Steaming Piping for Removal

[RJB32482]

We are looking at steaming a line that is used for normal pentane service. The line will be fully evacuated (drained) but we want to steam it to removor flammable vapours and liquid residue. Questions I have are:
-any method to calculate amount of time we need to steam the line? What info do I need (steam tables?)
-how would someone vent the line so it doesn't collapse under full vacuum (piping might be able to handle full vacuum)?
-hooking up to steam the line, need a valve to block off the steam connection?

Thanks

 

[Longinthetooth]

Generaly speaking, all process pressure piping (B31.3) under 12" diameter can handle full vacuum. You should steam the line and gas test at the vent until you are 20% of LEL. Maintain a steam velocity of 3 ft/sec. Be sure to check if the line was designed for "steam out conditions". If not, and it is a long line, you could pull the pipe shoes off the piperack or worse when the pipe expands to 100 DegC. An alternative would be to use nitrogen (no expansion or vacuum problems).

 

[RJB32482]

Thanks for the reply.

Any worries about draining the condensate that accumilates in the line? Or do you think the steam pressure (80 psig) will be able to push the condensate through the pipe line.

Problem with nitrogen is that it would not vaporize the pentane left in the line.

Why would the steam velocity need to be 3 ft/sec?

Thanks.

 

[Longinthetooth]

3 ft/sec or so will keep turbulent flow with a good scouring action to keep liquids moving. You will almost certainly end up with some condensate in the line. You can blow with warm compressed air after purging your pentanes.

We use warm nitrogen from a pumper truck eqipped with a heater (if we want to work on the line and then put it straight back into service).

 

[sshep]

Steaming a pentane line will be very easy to get hydrocarbon free. Generally this is prep work which can be done at night. A reasonable "in-plant" line can be steamed via a 1" steam hose for a few hours. A few hours before morning you can cut the steam and put under N2 purge to dry. My opinion is that 20% LEL at the vent is not good enough for cold cutting or hot work. With pentane you should be at zero shortly after steam appears at the end and that is what you need to be at when you sniff- anything else probably indicates a pocket of hydrocarbon residue somewhere that did not get benifit of the steam. Pockets can occure anywaywhere there was not a good steam flow, even high point branch lines (i.e. no high point vent usually means liquid won't drain well). Sniff in the pipe at a few low points, you can never be too careful with hot work.

The condensate created during initial warm up can be fairly significant in some cases, but your steam will keep it hot push it out of the way to the extent needed to get through. If you N2 dry then you can get the rest during dry out. Pressure and depressure at low points is the best way to dry, but operators rarely have the time for this so they usually leave it as a continuous blow which can take longer.

The two hazards with condensate are: 1) hot condensate can burn, so be sure to direct the discharge drain safely away rather than blowing wildly; 2) introducing large volume of uncontrolled steam into a cold line can create a long slug of fast moving condensate which can hammer and even rupture a pipe. The latter hazard is why you want to use a 1" steam hose, and warm the pipe up in a controlled manner. If you are steaming from a steam hose station which has a valve to control the steam flow, then you don't really need one at the pipe you are steaming.

These are just my opinions from general experience with preparing equipment, others may have different experience. best wishes as always, sshep

 

[MJCronin]

RJB....

"Geordie87" offers good advice on vacuum limitations, however, he was not specific enough in his limitations on piping.....

You do not have to worry about vacuum collapse of carbon steel or SS piping of at least 12"NPS and smaller when it is Schedule 40 or greater.

Schedule 10S and 5S piping systems are another matter, however


_MJC