Posted by Tony Conner on August 16, 2001 at 12:03:55:
In Reply to: Re: A report from the other end of the steam line. posted by Balut on August 14, 2001 at 20:17:42:
The confined space aspect is certainly important. Having spent a lot of time down steam manholes, I would rate the double block & bleed quite a bit further down the list. Lot's of times, there's no room. Water hammer is the BIG concern. Most people know about sending a slug of water shooting down a steam line at 100 MPH. This is bad. Most people don't understand "condensation induced water hammer". This is another kind of water hammer, and it's usually WORSE than the first kind. The Jan/99 issue of HPAC (Heating/Piping/Air Conditioning) magazine had an investigation report on an accident at a military base in Alaska. Having ALL of the condensate drained out of steam lines before they go into service is critical. Relatively cold condensate can form waves in the steam line. The peaks of these waves can trap a steam pocket. Without a steam supply, this bubble collapses instantly in the cold water, pulling a vacuum. The condensate rushing in to fill the void can generate a HUGE pressure spike. I saw this happen years ago when a low pressure (10 PSIG) section of line was being valved in after the summer. The guy down the hole just barely (slowly & carefuly) cracked that 6" valve and KA-POW! Noboby was hurt, and there was no damage, but the noise and that line jumping around was very impressive. Not even a by-pass would have helped. If I hadn't been standing there at the time, I wouldn't have believed how much that line jumped. I knew it was water hammer, but at that time, I didn't fully understand WHY it happened. (In the years since, I've figured it out.) I have read articles on this phenomenon several times since, but the one in HPAC provides the best explaination of the ones I've seen.