Breather pipe - Tank design

Well, My boss told me that the design is governed by API 2000. I tried to read it, but it's no fun. So I downloaded a spreadsheet, which does all the calculations. It calculates the breather valve diameter, based on the limiting case, I think fire case is the limiting case. API 2000 has some correlations for different cases. I found that the current diameter for the breather pipe is sufficient.

Now my company specs says all tanks must have a breather pipe of good neck type or mushroom cap type. Since we are specifying something difference (a straight pipe without mushroom cap), we need to issue a technical deviation (TDS).

A good resource

http://www.cheresources.com/invision/topic/7327-tank-vent-sizing/

Spreadsheet calculations (credit to original author) here

Equations for normal venting, which should help:

Inbreathing:

If tank capacity <= 3500 m³

Total Inbreathing = Pumpout + 0.178*tank capacity (m³)

If tank capacity >3500 m³

Total Inbreathing = Pumpout + 3.2*(tank capacity (m³))^0.651

Outbreathing:

Liquids with flashpoint >37.8 deg C or NBP above 149 deg C

If tank capacity <= 3500 m³

Total Outbreathing = 1.069*Pump-in + 0.107*Tank Capacity(m³)

If tank capacity > 3500 m³

Total Outbreathing = 1.069*Pump-in + 1.92*(Tank Capacity(m³))^0.651

Liquids with flashpoint <37.8>

If tank capacity <= 3500 m³

Total Outbreathing = 2.138*Pump-in + 0.178*Tank Capacity(m³)

If tank capacity > 3500 m³

Total Outbreathing = 2.138*Pump-in + 3.2*(Tank Capacity(m³))^0.651

Note: The Pumpout and Pump-in are given in m³/h.

Vent to be sized for the maximum of the inbreathing or outbreathing rate.

(All credits to ankur2061)