Saturday, March 27, 2010
Earlier post "API Std 520 Part 1 Dec 2008 is Released" highlighted one of the main changes was to use of isentropic nozzle flow method and/or ideal gas specific heat ratio for vapor/gas relief flow calculation.
The detailed method involve the fluid expanded isentropically across multiple expander with fix differential pressure (dP). Entire model is simulated using HYSYS with Isentropic expansion using Expanders. Stream 1 is the feed to PRV nozzle. Stream 1 is expanded isentropically (polytropic efficiency =100%) to stream 2 with fix differential pressure. Stream 2 is further expanded isentropically to stream 3, Stream 4 is further expanded isentropically to stream 4,.............
Mass density is extracted from each stream in order to use for integration of specific volume (v) time differential pressure (dP). Following is the example of specific volume (Inverse of Mass density).
Simple MethodA simple method is proposed hereafter. It basically utilize the capability of Property Table.
Step 1 : Enter stream 1 and open the Property Table Utility in Attachment tab.
Step 2 :From the Connections tab, select Variable 1 as Pressure with Incremental pressure from Outlet (e.g. 1 barg) to Inlet pressure (e.g. 12 barg).
Step 3 :From the Connections tab, select Variable 2 as Entropy with State mode. Enter the Entropy of Stream 1 here.
Step 4 : In Dep. Prop. tab, Add Mass Density. Now the system is ready for calculation. Click the Calculate button.
Now Mass density change according to pressure (with isentropic expansion) is ready.
Above method can be easily adopted for subcooled flashing, two phase flashing flow across the PRV nozzle.
Do you have better idea ?
- API Std 520 Part 1 Dec 2008 is Released
- API Std 521 ADDENDUM, MAY 2008 - Check Out Revised Section
- Workbook for Chemical Reactor Relief System Sizing
- A must have book...Emergency Relief System Design Using DIERS Technology
- Requirement of overpressure protection devices on system design to PIPING code
- Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve