Steam Out analysis

[frontino]

Can anyone help about the correct analysis of a steam traced line? according to the specification i have to use in my work i must use as fluid temperature also 75% of maximum operating temperature of the steam used to trace, but the important problem for me is: the line is empty or full, if there is the fluid this is subjected to pressure?
Thank you

 

[JohnBreen]

Hello Frontino,

For your steam out case the pipe is essentially empty and the temperature will be that of the steam inside the pipe during the steam out. The tracing hardware will likely be added after the steam out so it need not be included in the steam out analysis. The steam out will have the pipe filled with steam, not product. The weight of the steam will be neglible.

Your operating condition will be with the dead weight of the pipe, insulation, (perhaps support hardware) and tracing hardware and with the live weight of the product inside the pipe. All this will be analyzed in concert with the temperature being that of 0.75 times the temperature of the tracing steam.

If you are to analyze the system for the hydrostatic test case, you will model the supported piping system without insulation or tracing hardware but with the pipe filled with the hydrostatic test media (likely water and therefore likely heavier than product). For this case the temperature will be the ambient temperature of the surrounding air (the test should be done at temperatures below 50 degrees F.). An important issue for this analysis will be to determine if additional temporary supports will be needed for the hydrostatic test case.

Regards, John.

 

[JohnBreen]

I should add, if your steam out temperature is greater than your operating temperature you should look at the loadings transferred to the supports, restraints and terminating equipment (pumps, turbines, vessels) to evaluate the resulting loadings.

If you go to the COADE, Caesar II, discussion forum you can search on "steamout" and find discussions that raise other issure.

 

[frontino]

Thank you but I was thinking that steam out is the not-working condition for steam traced lines, is it correct? it another thing and in that case what is it? is it a test like Hydrotest done for steam traced lines?
I think that the important thing for an engineer are not first the parameters of the problem, but simple what is happening, nothing else. In fact I don't know what is really steam out condition , when this happens and how. Can you help me above all thing on this: steam out is a test or a critical condition?

anyway, thank you for the collaboration
Frontino

 

[JohnBreen]

Hello Frontino,

The steam out typically happens only once. It is usually done to clean the construction debris (welding rod ends, "buckshot", temporary paper welding gas dams and lunch buckets) out of the piping before it is first placed into service. There is also a benefit in that it will (sometimes) remove the mill scale and oil that is a product of the manufacture of the pipe. However, in some refineries, steam outs of certain existing piping systems will be done as a normal part of the start-up procedure after routine turnarounds (this would remove most of the loose, hardened residual product from the piping before the start-up and thus prevent solid object impacts within the piping on start-up). If so, this will be a mode of operation which must be evaluated carefully as the total number of steam outs will affect the total maximum temperature cycles and therefore will affect the theoretical fatigue life of the system.

I agree with your view that the steam out IS NOT really an operating condition. It is an "out-of-service" loading and that is why I detailed the list of loadings that should be included (and excluded) when evaluating the system during steam out. In some regards, the steam out is similar to the hydrostatic leak test. However, keep in mind that for some process systems the steam out procedure MAY result in pipe temperatures that will exceed the "design temperature". So, if the steam out temperature is higher that the operating temperature, the piping must be designed with sufficient flexibility to comply with the piping Code regarding maximum allowable stress range DURING THE STEAM OUT. While it is not exactly "a test" the piping system must be designed to accommodate the steam out temperature IF IT IS THE HIGHEST TEMPERATURE THE PIPING SYSTEM WILL EXPERIENCE (as this temperature will affect the system total temperature range). Also, keep in mind that the steam must exit the piping system during the steam out and some erection contractors will allow the steam to escape at an open end of the pipe. This will cause a dynamic "jet" force to be applied at the open end of the pipe. The piping must be securely restrained at the steam exit point to assure that the loadings on the pipe are not excessive.

In my first post on this thread I describe three conditions, the steam out event, the operating condition and the hydrostatic test. I would make three different Caesar II (C2) models for these three conditions.

I sincerely hope this will help you make good decisions in modeling your piping systems for analysis.

Regards, John.

 

[NozzleTwister]

More on Steam Out.

Pretty much all Refinery jobs that I've worked have had steam out requirements for all Hydrocarbon lines. The condition of 50 psig at 250 deg. F. is what I see most often but each refinery normally establishes their own conditions.

The steam out is done to rid the piping of hydrocarbons before opening a line for maintenance or welding.

Many clients allow pump nozzle allowable forces and moments to be increased by 50% for this non operating condition.


NozzleTwister
Houston, Texas

 

[mylexicon]

NozzleTwister & JohnBreen
First of all, I wanted to thank you guys for the informative replies you've posted. One question though; Don't they usually detach the piping from equipment (especially rotary types) nozzles and blind it before conducting steam out. This way, we won't have to worry about excessive external loads on equipment nozzles.

many thanks,

drno

 

[NozzleTwister]

mylexicon,

Many operators may in fact detach the piping from the equipment but some don't. I would never assume the equipment is detached.

Each operator has their own procedures and this is certainly something that should be addressed on a project and documented.

We have some clients that do note detach unless the loads or stresses become a problem. Loads used for steam-out should be approved by the client.

We have one client with high steam-out temps that does not detach but is willing to review and possibly approve detaching piping to a particular pump if the amount of additional piping flexibility required is unreasonable to meet the loads. They do allow a bump up from the API loads for steam-out. They also require the P&ID to be noted at the equipment that the line is to be disconnected for steam-out. Hopefully operations & maintenance reads this note before steam-out as it is part of that client’s procedure.

The bottom line is, do not assume anything and have the steam-out philosophy used approved by the client.


NozzleTwister
Houston, Texas

 

[dcv]

I agree with the statements made about the steam-out case for piping, and I can add one - lines subject to steam-out should be able to withstand full vacuum. This is in case the line gets filled with steam and then inadvertently shut-in, the steam eventually cools and collapses to water, pullng a hard vacuum in the process.

Also, re-reading the original post, the whole steam-out discussion is good but I don't think is what the question was about, and only one post touched on this. Lines that are steam traced (or glycol traced, electric traced, etc.) should be designed for the case where the line is shut in with the tracing on, and the line starts to heat up to something approaching the tracing medium temperature, like 75% of the steam tracing temperature.

Dave