×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Choked Tank Depressurisation Calculation

## Choked Tank Depressurisation Calculation

(OP)
Hi Guys,

I have been trying to write a model to calculate the depressurisation of hydrogen out of a tank through a vent line. I've used the fanno flow calculations as the tanks are at about 350bar so the flow will be choked. The results are in the expected ballpark but no one in my company can validate the results. I'd really appreciate it if someone with more knowledge and experience in this field would check what I've done.

Much appreciated

Dan

### RE: Choked Tank Depressurisation Calculation

(OP)
Hi guys some information to help understand what I've done. First I used the Colebrook equation to find the frictions factor using an assumed Reynolds number, I used the goal seek function in excel to solve this. using that and the goal seek again i solved the Fanno flow equation and found the Mach number at the inlet of the vent pipe. These functions are coded in a macro and linked to the run button. from the Mach number the flow rate through the vent pip can be found and using a user defined time step the the new tank pressure and temperature can be calculated (adiabatic assumption). then assuming the Reynolds number hasn't changed significantly the friction factor and Mach number can be assumed to stay reasonably constant for the majority of the depressurisation process. the new pressure and temperature can be solved for each time step using the trapezium rule to approximate the correct depressurisation profile. I know that the Fanno flow equations breakdown at lower pressures but I think that for the majority of the depressurisation process it will be under the Fanno flow regime.

Thanks again

Dan

### RE: Choked Tank Depressurisation Calculation

Sounds like the right approach, except the governing flow restriction is either the location of smallest flow area or one which has the most "oblique shock waves" , which occurs due to sudden changes in flow direction, as with many valves. The net effect of the oblique shock waves is contained in the ISA handbook of control valves method, factor Fl. The bottom line is the actual flow rate will be lower than that based on minimum flow area.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!