Dear All,

Your advice is requested regarding my following query:

Please refer the standard API 520 Part I – Sizing and Selection (7th edition, Jan. 2000) item 3.6.2 (Sizing for Critical Flow) – General

Query # 1: In this article there are three equations mentioned as 3.2, 3.3 & 3.4, can we use any of these three equations for effective discharge area for critical flow? if not then what is the criteria of selection among three?

Query # 2: In equation above referred equations “W” is referred as required flow through the device in kg/hr. and “V” is referred as required flow through device. Value of “W” is taken as 24,260 kg / hr. in the example 3.6.2.2 and it is assumed that it is in the given data.

My question: How one can know the required flow rate through the device, which will enable one to put in the equations referred above. I also tried to get my answer from the standard but not succeeded.

Looking forward for your expert opinion regarding above.

Regards

Karamat

With regards to your Q-1, the three equations are variations on a theme. Given that the mass is a function of the volume and temperature and pressure for a gas, one equation is using W for the mass flow rate and the others are using V for the volume flow rate.

With regards to Q-2, you need to study the rest of the API documents, e.g., API 520 parts I and II and API 521 to know the whole story. Together they will give you a good idea of the different scenarios and calculation methods. A classic case is going to be fire relief. You need to know the heat flux and the properties of the material to estimate/calculate/determine the mass or volume you need to relieve. Then you can use that number to calculate the relief device size.

you have a lot of reading and studying to do!