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Air System Blowdown Analysis Struggles

Air System Blowdown Analysis Struggles

Air System Blowdown Analysis Struggles

tl;dr: Having trouble estimating final properties for the 1 minute depressurization of an air system. I know the potential max's/min's based on whether it is an isothermal/isentropic system but am struggling to estimate where it will actually fall within those ranges. Any suggestions for resources, tools, and help would be much appreciated!


I am working on an air delivery system currently, and I am having some trouble coming up with some approximate final conditions of the air tanks/system after it depressurizes. I am working with a 2200in3 tank system that has an initial pressurization of 4000psia, and the tanks will depressurize over the course of 1 minute.

Through some initial calculations, I found a range for the final temperature and pressure of the system using rapid isentropic and slow isothermal processes as min. and max. boundaries, but I'm struggling to deduce where within these ranges the system might actually fall in a more realistic scenario. A lot of the resources I find only give vague descriptions and do not provide a frame of reference for how rapid or slow these processes are. Are there any tools/resources I could use to calculate/estimate some results for my system?

After some researching, I saw a lot of comments about using Aspen HYSYS for blowdown analyses such as these; however, my 1-day free trial kept crashing and wouldn't work long enough to make a quick mockup of the system. Are there any tools like this that anyone might be able to recommend or use to find some quick figures for me?

I appreciate any ideas/suggestions you might be able to offer, and thanks for taking the time to help!


RE: Air System Blowdown Analysis Struggles

A blowdown analysis is certainly not very easy to do. There are other packages other than HYSYS that will do it for you though - I'm pretty sure that Schlumberger Symmetry is able to simulate it.

I reckon that in your case you will be closer to isentropic than isothermal.

Rio de Janeiro - Brazil

RE: Air System Blowdown Analysis Struggles

Do you have any feel for how fast it will actually depressurize?
I am thinking that the first 75% of the pressure drop will be isentropic, with the rest somewhere in between.
You are going to venting some cold air.
Keeping valves and lines from icing could be an issue depending on how dry the air is.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Air System Blowdown Analysis Struggles

Well this is a transient thing so you need a good transient program. Or break it down into say 60 steps in an excel sheet or similar.

But that's some pressure to lose in 60 seconds so first I think you need to define where your end point is as the last few 10s of psi might take a loooong time. Don't know what your system is for depressurisation but it looks like you're into choked flow somewhere.

One minute gives very little time for any energy transfer to or from the outside into your tank.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air System Blowdown Analysis Struggles

What final props are you looking for ? Final air temp in the tank or final metal wall temp, or both?
Given P1=4000psi, you may be better off using a simulator. Given this is over 1min only, heat ingress into the tank from ambient may be neglected. But you still have the thermal content of the tank metal wall.
If you want metal wall temp, Hysis or Pro II from Simsci would given you some idea for fully isentropic blowdown, but it may not be accurate, since these simulators lump all the metal mass as a single mass. In reality, metal wall temp is lowest at the blowdown inlet nozzle. If you need individual reports on the metal wall temp for vessel and feed nozzle, you'll have to dig deeper into your pocket for specialist blowdown simulation software.
Suspect you are after values for min metal wall temp. Given the thick walls on this vessel, you may be okay with regular silicon killed CS, which should be good down to -20degC. But guesses wont pass muster at a technical audit. You may need to decide, at some time or other, if you want to restrict this only to hot depressure, or the min metal temp/lower design temp of -20degC is good even for cold depressure when contents have reached min normal ambient temp.

RE: Air System Blowdown Analysis Struggles

There are not data about the design of the blowdown line to depressurize the tank. If it is a short pipe with a valve that has a inner pass section little than the pipe, consider only the valve where the flow of air chokes. To calculate the air flow rate through the valve, use the equations and expansion factor Y of ISA 75.01.
To calculate the final conditions in the tank (P2 and T2) from the known initial conditions (P1 and T1), I suggest apply the following process:
1. Calculate with P1 and choking conditions in the valve, the air flow rate W1
2. After t = 1 minute, assume a pressure P2 in the tank that will be < P1. In the tank, the expansion of the air from P1 to P2 will be isentropic because there is no friction. The isentropic expansion coefficient of the air is 1.4 and now calculate the specific volume of the air ve2 with the equation ve2 = ve1(P1/P2)**1/1.4. 3. Calculate as in the step before, now with P2 and ve2, the air flow rate W2 that will be < W1.
The average flow rate of discharged air will be (W1 + W2)/2 = W and the mass of air discharged will be M = W/60 with W in lb/h.
4. With V = 2200 in3 = 1.27 ft3 and ve1, obtain the initial mass of air in the tank, M1 = V/ve1
5. Finally, the mass of air in the tank after 1 minute will be M1 - M and the specific volume ve2 = V/(M1 - M)
If this value of ve2 is the same as the value calculated in the step 2, the assumed pressure P2 is correct. If not, repeat the process with other value of P2.
The final temperature of the air T2, will be T2 = T1(P2/P1)**(1.4-1)/1.4

RE: Air System Blowdown Analysis Struggles

Hi everyone!

I am so sorry for the delayed response!! I want to thank each of you for getting back to me with your ideas; everything you all had to say is super helpful as I carry out this analysis, and I can't thank you enough for taking the time to read and respond to my post. I will reply to each of you below.

Thank you for the insight! I agree that it will probably lean more isentropic than isothermal; I will try and keep that in mind as I continue my analysis. And thanks for the suggestion on Schlumberger Symmetry; I’ll look into that and see if I can use it!

It won’t be a consistent depressurization as various legs of the system will become active/inactive throughout the operation window, but it will for sure be taking place over the course of 60 seconds.
I’m trying to currently find which system will be the limiting factor that will determine how low the tanks will depressurize to before being unable to drive any of the systems further, but until then, I’ve just been assuming it’ll depressurize down to somewhere between 1000-1500 psia. Your comment has definitely brought the temperature issue to the forefront of my mind though. It will get pretty cold like you said, so I’ve been trying to select appropriate valves and tube lines with that in mind so that I don’t cause a failure anywhere at any point. It’s hard to gauge, though, until I get some better rough approximations/determinations.

Breaking it down on excel is a good idea if I can’t get ahold of a transient program; that’s a good idea. Just did something like that for a related system, so perhaps I can figure out a way to do it in this case. Thanks!
Yeah I am currently working with a coworker on determining what some of the limiting systems would be for the overall air system in order to determine what would be the ending pressure. I am anticipating that it’ll fall somewhere between 1500-1000 psia, but I would like to get a more exact number. And yes, there will definitely be some choked flow at various points in the system.
I appreciate your insight into the system having little time for heat transfer to take place. I was wondering if this would be the case, so this helps further my understanding/intuition on the system. Thank you!

I am primarily concerned with the final air temperature, as that will affect a lot of estimations/restrictions for the system as a whole, but now that you mention it, I think knowing the final metal wall temp will be useful as well. I appreciate your recommendation as to various simulators and materials to look into as well! I’ll make sure to weigh these options in with the others I have while I further design and analyze this system and come up with more hard numbers for things. Thank you!

Thank you so much for the link! I’m gonna try and see if I can use this with my system; I really appreciate the help!

Thank you for walking me through this calculation process!! That is super helpful, and I’ll be sure to run through this exercise and see what my results are now that I have some more time and can return to this part of my objectives. Thank you so much!

Thank you all again for all of your help!

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