So normal venting out is the same for atmospheric tanks or pressure vessels.
API2000 applies ONLY to "atmospheric" storage tanks; it does not apply to pressure vessels which are covered in ASME Section VIII. That said, it may be possible to glean information about the rate at which gases must be vented by studying API2000 or other codes, even though they are not specifically meant for your application. Doing so is 100% at your risk, since you would not be using the codes as intended. My answers to your questions are as follows:
You and you alone must determine the composition of vent gases. It sounds like they could be the product of some reaction, and no code will have definitive procedures for those calculations. If this is true, then you and you alone must also determine how quickly gases are evolved from your system. (If it's simply a problem of displacing vapors at the top of the vessel, then Ankur has provided you with the path forward.) The effect of generated gases on the vessel's internal pressure is obtained from thermodynamic/physical chemistry principles. As a simplification for a first estimate, you can use the ideal gas law. You know the initial volume, gas MW, temperature, and pressure. Now determine what can cause the pressure to build to the PSV set pressure. You must have good knowledge of the system to do this, and we certainly are not in a good position to assist. Now calculate the situation right before a relief event. Get the vapor space temperature and gas MW. You should know the volume and the pressure is just the PSV setpoint, plus whatever overpressure you allow (say 10%). As additional gas is generated or evolved, it must be relieved. Your understanding of the system should allow for the relatively easy calculation of both the required relief rate and the properties (including composition) of the gas to be relieved. If you still do not see a soluition, you will need to supply much more detail regarding the problem to get meaningful help.
Edited by djack77494, 31 August 2010 - 02:05 PM.