CONDENSATE AT START-UP OF SUPERHEATED 900# STEAM LINE

[FDS2008]

My fellow engineers,

I have an issue that is somewhat related to the thread "Condensate in a superheated steam line (#391-142624 (16-Dec-05)". No I don't want to recesitate this thread, but it relates to my problem slightly. Here goes......

We designed a steam downcomer (14" A-335 P11 Sch 80 for 850#-900°F operating steam) from a boiler nozzle at the top with a horizontal expansion loop at mid elevation and a drip pocket at the bottom after it elbows horizontally. The problem is we did not indicate (on drawings) a slope on the horizontal lines of the expansion loop to allow vertical thermal growth without introducing low points. There are no drains or drip pockets designed since our intention was to provide the require slope.

I know there will be condensate build at the low points of the expansion loop during start-up, but can we count on the superheated steam to evaporate the condenaste (without generating CAWH) or do I need to provide some type of manual drainage at the low points for the start-up condition. I am not worried about the operating condition since we have ample superheat to absorb any condensate.

The line is about to be pressure tested next week....so if anyone has any good advice or has experienced a similar scenario, I want to hear about it ASAP.

Thanks.

 

[MintJulep]

Drain.

 

[unclesyd]

We have a superheated steam system 600# @ 750F even where we have installed traps in case of a low flow situation. If any part of this line is down and cools off it is restarted using blowdown vents where the SOP calls for them to be opened for certain time and close in a certain sequence. The drains are in drip legs.

 

[metengr]

Without the above drip legs you will have perfect conditions for a waterhammer event that will shake the hell out of your piping system.

 

[unclesyd]

I might add to the post by metengr you will see what expansion loops are for.

i also should have added that the traps are on the drip legs.

 

[FDS2008]

Appreciate all your inputs.

After careful thought though, and the fact that I may have many restless nights worrying if it was done right, I am leaning towards requesting that the contractor modify the piping to provide the slope we require. It will cost several dollars but it will be done right (i.e., no low points at the expansion loop). The steam is supplied to a turbine and I don't want to take any chances as the velocities will be very high.

Thanks again and if you have any more thoughts by all means keep the thread going.