1. Ron Darby, "Size Safety-Releif Valves for Any Conditions", Chemical Engineering Magazine, September 2005
2. Ron Darby, F.E. Self, V.H. Edwards, "Properly Size Pressure-Relief Valves for Two-Phase Flow", Chemical Engineering Magazine, June 2002
3. J.C. Leung, "Easily Size Relief Devices and Piping for Two-Phase Flow", Chemical Engineering Progress, December 1996
The method given by Darby et. al. has been adapted from work done by DIERS. The method by Leung (the OMEGA method) is another way to obtain the same thing. Darby discusses this method as well as some others in the articles I reference. Currently, the Omega method is what is given in API RP520. Darby solves the nozzle equation for mass flow rate via direct numerical integration. These methods are accepted as good engineering practice by most standardards.
Leung's method IS based on the HEM model. All Leung has done was simplify the calcualtions by creating a pseudo-linear equation of state that can easily be integrated for both choked and non-choked flow.
The HEM model assumes equal velocities between the vapor and the liquid (no slip) and thermal equilibrium between the two phases, i.e. they are at the same temperature. The HEM model is found to be conservative in that it slightly underpredicts the mass flux (lb/hr/in2) through the valve and is the current preferred model to use. Read reference #3 I give in my post above.
Other models introduce a slip ratio into the equation for volume fraction of the gas phase and some introduce a non-equilibrium "quality value" into the two-phase density calculation. The final calculation method to determine mass flux through the relief valve is performed the same way with just these modifications added. Read references #1 and 2 I give in my post above.
The old school method is to solve for the liquid and gas separately, add them togehter, and add a bit of safety factor.
In most of my applications, the tricky part isn't the calculations, it's the $ of gas/liq.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
The Leung method IS the Omega method as I state in my first post. Leung basically proposes a single point (pressure vs volume) to determine Omega. API RP520 uses a two point basis to calculate Omega, one at the stagnation conditions and one at 0.9 stagnation.
However, I hear that the next revision of API 520 (whenever that will be) will be emphasizing the use of the Direct Integration Method to determine the mass flux through the relief valve. This is also the method most preferred by the DIERS Users Group. The Omega method will still be very useful for single component systems away from the critical point and for use in pipe sizing/pressure drop determination.
Have any of you had experience with "The AIChE / ASME / DUG course “Emergency Relief System Design Using DIERS Technology”", and if so is it worth the money? I have an opportunity to go, but don't want to waste my employer's money. What do you think?
I took it and thought it was great. However, for it to be meaningful in anyway, you should be involved with doing PSV analysis and sizing at your site AND at least some of your relieving scenarios will be two-phase.
The course emphasizes the latest information on the DIERS methodology for sizng relief systems for two-phase flow, this includes the PSV itself, piping and downstream equipment.
