5 replies to this topic

Hi,

I'm a final year master's student doing a study on relief valve tailpipe design for my final year project. The system which the relief valve is fitted to is a high pressure breathing air gas cylinder (300 bar) on board a large ship and it vents to atmosphere via piping which can be between 30 and 100 metres long and include bends and changes in section. I am looking for any advice on how to go about this study... From what I have read etc so far I think that when overpressure occurs and the relief valve opens the gas will flow through the tailpipe and increase in velocity until the flow is choked. This choked flow will create large resultant forces against the inner walls of the piping and it is the magnitude of this force that I should be trying to work out in order to determine pipe diameters/mountings requirements etc. Does this sound like I am along the correct tracks? Also I am working under the assumption that the gas involved is compressible and unsteady and therefore I will have to use isothermal gas laws rather than bernoullis? Any suggestions on literature/resources etc would be gratefully appreciated since I am a little lost with this project so far!

Cheers.

[tslim]

Hi,

I'm a final year master's student doing a study on relief valve tailpipe design for my final year project. The system which the relief valve is fitted to is a high pressure breathing air gas cylinder (300 bar) on board a large ship and it vents to atmosphere via piping which can be between 30 and 100 metres long and include bends and changes in section. I am looking for any advice on how to go about this study... From what I have read etc so far I think that when overpressure occurs and the relief valve opens the gas will flow through the tailpipe and increase in velocity until the flow is choked. This choked flow will create large resultant forces against the inner walls of the piping and it is the magnitude of this force that I should be trying to work out in order to determine pipe diameters/mountings requirements etc. Does this sound like I am along the correct tracks? Also I am working under the assumption that the gas involved is compressible and unsteady and therefore I will have to use isothermal gas laws rather than bernoullis? Any suggestions on literature/resources etc would be gratefully appreciated since I am a little lost with this project so far!

Cheers.

Hi,

I would suggest some reading as below:
- perry's handbook (process safety)
- API 521

The tail pipe result with the back pressure could be factored in as a correction factor in the relief valve sizing. good luck!

[sandipanlaskar]

Hi,

I'm a final year master's student doing a study on relief valve tailpipe design for my final year project. The system which the relief valve is fitted to is a high pressure breathing air gas cylinder (300 bar) on board a large ship and it vents to atmosphere via piping which can be between 30 and 100 metres long and include bends and changes in section. I am looking for any advice on how to go about this study... From what I have read etc so far I think that when overpressure occurs and the relief valve opens the gas will flow through the tailpipe and increase in velocity until the flow is choked. This choked flow will create large resultant forces against the inner walls of the piping and it is the magnitude of this force that I should be trying to work out in order to determine pipe diameters/mountings requirements etc. Does this sound like I am along the correct tracks? Also I am working under the assumption that the gas involved is compressible and unsteady and therefore I will have to use isothermal gas laws rather than bernoullis? Any suggestions on literature/resources etc would be gratefully appreciated since I am a little lost with this project so far!

Cheers.

Hi,

I would suggest some reading as below:
- perry's handbook (process safety)
- API 521

The tail pipe result with the back pressure could be factored in as a correction factor in the relief valve sizing. good luck!

Hi!
I would recommend you to first read

API 520: Part I & II this deals with the sizing, selection and installation of the relief devices
API 521: is more deals with the overpressure scenario hence that wont be helpful with you area of research.

First thing that you have to consider is that what type of relief valve you are considering ( conventional, balanced bellows, pilot operated). Since while operating or even designing a relief system, back pressure plays an important role and there is certain back pressure acceptable limit which depends on the type of valve ( namely 10% for a conventional, 30-50% for balanced bellows and 75% for pilot operated type valves). Back pressure will dictate amount of flow through the relief valve.

Other important factor is the equivalent length of the outlet piping so what I would suggest you to do is to clearly define your problem since from the question above what I understood is that you are trying to define the magnitude of the force whereas that force is a variable entity and it depends on the many factors. I could help you more with your problem if you could enunciate your thoughts a little more.

Hi,

Thanks for your reply... I realise I wasn't very specific with my first post to say the least!

I have read all the relevant API Standards you listed and also ASME b31.3 and have had a quick look through Crane's Technical Paper 410, so I understand that aspect of the problem quite well (I hope).

However, I realise now that the company I am carrying out this study for are more interested in what happens downstream of the relief valve, and I believe that there exists some kind of pressure transient or oscillating pressure surges due to the choking (kind of like water hammering), and it is the magnitudes of the forces generated by this that my study aims to find. I think that that is the difficult part and from there on its a standard 'calculate thickness of piping for given internal pressure'... with pressure loss due to friction and bends etc along the way.

As you can no doubt tell it is still very early days in this project for me...

[sheiko]

Hi,

Thanks for your reply... I realise I wasn't very specific with my first post to say the least!

I have read all the relevant API Standards you listed and also ASME b31.3 and have had a quick look through Crane's Technical Paper 410, so I understand that aspect of the problem quite well (I hope).

However, I realise now that the company I am carrying out this study for are more interested in what happens downstream of the relief valve, and I believe that there exists some kind of pressure transient or oscillating pressure surges due to the choking (kind of like water hammering), and it is the magnitudes of the forces generated by this that my study aims to find. I think that that is the difficult part and from there on its a standard 'calculate thickness of piping for given internal pressure'... with pressure loss due to friction and bends etc along the way.

As you can no doubt tell it is still very early days in this project for me...

Hope the following will be useful:

http://www.apcmedia....Y6TEA_R0_EN.pdf

http://www.tycovalve... Sem Manual.pdf

Rgds

[katmar]

You have not been at all clear about whether you are trying to determine the required diameter of the tail pipe, or if you are concerned about how to safely bracket the pipe. These are two very different problems.

The flow through the pipe will not necessarily be choked. Your flowrate is fixed by the size of the relief valve you have and the pressure in the cylinder (and the assumed allowable back pressure). You say you have Crane 410 - this is the right tool to use to calculate the required pipe size. Since you know the flowrate, all you do is try progressively larger standard pipe sizes until the calculated pressure drop is less than the allowable back pressure. Theoretically, an adiabatic model will probably be closer to the truth than isothermal. But isothermal is easier to calculate and will give a slightly conservative result (i.e. higher pressure drop).