Nagara, may I suggest that you don't spend your Friday afternoon on a sophisticated calc, but just send a guy outside with hearing protection and a dB meter?
Sorry if this does not sound like an engineer writing, but engineering time is expensive, managers don't always realise.

I am evaluating the convenience of opening the vent. The engineering department will not do the job, unless I guarantee that 85 dBA limit will not be surpassed.

Go to

http://www.vent-axia.com/sharing/noise.asp

Maybe you found there some thing useful, but before venting your system put ears protections.

Cheers

Luis

Nagara:
I am reading this after a long time. I will give you my suggestions.
A vent valve need not have a 85dB(A) limit. The OSHA requirement, specifies 85dB(A) for a 8 hr exposure. Vent valves are always located several meters away from the vicinity of people working in a plant. Also they are intermittent operation valves, open for a few minutes until the process returns back to normal mode. Calculating noise from a control valve is a complex calculation. Also it depends on what standards you want to follow, IEC, ISA, Manufacturer std etc...
The valve vendor from whom you will purchase the valve must be able to give you a approximate value. The only thing you must bear in mind is to have a good support for the valve to minimise damage due to vibrations.
Inspite of all these if you still insist on 85dB(A) a vent silencer could be a good option. The valve vendor may be able to quote the valve and silencer.

Thanks consideritsolved!!. Your reply is very useful.

Based on the above post you might want to look a Maxim Silencer to be on the conservative side. We use quite a number of these at our site to silence a steam vapor vent from our  process if the heat recovery system if off line.

http://www.maximsilencers.com/products/vent.html