Pipe Stress Expansion Joint vs Flex Hose 2

[Doiscond]

Hello All,

We are looking at a process line from the bottom of a pressure vessel to the suction side of a PD pump. Here's a snapshot of the stress model. 20" OD vessel, 6" piping, reduces down to 5" pump nozzle. You can see the vessel at the top anchored from it's support lugs, and the PD pump is modeled as a rigid element at the bottom. Looking at the suction flange on the PD pump, I have used API 676 allowable nozzle loads to evaluate whether the system is acceptable. I've got the whole length between the two yellow highlighted flanges to use for any kind of in-line flexibility element.

Operating Temp. - 415 deg. F
Operating Pressure - 0.1 PSIA
Design Temp. - 415 deg. F
Design Pressure - 150 PSIG

[li]A Tied Expansion Joint doesn't allow for axial thermal displacement and will overload the vessel and pump.[/li]
[li]An Untied Expansion Joint will create a pressure thrust in the design case that will overload the vessel and pump. I will see if the design pressure actually needs to be this high for this line, I suspect it's just from the fact that is a Class 150# system.[/li]
[li]A Pressure Balanced Joint should work, but I'd rather avoid a more complex piece of equipment than needed if possible.[/li]

Could a flex hose be used in this application if it can handle the design/operating temperatures, pressures, and displacements?

My main reason for asking this question is that it seems like a flex hose could be used sometimes when expansion joints are called out, but I'm not aware of all the possibilities of when to use one versus the other.

If anyone could provide more insight or resources into how to decide between expansion joints or Flex Hoses, I'd greatly appreciate the knowledge share.

Thanks!

 

[GBTorpenhow]

Just like an expansion joint with tie rods, flex hose is not meant to be flexible axially. The braid serves the same function as the tie rods and restrains the pressure thrust. An expansion loop style pair of flex hoses with two ells and a 180 return may be an option for you, or introduce and offset to get a horizontal run.

 

[LittleInch]

Is that 415F or -415F ??

But agree with GBT - a hose won't do you any good here.

But you can't have flexibility / expansion and no pressure thrust I don't think. But class 150 isn't 150 psi, although the flange P-T rating might be so that's your first port of call for what seems to be a vacuum system....



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

 

[Doiscond]

@GBTorpenhow

I should've done a little more research on flex hoses before posting, and I could've seen that parts of my question have been answered in old threads or manufacturer websites.

Thanks for the response though, that definitely helps narrow down the options moving forward!

 

[Doiscond]

@LittleInch

It's +415 deg F, I should've dropped the - to make things easier.

It's a group 2.2 material from asme 16.5 so at the 415 F design temperature, the working pressure is higher than 150 psig. 150 is the stated design pressure so that's the max we'll look at for stress. They will do steam outs in this process, so we do anticipate some positive pressures in the system even though it's operating in a vacuum typically.

 

[StressGuy]

You really need to have a conversation with your project team and client folks about the conditions for this system. An untied joint is what you need here, but not with a fake 150psig design condition.

People think that setting the design pressure equal to the flange rating is free. It's never free and in a case like this pump suction, completely unreasonable. As the others have mentioned, if the team really wants a 150psig design, then you are looking at the expense of an in-line pressure balanced joint, or something like the "flex hose expansion loop" like Flexicraft makes:

https://www.flexicraft.com/ML_Loops

Which again, is overkill realistically, but would at least employ hoses in a manner that makes use of their flexibility without causing the braiding to disengage.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[LittleInch]

" working pressure is higher than the design pressure"

Errr, don't think so.

150 psi seems very high for what seems to be avacuum system....

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

 

[Andrew ONeill]

Coming from an Operating Plant background, I'd suggest you do everything you can to avoid using flexible joints, they are a maintenance nightmare. I'd suggest look at some form of expansion loop / leg in the pipework, before defaulting to a flexible joint.



Andrew O'Neill
Specialist Mechanical Engineer
Australia