 

[coolniks37]

In the ASME formula, 0.005D is not corrosion allowance.

 

[metengr]

The 0.005D is an additive term by Section I for conservatism.