piping for superheated steam 2

[garfio]

I have 3 interrelated questions about superheated steam piping:

1) Good piping practice for saturated steam is to connect the branches on top of the headers. However, considering that in superheated steam lines there is not condensate, is this also a recommended practice or connections can be done at sides or bottom?

2) Is is necessary to install steam traps in superheated steam lines?

3) Can vertical loops be installed in a main superheated steam line?


I understand that condensate can be formed in a superheated steam line during startup, but it will be re-vaporized once superheated vapor starts flowing, so it may be that this will take care of the condensate problem in the above 3 cases.

I'll appreciate your opinion.

 

[rmw]

If your line is always going to be superheated and will never ever be anything but superheated, and will be well, no, let's say perfectly insulated, then you have no worries.

But I don't know how you will ever get it started up.

While you are right about the condensate re-evaporating, it can and most likely will be be entrained by the flow and transported downstream before it evaporates and do significant damage.

Design your piping correctly and you won't have to worry about it.

rmw

 

[garfio]

rmw, when yo say "design your piping correctly", what would be the correct design in the 3 specific issues I asked?

 

[katmar]

If an analysis of the heat losses during normal operation shows that the steam remains superheated for the full length of the line then you can design it as you would any other non-condensing gas line. But it all depends on what "normal operation" is in your case. Is the flow steady and continuous? How often do you have shut downs and start ups?

If your shuts are infrequent then it may not be safe to rely on a steam trap that has been sitting closed for many months. Can you be sure it will work again? It might be better to use manual drain valves for start up.

I would not rely on the superheat in the steam to re-evaporate any collected condensate and the line should be drained at start up either automatically with traps or manually via hand valves on the drip legs. During start up the line should be treated like a saturated steam line with all the normal precautions against water hammer and so on. In other words - start slowly.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[trtom59]

Since insulation is not 100% efficient, the steam will lose superheat over time & distance. You do not mention how much superheat is available at the start & the length of the distribution run.

In general, follow steam best practices and take off from the top of the pipe. Install drip pockets at suitable locations - changes in direction, particularly vertical up & down (takeoffs), end of mains, and periodically in long horizontal runs.

The steam traps should be sized for the running load, not the warm-up load. As stated in another reply, the start-up should be supervised and the cold condensate drained manually via the mud leg vales.

The steam traps can either be thermodynamic or bimetallic thermostatic, both of which will work on SH mains.

 

[nhliew]

If you consider steam flow at 10m/s, a 10 meter section pipeline filled with, say, 1 liter of water, needs approximately 2,500 kJ/s to heat up from ambient and vaporize (at 1 barg pressure). This needs to be done almost instantaneously upon contact. That's a lot of superheat. If you start slow, there isn't enough heat, bearing in mind that while water is being heated, the steam is also being cooled. If the heat transfer cannot happen fast enough, what the startup SH steam does when you open the main valve is push a growing slug of water along the pipeline. Result will be hammer.

Safe to say that some precautions should be taken for above reason, in addition to:
(i) dewatering after hydrotest;
(ii) dewatering after shutdown and getting ready for startup.

I would go for designing like saturated steam. The use of SH steam is for satisfying process conditions, not for pipeline designers to save money in traps or drains.

 

[ione]

Superheated steam can be regarded as a dry gas and so moisture is practically absent, in running conditions. What above allows higher velocities for superheated steam (40-60 m/s) if compared with saturated steam line (usually 25 m/s) since erosion problems related to water droplets are virtually absent. Anyway at start-up especially when dealing with large diameter pipes, condensate load can be an issue. Manual removal of the condensate is possible, as already noticed by Katmar, but steam trapping remains an option too.
I’d dare to add, even if it is not a normal condition, what if one should handle an emergency situation which asks for operation on saturated steam? Keeping in mind this scenario I’d consider steam trapping and I’d also foresee manifolds with the same configuration as that for saturated steam

 

[steamdog]

I have never seen a superheated steam line without traps and that was not piped as a saturated line. This is done NOT for normal operation but for upset/start-up conditions for safety sake. I worked with steam trap manufacturers whose basic idea is that the steam trap is not needed during normal operation (the drip legs actually create their own condensate) but in the rare upset condition so that the line is safe from catastrophic water hammer.

 

[athomas236]

Is the steam pipe supplying steam to a steam turbine. If so you may like to look at the requirements of ASME TDP1 Prevention of Water Ingress which I think requires steam traps even on superheated steam pipes.

Regards,

athomas236

 

[garfio]

All comments are very interesting, thanks. Any additional comment about the use of vertical loops? Will they be ok if drip legs and/or traps are included?

 

[MJCronin]

With regard to the question about vertical expansion loops, it is my experience that they are acceptable but not preferred.

They will divide a horizontal segment into multiple areas for drainage.

IMHO, they are also more difficult to support and less "aestetic: than nested horizontal loops.

My opinions only....

-MJC