Hello friends,

I think all of you know well the criteria of the velocity limit in pipes: The choked flow is unexpected. But I don't know the impacts of the choked flow to flare system. Even though the velocity of fluid is fixed at 1 Mach, we can evacuate well the flow with a higher back pressure. Because the mass flow rate is a fonction of three basic parameters as velocity (consant), density ( bigger with a higher pressure) and orifice area. Do the choked flow cause some more vibration of pipes? Could you tell me why we should limit the velocity and how to do it? Thank you in advance.

Regards,

Tony

People shy away from designing their piping for choked flow for a number of mechanical reasons. They are high pipe vibrations, noice and stress forces. These can be accounted for in the piping design by an experienced piping design engineer so I'm so big on eliminating choked flow is certain areas; one of which is in a PSV tail pipe. These are typically short and small in comparison to the overall header going to a flare. I might limit the flow in the header because of their relative size and length; it gets expensive to design this section.

To limit the velocity is actually quite simple. Calculate the flow based on choked conditions and choose the next one or two larger pipe sizes.

I think it's not so easy as choosing the next one or two larger pipes sizes. Because you might cause the choked flow in upstream pipes. The problom isn't solved but displaced. Do you have some experience in such case?

Regards,

Tony

As I pointed out, sizing for choked flow is not an issue with me and yes, I've done it for many projects (I've worked at E&Cs for most of my career).

Per your original post, you asked about vibration and I gave you an answer; design for it. You asked about limiting velocity and I answered that too. Again, to limit velocity you are faced with fighting physics. If I have a set volumetric flow then the only way to change velocity is to change the pipe size. If you want to change the volumetric flow then either change the mass flow or change the conditions (pressure, density, etc.).

These are generalities becasue we do not have the specifics on the system you are concerned about. Perhaps if you give us detailed information on your system we can come up with some specifics. But if you are asking in a general manner, the answers you receive will only be general.

Tony,
Phil has pointed out that sizing header for choked flow is not an issue and he has done it for many projects. I would believe Phil's statement is TRUE. I have used SONIC flare tip for numbers of project. The tip is just PIPE. Nothing more than that. It has been designed to create SONIC flow. I believe the design has serious consideration on FIV, AIV, dynaimic stress analysis, noise, welding type, procedures, etc. I have not seen one failed.

However, i personally have not designed choke flow in header before and dare to this now with the limited experience that i have . Reason being there are many binding conditions behind success. We have to ensure the pipe design passing all criterias (FIV, AIV, Dynamic stress), noise (certainly the noise is tremendous and acoustic insulation is required in some part of header) welding is 100% good, 100% X-ray passed, etc. Human factor, working environment, stringent QA requirement, etc have to be taken into account. That's one of the main reason most of the COMPANY e.g.SHELL, DOW, EXXONMOBIL, etc limit it to MACH no of 0.5-0.8. Those all these factors has caused me take a conservative approach (of course always expensive).



Phil's statement is reference to TAIL pipe. Those increase by one or two size bigger will definitely get system away from MACH NO criteria.

You are also certainly right on the second part of the statement. Choked flow occured at MAIN HEADER (or SUBHEADER), increase the CHOKED header size will result lower pressure drop on the line and back pressure to the upstream subheader and tail pipes. The may end-up more upstream pipes/headers experience choke flow. you have to do the same for the usptream untill all pipe not experience choke flow. This could results an EXPENSIVE design as Phil mentioned in earlier post.

JoeWong

Hi,

I was just referring your choked flow problem, and thought i also facing similar problem,

Well in our case we are looking for evacuation system to -

evacuate 2500 Kg of Steam from 200 m3 of vessel from 0.1 Bar-G 104'C to final pressure of 0.9 bar -A ( vacuum ) at vessel with in 10 min.



Consider 0.15 Bar Pr.Drop across Vacuum producing equipment and our vessel ( between pipes /valves etcc )

Air Leakage expected is - 50 Kg/hr

We have no idea how to size the capacity for this vacuum system & piping,

Can anyone share his experience for sizing such vacuum system,

we need to know- Capacity of Vacuum Pump or Capacity of Steam ejector for this case
- In between we are expecting to put Pre condenser, Then how to decide / Calculate C.W Requirement.