Anup,
Two totally different things (API STD 2000 & API 42)
Vent sizing to be done according to API STD 2000 to prevent tank damage (bulging by overpressure and buckling by vacuum). Venting connected system design to ensure that back-pressure in the tank does not exceed design pressure and / or vacuum does not cause buckling of the tank. Also API 2000 is applicable only to atmospheric and low-pressure storage tanks with design pressure from Full Vacuum up to 103.4 kPag (15 psig).
API 42 is specifically used for emission losses from tanks without binding of design pressure. These losses include standing losses, losses due to liquid fill. It does not account for any losses due to flashing of dissolved gases when liquid is filled and pressure drops. API 42 is also applicable to floating roof tanks (IFRT and EFRT) for emissions whereas API STD 2000 specifically mentions that it does not apply for External Floating Roof Tanks (EFRT).
If tanks (atm and low pressure) have blanketing, it is clear that blanketing rate calculations are as per API STD 2000 and that needs to be accounted for the design of the vapor combustion system.
If tanks are not blanketed and only emission losses are to be considered due to thermal effects and filling operations, the vapor combustion system can be designed for the calculated emission rates and using a margin on the calculated emission rates. However, care needs to be taken that the sum of the pressure drop from the vent nozzle to the vapor combustion system piping, the vapor combustion system and the exhaust from the vapor combustion system does not exceed the design pressure of the tank. To ensure this, vent nozzle size and connected piping size would require sizing according to API STD 2000.
Also refer the attachment.
Hope this helps.
Regards,
Ankur