Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
(OP)
Hi everyone,
I am a process engineer and I've been trying to find a method for sizing relief devices for two-phase vapor-liquid venting without the need for software/equipment like SuperChems, VSP2, or RSST (I know I would probably need these for reactive systems, but for now I am only considering nonreactive systems). The most promising methods I've seen assume homogenous flow. A problem I keep running into with every method I try is that you need to know the vapor quality for at least one (T,P) point. The Pv equation that DIERs uses to find two-phase density (which is then used to find mass flux) requires performing at least two flash calculations in order to find two parameters, a and b, used in the Pv equation. The omega method is a simplification of this method that assumes a=ω and b=0. ω can be found using a one-, two-, or three-point method. Annex C of API 520 part 1 uses a two-point omega method.
API 520 also recommends a numerical integration method for calculating maximum mass flux. Numerous flash calculations are performed at constant entropy while lowering the pressure from the peak relieving pressure, until either a maximum mass flux is reached or the the backpressure is reached. I have constructed an excel spreadsheet for performing these flash calculations. It uses the Peng-Robinson EOS and Clapeyron equation to find the entropy (relative to a reference state) for the vapor and liquid phases separately. However, I still need to know the vapor quality for one of the (T,P) points in order to find the two-phase entropy value that is held constant throughout all the flash calculations. I thought of using the initial vessel average void fraction at set conditons, α = 1 - [(liquid volume at set conditions)/(total vessel volume)], and from that calculating the vapor mass quality and then the two-phase entropy (since homogenous equilibrium methods assume the vapor/liquid ratio in the vessel is the same as in the relief device). Would this be a valid method?
Basically, I just don't know how to do this without knowing the vapor quality at any (T,P) points. Maybe there's an assumption I can make about the quality or a way around it; another way to tackle this (I've read a 1995 article called “Protection of Storage Tanks from Two-Phase Flow Due to Fire Exposure” by H.G. Fisher and H.S. Forrest that assumes the vapor wt% is less than 2%, but I'm not sure how valid this assumption is). I've hit a snag and was hoping someone here could lend their experience with two-phase sizing. Thank you. Any help is much appreciated.
I am a process engineer and I've been trying to find a method for sizing relief devices for two-phase vapor-liquid venting without the need for software/equipment like SuperChems, VSP2, or RSST (I know I would probably need these for reactive systems, but for now I am only considering nonreactive systems). The most promising methods I've seen assume homogenous flow. A problem I keep running into with every method I try is that you need to know the vapor quality for at least one (T,P) point. The Pv equation that DIERs uses to find two-phase density (which is then used to find mass flux) requires performing at least two flash calculations in order to find two parameters, a and b, used in the Pv equation. The omega method is a simplification of this method that assumes a=ω and b=0. ω can be found using a one-, two-, or three-point method. Annex C of API 520 part 1 uses a two-point omega method.
API 520 also recommends a numerical integration method for calculating maximum mass flux. Numerous flash calculations are performed at constant entropy while lowering the pressure from the peak relieving pressure, until either a maximum mass flux is reached or the the backpressure is reached. I have constructed an excel spreadsheet for performing these flash calculations. It uses the Peng-Robinson EOS and Clapeyron equation to find the entropy (relative to a reference state) for the vapor and liquid phases separately. However, I still need to know the vapor quality for one of the (T,P) points in order to find the two-phase entropy value that is held constant throughout all the flash calculations. I thought of using the initial vessel average void fraction at set conditons, α = 1 - [(liquid volume at set conditions)/(total vessel volume)], and from that calculating the vapor mass quality and then the two-phase entropy (since homogenous equilibrium methods assume the vapor/liquid ratio in the vessel is the same as in the relief device). Would this be a valid method?
Basically, I just don't know how to do this without knowing the vapor quality at any (T,P) points. Maybe there's an assumption I can make about the quality or a way around it; another way to tackle this (I've read a 1995 article called “Protection of Storage Tanks from Two-Phase Flow Due to Fire Exposure” by H.G. Fisher and H.S. Forrest that assumes the vapor wt% is less than 2%, but I'm not sure how valid this assumption is). I've hit a snag and was hoping someone here could lend their experience with two-phase sizing. Thank you. Any help is much appreciated.





RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
there have been many discussions about this topic and you may find an Excel page for solving the HEM model in this thread
http://eng-tips.com/viewthread.cfm?qid=329093
out of curiosity, the iterative procedure written in VBA code runs slow and is less accurate than std. ISPF() method available in Prode Properties (see prode.com for additional info) as you know VBA code is not the best option if you need speed, but is ok to create a model and understanding how it works.
also the ISPF() method in Prode which you can access from VBA and Excel (as a macro) includes several models (four or more)
1) HEM Homogeneous Equilibrium (Solution of Mass Flux integral)
2) HNE Homogeneous Non-equilibrium (HEM with Boling Delay and Gas-Liquid Slip Contributes)
3) HNE-DS , Homogeneous Non-equilibrium
4) NHNE Non-homogeneous Non-equilibrium
every model offers some advantages in specific areas of application and you should investigate which is the more suitable in your case, there is also a page which allows to compare the results from different models
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
Good luck,
Latexman
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
Good luck,
Latexman
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
Good luck,
Latexman
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
http://eng-tips.com/viewthread.cfm?qid=329093
requires that you solve a flash at constant entropy which is the basis of the model,
for the constant entropy flash the procedure adopts a method available in Prode Properties library,
for a single component you can easily write your own code for solving a constant entropy flash,
for mixtures things may become very difficult and that is the main reason why I prefer a process library.
So, the first thing to do is to code the flash or use the method in the Prode library (the Excel page provided as example does that).
You may calculate latent heat (for a pure component) starting from saturation pressure but the ususal approach with simulators is to calculate the departure with a EOS (in your case Peng Robinson).
You can apply HEM (homogeneous equilibrium model) in many areas, for example there are methods in Prode Properties for calculating the two phase (vapor+liquid) speed of sound with HEM assumption.
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
@ PaoloPemi, I'll have to read over the user's manual before I can begin to understand how Prode Properties works, let alone how to use it. I've installed it and played around with it a bit but I really have no clue how to use it at the moment. I've already got methods for calculating the properties for each phase. My problem now is finding at least one vapor fraction for one of my flash calculations so that I can find the two-phase entropy.
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known
hin+1/2*vin^2 = ho+1/2*vo^2
(in = inlet, o = orifice)
where vo (for a critical flow) is the speed of sound
ho, vo calculated at vena contracta conditions
the vena contracta conditions are determined with a series of constant entropy flash operations,
to solve the constant entropy flash (i.e. given initial t,p and final p (or p) the procedure finds tout (or pout) which gives Sout=Sin), as said in previous post I prefer to use a commercial product (Prode),
you can use another product or write your code.
important advantages of this approach
-rigourous (not simplified as models based on two points evaluations)
-can be applied to all conditions including critical (gas,gas+liquid)
what surprises me is your reluctance to consider a well tested approach
when attempting (with the reported difficulties) to create your own,
the model is documented (VBA code available) and can be utilized for testing purposes,
anyway, good luck
RE: Sizing for Two-Phase Liquid/Vapor Relief - What to do if vapor quality is not known