How Relief Valve Sizing is done in Visual Flow

Relief Valve Sizing in Visual Flow.

This is one exercise which i have done in Visual Flow and to compare the results with under development software called SimCentral. It gave me great exposure to API520 standards and API 3 % Rule.So i thought to share my experience here.

Background of what Visual Flow is:

• Relief valve sizing,Network analysis,depressurization etc can be done in Visual Flow. This is a product from Invensys/Schnider.
• Visual Flow is a software which will give an error message to user if the valves doesn't meet API 3 % rule during rating.
• Visual Flow is a software which have a great interactive ISOMETRIC drawing layout. Means we can exactly draw (changes in directions of  piping's and fittings) from ISOMETRIC drawings.
• I am not that great in understanding  DIERS and OSHA compliance in software. But one thing i know is OSHA(Occupation Safety And Health Administration) will penalize companies/industries where relief valves doesn't meet the 3% rule during their inspection. This is the point which created interest in me.

Exercise:

• I need to size a relief valve for 3 vessels which are maintaining at different pressures(LOW,MID,HIGH) for certain gas mixture.
• Flow sheeting, Components, K Value method , Beggs and Brill for Pressure drop calculations method are selected same for both software's.

Differences in software's at that stage:

• Inlet and exit losses of vessels are also selected k=0.5 & 1 (standard if i remember). and it is not present in Simcentral
• ISOMETRICS interactive interface is present is Visual flow but not in Simcentral.
• Standard Relief valve selection is present in Visual Flow based on Codes like 6Q8, Q stands for orifice area. But orifice area is input in SimCentral and no standard charts available. 6 is suction line size and 8 is the discharge line size. Check appendix for more information.
• Sizes selected are different for all three vessels because sink/flare pressure is same but not the sources pressure.So when Delta P is large you need a big size valve to blow down that volume.
• Visual Flow is having Sim4Me Thermo but Simcentral doesn't have means both have their local thermo systems so used local thermo for both software's.

The problems that i faced during this exercise is.

• At the time of doing this exercises i don't have a proper ISOMETRIC drawings. So assumptions and API 3 % rule made me realize that why suction lines should be smaller and discharge lines in general larger. 
• Point 1) We cannot have so many fittings in suction line.
• Point 2) There has to be proper ratio maintained in Suction and Discharge lines.
• Finally converged the flow-sheet.

Apart from this:

• All i need to match is density,Molecular weight as a main property because Enthalpy datum will differ from software to software.
• And most importantly blow volume. Slight differences observed in velocities hence slight differences observed in pressure drop in suction line and discharge because of unavailability of fittings with directions in Simcentral.
• Problems faced at high pressure system so modified the Beggs and brill to modified Beggs and Brill method.

Who size relief valves:

• There are EPC companies which maintains teams for sizing of relief valves in separate department with much more standards,Codes and using different software.

OTS Point of view :

• From OTS point of view relief valve datasheet is used for modeling. mostly blow start pressure,density.
• And testing will be done whether it is popping up or not when blow start pressure reaches and also testing will be done when blow is stopping or not when it reaches blow stop pressure(85% of blow start pressure based on standards)

Appendix: