I need some opinion on the blowdown system for liquid petroleum gas storage. Some designer do not used any BD mechanisme except the PSV. However if I used PCV as BDV for blowdown any restriction from API 521?. or other codes?.

The PSV is not my choice for the blowdown valve.  In low pressure classes such as 150 or 300 consider a shutdown valve with an orifice plate to control the release rate.  In class 600 or higher consider a shutdown valve and choke to take the drop.  I normally use an API 6D ball valve for the shutdown valve.

I think that API requirements for the blowdown duration exist in the API 14 series offshore standards.  However I did not find it in a quick word search.  Perhaps they use depressure or something else in the standards.

Modulating pilot operated PSVs do not go fully open when the set pressure is reached.  Rather, the pilot adjusts the pressure on top of the dome to maintain, essentially, the inlet pressure at set pressure, opening and closing the disk as needed.  In effect, they act akin to a control valve.

Anderson Greenwood has a feature that will allow you to dump the pressure on top of the dome remotely causing the PSV to go fully open which would allow the PSV to function effectively as a blowdown valve (they call it a 'remote unloader').  However, blow down valves are usually quite a bit smaller than PSVs in my experience as they aren't typically sized for the same flow rates.  A PSV could be sized for a large vapor flow rate as a result of a fire or gas blowby while a depressurizing valve is typically sized for the gas inventory in the unit that needs to be vented as a result of an ESD (emergency shut-down).  Common requirements that I've seen for a blowdown valve is to depressure a unit to 100 psig (or 1/2 of design pressure, whichever is lowest) in 15 minutes.  

Remember if you use the PSV as a depessurizing valve to consider low temperatures in the system if you have any liquids that will autorefrigerate.  That comment also applies to a depressuring valve.

I don't see how you'd use a PCV as a blowdown valve. Acting as a backpressure device, the PCV would be designed to maintain pressure in the system. It would maintain the normal pressure and any higher pressure that were still below the point where you'd want to move into isolate and blowdown mode. If this is true, the setpoint would be up high - at the point where you're running into trouble. When the PCV starts to open and bleed off pressure, it will soon close. It will not lead to a low pressure condition, which is what I assume you want to achieve. Perhaps you'd better tell us by what you mean by blowdown.

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