Vent stack design input change

[Timetraveller007]

Hello Folks;

We have an existing standalone self-supported 30 in Dia x 40 ft length vertical stack with a pipe entrance at 45 degree to vent the natural gas to the atmosphere.

It is designed for 10 psig /180 F and supported by the foundation. The material is SA 516GR70N. The baseplate is 1.5 in thk.

Now client decided to increase the flow rate by adding more blowdown valves to the blowdown system.

With additional flow, we have found that we may see considerably higher thrust load due to venting applied to the bottom, and also we may see the dynamic load on the entrance nozzle.

We know that the flow won't be sonic at the stack tip so I don't imagine having any flow-induced vibration.

Now the question is that how we can assess the new flow and its effect on the mechanical integrity of the vent stack. How can we make sure the stack won't fail due to this additional blowdown rate and its additional forces and momentum.

Thank you for your help.

 

[JStephen]

First question, given the proposed flow rates, what will be the forces and vibration issues with the stack? That sounds like basically a fluid dynamics and piping problem, and I'd suggest the "Pipelines, Piping, and Fluid Mechanics engineering" subforum for that part of it.
Second question, once having gotten the forces and vibration effects, how should the existing stack be evaluated, analyzed, strengthened, etc. That question might fit in here. It would help to have some diagrams or photos showing the general arrangement of the elements. It would help if you have the original drawings, design data, etc., to work from.
In my ignorance, when I read "stack", I'm thinking puffy white smoke wafting into the atmosphere, but from the description, it's sounding more like a jet engine and you're about to crank the throttle up, so it's hard for me to visualize exactly what this is like.

 

[LittleInch]

I agree might be better off in the pipelines and piping forum, but you're here now and some of this is structural.

30" x ~5m high is a decent vent pipe but you need to let us know what sort of velocities you're looking at here. That isn't very high to me for such a big pipe so I can only assume its got a big unoccupied area around it.

Just because it's not sonic doesn't mean you won't get vibrations.

But we have no idea what this looks like so you need to send in a drawing, sketch or picture to figure out where it might need some more supports.

If this isn't wire guided, that might be a good place to start.

when you say "baseplate", do you mean metal baseplate or concrete pad?

Having difficulty seeing where the extra end force load is coming from if design pressure is 10 psi



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

 

[Timetraveller007]

JStephen:

This is a vent stack. Imagine it as a tubular column on a baseplate with a nozzle attached to the piping.
I know that the original one has been designed as per CICIND which is a design code for steel chimney and so they have not considered the thrust load due to venting.
The loads which have been considered are:

Overturning load due to wind
Weight
and snow load.

Per our calculation, another set of loads that act as oppose to venting direction will be transmitted to the stack bottom (baseplate).It can be angular (with 10 degrees lateral)

So my question is that can I simply deal with this load as I deal with weight? of course, I need to consider the lateral as well.
I guess before sending this tread I had to review the CICIND code. I think it is not applicable for our case because in vent stack we need to deal with dynamic loads (flow-induced) but in steel chimney, there is not any thrust load due to venting.

 

[georgeverghese]

Thumb rule for upper limit on well designed flare and vent piping systems is rho-V2 of approx 200e3 in SI units. Beyond that, further reinforcements required.
Other concerns would be increased radiation load due to accidental ignition and impact on sterile area radius around the vent stack; potential for low temp brittle fracture from cold blowdown streams, flow overload on vent KOD.